<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e8fff-af38-7cdc-b430-ef714551b05b,g:fcf0a67c-6bab-4e85-a416-9bc4601066ed-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.arq.com/role/CondensedConsolidatedStatementsofChangesinStockholdersEquity" xlink:type="simple" xlink:href="arq-20260630.xsd#CondensedConsolidatedStatementsofChangesinStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/CondensedConsolidatedStatementsofChangesinStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e8fff-b090-7a1d-b59c-31fa56f3c7e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e8fff-b090-787e-84b1-540ed09d1e43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e8fff-b090-7a1d-b59c-31fa56f3c7e2" xlink:to="loc_us-gaap_StatementTable_019e8fff-b090-787e-84b1-540ed09d1e43" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e8fff-b090-7efb-b53f-98d021cf1422" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e8fff-b090-787e-84b1-540ed09d1e43" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e8fff-b090-7efb-b53f-98d021cf1422" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7efb-b53f-98d021cf1422_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e8fff-b090-7efb-b53f-98d021cf1422" xlink:to="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7efb-b53f-98d021cf1422_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7695-862d-b7b41bc0a06c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e8fff-b090-7efb-b53f-98d021cf1422" xlink:to="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7695-862d-b7b41bc0a06c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e8fff-b090-72a9-b696-cf130445e8b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7695-862d-b7b41bc0a06c" xlink:to="loc_us-gaap_CommonStockMember_019e8fff-b090-72a9-b696-cf130445e8b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019e8fff-b090-7bb8-bad4-2900a4412744" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7695-862d-b7b41bc0a06c" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019e8fff-b090-7bb8-bad4-2900a4412744" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e8fff-b090-7214-8cdf-1940ad077707" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7695-862d-b7b41bc0a06c" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e8fff-b090-7214-8cdf-1940ad077707" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e8fff-b090-7911-82fa-209d75e8773f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e8fff-b090-7695-862d-b7b41bc0a06c" xlink:to="loc_us-gaap_RetainedEarningsMember_019e8fff-b090-7911-82fa-209d75e8773f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e8fff-b090-7a1d-b59c-31fa56f3c7e2" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e8fff-b090-7ad7-841b-3d2abb540eb3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_SharesOutstanding_019e8fff-b090-7ad7-841b-3d2abb540eb3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e8fff-b090-7c01-a1a5-2084c8c4fffb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_StockholdersEquity_019e8fff-b090-7c01-a1a5-2084c8c4fffb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019e8fff-b090-7995-a1e6-1e2eb74d88d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_TreasuryStockCommonShares_019e8fff-b090-7995-a1e6-1e2eb74d88d0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e928e-8564-7e8b-a91d-6ee1f7dd1c73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e928e-8564-7e8b-a91d-6ee1f7dd1c73" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e928e-8564-7715-948b-666d2130eb46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e928e-8564-7715-948b-666d2130eb46" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e928e-8564-7b2f-b409-e0a76684632f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e928e-8564-7b2f-b409-e0a76684632f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e928e-8564-7380-a2eb-b820c75db4c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e928e-8564-7380-a2eb-b820c75db4c3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e928e-8564-7169-b06b-b5e57b09d034" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e8fff-b090-747e-83de-37604dfec2a3" xlink:to="loc_us-gaap_NetIncomeLoss_019e928e-8564-7169-b06b-b5e57b09d034" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e8fff-b090-7692-a079-3d6fe1c9e2d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e8fff-b090-799f-b4cb-974012e3115b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019e8fff-b090-750c-ba8f-376fe8df4481" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonShares"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/RevenueNarrativeDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#RevenueNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/RevenueNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019fd8da-3801-7770-8b05-f1934858c2d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e8fff-b090-722e-a103-402833e1d83c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019fd8da-3801-7770-8b05-f1934858c2d9" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e8fff-b090-722e-a103-402833e1d83c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019fd8da-3801-743b-8617-a16d850b25d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e8fff-b090-722e-a103-402833e1d83c" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019fd8da-3801-743b-8617-a16d850b25d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019fd8da-3801-743b-8617-a16d850b25d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019fd8da-3801-743b-8617-a16d850b25d4" xlink:to="loc_us-gaap_ReceivableTypeDomain_019fd8da-3801-743b-8617-a16d850b25d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019fd8da-3801-7a35-867d-75fb01dcc482" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019fd8da-3801-743b-8617-a16d850b25d4" xlink:to="loc_us-gaap_ReceivableTypeDomain_019fd8da-3801-7a35-867d-75fb01dcc482" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeAccountsReceivableMember_019fd8da-3801-7ddb-acfa-1d383610b160" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TradeAccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019fd8da-3801-7a35-867d-75fb01dcc482" xlink:to="loc_us-gaap_TradeAccountsReceivableMember_019fd8da-3801-7ddb-acfa-1d383610b160" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019fd8da-3801-7771-b4a1-b6eb51642258" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019fd8da-3801-7770-8b05-f1934858c2d9" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019fd8da-3801-7771-b4a1-b6eb51642258" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherReceivablesNetCurrent_019fd8da-3801-7e8f-8125-5046f66d1233" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherReceivablesNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019fd8da-3801-7770-8b05-f1934858c2d9" xlink:to="loc_us-gaap_OtherReceivablesNetCurrent_019fd8da-3801-7e8f-8125-5046f66d1233" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/RevenueScheduleofAccountsReceivablesDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#RevenueScheduleofAccountsReceivablesDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/RevenueScheduleofAccountsReceivablesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e8fff-b090-7f95-9403-504683e56d32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesAndLoansReceivableLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019e8fff-b090-7226-b82e-412cf93ae678" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e8fff-b090-7f95-9403-504683e56d32" xlink:to="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019e8fff-b090-7226-b82e-412cf93ae678" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e8fff-b090-75be-a4fd-f96b787f1d43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019e8fff-b090-7226-b82e-412cf93ae678" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e8fff-b090-75be-a4fd-f96b787f1d43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019e8fff-b090-75be-a4fd-f96b787f1d43_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e8fff-b090-75be-a4fd-f96b787f1d43" xlink:to="loc_us-gaap_ReceivableTypeDomain_019e8fff-b090-75be-a4fd-f96b787f1d43_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019e8fff-b090-7404-9349-8f5c42b50898" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019e8fff-b090-75be-a4fd-f96b787f1d43" xlink:to="loc_us-gaap_ReceivableTypeDomain_019e8fff-b090-7404-9349-8f5c42b50898" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeAccountsReceivableMember_019e8fff-b090-7e4c-9084-28c08f4eca77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TradeAccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019e8fff-b090-7404-9349-8f5c42b50898" xlink:to="loc_us-gaap_TradeAccountsReceivableMember_019e8fff-b090-7e4c-9084-28c08f4eca77" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019e8fff-b090-752c-923e-0355572115b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e8fff-b090-7f95-9403-504683e56d32" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019e8fff-b090-752c-923e-0355572115b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherReceivablesNetCurrent_019e8fff-b090-7187-8d67-244f104923dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherReceivablesNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e8fff-b090-7f95-9403-504683e56d32" xlink:to="loc_us-gaap_OtherReceivablesNetCurrent_019e8fff-b090-7187-8d67-244f104923dc" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/DebtObligationsScheduleofDebtDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#DebtObligationsScheduleofDebtDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/DebtObligationsScheduleofDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-78b8-9d1d-9920710d1e17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-78b8-9d1d-9920710d1e17" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-75c3-b707-ea08c6e98464" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-78b8-9d1d-9920710d1e17" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-75c3-b707-ea08c6e98464" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-75c3-b707-ea08c6e98464_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-75c3-b707-ea08c6e98464" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-75c3-b707-ea08c6e98464_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-73c5-b474-62afa8f33b5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-75c3-b707-ea08c6e98464" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-73c5-b474-62afa8f33b5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_SecuredRevolvingCreditFacilityMember_019e8fff-b090-76c6-b369-f1a497d0f130" xlink:href="arq-20260630.xsd#arq_SecuredRevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-73c5-b474-62afa8f33b5b" xlink:to="loc_arq_SecuredRevolvingCreditFacilityMember_019e8fff-b090-76c6-b369-f1a497d0f130" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_CTBLoanDueJanuary2036Member_019e8fff-b090-783a-8a21-77f795073ebd" xlink:href="arq-20260630.xsd#arq_CTBLoanDueJanuary2036Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-73c5-b474-62afa8f33b5b" xlink:to="loc_arq_CTBLoanDueJanuary2036Member_019e8fff-b090-783a-8a21-77f795073ebd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e8fff-b090-7290-ba66-daaf69f73cb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e8fff-b090-7290-ba66-daaf69f73cb2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_019e8fff-b090-75ef-be88-adb984b2682e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_FinanceLeaseLiabilityStatementOfFinancialPositionExtensibleList_019e8fff-b090-75ef-be88-adb984b2682e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_019e8fff-b090-77ae-912c-c515f73ec0d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_FinanceLeaseLiability_019e8fff-b090-77ae-912c-c515f73ec0d4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLongTermDebt_019e8fff-b090-7fd9-955f-ed597bf03919" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_OtherLongTermDebt_019e8fff-b090-7fd9-955f-ed597bf03919" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e8fff-b090-71cc-abb6-5f87b35b4606" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_LongTermDebt_019e8fff-b090-71cc-abb6-5f87b35b4606" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e8fff-b090-7293-b8e0-14a4466f4982" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e8fff-b090-7293-b8e0-14a4466f4982" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnamortizedDebtIssuanceExpense_019e8fff-b090-756d-a346-fdca51a08100" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnamortizedDebtIssuanceExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_UnamortizedDebtIssuanceExpense_019e8fff-b090-756d-a346-fdca51a08100" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019e8fff-b090-701c-a862-9d9983274fe2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_019e8fff-b090-701c-a862-9d9983274fe2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019e8fff-b090-7df4-bee8-1be55203e35d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_LongTermDebtCurrent_019e8fff-b090-7df4-bee8-1be55203e35d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019e8fff-b090-739f-bed9-e91319db8f7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-7aac-aa34-1dca390885d9" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019e8fff-b090-739f-bed9-e91319db8f7d" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/DebtObligationsNarrativeDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#DebtObligationsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/DebtObligationsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e8fff-b090-7f08-81bf-0f48af8dcf21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:to="loc_us-gaap_CreditFacilityAxis_019e8fff-b090-7f08-81bf-0f48af8dcf21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e8fff-b090-7f08-81bf-0f48af8dcf21_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e8fff-b090-7f08-81bf-0f48af8dcf21" xlink:to="loc_us-gaap_CreditFacilityDomain_019e8fff-b090-7f08-81bf-0f48af8dcf21_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e8fff-b090-7b1e-95c8-2e121635a19b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e8fff-b090-7f08-81bf-0f48af8dcf21" xlink:to="loc_us-gaap_CreditFacilityDomain_019e8fff-b090-7b1e-95c8-2e121635a19b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e8fff-b090-7603-830f-732d7beed5c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e8fff-b090-7b1e-95c8-2e121635a19b" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e8fff-b090-7603-830f-732d7beed5c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b090-742e-8bd1-b25de2d19c87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b090-742e-8bd1-b25de2d19c87" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b090-742e-8bd1-b25de2d19c87_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b090-742e-8bd1-b25de2d19c87" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b090-742e-8bd1-b25de2d19c87_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b090-7305-94f0-9b99995dff52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b090-742e-8bd1-b25de2d19c87" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b090-7305-94f0-9b99995dff52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019e8fff-b090-7faa-9748-cc7c5240d045" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b090-7305-94f0-9b99995dff52" xlink:to="loc_us-gaap_SecuredDebtMember_019e8fff-b090-7faa-9748-cc7c5240d045" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019e8fff-b090-7eb5-9cb4-0d8951eaea51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:to="loc_us-gaap_VariableRateAxis_019e8fff-b090-7eb5-9cb4-0d8951eaea51" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019e8fff-b090-7eb5-9cb4-0d8951eaea51_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019e8fff-b090-7eb5-9cb4-0d8951eaea51" xlink:to="loc_us-gaap_VariableRateDomain_019e8fff-b090-7eb5-9cb4-0d8951eaea51_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019e8fff-b090-7e04-b16c-38cdd4d866eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019e8fff-b090-7eb5-9cb4-0d8951eaea51" xlink:to="loc_us-gaap_VariableRateDomain_019e8fff-b090-7e04-b16c-38cdd4d866eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019e8fff-b090-7eb4-808e-738d2e26a21d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019e8fff-b090-7e04-b16c-38cdd4d866eb" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019e8fff-b090-7eb4-808e-738d2e26a21d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrimeRateMember_019e8fff-b090-798a-8647-eaacd0a754d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrimeRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019e8fff-b090-7e04-b16c-38cdd4d866eb" xlink:to="loc_us-gaap_PrimeRateMember_019e8fff-b090-798a-8647-eaacd0a754d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TriggeringEventAxis_019e8fff-b090-779d-b97f-dd0a5deb64cf" xlink:href="arq-20260630.xsd#arq_TriggeringEventAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:to="loc_arq_TriggeringEventAxis_019e8fff-b090-779d-b97f-dd0a5deb64cf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TriggeringEventDomain_019e8fff-b090-779d-b97f-dd0a5deb64cf_default" xlink:href="arq-20260630.xsd#arq_TriggeringEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_arq_TriggeringEventAxis_019e8fff-b090-779d-b97f-dd0a5deb64cf" xlink:to="loc_arq_TriggeringEventDomain_019e8fff-b090-779d-b97f-dd0a5deb64cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:href="arq-20260630.xsd#arq_TriggeringEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_arq_TriggeringEventAxis_019e8fff-b090-779d-b97f-dd0a5deb64cf" xlink:to="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TrancheYearOneMember_019e8fff-b090-73d8-a4e0-040ee6f110db" xlink:href="arq-20260630.xsd#arq_TrancheYearOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:to="loc_arq_TrancheYearOneMember_019e8fff-b090-73d8-a4e0-040ee6f110db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TrancheYearTwoMember_019e8fff-b090-7468-ac2a-1b2909302317" xlink:href="arq-20260630.xsd#arq_TrancheYearTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:to="loc_arq_TrancheYearTwoMember_019e8fff-b090-7468-ac2a-1b2909302317" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TrancheYearThreeMember_019e8fff-b090-7ffb-b4c7-31db8cbc0c02" xlink:href="arq-20260630.xsd#arq_TrancheYearThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:to="loc_arq_TrancheYearThreeMember_019e8fff-b090-7ffb-b4c7-31db8cbc0c02" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TrancheYearThereafterMember_019e8fff-b090-719e-9e40-d16d580651ec" xlink:href="arq-20260630.xsd#arq_TrancheYearThereafterMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:to="loc_arq_TrancheYearThereafterMember_019e8fff-b090-719e-9e40-d16d580651ec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_ThroughJanuary2026Member_019e8fff-b090-74b4-a885-4dae13ab8957" xlink:href="arq-20260630.xsd#arq_ThroughJanuary2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:to="loc_arq_ThroughJanuary2026Member_019e8fff-b090-74b4-a885-4dae13ab8957" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_AfterJanuary2026Member_019e8fff-b090-77ea-bf06-36c99e58c6e8" xlink:href="arq-20260630.xsd#arq_AfterJanuary2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arq_TriggeringEventDomain_019e8fff-b090-7294-a600-a1797b9ecd94" xlink:to="loc_arq_AfterJanuary2026Member_019e8fff-b090-77ea-bf06-36c99e58c6e8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019e8fff-b090-7a93-a5e0-389b7ebe0a7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019e8fff-b090-7a93-a5e0-389b7ebe0a7a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e8fff-b090-7a93-a5e0-389b7ebe0a7a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e8fff-b090-7a93-a5e0-389b7ebe0a7a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e8fff-b090-7a93-a5e0-389b7ebe0a7a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e8fff-b090-70d6-ac99-a69cf89aeb57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e8fff-b090-7a93-a5e0-389b7ebe0a7a" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e8fff-b090-70d6-ac99-a69cf89aeb57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_ArqLimitedMember_019e8fff-b090-7434-985d-652e09da6327" xlink:href="arq-20260630.xsd#arq_ArqLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e8fff-b090-70d6-ac99-a69cf89aeb57" xlink:to="loc_arq_ArqLimitedMember_019e8fff-b090-7434-985d-652e09da6327" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-7e4a-b931-8ec139401d74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-7e4a-b931-8ec139401d74" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-7e4a-b931-8ec139401d74_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-7e4a-b931-8ec139401d74" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-7e4a-b931-8ec139401d74_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-77b5-bfda-196cd7aeab29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e8fff-b090-7e4a-b931-8ec139401d74" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-77b5-bfda-196cd7aeab29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_CTBLoanMember_019e8fff-b090-7132-abbe-71ad2af59479" xlink:href="arq-20260630.xsd#arq_CTBLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-77b5-bfda-196cd7aeab29" xlink:to="loc_arq_CTBLoanMember_019e8fff-b090-7132-abbe-71ad2af59479" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_CFGLoanMember_019e8fff-b090-7382-9ec0-36ca0ad15286" xlink:href="arq-20260630.xsd#arq_CFGLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-77b5-bfda-196cd7aeab29" xlink:to="loc_arq_CFGLoanMember_019e8fff-b090-7382-9ec0-36ca0ad15286" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_PromissoryNoteAMember_019e8fff-b090-776c-b48e-c9400632ad0d" xlink:href="arq-20260630.xsd#arq_PromissoryNoteAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-77b5-bfda-196cd7aeab29" xlink:to="loc_arq_PromissoryNoteAMember_019e8fff-b090-776c-b48e-c9400632ad0d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_PromissoryNoteBMember_019e8fff-b090-7130-88a9-254eb6db12d1" xlink:href="arq-20260630.xsd#arq_PromissoryNoteBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e8fff-b090-77b5-bfda-196cd7aeab29" xlink:to="loc_arq_PromissoryNoteBMember_019e8fff-b090-7130-88a9-254eb6db12d1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019e8fff-b090-7acb-8817-fde2049bed63" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b090-77ad-89a5-466e12d5569c" xlink:to="loc_srt_StatementScenarioAxis_019e8fff-b090-7acb-8817-fde2049bed63" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019e8fff-b090-7acb-8817-fde2049bed63_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019e8fff-b090-7acb-8817-fde2049bed63" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019e8fff-b090-7acb-8817-fde2049bed63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019e8fff-b090-7efa-acca-9ed0de375aa4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019e8fff-b090-7acb-8817-fde2049bed63" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019e8fff-b090-7efa-acca-9ed0de375aa4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_019e8fff-b090-757d-9eba-377ce5fb8e5e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_019e8fff-b090-7efa-acca-9ed0de375aa4" xlink:to="loc_srt_ScenarioForecastMember_019e8fff-b090-757d-9eba-377ce5fb8e5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019e8fff-b090-7011-8b89-238ad0b0ad54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentTerm_019e8fff-b090-7011-8b89-238ad0b0ad54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e8fff-b090-7b52-b5c5-4ec985d5d0b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e8fff-b090-7b52-b5c5-4ec985d5d0b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e8fff-b090-7631-ab40-bc7adf11edfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e8fff-b090-7631-ab40-bc7adf11edfe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_DebtInstrumentBasisSpreadOnVariableRateFloor_019e8fff-b090-7e38-a783-9e07fd468098" xlink:href="arq-20260630.xsd#arq_DebtInstrumentBasisSpreadOnVariableRateFloor"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_arq_DebtInstrumentBasisSpreadOnVariableRateFloor_019e8fff-b090-7e38-a783-9e07fd468098" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e8fff-b090-7c7f-a68e-58b3e7560bc8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e8fff-b090-7c7f-a68e-58b3e7560bc8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_LineOfCreditFacilityPercentageOfCollateralManagementFeeOfOutstandingBalance_019e8fff-b090-770d-b232-7da17d4f8fa8" xlink:href="arq-20260630.xsd#arq_LineOfCreditFacilityPercentageOfCollateralManagementFeeOfOutstandingBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_arq_LineOfCreditFacilityPercentageOfCollateralManagementFeeOfOutstandingBalance_019e8fff-b090-770d-b232-7da17d4f8fa8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_LineOfCreditFacilityPreclosureFeePercentage_019e8fff-b090-71d1-b709-d8027e3e3a4c" xlink:href="arq-20260630.xsd#arq_LineOfCreditFacilityPreclosureFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_arq_LineOfCreditFacilityPreclosureFeePercentage_019e8fff-b090-71d1-b709-d8027e3e3a4c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_DebtCovenantMinimumCashBalanceRequirement_019e8fff-b090-76ec-9511-3f6abb9b2ed8" xlink:href="arq-20260630.xsd#arq_DebtCovenantMinimumCashBalanceRequirement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_arq_DebtCovenantMinimumCashBalanceRequirement_019e8fff-b090-76ec-9511-3f6abb9b2ed8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e8fff-b090-772a-b840-129677b1bab4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e8fff-b090-772a-b840-129677b1bab4" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e8fff-b090-76f5-890e-4ea407e503ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e8fff-b090-76f5-890e-4ea407e503ad" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_019e8fff-b090-70b8-b818-feb199ccdb79" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentFairValue_019e8fff-b090-70b8-b818-feb199ccdb79" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e8fff-b090-71b6-a468-3e06d2e32050" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019e8fff-b090-71b6-a468-3e06d2e32050" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_DebtInstrumentNumberOfInstruments_019e8fff-b090-75fb-a84c-0cf0c50e4148" xlink:href="arq-20260630.xsd#arq_DebtInstrumentNumberOfInstruments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_arq_DebtInstrumentNumberOfInstruments_019e8fff-b090-75fb-a84c-0cf0c50e4148" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e8fff-b090-733d-a368-0e7cb8f10069" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e8fff-b090-733d-a368-0e7cb8f10069" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentPeriodicPaymentPrincipal_019e8fff-b090-78d3-b4ec-6987ae66b916" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentPeriodicPaymentPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentPeriodicPaymentPrincipal_019e8fff-b090-78d3-b4ec-6987ae66b916" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_DebtInstrumentCovenantIndebtednessToNetWorth_019e8fff-b090-71f0-8a12-1f140c0469d2" xlink:href="arq-20260630.xsd#arq_DebtInstrumentCovenantIndebtednessToNetWorth"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_arq_DebtInstrumentCovenantIndebtednessToNetWorth_019e8fff-b090-71f0-8a12-1f140c0469d2" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateIncreaseDecrease_019e8fff-b090-7409-8319-c6b3da7056ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateIncreaseDecrease_019e8fff-b090-7409-8319-c6b3da7056ff" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e8fff-b090-7de6-89d4-7461ae2c4e70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b090-763e-88a9-a4ffa59bf51e" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e8fff-b090-7de6-89d4-7461ae2c4e70" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/LeasesNarrativeDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#LeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/LeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019e8fff-b091-7989-a3a7-e100d256cace" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019e8fff-b091-7989-a3a7-e100d256cace" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e8fff-b091-7994-a6e0-7999229721d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e8fff-b091-7989-a3a7-e100d256cace" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e8fff-b091-7994-a6e0-7999229721d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e8fff-b091-7989-a3a7-e100d256cace" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7482-9184-2c6d7a82b290_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7482-9184-2c6d7a82b290_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7101-9eef-0874beb5c891" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7101-9eef-0874beb5c891" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_CorbinFacilityMember_019e8fff-b091-7f6c-b947-918f50d0d76f" xlink:href="arq-20260630.xsd#arq_CorbinFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7101-9eef-0874beb5c891" xlink:to="loc_arq_CorbinFacilityMember_019e8fff-b091-7f6c-b947-918f50d0d76f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e8fff-b091-7842-9c4e-f08ff729f9e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e8fff-b091-7842-9c4e-f08ff729f9e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-79f3-b974-a618f0979047" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-79f3-b974-a618f0979047" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-7478-be7b-7c564e6e4a15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-7478-be7b-7c564e6e4a15" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseExpense_019e8fff-b091-74ef-9735-8ecfbba0ca6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseExpense_019e8fff-b091-74ef-9735-8ecfbba0ca6c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019e8fff-b091-7b07-8b63-db7271a1779f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019e8fff-b091-7b07-8b63-db7271a1779f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_019e8fff-b091-7733-991d-e0fdea45e33e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_019e8fff-b091-7733-991d-e0fdea45e33e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_OperatingLeaseRightOfUseLiabilityPeriodicReduction_019e8fff-b091-7d96-8262-b3b7e9e18533" xlink:href="arq-20260630.xsd#arq_OperatingLeaseRightOfUseLiabilityPeriodicReduction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_arq_OperatingLeaseRightOfUseLiabilityPeriodicReduction_019e8fff-b091-7d96-8262-b3b7e9e18533" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/LeasesNarrativeDetails_1" xlink:type="simple" xlink:href="arq-20260630.xsd#LeasesNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/LeasesNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019e8fff-b091-7989-a3a7-e100d256cace" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019e8fff-b091-7989-a3a7-e100d256cace" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e8fff-b091-7989-a3a7-e100d256cace" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7482-9184-2c6d7a82b290_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7482-9184-2c6d7a82b290_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7101-9eef-0874beb5c891" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e8fff-b091-7482-9184-2c6d7a82b290" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7101-9eef-0874beb5c891" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_CorbinFacilityMember_019e8fff-b091-7f6c-b947-918f50d0d76f" xlink:href="arq-20260630.xsd#arq_CorbinFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e8fff-b091-7101-9eef-0874beb5c891" xlink:to="loc_arq_CorbinFacilityMember_019e8fff-b091-7f6c-b947-918f50d0d76f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e8fff-b091-7842-9c4e-f08ff729f9e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e8fff-b091-7842-9c4e-f08ff729f9e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-79f3-b974-a618f0979047" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-79f3-b974-a618f0979047" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-7478-be7b-7c564e6e4a15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrentStatementOfFinancialPositionExtensibleList_019e8fff-b091-7478-be7b-7c564e6e4a15" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseExpense_019e8fff-b091-74ef-9735-8ecfbba0ca6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseExpense_019e8fff-b091-74ef-9735-8ecfbba0ca6c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019e8fff-b091-7b07-8b63-db7271a1779f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019e8fff-b091-7b07-8b63-db7271a1779f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_019e8fff-b091-7733-991d-e0fdea45e33e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_019e8fff-b091-7733-991d-e0fdea45e33e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_OperatingLeaseRightOfUseLiabilityPeriodicReduction_019e8fff-b091-7d96-8262-b3b7e9e18533" xlink:href="arq-20260630.xsd#arq_OperatingLeaseRightOfUseLiabilityPeriodicReduction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e8fff-b091-793b-ab91-c0fca2f7545f" xlink:to="loc_arq_OperatingLeaseRightOfUseLiabilityPeriodicReduction_019e8fff-b091-7d96-8262-b3b7e9e18533" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_019e8fff-b14f-7c3e-bb5f-aa938c643665" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_us-gaap_OtherCommitmentsTable_019e8fff-b14f-7c3e-bb5f-aa938c643665" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019e8fff-b14f-7f85-b178-0e9c1f1d2df9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019e8fff-b14f-7c3e-bb5f-aa938c643665" xlink:to="loc_srt_CounterpartyNameAxis_019e8fff-b14f-7f85-b178-0e9c1f1d2df9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e8fff-b14f-7f85-b178-0e9c1f1d2df9_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019e8fff-b14f-7f85-b178-0e9c1f1d2df9" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e8fff-b14f-7f85-b178-0e9c1f1d2df9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e8fff-b14f-7a0d-9d59-f40f64cddf4e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019e8fff-b14f-7f85-b178-0e9c1f1d2df9" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e8fff-b14f-7a0d-9d59-f40f64cddf4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_FiveForksMineMember_019e8fff-b14f-7000-be62-5ac03ffe26ae" xlink:href="arq-20260630.xsd#arq_FiveForksMineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e8fff-b14f-7a0d-9d59-f40f64cddf4e" xlink:to="loc_arq_FiveForksMineMember_019e8fff-b14f-7000-be62-5ac03ffe26ae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_CorbinFacilitiesMember_019e8fff-b14f-75a4-b964-b264f2840461" xlink:href="arq-20260630.xsd#arq_CorbinFacilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e8fff-b14f-7a0d-9d59-f40f64cddf4e" xlink:to="loc_arq_CorbinFacilitiesMember_019e8fff-b14f-75a4-b964-b264f2840461" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_WestVirginiaMine4Member_019e8fff-b14f-7e0a-a408-4826e91a473b" xlink:href="arq-20260630.xsd#arq_WestVirginiaMine4Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e8fff-b14f-7a0d-9d59-f40f64cddf4e" xlink:to="loc_arq_WestVirginiaMine4Member_019e8fff-b14f-7e0a-a408-4826e91a473b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_019e8fff-b14f-75a3-a3e4-23792de5b1ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019e8fff-b14f-7c3e-bb5f-aa938c643665" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_019e8fff-b14f-75a3-a3e4-23792de5b1ca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019e8fff-b14f-75a3-a3e4-23792de5b1ca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019e8fff-b14f-75a3-a3e4-23792de5b1ca" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019e8fff-b14f-75a3-a3e4-23792de5b1ca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019e8fff-b14f-758e-87d5-99df7b83952b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019e8fff-b14f-75a3-a3e4-23792de5b1ca" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019e8fff-b14f-758e-87d5-99df7b83952b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-7bdc-816c-ffda71c5d7ea" xlink:href="arq-20260630.xsd#arq_TinuumGroupLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e8fff-b14f-758e-87d5-99df7b83952b" xlink:to="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-7bdc-816c-ffda71c5d7ea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-76b1-b8db-4ae7dbfdcff3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019e8fff-b14f-7c3e-bb5f-aa938c643665" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-76b1-b8db-4ae7dbfdcff3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-76b1-b8db-4ae7dbfdcff3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-76b1-b8db-4ae7dbfdcff3" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-76b1-b8db-4ae7dbfdcff3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7003-bad8-b2089165d4f9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-76b1-b8db-4ae7dbfdcff3" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7003-bad8-b2089165d4f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-78c3-b709-6c87dd9c8590" xlink:href="arq-20260630.xsd#arq_TinuumGroupLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7003-bad8-b2089165d4f9" xlink:to="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-78c3-b709-6c87dd9c8590" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_SuretyAgreementBondAmount_019e8fff-b14f-749e-80f2-40e6547024e1" xlink:href="arq-20260630.xsd#arq_SuretyAgreementBondAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_arq_SuretyAgreementBondAmount_019e8fff-b14f-749e-80f2-40e6547024e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashNoncurrent_019e8fff-b14f-791e-9204-e8b6fa042b51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_us-gaap_RestrictedCashNoncurrent_019e8fff-b14f-791e-9204-e8b6fa042b51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_SupplyAgreementPerformanceBond_019e8fff-b14f-706d-ad8f-e5c75d45b792" xlink:href="arq-20260630.xsd#arq_SupplyAgreementPerformanceBond"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_arq_SupplyAgreementPerformanceBond_019e8fff-b14f-706d-ad8f-e5c75d45b792" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_SupplyAgreementPerformanceBondOutstandingAmount_019e8fff-b14f-7b8f-9a60-e5a89e573fe9" xlink:href="arq-20260630.xsd#arq_SupplyAgreementPerformanceBondOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_arq_SupplyAgreementPerformanceBondOutstandingAmount_019e8fff-b14f-7b8f-9a60-e5a89e573fe9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashReserveDepositRequiredAndMade_019f93ae-3a67-70fe-b85c-c08b0e5e6ea7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashReserveDepositRequiredAndMade"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_us-gaap_CashReserveDepositRequiredAndMade_019f93ae-3a67-70fe-b85c-c08b0e5e6ea7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_LimitedGuarantees.Percent_019e8fff-b14f-7a29-a55a-6e645a2fe582" xlink:href="arq-20260630.xsd#arq_LimitedGuarantees.Percent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_arq_LimitedGuarantees.Percent_019e8fff-b14f-7a29-a55a-6e645a2fe582" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_ContingentObligationLiabilityAndExpenseProvisionAmount_019fbe66-7039-720d-83cb-5e529821e7a3" xlink:href="arq-20260630.xsd#arq_ContingentObligationLiabilityAndExpenseProvisionAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e8fff-b14f-7f79-8fd8-cab8459cf07c" xlink:to="loc_arq_ContingentObligationLiabilityAndExpenseProvisionAmount_019fbe66-7039-720d-83cb-5e529821e7a3" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/SupplementalFinancialInformationScheduleofOtherLongTermAssetsDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#SupplementalFinancialInformationScheduleofOtherLongTermAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/SupplementalFinancialInformationScheduleofOtherLongTermAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b14f-714f-a09d-e627ca88f59b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e8fff-b14f-7bcf-9ac2-8af364a53939" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b14f-714f-a09d-e627ca88f59b" xlink:to="loc_us-gaap_DebtInstrumentTable_019e8fff-b14f-7bcf-9ac2-8af364a53939" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e8fff-b14f-7322-b022-c0c2a4b9f4f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b14f-7bcf-9ac2-8af364a53939" xlink:to="loc_us-gaap_CreditFacilityAxis_019e8fff-b14f-7322-b022-c0c2a4b9f4f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e8fff-b14f-7322-b022-c0c2a4b9f4f6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e8fff-b14f-7322-b022-c0c2a4b9f4f6" xlink:to="loc_us-gaap_CreditFacilityDomain_019e8fff-b14f-7322-b022-c0c2a4b9f4f6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e8fff-b14f-752d-802a-853877aac6fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e8fff-b14f-7322-b022-c0c2a4b9f4f6" xlink:to="loc_us-gaap_CreditFacilityDomain_019e8fff-b14f-752d-802a-853877aac6fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e8fff-b14f-759a-ab1a-ff1e258ee854" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e8fff-b14f-752d-802a-853877aac6fd" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e8fff-b14f-759a-ab1a-ff1e258ee854" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b14f-7718-8af5-6f0f6be2c027" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b14f-7bcf-9ac2-8af364a53939" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b14f-7718-8af5-6f0f6be2c027" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b14f-7718-8af5-6f0f6be2c027_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b14f-7718-8af5-6f0f6be2c027" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b14f-7718-8af5-6f0f6be2c027_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b14f-763d-8130-46a30a99ee1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e8fff-b14f-7718-8af5-6f0f6be2c027" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b14f-763d-8130-46a30a99ee1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019e8fff-b14f-7c06-9884-79427342cd51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e8fff-b14f-763d-8130-46a30a99ee1b" xlink:to="loc_us-gaap_SecuredDebtMember_019e8fff-b14f-7c06-9884-79427342cd51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerDurationAxis_019fbe6d-63c0-7805-9d79-f0d1931b211e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerDurationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e8fff-b14f-7bcf-9ac2-8af364a53939" xlink:to="loc_us-gaap_ContractWithCustomerDurationAxis_019fbe6d-63c0-7805-9d79-f0d1931b211e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerDurationDomain_019fbe6d-63c0-7805-9d79-f0d1931b211e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerDurationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerDurationAxis_019fbe6d-63c0-7805-9d79-f0d1931b211e" xlink:to="loc_us-gaap_ContractWithCustomerDurationDomain_019fbe6d-63c0-7805-9d79-f0d1931b211e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerDurationDomain_019fbe6d-63c0-75fa-943e-e2633d8118ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerDurationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerDurationAxis_019fbe6d-63c0-7805-9d79-f0d1931b211e" xlink:to="loc_us-gaap_ContractWithCustomerDurationDomain_019fbe6d-63c0-75fa-943e-e2633d8118ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_A15YearsContractualPeriodMember_019fbe6d-d204-7a2d-8cbb-9463bc1d322a" xlink:href="arq-20260630.xsd#arq_A15YearsContractualPeriodMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerDurationDomain_019fbe6d-63c0-75fa-943e-e2633d8118ff" xlink:to="loc_arq_A15YearsContractualPeriodMember_019fbe6d-d204-7a2d-8cbb-9463bc1d322a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_A2YearsContractualPeriodMember_019fbe6e-08b6-7a82-a490-6c7ce3ffa57c" xlink:href="arq-20260630.xsd#arq_A2YearsContractualPeriodMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerDurationDomain_019fbe6d-63c0-75fa-943e-e2633d8118ff" xlink:to="loc_arq_A2YearsContractualPeriodMember_019fbe6e-08b6-7a82-a490-6c7ce3ffa57c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_019e8fff-b14f-746d-8eee-e55346b1a11e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b14f-714f-a09d-e627ca88f59b" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_019e8fff-b14f-746d-8eee-e55346b1a11e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseCurrent_019e8fff-b14f-7062-b300-1efb9bc62e54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidExpenseCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_019e8fff-b14f-746d-8eee-e55346b1a11e" xlink:to="loc_us-gaap_PrepaidExpenseCurrent_019e8fff-b14f-7062-b300-1efb9bc62e54" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_PrepaidLenderFeesCurrent_019e8fff-b14f-7a42-a35b-d0a99ef5eed0" xlink:href="arq-20260630.xsd#arq_PrepaidLenderFeesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_019e8fff-b14f-746d-8eee-e55346b1a11e" xlink:to="loc_arq_PrepaidLenderFeesCurrent_019e8fff-b14f-7a42-a35b-d0a99ef5eed0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidTaxes_019e8fff-b14f-7496-8ff6-aa8adb97ffc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_019e8fff-b14f-746d-8eee-e55346b1a11e" xlink:to="loc_us-gaap_PrepaidTaxes_019e8fff-b14f-7496-8ff6-aa8adb97ffc3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsCurrent_019e8fff-b14f-7de6-80dd-797feda0bde3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_019e8fff-b14f-746d-8eee-e55346b1a11e" xlink:to="loc_us-gaap_OtherAssetsCurrent_019e8fff-b14f-7de6-80dd-797feda0bde3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019e8fff-b14f-775a-8cea-20ef292ffe33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_019e8fff-b14f-746d-8eee-e55346b1a11e" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019e8fff-b14f-775a-8cea-20ef292ffe33" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsNoncurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e8fff-b14f-714f-a09d-e627ca88f59b" xlink:to="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInventoriesSpareParts_019e8fff-b14f-71a7-baf4-dafe453db390" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherInventoriesSpareParts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_us-gaap_OtherInventoriesSpareParts_019e8fff-b14f-71a7-baf4-dafe453db390" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_MineDevelopmentCost_019e8fff-b14f-7644-824b-ecf4dede69dd" xlink:href="arq-20260630.xsd#arq_MineDevelopmentCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_arq_MineDevelopmentCost_019e8fff-b14f-7644-824b-ecf4dede69dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_019e8fff-b14f-7488-adcf-a448d3b6be8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_019e8fff-b14f-7488-adcf-a448d3b6be8b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_UpfrontPaymentsFromContractWithCustomer_019e8fff-b14f-7919-a275-45873aac245d" xlink:href="arq-20260630.xsd#arq_UpfrontPaymentsFromContractWithCustomer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_arq_UpfrontPaymentsFromContractWithCustomer_019e8fff-b14f-7919-a275-45873aac245d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_DeferredPlantTurnaroundCostsNet_019fb821-9859-73ed-9f68-d6dc42f17f47" xlink:href="arq-20260630.xsd#arq_DeferredPlantTurnaroundCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_arq_DeferredPlantTurnaroundCostsNet_019fb821-9859-73ed-9f68-d6dc42f17f47" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_MineReclamationAsset_019e8fff-b14f-7e67-95de-8333944f4db2" xlink:href="arq-20260630.xsd#arq_MineReclamationAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_arq_MineReclamationAsset_019e8fff-b14f-7e67-95de-8333944f4db2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019e8fff-b14f-7191-a4fe-bcf79d7b52cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019e8fff-b14f-7191-a4fe-bcf79d7b52cf" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e8fff-b14f-74c1-a2ce-9726db5adfe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e8fff-b14f-74c1-a2ce-9726db5adfe9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_OtherAssetsNoncurrentOther_019e8fff-b14f-7856-94bf-7f676c4d506b" xlink:href="arq-20260630.xsd#arq_OtherAssetsNoncurrentOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_arq_OtherAssetsNoncurrentOther_019e8fff-b14f-7856-94bf-7f676c4d506b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_019e8fff-b14f-72dd-b933-2d3da2c280d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_019e8fff-b14f-72dd-b933-2d3da2c280d6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019e8fff-b14f-7f89-bfde-d1e390a234b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_us-gaap_DebtInstrumentTerm_019e8fff-b14f-7f89-bfde-d1e390a234b3" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_ContractWithCustomerTermOfContract_019e8fff-b14f-72af-afcb-f2bc187c413e" xlink:href="arq-20260630.xsd#arq_ContractWithCustomerTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherAssetsNoncurrentAbstract_019e8fff-b14f-7d55-ba72-06c4114221b6" xlink:to="loc_arq_ContractWithCustomerTermOfContract_019e8fff-b14f-72af-afcb-f2bc187c413e" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/SupplementalFinancialInformationNarrativeDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#SupplementalFinancialInformationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/SupplementalFinancialInformationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e8fff-b14f-7bf0-9718-b71c5cc98baa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e8fff-b14f-7bf0-9718-b71c5cc98baa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e8fff-b14f-7bf0-9718-b71c5cc98baa" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-716b-9958-d4fb1194b7de_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-716b-9958-d4fb1194b7de_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_HighviewEnterprisesLimitedMember_019e8fff-b14f-71f3-9320-9d730a80e2cf" xlink:href="arq-20260630.xsd#arq_HighviewEnterprisesLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:to="loc_arq_HighviewEnterprisesLimitedMember_019e8fff-b14f-71f3-9320-9d730a80e2cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-7ec4-9002-aa20ecd1f669" xlink:href="arq-20260630.xsd#arq_TinuumGroupLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:to="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-7ec4-9002-aa20ecd1f669" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e8fff-b14f-7975-b6cd-ec4330720886" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e8fff-b14f-7bf0-9718-b71c5cc98baa" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e8fff-b14f-7975-b6cd-ec4330720886" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019e8fff-b14f-7fa5-8f2d-e70260d5f07d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_EquityMethodInvestments_019e8fff-b14f-7fa5-8f2d-e70260d5f07d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019e8fff-b14f-7039-ac8d-eadb9410f8cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019e8fff-b14f-7039-ac8d-eadb9410f8cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019e8fff-b14f-7bdb-b3c4-5cd04eb50faa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019e8fff-b14f-7bdb-b3c4-5cd04eb50faa" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/SupplementalFinancialInformationNarrativeDetails_1" xlink:type="simple" xlink:href="arq-20260630.xsd#SupplementalFinancialInformationNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/SupplementalFinancialInformationNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e8fff-b14f-7bf0-9718-b71c5cc98baa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e8fff-b14f-7bf0-9718-b71c5cc98baa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e8fff-b14f-7bf0-9718-b71c5cc98baa" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-716b-9958-d4fb1194b7de_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-716b-9958-d4fb1194b7de_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019e8fff-b14f-716b-9958-d4fb1194b7de" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_HighviewEnterprisesLimitedMember_019e8fff-b14f-71f3-9320-9d730a80e2cf" xlink:href="arq-20260630.xsd#arq_HighviewEnterprisesLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:to="loc_arq_HighviewEnterprisesLimitedMember_019e8fff-b14f-71f3-9320-9d730a80e2cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-7ec4-9002-aa20ecd1f669" xlink:href="arq-20260630.xsd#arq_TinuumGroupLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019e8fff-b14f-7594-b3bd-e1520829e609" xlink:to="loc_arq_TinuumGroupLLCMember_019e8fff-b14f-7ec4-9002-aa20ecd1f669" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_019e8fff-b14f-7fa5-8f2d-e70260d5f07d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_EquityMethodInvestments_019e8fff-b14f-7fa5-8f2d-e70260d5f07d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019e8fff-b14f-7039-ac8d-eadb9410f8cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019e8fff-b14f-7039-ac8d-eadb9410f8cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019e8fff-b14f-7bdb-b3c4-5cd04eb50faa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e8fff-b14f-7fa9-8fb3-14327ca40121" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019e8fff-b14f-7bdb-b3c4-5cd04eb50faa" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/StockholdersEquityDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#StockholdersEquityDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/StockholdersEquityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019e8fff-b14f-7849-abf9-68bcbc174a5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019e8fff-b14f-7381-a944-f8d063781c8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e8fff-b14f-7849-abf9-68bcbc174a5b" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019e8fff-b14f-7381-a944-f8d063781c8c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e8fff-b14f-7bfe-af4f-8de8414f1a5d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e8fff-b14f-7381-a944-f8d063781c8c" xlink:to="loc_srt_RangeAxis_019e8fff-b14f-7bfe-af4f-8de8414f1a5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e8fff-b14f-7bfe-af4f-8de8414f1a5d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e8fff-b14f-7bfe-af4f-8de8414f1a5d" xlink:to="loc_srt_RangeMember_019e8fff-b14f-7bfe-af4f-8de8414f1a5d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e8fff-b14f-7db4-829f-0bd431036eef" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e8fff-b14f-7bfe-af4f-8de8414f1a5d" xlink:to="loc_srt_RangeMember_019e8fff-b14f-7db4-829f-0bd431036eef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e8fff-b14f-7c65-8ea2-eb499f435341" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e8fff-b14f-7db4-829f-0bd431036eef" xlink:to="loc_srt_MaximumMember_019e8fff-b14f-7c65-8ea2-eb499f435341" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_RequirementToOwnSharesOutstandingAsPercent_019e8fff-b14f-796e-a147-76d57f231499" xlink:href="arq-20260630.xsd#arq_RequirementToOwnSharesOutstandingAsPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e8fff-b14f-7849-abf9-68bcbc174a5b" xlink:to="loc_arq_RequirementToOwnSharesOutstandingAsPercent_019e8fff-b14f-796e-a147-76d57f231499" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/StockBasedCompensationScheduleofAllocationofCompensationExpenseDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#StockBasedCompensationScheduleofAllocationofCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/StockBasedCompensationScheduleofAllocationofCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7ea1-b4c3-7b2a6f83229a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-72a6-bd97-b0e8493c5da9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7ea1-b4c3-7b2a6f83229a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-72a6-bd97-b0e8493c5da9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7896-aedc-604ea18a2d8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-72a6-bd97-b0e8493c5da9" xlink:to="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7896-aedc-604ea18a2d8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7896-aedc-604ea18a2d8b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7896-aedc-604ea18a2d8b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7896-aedc-604ea18a2d8b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73e8-b6f1-284ca53d683f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7896-aedc-604ea18a2d8b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73e8-b6f1-284ca53d683f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019e8fff-b150-724b-b085-28f21b28797f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73e8-b6f1-284ca53d683f" xlink:to="loc_us-gaap_RestrictedStockMember_019e8fff-b150-724b-b085-28f21b28797f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-736e-9518-0a9c3a1b557d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73e8-b6f1-284ca53d683f" xlink:to="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-736e-9518-0a9c3a1b557d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-72b1-b06b-2e0468114b27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73e8-b6f1-284ca53d683f" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-72b1-b06b-2e0468114b27" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e8fff-b150-7b8a-9b48-eda714db1158" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7ea1-b4c3-7b2a6f83229a" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e8fff-b150-7b8a-9b48-eda714db1158" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/StockBasedCompensationScheduleofUnrecognizedCompensationCostDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#StockBasedCompensationScheduleofUnrecognizedCompensationCostDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/StockBasedCompensationScheduleofUnrecognizedCompensationCostDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-730a-8879-c00f9af59edd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-7306-a7da-67e6218eb9a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-730a-8879-c00f9af59edd" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-7306-a7da-67e6218eb9a4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7e4a-a0a2-1be1ab74463e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-7306-a7da-67e6218eb9a4" xlink:to="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7e4a-a0a2-1be1ab74463e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7e4a-a0a2-1be1ab74463e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7e4a-a0a2-1be1ab74463e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7e4a-a0a2-1be1ab74463e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7bc7-80d1-9b8f22ac91ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7e4a-a0a2-1be1ab74463e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7bc7-80d1-9b8f22ac91ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019e8fff-b150-762c-968e-ed0ea6d71beb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7bc7-80d1-9b8f22ac91ad" xlink:to="loc_us-gaap_RestrictedStockMember_019e8fff-b150-762c-968e-ed0ea6d71beb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-7ce4-a000-ea3db308e62e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7bc7-80d1-9b8f22ac91ad" xlink:to="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-7ce4-a000-ea3db308e62e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-7f79-98f3-c4446962fcd3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7bc7-80d1-9b8f22ac91ad" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-7f79-98f3-c4446962fcd3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019e8fff-b150-7b96-9c8c-974a3144436b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-730a-8879-c00f9af59edd" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019e8fff-b150-7b96-9c8c-974a3144436b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e8fff-b150-7984-b299-3b4f87dc5ef3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-730a-8879-c00f9af59edd" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e8fff-b150-7984-b299-3b4f87dc5ef3" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-76fd-b6c7-4525c967da33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75db-a3a4-a655f0ad01bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-76fd-b6c7-4525c967da33" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75db-a3a4-a655f0ad01bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7ada-96a4-ffbd2f29b08a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75db-a3a4-a655f0ad01bc" xlink:to="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7ada-96a4-ffbd2f29b08a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7ada-96a4-ffbd2f29b08a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7ada-96a4-ffbd2f29b08a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7ada-96a4-ffbd2f29b08a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73a4-b8a5-70ec3cf1229b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7ada-96a4-ffbd2f29b08a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73a4-b8a5-70ec3cf1229b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019e8fff-b150-7cd4-8cdd-0ea461b892ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73a4-b8a5-70ec3cf1229b" xlink:to="loc_us-gaap_RestrictedStockMember_019e8fff-b150-7cd4-8cdd-0ea461b892ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-7b72-94dd-4c0ddbcd377d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73a4-b8a5-70ec3cf1229b" xlink:to="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-7b72-94dd-4c0ddbcd377d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-7218-b07a-2aa19a022635" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-73a4-b8a5-70ec3cf1229b" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-7218-b07a-2aa19a022635" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e8fff-b150-733a-b1d6-88380de86900" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75db-a3a4-a655f0ad01bc" xlink:to="loc_srt_RangeAxis_019e8fff-b150-733a-b1d6-88380de86900" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e8fff-b150-733a-b1d6-88380de86900_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e8fff-b150-733a-b1d6-88380de86900" xlink:to="loc_srt_RangeMember_019e8fff-b150-733a-b1d6-88380de86900_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e8fff-b150-7204-b6b7-728a5ef8ac54" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e8fff-b150-733a-b1d6-88380de86900" xlink:to="loc_srt_RangeMember_019e8fff-b150-7204-b6b7-728a5ef8ac54" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e8fff-b150-7024-86cc-ea3690c28456" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e8fff-b150-7204-b6b7-728a5ef8ac54" xlink:to="loc_srt_MaximumMember_019e8fff-b150-7024-86cc-ea3690c28456" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e8fff-b150-77e3-9ffc-425826254ddf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-76fd-b6c7-4525c967da33" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e8fff-b150-77e3-9ffc-425826254ddf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e8fff-b150-7075-9425-ffc45c061363" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-76fd-b6c7-4525c967da33" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e8fff-b150-7075-9425-ffc45c061363" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e8fff-b150-7570-b077-6ea52703a468" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-76fd-b6c7-4525c967da33" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e8fff-b150-7570-b077-6ea52703a468" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/StockBasedCompensationScheduleofRestrictedStockActivityDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#StockBasedCompensationScheduleofRestrictedStockActivityDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/StockBasedCompensationScheduleofRestrictedStockActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7790-b3ae-591a3d5f5463" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-7c01-b24e-19c0129ab8b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7790-b3ae-591a3d5f5463" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-7c01-b24e-19c0129ab8b7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e8fff-b150-76cf-a1b1-6101b2e1ee88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-7c01-b24e-19c0129ab8b7" xlink:to="loc_us-gaap_AwardTypeAxis_019e8fff-b150-76cf-a1b1-6101b2e1ee88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-76cf-a1b1-6101b2e1ee88_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-76cf-a1b1-6101b2e1ee88" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-76cf-a1b1-6101b2e1ee88_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7414-b625-8d4f3bba961c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-76cf-a1b1-6101b2e1ee88" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7414-b625-8d4f3bba961c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019e8fff-b150-7ead-aee1-11dbc11295f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7414-b625-8d4f3bba961c" xlink:to="loc_us-gaap_RestrictedStockMember_019e8fff-b150-7ead-aee1-11dbc11295f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7874-a200-3e7dda4c4b10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7790-b3ae-591a3d5f5463" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7874-a200-3e7dda4c4b10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e8fff-b150-770c-aebd-709bc5331fcc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7874-a200-3e7dda4c4b10" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e8fff-b150-770c-aebd-709bc5331fcc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e8fff-b150-7a1b-ab45-a980a4d3b7d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7874-a200-3e7dda4c4b10" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e8fff-b150-7a1b-ab45-a980a4d3b7d9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e8fff-b150-74fa-9f24-e174a68d716b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7874-a200-3e7dda4c4b10" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e8fff-b150-74fa-9f24-e174a68d716b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e8fff-b150-7a7c-af1f-8898e1541250" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7874-a200-3e7dda4c4b10" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e8fff-b150-7a7c-af1f-8898e1541250" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e8fff-b150-79c3-86ca-435ec3fe3c0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-73fa-a0e8-4b2cb87251af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7790-b3ae-591a3d5f5463" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-73fa-a0e8-4b2cb87251af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e8fff-b150-7b92-a539-2691b3b54381" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-73fa-a0e8-4b2cb87251af" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e8fff-b150-7b92-a539-2691b3b54381" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-7d99-b3f4-786182578075" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-73fa-a0e8-4b2cb87251af" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-7d99-b3f4-786182578075" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-7a40-a93b-5385d0da195b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-73fa-a0e8-4b2cb87251af" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-7a40-a93b-5385d0da195b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e8fff-b150-7ac4-8ac6-4dea2efd90ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-73fa-a0e8-4b2cb87251af" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e8fff-b150-7ac4-8ac6-4dea2efd90ec" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e8fff-b150-72dc-8af2-2f0f08a3fca2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/StockBasedCompensationScheduleofPerformanceSharesUnitsDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#StockBasedCompensationScheduleofPerformanceSharesUnitsDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/StockBasedCompensationScheduleofPerformanceSharesUnitsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7bfc-b6c3-b5f0643ffec3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75a7-91db-7f7b57e9fd8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7bfc-b6c3-b5f0643ffec3" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75a7-91db-7f7b57e9fd8f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7064-b08b-261d2d323c16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75a7-91db-7f7b57e9fd8f" xlink:to="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7064-b08b-261d2d323c16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7064-b08b-261d2d323c16_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7064-b08b-261d2d323c16" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7064-b08b-261d2d323c16_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7079-98c3-c5628b94aeca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-7064-b08b-261d2d323c16" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7079-98c3-c5628b94aeca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-75ee-8e67-7a71847c63f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7079-98c3-c5628b94aeca" xlink:to="loc_us-gaap_PerformanceSharesMember_019e8fff-b150-75ee-8e67-7a71847c63f4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TriggeringEventAxis_019e8fff-b150-775a-82b0-949a09873677" xlink:href="arq-20260630.xsd#arq_TriggeringEventAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75a7-91db-7f7b57e9fd8f" xlink:to="loc_arq_TriggeringEventAxis_019e8fff-b150-775a-82b0-949a09873677" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TriggeringEventDomain_019e8fff-b150-775a-82b0-949a09873677_default" xlink:href="arq-20260630.xsd#arq_TriggeringEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_arq_TriggeringEventAxis_019e8fff-b150-775a-82b0-949a09873677" xlink:to="loc_arq_TriggeringEventDomain_019e8fff-b150-775a-82b0-949a09873677_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_TriggeringEventDomain_019e8fff-b150-777c-841e-eaf2a309bec0" xlink:href="arq-20260630.xsd#arq_TriggeringEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_arq_TriggeringEventAxis_019e8fff-b150-775a-82b0-949a09873677" xlink:to="loc_arq_TriggeringEventDomain_019e8fff-b150-777c-841e-eaf2a309bec0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_IssuedUponVestingOfPSUsMember_019e8fff-b150-7e73-8bf3-d67496e9d33b" xlink:href="arq-20260630.xsd#arq_IssuedUponVestingOfPSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arq_TriggeringEventDomain_019e8fff-b150-777c-841e-eaf2a309bec0" xlink:to="loc_arq_IssuedUponVestingOfPSUsMember_019e8fff-b150-7e73-8bf3-d67496e9d33b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e8fff-b150-78bd-80b0-74839728074d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-75a7-91db-7f7b57e9fd8f" xlink:to="loc_srt_RangeAxis_019e8fff-b150-78bd-80b0-74839728074d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e8fff-b150-78bd-80b0-74839728074d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e8fff-b150-78bd-80b0-74839728074d" xlink:to="loc_srt_RangeMember_019e8fff-b150-78bd-80b0-74839728074d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e8fff-b150-7d61-a147-3fa95e22ed37" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e8fff-b150-78bd-80b0-74839728074d" xlink:to="loc_srt_RangeMember_019e8fff-b150-7d61-a147-3fa95e22ed37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e8fff-b150-7b45-b41f-e502fab15eee" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e8fff-b150-7d61-a147-3fa95e22ed37" xlink:to="loc_srt_MinimumMember_019e8fff-b150-7b45-b41f-e502fab15eee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e8fff-b150-7f43-9ec7-3d5258da7b83" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e8fff-b150-7d61-a147-3fa95e22ed37" xlink:to="loc_srt_MaximumMember_019e8fff-b150-7f43-9ec7-3d5258da7b83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7e78-bb0e-6f3d0048a495" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7bfc-b6c3-b5f0643ffec3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7e78-bb0e-6f3d0048a495" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e8fff-b150-7496-be78-9f0d695c74d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7e78-bb0e-6f3d0048a495" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e8fff-b150-7496-be78-9f0d695c74d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e8fff-b150-76d2-a822-3f103e55a4a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7e78-bb0e-6f3d0048a495" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e8fff-b150-76d2-a822-3f103e55a4a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e8fff-b150-794b-8161-e99cc30e5eb0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7e78-bb0e-6f3d0048a495" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e8fff-b150-794b-8161-e99cc30e5eb0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e8fff-b150-7f1f-94c3-662d8807bb8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e8fff-b150-7e78-bb0e-6f3d0048a495" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e8fff-b150-7f1f-94c3-662d8807bb8f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e8fff-b150-7b33-8efb-3a7e4d881ee6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-741d-9715-8b295fbc4616" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7bfc-b6c3-b5f0643ffec3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-741d-9715-8b295fbc4616" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e8fff-b150-7795-8b51-277a2796e45e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-741d-9715-8b295fbc4616" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e8fff-b150-7795-8b51-277a2796e45e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-7995-a92a-c5668a8fdf84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-741d-9715-8b295fbc4616" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-7995-a92a-c5668a8fdf84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-70b5-a46e-9bbbc048ae2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-741d-9715-8b295fbc4616" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e8fff-b150-70b5-a46e-9bbbc048ae2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e8fff-b150-7d09-9eda-4f5ddc3f9f2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e8fff-b150-741d-9715-8b295fbc4616" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e8fff-b150-7d09-9eda-4f5ddc3f9f2a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e8fff-b150-7bb7-aac9-729172224d01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019e8fff-b150-7d8a-888a-85433ea21b60" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7bfc-b6c3-b5f0643ffec3" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019e8fff-b150-7d8a-888a-85433ea21b60" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019e8fff-b150-7dd9-a723-8aef2d2a208f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7bfc-b6c3-b5f0643ffec3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019e8fff-b150-7dd9-a723-8aef2d2a208f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019e8fff-b150-7ccc-b064-844f628a3e2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7bfc-b6c3-b5f0643ffec3" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019e8fff-b150-7ccc-b064-844f628a3e2c" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/StockBasedCompensationScheduleofOptionActivityDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#StockBasedCompensationScheduleofOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/StockBasedCompensationScheduleofOptionActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-725f-966a-5efaa3b1524b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-725f-966a-5efaa3b1524b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e8fff-b150-778d-8035-55a55e616127" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e8fff-b150-725f-966a-5efaa3b1524b" xlink:to="loc_us-gaap_AwardTypeAxis_019e8fff-b150-778d-8035-55a55e616127" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-778d-8035-55a55e616127_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-778d-8035-55a55e616127" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-778d-8035-55a55e616127_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7149-a097-fa0c3b1199f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e8fff-b150-778d-8035-55a55e616127" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7149-a097-fa0c3b1199f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-7917-bf9e-0ac9a9a83ac3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e8fff-b150-7149-a097-fa0c3b1199f4" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e8fff-b150-7917-bf9e-0ac9a9a83ac3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e8fff-b150-701c-9c0a-1407e3a5cc16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e8fff-b150-701c-9c0a-1407e3a5cc16" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e8fff-b150-7ade-9572-9175e48a555f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e8fff-b150-701c-9c0a-1407e3a5cc16" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e8fff-b150-7ade-9572-9175e48a555f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019e8fff-b150-7efa-bfe6-9256c1115e10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e8fff-b150-701c-9c0a-1407e3a5cc16" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019e8fff-b150-7efa-bfe6-9256c1115e10" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e8fff-b150-7f2e-8a5f-cf5b13835c36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e8fff-b150-701c-9c0a-1407e3a5cc16" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e8fff-b150-7f2e-8a5f-cf5b13835c36" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_019e8fff-b150-7129-bb12-0c66172d183b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e8fff-b150-701c-9c0a-1407e3a5cc16" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_019e8fff-b150-7129-bb12-0c66172d183b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e8fff-b150-76c9-998f-35d79816f81d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019e8fff-b150-7d8b-9ebd-36b91529db3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019e8fff-b150-7d8b-9ebd-36b91529db3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e8fff-b150-753a-992a-9a1797659289" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e8fff-b150-753a-992a-9a1797659289" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e8fff-b150-7b4f-919c-602fcc5d0507" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e8fff-b150-7b4f-919c-602fcc5d0507" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e8fff-b150-7535-8c82-663f0165ffbe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e8fff-b150-7b4f-919c-602fcc5d0507" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e8fff-b150-7535-8c82-663f0165ffbe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e8fff-b150-7031-aa1d-48fd82d89f84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e8fff-b150-7b4f-919c-602fcc5d0507" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e8fff-b150-7031-aa1d-48fd82d89f84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019e8fff-b150-7c0d-84af-8c57ca5d5e3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e8fff-b150-7b4f-919c-602fcc5d0507" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019e8fff-b150-7c0d-84af-8c57ca5d5e3c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_019e8fff-b150-73bd-a0e3-81198a961801" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e8fff-b150-7b4f-919c-602fcc5d0507" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_019e8fff-b150-73bd-a0e3-81198a961801" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e8fff-b150-7f14-9b88-ff64aacba22e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019e8fff-b150-7612-9fe5-1636b5108e27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019e8fff-b150-7612-9fe5-1636b5108e27" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019e8fff-b150-7819-bda9-4578f7420938" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019e8fff-b150-7819-bda9-4578f7420938" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019e8fff-b150-7ad6-be66-f35becf54d9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019e8fff-b150-7ad6-be66-f35becf54d9a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019e8fff-b150-7d94-b077-7c4a4ca2b410" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019e8fff-b150-7d94-b077-7c4a4ca2b410" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019e8fff-b150-7aa9-b478-b0b387391baa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019e8fff-b150-7aa9-b478-b0b387391baa" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019e8fff-b150-7d13-be82-adf24a815077" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019e8fff-b150-7d13-be82-adf24a815077" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019e8fff-b150-720c-985d-9f5a19476c20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019e8fff-b150-720c-985d-9f5a19476c20" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e8fff-b150-76f4-b6b2-c4cac7193266" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e8fff-b150-7c7e-a528-f44456396cdf" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e8fff-b150-76f4-b6b2-c4cac7193266" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/SegmentReportingScheduleofRevenuefromExternalCustomersAttributedbycountryDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#SegmentReportingScheduleofRevenuefromExternalCustomersAttributedbycountryDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/SegmentReportingScheduleofRevenuefromExternalCustomersAttributedbycountryDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e8fff-b150-7769-835b-df9a9477c932" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e8fff-b150-7609-89e6-2d4b605b0396" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e8fff-b150-7769-835b-df9a9477c932" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e8fff-b150-7609-89e6-2d4b605b0396" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e8fff-b150-7a91-898d-fbfd71b6f8b7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e8fff-b150-7609-89e6-2d4b605b0396" xlink:to="loc_srt_StatementGeographicalAxis_019e8fff-b150-7a91-898d-fbfd71b6f8b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e8fff-b150-7a91-898d-fbfd71b6f8b7_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e8fff-b150-7a91-898d-fbfd71b6f8b7" xlink:to="loc_srt_SegmentGeographicalDomain_019e8fff-b150-7a91-898d-fbfd71b6f8b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e8fff-b150-7c78-b189-380996a5a371" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e8fff-b150-7a91-898d-fbfd71b6f8b7" xlink:to="loc_srt_SegmentGeographicalDomain_019e8fff-b150-7c78-b189-380996a5a371" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e8fff-b150-7bcb-9d50-3cb3f08b3116" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e8fff-b150-7c78-b189-380996a5a371" xlink:to="loc_country_US_019e8fff-b150-7bcb-9d50-3cb3f08b3116" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_019e8fff-b150-7903-aefe-229e159e0b1c" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e8fff-b150-7c78-b189-380996a5a371" xlink:to="loc_country_CA_019e8fff-b150-7903-aefe-229e159e0b1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019e8fff-b150-7335-9a3a-a551bc1a2fa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e8fff-b150-7769-835b-df9a9477c932" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019e8fff-b150-7335-9a3a-a551bc1a2fa2" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.arq.com/role/ChangeinAccountingPrincipleScheduleofImpactofChangeonCertainFinancialStatementDetails" xlink:type="simple" xlink:href="arq-20260630.xsd#ChangeinAccountingPrincipleScheduleofImpactofChangeonCertainFinancialStatementDetails"/>
  <link:definitionLink xlink:role="http://www.arq.com/role/ChangeinAccountingPrincipleScheduleofImpactofChangeonCertainFinancialStatementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_019fb850-209f-787e-91e5-5e0f22585bbe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_019fb850-209f-787e-91e5-5e0f22585bbe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_019fb850-209f-70c2-a69f-61818cd716a0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_019fb850-209f-787e-91e5-5e0f22585bbe" xlink:to="loc_srt_RestatementAxis_019fb850-209f-70c2-a69f-61818cd716a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019fb850-209f-70c2-a69f-61818cd716a0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_019fb850-209f-70c2-a69f-61818cd716a0" xlink:to="loc_srt_RestatementDomain_019fb850-209f-70c2-a69f-61818cd716a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019fb850-209f-7f3f-9ff8-f59ab4d53d55" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_019fb850-209f-70c2-a69f-61818cd716a0" xlink:to="loc_srt_RestatementDomain_019fb850-209f-7f3f-9ff8-f59ab4d53d55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_019fb850-209f-7b38-a6f5-2616aab5dc0e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioPreviouslyReportedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019fb850-209f-7f3f-9ff8-f59ab4d53d55" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_019fb850-209f-7b38-a6f5-2616aab5dc0e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodChangeInAccountingPrincipleAdjustmentMember_019fb850-209f-7270-ae76-8f6283a3c4d7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodChangeInAccountingPrincipleAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019fb850-209f-7f3f-9ff8-f59ab4d53d55" xlink:to="loc_srt_RevisionOfPriorPeriodChangeInAccountingPrincipleAdjustmentMember_019fb850-209f-7270-ae76-8f6283a3c4d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arq_IncreaseDecreaseOtherLongTermAssetNetAndRetainedEarningsCumulativeEffectOfChangeInAccountingPrinciple_019fd9de-cc5c-79f8-9ce0-a6d9f677cd83" xlink:href="arq-20260630.xsd#arq_IncreaseDecreaseOtherLongTermAssetNetAndRetainedEarningsCumulativeEffectOfChangeInAccountingPrinciple"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_arq_IncreaseDecreaseOtherLongTermAssetNetAndRetainedEarningsCumulativeEffectOfChangeInAccountingPrinciple_019fd9de-cc5c-79f8-9ce0-a6d9f677cd83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_019fb850-209f-7e47-be07-1800384db485" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_019fb850-209f-7e47-be07-1800384db485" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019fb850-209f-74f3-88e0-edfb092a2cde" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_DepreciationAndAmortization_019fb850-209f-74f3-88e0-edfb092a2cde" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019fb850-209f-7ede-bbdd-f21da600db71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_NetIncomeLoss_019fb850-209f-7ede-bbdd-f21da600db71" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019fb850-209f-7bf5-99e9-5ef87b512724" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_EarningsPerShareBasic_019fb850-209f-7bf5-99e9-5ef87b512724" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019fb850-209f-7861-93b8-4453057687d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019fb850-209f-7861-93b8-4453057687d8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_019fd9d6-f2fc-7dea-965e-b272d73f7cfc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_InventoryNet_019fd9d6-f2fc-7dea-965e-b272d73f7cfc" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_019fb850-209f-798a-a0d1-5f79e364b47e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_019fb850-209f-798a-a0d1-5f79e364b47e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019fb850-209f-718c-874c-02212fc06502" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019fb850-209f-7bd7-90ce-b61233d28ade" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019fb850-209f-718c-874c-02212fc06502" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-roles-2026.xsd#eedm1"/>
  <link:definitionLink xlink:role="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract" xlink:to="loc_us-gaap_FinanceLeaseLiability" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract" xlink:to="loc_us-gaap_OtherAssetsNoncurrent" xlink:type="arc" order="101"/>
  </link:definitionLink>
</link:linkbase>
