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  <xsd:import namespace="http://xbrl.sec.gov/ecd/2025" schemaLocation="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd" />
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  <xsd:import namespace="http://fasb.org/us-roles/2025" schemaLocation="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd" />
  <xsd:import namespace="http://fasb.org/us-gaap/2025" schemaLocation="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd" />
  <xsd:import namespace="http://fasb.org/srt/2025" schemaLocation="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/stpr/2025" schemaLocation="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd" />
  <xsd:import namespace="http://xbrl.sec.gov/ecd-sub/2025" schemaLocation="https://xbrl.sec.gov/ecd/2025/ecd-sub-2025.xsd" />
  <xsd:element name="DenverRefrigerantsIncMember" id="hdsn_DenverRefrigerantsIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element id="hdsn_ScheduleOfOperatingLeaseInBalanceSheetClassificationTableTextBlock" name="ScheduleOfOperatingLeaseInBalanceSheetClassificationTableTextBlock" nillable="true" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" abstract="false" />
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  <xsd:element name="SignificantAccountingPoliciesTable" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_SignificantAccountingPoliciesTable" substitutionGroup="xbrldt:hypercubeItem" />
  <xsd:element name="SignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_SignificantAccountingPoliciesLineItems" substitutionGroup="xbrli:item" />
  <xsd:element name="ExpirationPeriodAxis" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_ExpirationPeriodAxis" substitutionGroup="xbrldt:dimensionItem" />
  <xsd:element name="ExpirationPeriodDomain" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_ExpirationPeriodDomain" substitutionGroup="xbrli:item" />
  <xsd:element name="CylindersMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_CylindersMember" substitutionGroup="xbrli:item" />
  <xsd:element name="BalanceSheetClassificationAbstract" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_BalanceSheetClassificationAbstract" substitutionGroup="xbrli:item" />
  <xsd:element name="StockOptionPlanMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_StockOptionPlanMember" substitutionGroup="xbrli:item" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceAbstract" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceAbstract" substitutionGroup="xbrli:item" />
  <xsd:element name="TwoThousandAndEighteenStockIncentivePlanMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_TwoThousandAndEighteenStockIncentivePlanMember" substitutionGroup="xbrli:item" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageFromFairMarketPrice" type="dtr-types:percentItemType" abstract="false" xbrli:periodType="duration" nillable="true" id="hdsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageFromFairMarketPrice" substitutionGroup="xbrli:item" />
  <xsd:element name="FCCRMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_FCCRMember" substitutionGroup="xbrli:item" />
  <xsd:element name="SwingLineLoanMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="hdsn_SwingLineLoanMember" substitutionGroup="xbrli:item" />
  <xsd:element id="hdsn_FixedChargesCoverageRatio" name="FixedChargesCoverageRatio" nillable="true" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="hdsn_WaiverAndThirdAmendmentToTermLoanCreditAndSecurityAgreementMember" name="WaiverAndThirdAmendmentToTermLoanCreditAndSecurityAgreementMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_PropertyLocationAxis" name="PropertyLocationAxis" nillable="true" type="xbrli:stringItemType" substitutionGroup="xbrldt:dimensionItem" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_PropertyLocationDomain" name="PropertyLocationDomain" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_MinimumAggregateUndrawnLoanAvailability" name="MinimumAggregateUndrawnLoanAvailability" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" />
  <xsd:element id="hdsn_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFour" name="LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFour" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" />
  <xsd:element id="hdsn_WellsFargoMember" name="WellsFargoMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_LineOfCreditFacilityCovenantComplianceMinimumLiquidity" name="LineOfCreditFacilityCovenantComplianceMinimumLiquidity" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" />
  <xsd:element id="hdsn_LineOfCreditFacilityCovenantComplianceMinimumLiquidityToBeDerivedFromAvailability" name="LineOfCreditFacilityCovenantComplianceMinimumLiquidityToBeDerivedFromAvailability" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" />
  <xsd:element id="hdsn_PeriodForFixedChargeCoverageRatio" name="PeriodForFixedChargeCoverageRatio" nillable="true" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="hdsn_FourthAmendmentMember" name="FourthAmendmentMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="hdsn_TwoThousandAndTwentyStockIncentivePlanMember" name="TwoThousandAndTwentyStockIncentivePlanMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element name="AccruedExpensesAndOtherCurrentLiabilitiesAbstract" id="hdsn_AccruedExpensesAndOtherCurrentLiabilitiesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="PrimeCommercialLendingRateOfWellsFargoMember" id="hdsn_PrimeCommercialLendingRateOfWellsFargoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="NatureOfOperationsPolicyPolicyTextBlock" id="hdsn_NatureOfOperationsPolicyPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="CylinderDepositLiabilityPolicyPolicyTextBlock" id="hdsn_CylinderDepositLiabilityPolicyPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="ContractTerm" id="hdsn_ContractTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="RenewalTerm" id="hdsn_RenewalTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueNonvested." id="hdsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueNonvested." type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="LeaseObligationsCurrent" id="hdsn_LeaseObligationsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="ScheduleOfSupplementalCashFlowAndNonCashInformationRelatedToLeasesTableTextBlock" id="hdsn_ScheduleOfSupplementalCashFlowAndNonCashInformationRelatedToLeasesTableTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="AccruedLiabilitiesAndOtherLiabilitiesCurrent" id="hdsn_AccruedLiabilitiesAndOtherLiabilitiesCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="UnitedSuppliersOfAmericaIncAndBBJobberServicesIncMember" id="hdsn_UnitedSuppliersOfAmericaIncAndBBJobberServicesIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="BusinessCombinationContingentConsiderationPaymentPeriod" id="hdsn_BusinessCombinationContingentConsiderationPaymentPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="StockIncentivePlan2024Member" id="hdsn_StockIncentivePlan2024Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="LineOfCreditFacilityCovenantTermsMaximumShareRepurchasePerCalendarYearAmount" id="hdsn_LineOfCreditFacilityCovenantTermsMaximumShareRepurchasePerCalendarYearAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="LineOfCreditFacilityCovenantTermsMaximumShareRepurchaseOverFacilityTermAmount" id="hdsn_LineOfCreditFacilityCovenantTermsMaximumShareRepurchaseOverFacilityTermAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="IncentiveStockOptionsMember" id="hdsn_IncentiveStockOptionsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="LineOfCreditFacilityCovenantTermsMaximumShareRepurchasePerCalendarYearAfterYearOneAmount" id="hdsn_LineOfCreditFacilityCovenantTermsMaximumShareRepurchasePerCalendarYearAfterYearOneAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="ShareRepurchaseProgramAuthorizedPerCalendarYearAmount" id="hdsn_ShareRepurchaseProgramAuthorizedPerCalendarYearAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="UnitedStatesDefenseLogisticsAgencyMember" id="hdsn_UnitedStatesDefenseLogisticsAgencyMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="SingleReportingSegmentMember" id="hdsn_SingleReportingSegmentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="BaseRateLoansMember" id="hdsn_BaseRateLoansMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="SofrLoansMember" id="hdsn_SofrLoansMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="FedFundsRateMember" id="hdsn_FedFundsRateMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="VestingOnFirstAnniversaryOfGrantMember" id="hdsn_VestingOnFirstAnniversaryOfGrantMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="VestingOnSecondAnniversaryOfGrantMember" id="hdsn_VestingOnSecondAnniversaryOfGrantMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="AdjustmentsToAdditionalPaidInCapitalExcessTaxBenefitsFromStockOptionExercises" id="hdsn_AdjustmentsToAdditionalPaidInCapitalExcessTaxBenefitsFromStockOptionExercises" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
</xsd:schema>
