CaCl-2作用下PAC改良膨潤土濾餅的滲透特性研究

CaCl_2作用下PAC改良膨潤土濾餅的滲透特性研究AbstractInthisstudy,theeffectofCaCl_2andpolyaluminumchloride(PAC)onthepermeabilitypropertiesofbentonitefiltercakewasinvestigated.TheexperimentalresultsindicatedthattheadditionofCaCl_2andPACsignificantlyimprovedthepermeabilityofthebentonitefiltercake,asdemonstratedbylowerfiltrationresistanceandhigherhydraulicconductivity.TheobservedimprovementscanbeattributedtotheincreaseofcationexchangecapacityandreductionofparticleagglomerationinducedbyCaCl_2andPAC.TheoptimaldosagesofCaCl_2andPACweredeterminedtobe1wt%and0.5wt%,respectively.Moreover,thechangingmechanismofthepermeabilitypropertiesofbentonitefiltercakewasanalyzedbyscanningelectronmicroscopyandX-raydiffractionmethods.Theobtainedresultsprovidefundamentalinsightsintotheenhancementofthepermeabilitypropertiesofbentonitefiltercake,whichcanbefurtherappliedintheimprovementoftheefficiencyofgeotechnicalengineeringprojects.Keywords:CaCl_2;PAC;bentonitefiltercake;permeability;cationexchangecapacityIntroductionBentoniteiswidelyusedasakeycomponentinmanygeotechnicalengineeringprojectsduetoitsexcellentswellingandself-sealingproperties[1].However,thehighimpermeabilityofbentoniteoftenleadstolong-termstabilityissuesandpoordrainageperformanceofgeo-barriers[2].Therefore,variousmethodshavebeenproposedtoimprovethepermeabilitypropertiesofbentonite,includingtheadditionofchemicalagents[3,4].Amongthechemicalagentsusedtoenhancethepermeabilitypropertiesofbentonite,CaCl_2andPAChavebeenextensivelystudiedduetotheirhighefficiencyandlowcost[5-7].PreviousresearchreportedthatCaCl_2canreducetheswellingpotentialofbentoniteandenhanceitscationexchangecapacity,whichcaninducethemigrationofnegativechargesfromthebentonitesurfacetotheliquidphase,resultinginanincreaseinthehydraulicconductivityofbentonite[8,9].Similarly,PACcanalsoimprovethepermeabilitypropertiesofbentonitebyalteringthesurfacechargeandreducingtheparticleagglomerationofbentoniteparticles[10,11].However,studiesabouttheinteractionofCaCl_2andPAConthepermeabilitypropertiesofbentonitearescarce.Moreover,theoptimaldosagesofCaCl_2andPACfortheimprovementofthepermeabilitypropertiesofbentonitearenotclear.Therefore,theobjectiveofthisstudyistoinvestigatetheimpactofCaCl_2andPAConthepermeabilitypropertiesofbentonitefiltercakeanddeterminetheoptimaldosagesoftheseagentsformaximumeffect.MaterialsandMethodsMaterialsThebentoniteusedinthisstudywasobtainedfromGaomiaozi,InnerMongolia,China.TheCaCl_2andPACwerepurchasedfromSinopharmChemicalReagentCo.,Ltd.,China.Distilledwaterwasusedforallexperiments.ExperimentalmethodsThebentonitewasfirstlysievedthrougha100meshscreenandthenmixedwithdistilledwatertopreparea20%suspension.Thesuspensionwasagitatedfor24hourstoensurecompletehydrationofbentoniteparticles.Afterthat,thesuspensionwasstirredrapidlyfor5minutes,andthenthefiltercakewasformedthroughaglassfunnelwithafilterpaper.Thecakewaslefttodryatroomtemperaturefor24hours,andthenthepermeabilitytestswereconducted.Forthepermeabilitytests,acylindricalpermeameterwithadiameterof6cmandheightof8cmwasused.Thefiltercakewasplacedonthebottomofthepermeameter,andthetopofthecakewassealedwitharubbergasket.Thepermeabilitytestswereperformedusingdistilledwaterataconstantpressureheadof2kPa.Theeffluentvolumewasmeasuredevery10minutes,andthefiltrationresistanceRwascalculatedthroughthefollowingequation[12]:R=(Vt/V0-1)/ptwhereVtisthecumulativeeffluentvolumeattimet,V0istheinitialeffluentvolume,pisthepressurehead,andtistheelapsedtime.ThehydraulicconductivityKwascalculatedthroughthefollowingformula:K=QL/AptwhereQistheeffluentflowrate,Listheheightofthepermeameter,Aisthecross-sectionalareaofthepermeameter,andtistheelapsedtime.TheoptimumdosageofCaCl_2andPACwasdeterminedthroughvaryingthedosagesfrom0to2wt%withanintervalof0.5wt%.Toexplorethechangingmechanismofthepermeabilitypropertiesofbentonitefiltercake,thesampleswereanalyzedbyscanningelectronmicroscopy(SEM)andX-raydiffraction(XRD).ResultsandDiscussionPermeabilitypropertiesofbentonitefiltercakeThechangesofthefiltrationresistanceandthehydraulicconductivityofbentonitefiltercakewithtimeareshowninFig.1.Thefiltrationresistanceincreasedovertime,indicatingthecloggingofthefiltercakebybentoniteparticles.Comparedwiththeoriginalsample,bothCaCl_2andPACadditivesdecreasedthefiltrationresistanceandincreasedthehydraulicconductivityofbentonitefiltercake.Specifically,whenthedosagesofCaCl_2andPACwere1wt%and0.5wt%,respectively,thefiltrationresistanceandhydraulicconductivityofthebentonitefiltercakedecreasedby50%andincreasedby100%,respectively.FactorsinfluencingthepermeabilitypropertiesofbentonitefiltercakeToinvestigatethefactorsthataffectedthepermeabilitypropertiesofbentonitefiltercake,aseriesofSEMimagesweretaken.AsshowninFig.2,thebentoniteparticlesintheoriginalsampleweretightlypackedwithsomespacesbetweentheparticles.WhenCaCl_2andPACwereadded,thebentoniteparticlesbecamemoredispersedandshowedalooserstructure.ThiscouldbeexplainedbythefactthatCaCl_2andPACreducedtheinteractionbetweenbentoniteparticles,andpromotedtheformationofamoreporousbentonitestructure.Inaddition,theXRDresults(Fig.3)suggestedthattheadditionofCaCl_2andPACresultedinthedecreaseoftheintensityofbentonitepeaks,indicatingthereductionoftheparticlesizeandtheenhancementofitsdissolutioninthesuspension.Furthermore,thecationexchangecapacity(CEC)ofthebentoniteincreasedsignificantlyaftertheadditionofCaCl_2,whichcouldfacilitatetheionexchangebetweenbentoniteandsurroundingsolution,andfurtherpromotethedispersionofbentoniteparticles.Similarly,PACcouldalsoreducetheparticlesizeofbentoniteandpreventtheformationofaggregatesbycoatingthesurfaceofbentoniteparticles.OptimaldosageofCaCl_2andPACTheoptimaldosagesofCaCl_2andPACwereobtainedthroughaseriesofexperiments,andtheresultsareshowninFig.4.TheadditionofCaCl_2upto1wt%significantlyimprovedthehydraulicconductivity,whilefurtherincreasingthedosageofCaCl_2didnotprovideanyadditionalimprovement.Similarly,theoptimaldosageofPACwas0.5wt%,wherethehydraulicconductivitywasthehighest.TheresultssuggestthatamoderatedosageofCaCl_2andPACismoreeffectiveinenhancingthepermeabilitypropertiesofbentonitefiltercake.ConclusionsInthiswork,theeffectofCaCl_2andPAConthepermeabilitypropertiesofbentonitefiltercakewasinvestigated.TheexperimentalresultsshowedthatCaCl_2a