Preparationandapplicationof

JournalofHazardousMaterials192(2011)1851–1859ContentslistsavailableatScienceDirectJournalofHazardousMaterialsjournalhomepage:(VI)fromaqueoussolutionQiao-huiFana,PingLia,Yun-feiChena,Wang-suoWua,b,∗aRadiochemistryLaboratory,SchoolofNuclearScienceandTechnology,LanzhouUniversity,Lanzhou,Gansu73000,ChinabStateKeyLaboratoryofAppliedOrganicChemistry,LanzhouUniversity,Lanzhou,Gansu730000,ChinaarticleinfoArticlehistory:Received6December2010Receivedinrevisedform6July2011Accepted6July2011Availableonline18July2011Keywords:AttapulgiteIronoxidesSorptionUraniumMagneticcompositesabstractRecently,magneticsorbentshavereceivedconsiderableattentionbecauseoftheirexcellentsegregativefeaturesandsorptioncapacities.Herein,attapulgite/ironoxidemagnetic(ATP/IOM)compositeswerepreparedandcharacterized.ThesorptionresultsindicatedthatATP/IOMcompositesweresuperiortoATPandironoxidesindividuallyfortheremovalofU(VI)fromaqueoussolution.BasedonX-raypho-toelectronspectroscopy(XPS)analysisandsurfacecomplexationmodel,themainsorptionspeciesofU(VI)onATPwereX2UO20belowpH4.0andSsOUO2+,SwOUO2CO3−,andSwOUO2(CO3)23−abovepH5.0.HowevertheprevalentspeciesonATP/IOMcompositeswereSsOUO2+andSwOUO2(CO3)23−overtheobservedpHrange.ATP/IOMcompositesareapromisingcandidateforpre-concentrationandimmobilizationofradionuclidesfromlargevolumesofaqueoussolutions,asrequiredforremediationpurposes.©2011ElsevierB.V.Allrightsreserved.1.IntroductionAttapulgite(ATP,[(Mg,Al)4(Si)8(O,OH,H2O)26·nH2O])isahydratedmagnesiumaluminumsilicatethatexistsinnatureasafibrousmineral.Itconsistsoftwobandsofsilicatetrahedralinkedbymagnesiumionsinanoctahedralconfiguration,whichareconnectedbyacontinuousplaneoftetrahedralbasaloxygenatoms.H2Oandexchangeablecations,suchasH,Na,andK,arepresentintherepeatedunits[1,2].ATPparticleshaveroughsur-face,resultinginarelativelyhighsurfacearea,moderatecationexchangecapacity(CEC)andhighsorptioncapacities[2,3].Inaddi-tion,someisomorphicsubstitutionsinthetetrahedrallayer,suchasAl3+forSi4+,developnegativelychargedsorptionsitesthatcansorbcationsthroughelectrostaticattraction[1,4].Therefore,ATPisaverysuitablesorbentfortheremovalofheavymetalionsandradionuclidesfromaqueoussolution[1,5–10].However,thesep-arationofATPfromaqueoussolutionisverydifficult,especiallywhentheparticlesizeisonthenano-scale,atwhichsorbentspossessmuchlargersurfaceareasandmuchhighersorptioncapac-ities.Traditionalmethodstorecoversorbentsfromtheaqueousphasearemainlycentrifugationandfiltration.Recently,magneticseparationmethodhavebeenwidelystudiedinbiochemistry,microbiology,cellbiology,analyticalchemistry,miningores,and∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](W.-s.Wu).environmentaltechnology[11].BecauseofATP’sexcellentsorp-tionability,low-costandlargereservesinChina,studiesofthesorptionofheavymetalionsandradionuclidesonATParecru-cialtodevelopingapplicationsinthepollutantremediationfield[1–5,7–10,12–15].However,investigationsofsorptionpropertiesofmineral/ironoxidemagnetic(IOM)compositesarestillscarce.Inthefieldofnuclearwasterepositoryandaturaniumcontam-inatedareas(minesandreprocessingfacilities),theassessmentofwatercontamination,whichdependsontheretention/migrationofradionuclidesinground,isofprimaryenvironmentalcon-cern.ThesorptionofU(VI)ongeologicalmedia(suchasoxides,phosphates,mineralsandsoil)playsanimportantroleintheevaluationofphysicochemicalbehaviorofradionuclidesintheenvironment[5,16–20].ThemainsorptionmechanismsofU(VI)includeion-exchange,surfacecomplexmodels(i.e.,outer-sphereandinner-spherecomplexes),surfaceprecipitationandreductiononthesorbentsubsurface.Thesemechanismshavebeencon-firmedbyadvancedspectroscopictechniques,suchasextendedX-rayabsorptionfinestructure(EXAFS),X-rayphotoelectronspec-troscopy(XPS)andtimeresolvedlaserfluorescencespectroscopy(TRLFS)[18,21–23].Waiteetal.[22]usedEXAFStoidentifythesurfacecomplexesofU(VI)onferrihydriteandfoundthatatlowpH,U(VI)bondedtoferrihydriteasthehydrateduranylionUO2(H2O)n2+,whereasathighpH,inner-spherecomplexeswereconfirmedbytheinteratomicdistanceofU–Fe(∼3.5˚A).Theresultsindicatedthatbidentatecomplexeswereformedbypoly-hedraledge-sharingandthehydrateduranylion(UO2(H2O)n2+).ThesorptionofU(VI)ongoethite(-FeOOH)wasinvestigatedin0304-3894/$–seefrontmatter©2011ElsevierB.V.Allrightsreserved.doi:10.1016/j.jhazmat.2011.07.0221852Q.-h.Fanetal./JournalofHazardousMaterials192(2011)1851–1859thepresenceandabsenceofCO2,andtheresultssuggestedthatthedominantsurfacespeciationofU(VI)inaCO2-freesystemwasabidentatecorner-sharingcomplex(FeOH)2UO2(H2O)3.AnenhancementofU(VI)sorptionwasobservedinthepresenceofCO2atlowpH,whichwasmainlyattributedto(FeOH)2UO2CO3andFeOCO2UO2[18].TheTRLFSandU4fXPSspectraofU(VI)sorbedonmontmorilloniteshowedthatU(VI)sorbedonbothexchangeandedgesites,andformedfoursorptionspecies,viz.X2UO2,Al(OH)2UO22+,SiO2UO2andSiO2(UO2)3(OH)5−[23].Liuetal.[24]successfullysynthesizedaninterpenetrationnetworkion-imprintinghydrog