Reductionofhexachlorobenzenebynanoscalezerovalenti

SeparationandPurificationTechnology76(2011)268–274ContentslistsavailableatScienceDirectSeparationandPurificationTechnologyjournalhomepage::Kinetics,pHeffect,anddegradationmechanismYang-hsinShih∗,Chung-yuHsu,Yuh-fanSuDepartmentofAgriculturalChemistry,NationalTaiwanUniversity,Taipei106,TaiwanarticleinfoArticlehistory:Received13September2009Receivedinrevisedform18October2010Accepted25October2010Keywords:Nanoscalezero-valentironHexachlorobenzeneActivationenergypHDechlorinationabstractNanoscalezero-valentiron(NZVI)particlesweresynthesizedinthelaboratoryandusedtoinvestigatethereductionkineticsanddegradationmechanismsofhexachlorobenzene(HCB)andtheenvironmentaleffects.TherapiddegradationofHCBbyNZVIfollowspseudo-first-orderkinetics.IncreasingthedoseofNZVIparticlesenhancedthedechlorinationratesofHCB.Withanincreaseintemperature,thedegra-dationrateincreases.Theactivationenergywasdeterminedtobe16.6kJmol−1.ThedechlorinationrateconstantsofHCBlinearlyincreasedfrom0.052to0.12h−1withdecreasingaqueouspHvaluesfrom9.2to3.2.ThedehalogenationofHCBwithNZVIisfavorableunderacidconditions.Thedegradationkineticsandefficiencyincreasedwithincreasingwatercontentinsolutions,indicatingthathydrogenionwasalsooneofthedrivingforcesofreaction.ThestepwisedechlorinationpathwayofHCBwithNZVIwasthedominantreaction.ThesefindingsindicatehightemperatureandacidconditionsarebeneficialtothecatalyticdechlorinationofpolychlorinatedbenzeneswithNZVIanditpotentialapplicationintotheenvironment.CrownCopyright©2010PublishedbyElsevierB.V.Allrightsreserved.1.IntroductionAhighlyhexachlorobenzene(HCB)contaminatedsitewasfoundincentralChina[1].HCBisstillemittedfromthewastestreamdur-ingthemanufactureofseveralchlorinatedhydrocarbonsandsomeelectricarcfurnaces[2]althoughthecommercialproductionofHCBisbannedinmanycountries.HCBisoneofthetwelvepersistentorganicpollutants,whichrequiredtheinventoryandtheactionplanbytheStockholmConventionduetoitstendencytoaccumu-latealongthefoodchain.Moreover,itisrecalcitranttodegradationandhasharmfuleffectsonhumanbeingsandtheenvironment.HCBhasbeenconfirmedasananimalcarcinogenandhasthepotentialtocausedamagetohumanorgans[3–7].Severalstudies[5–7]indi-catethatapossibleassociationbetweentheseHCBexposureandtheriskofhumanbreastcancer.Halogenatedorganiccompounds,presentinthesubsurfaceasdensenonaqueousphaseliquids,arealsocommonlong-termsourcestogroundwaterpollutantsintheenvironment.Theremediationofthesepollutantsrequirespower-fulandcost-effectivetechnologiestotreatthemandcleanthesitesup.Recently,zerovalentiron(ZVI)hasbeenintensivelystudiedfortheremediationofawiderangeofcontaminantsincludingchlorinatedorganiccompounds[8–10].Zerovalentironhasbeen∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](Y.-h.Shih).demonstratedtosuccessfullyremovethesecontaminants.Forhalo-genatedcompounds,thegeneraldehalogenationreactioncanberepresentedbyFe0+RX+H+→Fe2++RH+X−(1)Zerovalentironhasbeenusedtoremediatechlorinatedorganiccompoundcontaminatedgroundwaterthroughtheuseofperme-ablebarriers;however,theinvestigationsintoitsapplicationforpersistentorganicpollutantshavebeenlimited.AndtheparticlesizeandreactivityofconventionalZVImaylimititsapplicationintothesubsurfaceenvironment.Nanoscalezerovalentiron(NZVI)hasbeendevelopedtorapidlyandcompletelychemicallyreducehalogenatedhydrocarbons[11–13].Thechemicalreductionofpolychlorinatedbiphenyls(PCB)hasalsobeendemonstrated[11].TherapidchemicalreductionofPCBwithnanoscalezerovalentironwasobservedbecauseNZVIhasalargerspecificsurfaceareaandhighersurfaceactivitythanmicroscaleZVI[14].NanoscaleZVIhasbeenabettercandidateasaremedialagentthanmicroscaleZVIforpolyhalogenatedorganiccompounds.WehavepresentedthatnanoscaleironandbimetallicPd/FeparticlescanquicklydegradeHCB[15].However,theeffectsofgeochemicalconditions(e.g.,pHandtemperature)onchemi-calreductionkineticsandefficiencyofpolychlorinatedaromaticcompoundswithNZVIwerenotwellunderstood.Therefore,thisstudywasundertakentoassessthedegradationreactivityofHCBtowardnanoscaleZVI,toevaluateitsreductionkineticsandefficiencywithdifferentdosageofNZVIanddifferent1383-5866/$–seefrontmatter.CrownCopyright©2010PublishedbyElsevierB.V.Allrightsreserved.doi:10.1016/j.seppur.2010.10.015Y.-h.Shihetal./SeparationandPurificationTechnology76(2011)268–274269cosolventfractions,toexaminethekineticsofHCBtransformationwithNZVIunderdifferentpHconditionsanddifferenttemperatureconditions,andtoproposethedegradationpathway.ThechemicaldegradationbehaviorsofHCBwithnanoscalezerovalentironwillbeproposedtoevaluatethepossibilityofapplyingnanoscaleironparticlesintheenvironment.2.Materialsandmethods2.1.ChemicalsTwohighpuritychemicalpowders,sodiumborohydrideandferroussulfate,werepurchasedfromRiedel-deHaën(Seelze,Germany).HexachlorobenzenewasobtainedfromSigma–Aldrich(Milwaukee,WI,USA).ChlorobenzenestandardswerepurchasedfromUltraScientificCo(Hope,RI,USA).Methanolandn-hexanewereultraresi-analyzedgradeandpurchasedfromJTBakerChem-icalCo.(Ph