Sulfate-adsorption-and-surface-precipitation-on-a-volcanic-ash-soil-(allophanic-andisol)

JournalofColloidandInterfaceScience300(2006)504–510www.elsevier.com/locate/jcisSulfateadsorptionandsurfaceprecipitationonavolcanicashsoil(allophanicandisol)MunehideIshiguroa,∗,TomoyukiMakinob,YasunobuHattoriaaFacultyofEnvironmentalScienceandTechnology,OkayamaUniversity,Okayama700-8530,JapanbNationalInstituteforAgro-EnvironmentalSciences,Tsukuba,Ibaraki305-8604,JapanReceived31January2006;accepted10April2006Availableonline5June2006AbstractSulfatestronglyadsorbsonmetaloxidesandsoilswithvariablecharges.However,itssurfaceprecipitationhasnotbeenclearlyevaluatedanditsadsorptionmechanismhasbeenindispute.Inthepresentstudy,anallophanicandisol,atypicalvolcanicashsoilhavingbothnegativeandpositivevariablecharges,wasusedtoidentifytheadsorptionmechanismofsulfate.SulfateadsorptionisothermswereobtainedbyabatchmethodatpHvaluesof4,5,6,and7inawiderangeofconcentrationsinanNa–H–SO4–OHsystem.Theoreticalisothermswereappliedtothemeasuredvaluesfortheevaluation.Thesurfaceprecipitationwasdetectedbythemeasuredadsorptionisotherms,andtheBETisothermconfirmedthepresenceofmultilayeradsorption.StrongerandweakeradsorptionsitesweresuggestedbyusingtheLangmuirisothermforthemonolayeradsorption.TheadsorptionenergiesobtainedfromtheLangmuirequationandrecentspectroscopicanalysissuggestedthatthestrongeradsorptioncorrespondedtoaninner-spheresurfacecomplexandthattheweakeradsorptioncorrespondedtoouter-spheresurfacecomplexation.TheBETandLangmuirequationsshowedthreetypesofadsorptionmechanismsforthesulfateadsorptiononthesoil.©2006ElsevierInc.Allrightsreserved.Keywords:Surfaceprecipitation;Sulfateadsorption;BETequation;Langmuirequation;Allophanicsoil;Variablecharge;Adsorptionisotherm;Multilayeradsorption1.IntroductionSulfatestronglyadsorbsonmetaloxidesandsoilswithvari-ablecharges.However,itssurfaceprecipitationhasnotbeenclearlyevaluatedanditsadsorptionmechanismhasbeenindis-pute.Precipitationisenhancednearthesolid–solutioninterfaceunderundersaturatedconditions[1].Accordingtosurfacepre-cipitation,achemicalequilibriummo