Arsenitesorptionatthemagnetite–waterinterfaceduring aqueousprecipitationof magnetite: EXAFSevidencefora newarsenitesurfacecomplex
Résumé
TheinteractionofaqueousAs(III)withmagnetiteduringitsprecipitationfromaqueoussolutionatneutral pHhasbeen studiedasafunctionofinitial As/Feratio. Arseniteissequesteredviasurfaceadsorptionandsurfaceprecipitationreactions, whichinturninfluencethecrystal growthofmagnetite. SorptionsampleswerecharacterizedusingEXAFSspectroscopyat theAsK-edgeincombinationwithHRTEMobservations, energydispersiveX-rayanalysisatthenanoscale, electronenergy lossspectroscopyat theFeL3-edge, andXRD-Rietveldanalysesof reactionproducts. Ourresultsshowthat As(III) forms predominantlytridentatehexanuclear As(III)O3 complexes (3C), wheretheAs(III)O3 pyramids occupyvacant tetrahedral sites on{111}surfaces of magnetiteparticles. This is thefirst timesuchatridentatesurfacecomplexhas beenobserved for arsenic. This complex, withadominant As–Fedistanceof 3.53±0.02A ̊, occurs inall samples examinedexcept the onewiththehighestAs/Feratio(0.33). Inaddition, atthetwohighestAs/Feratios(0.133and0.333)arsenitetendstoform mononuclearedge-sharingAs(III)O3species(2E)withinahighlysolubleamorphousAs(III)–Fe(III,II)-containingprecipitate. AtthetwolowestAs/Feratios(0.007and0.033), ourresultsindicatethepresenceofadditional As(III)specieswithadom- inantAs–Fedistanceof3.30±0.02A ̊, forwhichapossiblestructural model isproposed. Thetridentate3CAs(III)O3com- plexes onthe{111}magnetitesurface, together withthis additional As(III) species, dramaticallylower thesolubilityof arseniteintheanoxicmodelsystemsstudied. Theymaythusplayanimportantroleinloweringarsenitesolubilityinputative magnetite-basedwatertreatmentprocesses, aswell asinnatural iron-richanoxicmedia, especiallyduringthereductivedis- solution-precipitationof ironmineralsinanoxicenvironments.