Arsenite sorption at the magnetite–water interface during aqueous precipitation of magnetite: EXAFS evidence fo ra new arsenite surface complex

Yuheng Wang (1) , Guillaume Morin (1) , Georges Ona-Nguema (1) , N. Menguy (1) , Farid Juillot (1) , Emmanuel Aubry (2) , François Guyot (1) , Gordon E. Brown Jr. (3, 4)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés
2 Bioemco - Biogéochimie et écologie des milieux continentaux
3 Surface and Aqueous Geochemistry Group [Stanford]
4 SSRL SLAC - Stanford Synchrotron Radiation Lightsource

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.

Domaines: Minéralogie

Yuheng Wang, Guillaume Morin, Georges Ona-Nguema, N. Menguy, Farid Juillot, et al.. Arsenite sorption at the magnetite–water interface during aqueous precipitation of magnetite: EXAFS evidence fo ra new arsenite surface complex. Geochimica et Cosmochimica Acta, 2008, 72, pp.2573-2586. ⟨10.1016/j.gca.2008.03.011⟩. ⟨hal-00282032⟩