Heavy metal adsorption on iron oxide and iron oxide-coated silica ...

21hematite (up to 9.9 limo' Pb ril-2). Combined with bond-valence determination, theauthors proposed that Pb(II) <strong>adsorption</strong> occurred primarily at [Fe F:4O -1/2 ] and[Fe-OH 2+1/2 ] sites. Similar surface complexes have been observed for Al oxides (Bargaret al. 1997a; Strawn et al. 1998). However, Pb binding has been found to change withincreasing sorption density: surface complexes on goethite shifted from bidentate cornersharingto bidentate edge-sharing as the loading increased from 0.83 to 4.9 μmol Pb m -2(Templeton et al. 2003). On the other hand, Pb(II) polynuclear complexes were observedon γ-Al2O3 (1.3 μmol Pb 111-2, Chisholm-Brause et al. 1990), a-Al2O3 (> 3.4 μmol Pb m-2,Bargar et al. 1997a), and goethite (9.6-19.2 μmol Pb m -2, Roe et al. 1991). Interestingly,Trivedi et al. (2003) found a mixture of bidentate edge- and monodentate corner-sharingPb(II) complexes on 2-line ferrihydrite at pH 4.5 above which only the former wasobserved despite the extent of surface coverage.Pb(II) uptake by goethite in the presence of various anions has also beeninvestigated. Bargar et al. (1998) proposed that the presence of Cl- has little effect on thePb(II)/goethite and Pb(II)/'y-alumina surface complexes at pH 7, while Pb(II)-C1 --goethiteternary complexes were formed at pH ... 6. Recent studies (Elzinga et al. 2001; Ostergrenet al. 2000 a, b) indicate that first and second-shell distances for Pb sorption to goethiteare essentially unchanged (AR < 0.03 A) by the presence of carbonate or sulfate. Whilemuch work has been conducted with crystalline oxide surfaces, limited EXAFS studieshave been reported on amorphous iron oxides. Manceau et al. (1992) observed thatPb(II) was adsorbed on HFO as mononuclear bidentate complexes on edges of FeO6octahedra based on a short-term contact time of three hours. Scheinost et al. (2001)found that Cu and Pb formed similar edge-sharing inner-sphere sorption complexes when