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

20authors concluded that hydroxyl-Al interlayered phyllosilicates act as a potential sink forZn in acidic soils. Furrer et al. (2001) proposed a similar binding mechanism for Ni.EXAFS studies helped to isolate the sorption mechanism where hydroxyl-Al interlayersof phyllosilicates may be important for the retention of 3d <strong>metal</strong>s (Scheinost et al. 2002).EXAFS has been extensively used to probe the local structure of adsorbed specieson model soil surfaces. From such studies, Axe et al. (1998) concluded that the strontiumion remains hydrated thus physically adsorbed to HFO surface. Sahai et al. (2000)observed that strontium sorbed to kaolinite, amorphous silica gel, and goethite surfacesby forming hydrated surface complexes and this structure did not change up to 57 days.Concomitant with a maximum of dissolved CO2, the authors observed an increase of Srsorption at approximately pH 8.5 where the local structure was consistent with SrCO3(s).Sahai et al. (2000) suggested that sorption of carbonate may nucleate the precipitation ofSrCO3 in the pH range in which carbonate species sorption on goethite is near amaximum. Additionally, Zn(II) was found to form bidentate surface complex on 2-lineferrihydrite (Waychunas et al. 2002, 2003; Trivedi et al. 2004), aluminum oxide(Trainor et al. 2000), and goethite (Schlegel et al. 1997; Trivedi et al. 2001a).Many studies have been conducted on Pb(II) <strong>adsorption</strong> mechanisms oncrystalline iron and aluminum oxides (e.g., Chisholm-Brause et al. 1989, 1990; Roe et al.1991; Bargar et al. 1997b; Strawn et al. 1998; Ostergren et al. 1999a, b, 2000a; Elzinga etal. 2001; Templeton et al. 2003). To construct an accurate description of Pb(II)-oxidesurface, Bargar et al. (1997b) studied Pb(II) sorption products and surface functionalgroups on iron oxides. They found that Pb(II) ions sorbed as mononuclear bidentatecomplexes to edges of FeO6 octahedra on both goethite (up to 4.2 μmol Pb m -2) and