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

31• Understand the effect of competing ions on heavy <strong>metal</strong> <strong>adsorption</strong> to ironoxides. The ability of models developed in this research to predict heavy <strong>metal</strong>competition on iron oxide will be tested.Specific hypotheses for this research include:First, surface functional groups and reactions are similar between crystalline andamorphous iron oxides (Davis and Leckie, 1978; Charlet and Manceau, 1993; Bargar etal. 1997b; Villalobos et al. 2001; Dixit and Hering, 2003). Therefore, XAS results for<strong>metal</strong> ions sorbed on HFO may be applied to that on goethite. Bargar et al. (1997a, b)reported that iron oxides and aluminum oxides have fundamentally similar crystalchemistry — both form Fe06 or A106 octahedra with very similar average Al-O and Fe-Obond lengths, therefore, they may exhibit comparable surface structures and functionalgroups. Bargar et al. (1997a, b) found that Pb(II) ions sorbed as mononuclear bidentatecomplexes to edges of FeO6 or A1O6 octahedra on goethite, hematite, and aluminumoxide. Also, Pb <strong>adsorption</strong> to HFO (three hours contact time) and 2-line ferrihydrite(Manceau et al. 1992; Scheinost et al. 2001; Trivedi et al. 2003 [for pH > 4.5]) weredominated by similar surface complexes. The first hypothesis will be tested by analyzingPb/HFO sorption samples (4 hours contact time) with XAS and comparing to that ofPb/goethite.Secondly, coupled with XAS data, intrinsic surface complexation constants can beachieved by calibrating the SCM with a variety of <strong>adsorption</strong> data including titration,isotherm, and edge data covering a wide range of conditions. SCMs offer the advantageof representing surface chemical reactions with a set of quasi-thermodynamic constants,which are independent of changes in solution conditions (Hayes, 1987). However, fewstudies have addressed both isotherm and edge data over a large range of conditions withthe same model (e.g. Müller and Sigg, 1992; Waite et al. 1994; Heidmann et al. 2005);