Surface complexation of Pb(II) at oxide-water interfaces: II. XAFS and bond-valence determination of mononuclear Pb(II) sorption products and surface functional groups on iron oxides
- Stanford Univ., CA (United States)
Pb(II) sorption on goethite and hematite powders was studied at room temperature as a function of pH (6-8), sorption density (2-10 {mu}moles/m{sup 2} ), and [Pb]{sub eq} (0.2 {mu}M - 1.2 mM) in 0.1 M NaNO{sub 3} electrolyte using XAFS spectroscopy. Pb(II) ions were found to be hydrolyzed and adsorbed as mononuclear bidentate complexes to edges of FeO{sub 6} octahedra on both goethite and hematite under all conditions. Hydrolysis of Pb(II) appears to be a primary source of proton release associated with surface complexation of Pb(II). A bond-valence model was used to relate the relative stabilities of iron-oxide surface functional groups and Pb(II) adsorption complexes to their structures and compositions. This combined approach suggests that Pb(II) adsorption occurs primarily at unprotonated [Fe {sub Fe}{sup Fe}(>)O{sup -{1/2}}] sites and at [Fe-OH{sub 2}{sup +}{sup +{1/2}}] sites. Several adsorption reactions are proposed. Comparison to EXAFS results from Pb(II) adsorption on aluminum oxides suggests that the edge lengths of surface AlO{sub 6} or FeO{sub 6} octahedra partially determine the reactivities and densities of available surface sites. The results of this study provide a basis for constructing chemically realistic descriptions of Pb(II) surface complexation reactions on Fe hydroxides. 46 refs., 7 figs., 4 tabs.
- DOE Contract Number:
- FG03-93ER14347
- OSTI ID:
- 576308
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 61, Issue 13; Other Information: PBD: Jul 1997
- Country of Publication:
- United States
- Language:
- English
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