Surface complexation of Pb(II) on amorphous iron oxide and manganese oxide: spectroscopic and time studies

Hydrous Fe and Mn oxides (HFO and HMO) are important sinks for heavy metals and Pb(II) is one of the more prevalent metal contaminants in the environment. In this work, Pb(II) sorption to HFO (Fe(2)O(3) x nH(2)O, n=1-3) and HMO (MnO(2)) surfaces has been studied with EXAFS: mononuclear bidentate surface complexes were observed on FeO(6) (MnO(6)) octahedra with PbO distance of 2.25-2.35 Angstrom and PbFe(Mn) distances of 3.29-3.36 (3.65-3.76) Angstrom. These surface complexes were invariant of pH 5 and 6, ionic strength 2.8 x 10(-3) to 1.5 x 10(-2), loading 2.03 x 10(-4) to 9.1 x 10(-3) mol Pb/g, and reaction time up to 21 months. EXAFS data at the Fe K-edge revealed that freshly precipitated HFO exhibits short-range order; the sorbed Pb(II) ions do not substitute for Fe but may inhibit crystallization of HFO. Pb(II) sorbed to HFO through a rapid initial uptake ( approximately 77%) followed by a slow intraparticle diffusion step ( approximately 23%) resulting in a surface diffusivity of 2.5 x 10(-15) cm(2)/s. Results from this study suggest that mechanistic investigations provide a solid basis for successful adsorption modeling and that inclusion of intraparticle surface diffusion may lead to improved geochemical transport depiction.