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 (Fe2O3⋅nH2O, n=1–3n=1–3) and HMO (MnO2) surfaces has been studied with EXAFS: mononuclear bidentate surface complexes were observed on FeO6 (MnO6) octahedra with PbO distance of 2.25–2.35 Å and PbFe(Mn) distances of 3.29–3.36 (3.65–3.76) Å. These surface complexes were invariant of pH 5 and 6, ionic strength 2.8×10−32.8×10−3 to 1.5×10−21.5×10−2, loading 2.03×10−42.03×10−4 to 9.1×10−3 molPb/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 (∼77%) followed by a slow intraparticle diffusion step (∼23%) resulting in a surface diffusivity of 2.5×10−15 cm2/s2.5×10−15 cm2/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.

EXAFS spectra and Fourier transform (magnitude and imaginary part) of Pb/HFO samples at pH 5 and IS 10−2 as a function of time at Pb LIII-edge. FT was performed over range 2.8–9.5 Å−1, fitted over range 0.45–3.58 Å. Dashed lines represent fits.Figure optionsDownload as PowerPoint slide