Sorption and speciation of heavy metals on hydrous Fe and Mn oxides. From microscopic to macroscopic

Abstract

The mobility, biodisponibility and toxicity of heavy metals are largely controlled by chemical reactions which take place at the hydrous oxide/water interface. The recent development of the synchrotron-based fluorescence yield X-ray absorption spectroscopy permits elucidation of these chemical reactions, at a molecular level. Six case studies are presented: the sorption mechanism of SeO 3 2−, UO 2 2+, Pb(II), Cr(III) on hydrous Fe oxide, Pb(II) on hydrous Mn oxide and the surface oxidation mechanism of the Cr(III) oxidation to Cr(VI) at the MnO 2/water interface. For all these examples the nature of the active site involved in the sorption process is described. On hydrous Fe oxide, cations [except Cr(III)] and anions adsorb on different surface sites: the former share edges and the latter double corners with Fe(O,OH) 6 octahedra. The sizes of these two surface sites are quite identical so that steric factors cannot account for the difference of sorption processes between cations and anions. The high reactivity of hydrous oxides results from the fact that hydroxyl groups of the two active sites form an ideal “template” for bridging heavy metals. Indeed, OH-OH distances match well with the geometry of the coordination polyhedra of heavy metals. As an application it is shown how the knowledge of sorption mechanisms, at a microscopic level, may help understanding aqueous chemical experiments conducted at a macroscopic level. More generally, this level of investigation is a key issue in evaluating the environmental impact of long term application of heavy metals to land.