Surface complexation of Hg2+ on goethite: Mechanism from EXAFS spectroscopy and density functional calculations

EXAFS spectra of Hg2+ sorbed on goethite at pH 4.60 has revealed the existence of an inner-sphere complex with an immediate coordination shell of 2 O atoms at 2.04 Angstrom and a further shell of 1 Fe atom at 3.28 Angstrom. Using density functional calculations, we determined the optimized geometries of small clusters representing surface complexes. These results shows that adsorption of Hg2+ occurs via two oxygen atoms bound to edge-sharing Fe sites on the (110) surface. This is the same surface site we find for Cd (1, 2) and Sr (3). The short Hg-O bond length shows that the oxygens bridging Hg and Fe atoms are deprotonated. (C) 1999 Academic Press.