Behavior of hexavalent chromium in a polluted groundwater: redox processes and immobilization in soils

This work describes the chemical mechanisms governing transport and reduction of hexavalent chromium in soils of a contaminated industrial waste landfill. Groundwater and soil analyses indicate that the main source of chromium is a slag heap essentially consisting of mill tailings. In the groundwater, downstream migration of Cr(IV) is limited thanks to a redox mechanism involving chromate ions and ferrous ions or Fe(II)-bearing minerals. High Fe2+ concentrations in the groundwater are a result of pyrite residues from old activities at the site. Analyses of soil samples reveal that chromium is preferentially located in the soil profile at the fluctuation of the groundwater level. Grain size fractionation of four soil samples was performed, and fraction analyses show that chromium is preferentially accumulated in the clay fraction (<2 microm) and more specifically associated with montmorillonite particles. This work is a demonstration of the reduction of Cr(VI) by Fe(II) studied previously in the laboratory (Buerge, I. J.; Hug, S. J. Environ. Sci. Technol. 1997, 31, 1426-1432; Fendorf, S. E.; Li, G. Environ. Sci. Technol. 1996, 30, 1614-1617; Sedlak, D. L.; Chan, P. G. Geochim. Cosmochim. Acta 1997, 11, 2185-2192) in a field setting. Cr(VI) migration into the groundwater is stopped vertically by the very thick green clay unit and horizontally by the presence of Fe(II) acting as a chemical barrier. The specific site conditions safely prevent any extension of the Cr pollution.