Reduction of chromium(VI) by pyrite in dilute aqueous solutions

Hexavalent chromium is a known toxic and mobile contaminant in the environment. Reduction of Cr(VI) using Fe(II) compounds is one of the most common practices in dealing with Cr(VI) as a contaminant. Among the various Fe(II) salts suggested for Cr(VI) treatment, pyrite, also a potential waste as well as a natural mineral, has been suggested. This study investigated the abiotic reduction of Cr(VI) by pyrite under dark and anaerobic conditions. The effect of pertinent chemical properties such as the initial Cr(VI) concentration, the amount of pyrite, temperature, and pH on the pyrite reduction of Cr(VI) was studied. The results showed that the rate of Cr(VI) reduction were strongly dependent on pH. A maximum rate at pH 3 was observed; at pH > 3.0, the initial rate of Cr(VI) reduction decreased as the pH increased and at pH < 3.0, the rates increased as pH increased. The rates of pyrite oxidation by Cr(VI) from pH 2.0 to 11.0 were found to be zero-order with respect to the concentration of initial Cr(VI) and second order with respect to the ratio of pyrite surface area to solution volume. The activation energy of the Cr(VI) reduction in the temperature range of 10-50 degrees C was 73 kJ/mol. The relatively high activation energy indicated that surface reaction controlled the reduction rate over this temperature range. Based on the results, it is seen that Cr(VI) reduction reaction took place in two distinct stages each involved with multiple steps. The first stage involved the dissolution of pyrite under acidic conditions that released Fe(II) and S-2(2-) ions which were then readily oxidized by Cr(VI) with the production of Cr(III), Fe(III), and SO42- species. In the presence of Cr(VI), direct oxidation of pyrite took place at rate slower than the above redox reactions. Immediately upon the depletion of Cr(VI), reaction between Fe(III) and excessive pyrite resulted in the generation of Fe(II) species, however. (C) 2008 Elsevier B.V. All rights reserved.