A kinetic study of Cr(VI) reduction by calcium polysulfide

The goal of this study was to determine the influence of pH and oxygen conditions on the reaction kinetics of hexavalent chromium (Cr(VI)) with calcium polysulfide (CPS). The observed kinetic reaction rate, k_obs, between Cr(VI) and CPS was evaluated for the pH range 5.5-8.5, aerobic open, aerobic closed and anaerobic conditions, and two Cr(VI) species (aqueous and adsorbed on goethite). The kinetic model followed a second-order reaction rate, unlike the first-order reaction rate of the Cr(VI)-sulfide reaction. k_obs increased exponentially with pH in the range of 8.5 to 5.5 under anaerobic conditions, while under aerobic conditions k_obs had a maximum at pH 7. Qualitative sulfur analyses showed that thiosulfates were present at and above pH 7 in the aerobic experiments, which can also reduce Cr(VI). Similarly, pure CPS in the presence of oxygen yielded sulfides and thiosulfates, while sulfide converted to a mixture of sulfite, thiosulfates, sulfites and sulfates. CPS is thus likely to have a longer residence time and greater reducing capacity in the subsurface compared to sulfide at neutral and basic pH environments.