Chromium(VI) reduction by hydrogen sulfide in aqueous media: Stoichiometry and kinetics

The objective of this work was to investigate the reaction stoichiometry, kinetics, and mechanism for Cr(VI) reduction by hydrogen sulfide in the aqueous phase. Batch experiments with excess [Cr(VI)] over [H2S](inverted perpendicular) indicated that the molar amount of sulfide required for the reduction of 1 M Cr(VI) was 1.5, suggesting the following stoichiometry: 2CrO(4)(2-) + 3H(2)S + 4H(+) --> 2Cr(OH)(3)(s) + 3S(s) + 2H(2)O. Further study with transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) confirmed that chromium hydroxide and elemental sulfur were the stable products. The kinetics of Cr(VI) reduction by hydrogen sulfide was measured under various initial concentrations of Cr(VI) and sulfide as well as pH values controlled by HEPES, phosphate, and berate buffers. Results showed that the overall reaction was second-order, i.e., first-order with respect to Cr(VI):and first-order to sulfide. The reaction rate increased as pH was decreased, and the pH dependence correlated well with the fraction of fully protonated sulfide (H2S) in the pH range of 6.5-10. The nature of buffers did not influence the reaction rate significantly in the homogeneous system. The reaction kinetics could be interpreted by a three-step mechanism: formation of an inner-sphere chromate-sulfide intermediate complex (((H2O4CrS)-S-VI)(2-)), intramolecular electron transfer to form Cr(IV) species, and subsequent fast reactions leading to Cr(III).