Effect of metals on the reduction of chromium (VI) with hydrogen sulfide

The influence of an extended set of metals (Ca²⁺, Mg²⁺, Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Zn²⁺, Mn²⁺, Fe³⁺) on the reduction rates of chromium (VI) with H₂S have been measured in NaCl solutions. Ca²⁺ and Zn²⁺ (0.01 to 0.02 M and 0.5 to 3.0 μM) did not show any significant effect. Mg²⁺ and Mn²⁺ (0.05 to 0.2 M and 1.8 to 14.2 μM) produced only moderate increases in the rates in 0.5 M NaCl that were not proportional to the metal concentrations. The metals studied (Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺) caused large increases in the reduction rates at micromolar concentrations. The effect was attributed to the formation of MeCrO₄ complexes which react faster with sulfide than free chromate. From measurements of the rates as a function of the added metal the rate constants for the metal chromate complexes were determined. The rates increased in the order k(NiCrO₄)>k(PbCrO₄)> k(CuCrO₄)>k(CdCrO₄). The addition of Fe(III) (0.2 to 1.8 μM) increased the rates considerably due to the catalytic cycling of the redox reactions of iron.