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.