EFFECT OF METALS ON THE REDUCTION OF CHROMIUM (VI) WITH HYDROGEN-SULFIDE

The influence of an extended set of metals (Ca2+, Mg2+, Pb2+, Cu2+, Cd 2+ 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 mu M) did not show any significant effect. Mg2+ and Mn2+ (0.05 to 0.2 M and 1.8 to 14.2 mu M) produced only moderate incre ases in the rates in 0.5 M NaCl that were not proportional to the meta l concentrations. The metals studied (Pb2+, Cu2+, Cd2+ Ni2+) caused la rge increases in the reduction rates at micromolar concentrations. The effect was attributed to the formation of MeCrO4 complexes which reac t faster with sulfide than free chromate. From measurements of the rat es as a function of the added metal the rate constants for the metal c hromate complexes were determined. The rates increased in the order k( NiCrO4)>k(PbCrO4)>(CuCrO4)>k(CdCrO4). The addition of Fe(III) (0.2 to 1.8 mu M) increased the rates considerably due to the catalytic cyclin g of the redox reactions of iron. (C) 1998 Elsevier Science Ltd. All r ights reserved.