Catalysis of elemental sulfur nanoparticles on chromium(VI) reduction by sulfide under anaerobic conditions

Chromate (CrVI) reduction by sulfide was conducted in anaerobic batch experimental systems. The molar ratio of the reduced CrVI to the oxidized S(-II) was 1:1.5 during the reaction, suggesting that the product of sulfide oxidation was elemental sulfur. Under the anaerobic condition, the reaction was pseudo first order initially with respect to CrVI, but the rate was dramatically accelerated at the later stage of the reaction. The rate acceleration was due to catalysis by elemental sulfur nanoparticles; dissolved species such as monomeric elemental sulfur and polysulfides appeared to be ineffective catalysts. Elemental sulfur nanoparticles were capable of adsorbing sulfide and such adsorbed sulfide exhibited much higher reactivity toward CrVI reduction than the aqueous-phase sulfide, resulting in the observed rate acceleration. Kinetic data under various reactant concentrations can be represented by the following empirical kinetic equation: -d[CrVI]/dt = k1 [CrVI][H2S]0.63 + k3[CrVI][triple bond S--SH]0.57. The first term on the right-hand side corresponds to the noncatalytic pathway, with k1 = 1.0 x 10(-3) (microM)(-0.63) min(-1) at pH 7.60 and 8.2 x 10(-5) (microM)-0.63 min(-1) at pH 8.10. The second term, k3[CrVI][triple bond S--SH]b, is the catalytic term with [triple bond S--SH] representing the adsorbed concentration of sulfide on the elemental sulfur nanoparticles (microM). The catalytic term is more important at the later stage of the reaction, as indicated by the observed kinetics and the enhancement of the reaction rate by externally added elemental sulfur nanoparticles. At pH 8.10, k3 = 0.0057 (microM)(-0.57) min(-1).