Mechanism of Hydrogen Sulfide Oxidation by Manganese(IV) Oxide in Aqueous Solutions

The kinetics of reductive dissolution of Mn(IV) oxides by hydrogen sulfide has been investigated. The concentration of total H 2 S(aq) in the reaction system ranged between 1.0 and 4.0 mM and was adjusted keeping a constant partial pressure of the gaseous H 2 S phase above the solution. The reaction products were identified as Mn(II), sulfate, elemental sulfur, and small amounts of thiosulfate. The distribution of products changes with pH; sulfate is the main product at low pH values while elemental sulfur is the main product at near neutral pH values. The rate constant decreased with an increase in pH. Experimental data indicate that the rate law is first order on both the surface sites and the H 2 S concentration. The reaction proceeds via the formation of two different inner-sphere surface complexes ⋮Mn IV S - and ⋮Mn IV SH and their further oxidation to products. A mechanism in agreement with the experimental results is proposed.