Reductive dissolution of iron(III) (hydr)oxides by hydrogen sulfide

The reductive dissolution of hematite by H2S was investigated under conditions of constant partial pressure of H2S (10-4 to 10-2 atm), pH = 3-7 and 25°C, and under conditions where the solubility product of iron sulfide is not exceeded. The reaction proceeds by an initial (relatively fast) formation of ≡FeS- and ≡FeSH surface complexes; subsequently, electron transfer and detachment of Fe (II) from the oxide surface occur. The rate law for the dissolution can be interpreted by assuming a surface-controlled reaction depending on the surface concentration of the surface complexes ≡FeS- and ≡FeSH: Rt = ke≡FeS- + ke′≡FeSH where ≡FeS- and ≡FeSH are surface concentrations in mol·m-2. The experimental data are compatible with the constants ke = 30 h-1 and ke′ = 400 h-1 and surface complex formation constants for ≡FeS- (≡FeOH + H2S (g) ⇄ ≡FeS- + H+ + H2O) and ≡FeSH (≡FeOH + H2S (g) ⇄ ≡FeSH + H2O) of log Ks = -2. 70 and 2. 82, respectively. Preliminary experiments with lepidocrocite and goethite give results on the reaction rates that show the same kind of dependence on solution variables as those with α-Fe2O3 and are compatible with the mechanism proposed here.