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

Abstract

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: <I>R</I>_t = <I>k</I>_e{≡FeS^-} + <I>k</I>_e′{≡FeSH} where {≡FeS^-} and {≡FeSH} are surface concentrations in mol·m^-2. The experimental data are compatible with the constants <I>k</I>_e = 30 h^-1 and <I>k</I>_e′ = 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 <I>K</I>^s = -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.