The initial reaction between hydrogen sulfide and the surface of lepidocrocite was studied in the pH range between 4 and 8.6 by monitoring the change of the emf of a pH₂S sensor. The rate of H₂S oxidation is pseudo first order with respect to H₂S and shows a strong pH dependence with a maximum at pH 7. Two rate laws were derived: R(5 < pH < 6) = ka[H⁺]⁻²[H₂S]totA and R(7 < pH < 8.6) = kb[H⁺]¹[H₂S]totA with ka = 1.5 x 10⁻¹³ M² min⁻¹ and kb = 2 x 10⁶ M⁻¹ min⁻¹. The pH maximum of the reaction rate can be explained by using a surface speciation model suggested for the reductive dissolution of hematite by H₂S (1): The oxidation rate of H₂S is proportional to the concentration of inner-sphere surface complexes of HS formed with either the neutral ferric oxide surface sites (>FeOH) or the protonated ferric oxide surface sites (>FeOH₂⁺). The amount of protons consumed per mole of H₂Stot suggests that polysulfides and elemental sulfur are the initial products formed during the experiments.he experiments.
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Peiffer et al. (1992) studied this question.