KINETICS AND MECHANISM OF THE REACTION OF H2S WITH LEPIDOCROCITE

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: F(5<pH<6) = k(a)[H+]-2[H2S]totA and R(7<pH<8.6) = k(b)[H+]1[H2S]totA with k(a) = 1.5 X 10(-13) M2 min-1 and k(b) = 2 X 10(6) 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 H2S(tot) suggests that polysulfides and elemental sulfur are the initial products formed during the experiments.