Kinetics and mechanism of the reaction of H2S with lepidocrocite

Ferric oxides had an important role in the cycling of elements in lakes due to their ability to scavenge reaction species by adsorption. The reaction of hydrogen sulphide with ferric oxide surface in the pH range 4-8.5 was studied by monitoring the electromotive force of a hydrogen sulphide-sensitive electrode cell using lepidocrocite (gamma-FeOOH) as a model compound. The reaction was a surface-controlled process. The rate of hydrogen sulphide oxidation was pseudo first order with respect to hydrogen sulphide and showed a strong pH dependency (maximum at pH 7.0). Two rate laws were derived. A model proposed for the reductive dissolution of haematite by hydrogen sulphide was used to explain the reaction of hydrogen sulphide with lepidocrocite. The oxidation rate of hydrogen sulphide was proportional to the concentration of inner-sphere surface complexes of the hydrogen sulphide ion formed with either the neutral ferric oxide surface sites of the protonated ferric oxide surface sites. The abiotic oxidation of hydrogen sulphide by reactive ferric oxides had a greater role in sediment diagenesis than previously thought. There are 34 references.