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The interaction of H 2 S with the surface of either R-Fe 2 O 3 or amorphous ferric hydroxide at low temperatures has been characterized using FTIR and quantified using volumetric methods. The mechanism of the interaction of H 2 S with either reagent is ionic and involves heterolytic dissociation of H 2 S and exchange of S 2and SHanions for O 2or OH -. Subsequently water is eliminated and, in coupled redox reactions, sulfide is oxidized to elemental sulfur and Fe 3+ cations are reduced to Fe 2+ . Initially, almost all the surface sites can interact readily with H 2 S, to form a layer of iron sulfides. Over longer reaction times reaction occurs with deeper sites, to form bulk sulfides. Regeneration of ferric (hydr)oxide by oxidation with oxygen converts Fe 2+ to Fe 3+ , and S 2anions are replaced by O 2with formation of elemental sulfur. The surface species involved in each reaction have been characterized using FTIR and, for the first time, direct evidence has been obtained for the presence of SHanions as very reactive intermediates.

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