Abstract

It is possible to demonstrate a heterogeneous reaction mechanism for both pyrite oxidation and reduction using a molecular orbital theory approach. The mechanism demonstrates that attachment to the FeS₂ surface by an oxidant or reductant requires that they have a vacant orbital (solution phase) or site (solid phase) to bind the oxidant or reductant to a sulfur from S²⁻₂ in FeS₂. The approach thus requires the formation of a persulfido (disulfide) bridge between the iron in pyrite and the oxidant (e.g. Fe³⁺) or the reductant (e.g. Cr²⁺).

The first electron transfer in oxidation occurs from the π^*; orbital (highest occupied molecular orbital, HOMO) of the S²⁻₂ in FeS₂ to the π orbital (lowest unoccupied molecular orbital, LUMO) of the oxidant. Electron transfer in reduction occurs from the σ^* orbital (HOMO) of the reductant to the σ_* orbital (LUMO) of the S²⁻₂ in FeS₂. The bridge formation between two metals by a common ligand (persulfido), and the electron transfer, is consistent with an inner sphere type mechanism. In FeS₂, however, the ligand S²⁻₂ acts as the electron source or sink rather the Fe²⁺. The strength of the sulfur-sulfur bond in the persulfido bridge after electron transfer is key to the understanding of FeS₂ oxidation and reduction.

Additional bridges can be formed on the FeS₂ surface during oxidation until the initial oxidation product S₂O²⁻₃ is produced. The proposed mechanism allows for facile pyrite oxidation by Fe³⁺ but not O₂. This is consistent with experimental observations. The mechanism does not require the formation of a free radical in solution. It does predict the formation of an ion radical on the pyrite surface. The proposed mechanism gives the same initial intermediate (FeS₂O) as recently proposed by Moses et al. (1987), but explains possible surface attachment by an oxidant and subsequent FeS₂ oxidation using a molecular orbital theory approach. The proposed surface mechanism is consistent with experimental observations of several investigators (Goldhaber, 1983; McKibben and Barnes, 1986; Wiersma and Rimstidt, 1984).

The importance of thiosulfate formation and reactivity with Fe³⁺ is discussed in light of pyrite oxidation and of previous reports on the presence of thiosulfate in marine porewaters