ABIOTIC OXIDATION RATE STUDIES OF SYNTHETIC PYRITE DOPED WITH AS, CO, AND NI: WHAT ARE THE EFFECTS OF MINOR ELEMENTS ON PYRITE OXIDATION?

Batch reactor oxidation experiments are in progress on synthetic pyrite doped with As, Co, and Ni as well as that synthesized with no intentionally added impurities. The electrical properties of these pyrites vary with impurity type. Pyrite containing only As typically exhibits p-type conductivity while the others are n-type. Co-doped pyrite exhibits conductivity several orders of magnitude above the other types behaving as a semi-metal. This may be due to the stabilization energy of low spin Co²⁺ substituting for Fe²⁺ in the pyrite structure, which places an e_g electron in a defect state very high in the forbidden zone overlapping the pyrite conduction band. By contrast, the stabilization energy for Ni²⁺ substituting for Fe²⁺ in pyrite is not as great, with the result of placing the two e_g electrons in a defect state within the forbidden zone far below the conduction band energy. Arsenic substituting for S seems to introduce empty defect states in the forbidden zone just above the valence band energy. Valence electrons should be able to occupy these defect states resulting in holes as charge carriers.

It has been suggested that redox reactions at one site on a semiconducting crystal lattice may affect the electronic structure and thus reactivity at remote sites due to electron transfer through the substrate. The high conductivity of Co bearing pyrite may affect its oxidization rate. The tendency for arsenian pyrite to be a p-type semiconductor may also influence its oxidation rate. In natural pyrites, As rich areas are found within grains rich in either Ni or Co or both. Thus, the potential for enhanced oxidation may exist due to voltage gradients arising from p-n conductivity junctions and variable resistivity. The homogeneity of our synthetic samples allows us to study the oxidation kinetics of pyrite with different minor elements and to compare these results with pyrite containing impurity concentrations of less than 20ppm.