Aqueous geochemical and surface science techniques were used to investigate the effect of phosphate on the oxidation of crushed and {100} pyrite. Studies showed that the presence of phosphate in solution significantly impeded the oxidation rate of crushed pyrite at pH values equal to or greater than 4. At pH 3, the presence of phosphate had almost no experimentally discernible effect on pyrite oxidation. X-ray photoelectron spectroscopy (XPS) studies showed that phosphate, at pH values greater than 4, became irreversibly bound to an Fe{sup}(3+)-bearing product on the pyrite surface during the oxidation process. Once bound to this region on the pyrite surface, the adsorbed phosphate inhibited further oxidation of the pyrite (based on XPS determinations of sulfur and iron oxide product concentrations) under our experimental conditions. These results suggested that the rate of pyrite oxidation in the absence of phosphate was facilitated at or near Fe{sup}(3+) - bearing oxidation phases on the surface. Phosphate bound on iron(III) oxide product either prevents O{sub}2 adsorption on this phase or electronically modifies these surface regions, but in either case it inhibits electron transfer from pyrite-Fe{sup}(2+) sites to molecular O{sub}2.
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