Synthesized sub-micron pyrite was oxidized in 0.2 μm-filtered Sargasso seawater in order to investigate the rate of oxidation and reaction end-products. Over the initial phase of the reaction, the sub-micron pyrite behaved as a soluble entity as the initial rate of oxidation was determined to be first order with respect to both pyrite and oxygen concentration, where the rate is described as −dFeS2/dt = k[FeS2][O2] and k, the rate constant, is 7.60 × 10−5 + 6.29 × 10−5 μM−1 day−1 at 25 °C. Oxidation proceeds inward from an initial surface oxidation of the pyrite and the formation of an amorphous mixed valence Fe(II)/(III) oxide surrounding the remaining pyrite core. The oxidation rates obtained through this study are up to two orders of magnitude slower than reported in previous pyrite oxidation studies using ground rather than synthesized pyrite at similar pH values. These results may be applied anywhere seawater and sub-micron pyrite are found, including hydrothermal vents, salt marshes and marine sediments.
