The kinetics of pyrite oxidation in marine systems

Pyrite oxidation is a significant process in many marine environments, including salt marshes, normal marine sediments, and the massive deposits and plumes of hydrothermal vents. The kinetics of pyrite oxidation was studied at 10 to 55$\sp\circ$C, pH 7 to 9, oxygen partial pressures of 0 to 100%, and ionic strength of 0 to 1 in various solutions approximating seawater. The effects of these parameters on the oxidation rate, solution sulfur speciation, and nature of the solid alteration products was assessed using various techniques, including ion chromatography, SEM/EDS, TEM/ED, and FTIR.The effects observed on the oxidation rate were as follows. Pretreatment of the pyrite grains that included either a hot HCl rinse or prolonged exposure to ethanol resulted in increased initial oxidation rates. Pyrite from two different sources exhibited identical oxidation rates, even though their grain morphologies and elemental impurities were significantly different. The rate was found to vary with (O$\sb2$) $\sp{0.5}$, and exhibited an activation energy of 83 kJ mol$\sp{-1}$. The rate was observed to increase slightly with increasing ionic strength and to increase slightly with increasing pH. Solution components determined to affect the rate were bicarbonate, divalent cations (specifically, Mg$\sp{2+}$ and Ca$\sp{2+}$), and phosphate. These results were modelled in terms of a dependence of the rate upon the concentration of free carbonate ion, (CO$\sb3\sp{2-}$) $\sb{\rm F}$, in solution. A model is proposed in which the formation of a Fe(II)CO$\sb3$ complex on the pyrite surface facilitates the oxidation of the pyritic sulfur by molecular oxygen. The model also implies that OH$\sp-$ and H$\sb2$O behave similarly to CO$\sb3\sp{2-}$, and may account for the weak pH dependence in this pH range. Study of the solution sulfur speciation indicated increasing levels of sulfoxy anion intermediates with increasing pH, and decreasing levels with increasing phosphate concentration. SEM/EDS analysis indicated that oxidation was concentrated at sites of high excess surface energy on the pyrite surface. The only solid phase alteration product conclusively identified was ferrihydrite. Phosphate was found to inhibit iron oxyhydroxide formation at high concentrations and to be incorporated into precipitated iron oxyhydroxides at low concentrations