Rates of Arsenopyrite Oxidation by Oxygen and Fe(III) at pH 1.8−12.6 and 15−45 °C

The oxidation rate of arsenopyrite by dissolved oxygen was measured using a mixed flow reactor at dissolved O2 concentrations of 0.007−0.77 mM, pH 1.8−12.6, and temperatures of 15−45 °C. As(III) was the dominant redox species (>75%) in the experimental system, and the As(III)/As(V) ratio of effluent waters did not change with pH. The results were used to derive the following rate law expression (valid between pH 1.8 and 6.4): r = 10(-2211±57)/T(mO2)0.45±0.05, where r is the rate of release of dissolved As in mol m-2 s-1 and T is in Kelvin. Activation energies (E a) for oxidation of arsenopyrite by O2 at pH 1.8 and 5.9 are 43 and 57 kJ/mol, respectively, and they compare to an E a value of 16 kJ/mol for oxidation by Fe(III) at pH 1.8. Apparent As release rates passed through a minimum in the pH range 7−8, which may have been due to oxidation of Fe2+ to hydrous ferric oxide (HFO) with attenuation of dissolved As onto the freshly precipitated HFO.