Rates of arsenopyrite oxidation by oxygen and Fe(III) at pH 1.8-12.6 and 15-45 degrees 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 degrees 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 (Ea) for oxidation of arsenopyrite by 02 at pH 1.8 and 5.9 are 43 and 57 kJ/mol, respectively, and they compare to an Ea 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.