EFFECTS OF IRON SOLIDS AND BACTERIA ON IRON OXIDATION RATES IN MINE DRAINAGE

Abstract

Published laboratory rate laws suggest an abiotic homogeneous mechanism, an abiotic heterogeneous mechanism, and a bacterial-heterogeneous mechanism for aqueous iron oxidation. This study attempts to groundtruth these laboratory rate laws against rates measured in the field and from mine drainage brought to the laboratory. Rates of oxidation were measured with varying pH and ferric oxyhydroxide concentrations. The addition of low concentrations of ferric oxyhydroxides from field sites to pH 5.7 and 6 mine water had little effect on oxidation rates, while it increased the rates at pH 3. At pH 6.5, the abiotic heterogeneous rate law overestimated the oxidation rate in the field when there is sediment present. At pH 5.7, the range of measured rates of oxidation fell between the predictions of the abiotic homogeneous and abiotic heterogeneous rate laws. At pH 3 the rate was very similar to those predicted by the bacterial-heterogeneous rate law. Data from this study suggest that acidophilic iron-oxidizing bacteria preferentially attach to the surface of the ferric oxyhydroxides or oxyhydroxysulfates, accelerating the rate of oxidation at low pH; few non-attached bacteria are found in the water column. The heterogeneous rate may not be needed to predict the rate of oxidation at pH greater than 5.7 with low concentrations of ferric oxyhydroxides, but other studies suggest that heterogeneous catalysis is important in this pH range. 1107