Heterogeneous oxidation of Fe(II) in AMD

Abstract

Laboratory and field experiments were used to measure the effects of several variables on the rate of heterogeneous oxidation in acid mine discharges (AMD). A wider range of variables were used compared to previously published studies, i.e., Fe(III)solid from 0.012 to 2.40 g L−1, pH 5.35 to 6.81, DO 0.11–0.36 mM, temperature 10.1–22.7 °C, and Fe(II)total 0.04–1.0 mM. The heterogeneous rate exceeded the predicted homogeneous rate for slightly acidic pH values when Fe(III)solid was greater than about 0.02 g L−1, with the cross-over concentration of Fe(III)solid a function of temperature. An equation for the heterogeneous oxidation rate was derived, d[Fe(II)aq]dt=−khetero[Fe(II)aq][Fe(III)solid]{O2}{H+}n where n was 1.4 for pH from 6.0 to 6.8 and 2.0 for pH from 5.3 to 6.0. All previous studies reported n equal to 1.0. One earlier study reported that the rate of heterogeneous oxidation was constant for Fe(III)solid greater than 0.4 g L−1. In our experiments the rate remained linear to at least 2.4 g L−1 Fe(III)solid. The predicted rate constants along with DO, pH, Fe(III)solid, and temperature can be used to predict the removal of Fe(II) in existing facilities and for design of new reactors. Application can result in design of smaller reactors for slightly acidic pH values and in production of re-usable Fe(III) oxide solids.