Rates and stoichiometry of pyrite dissolution at pH 3 under low O2 conditions

Abstract

This study experimentally examined the rates and stoichiometry of pyrite dissolution under low O2 conditions to obtain insights into the S and trace-metal cycles in the Precambrian. The pyrite dissolution experiments were conducted at ~10−7–0.21 atm of the partial pressure of O2 (PO2) in 1 mM HCl solutions at pH 3 and 23 ± 2 °C for ~10–40 days durations. The experiments were performed in a glove box to keep the low PO2 except for those at 0.21 atm which were conducted in ambient air. The rates of pyrite dissolution were determined as a function of PO2 based on the concentrations of total dissolved Fe. The rates at < ~10−4 atm of PO2 were found to be represented by 10–10.07[O2]0.16 (mol m−2 s−1) and to deviate from and be larger than those expected from the rate law at higher PO2 levels, 10–7.62[O2]0.55. The higher rates at < ~10−4 atm of PO2 were caused by the reactions of pyrite with intermediate O2 species, HO2•, H2O2 and OH•, additionally produced from O2 and/or water in the presence of pyrite. The stoichiometry of pyrite dissolution was examined based on the concentration ratios among SO42− and total dissolved S and Fe, namely, [SO42−]/[Fe], [Stot]/[Fe] and [SO42−]/[Stot]. The ratios of [SO42−]/[Fe] and [SO42−]/[Stot] were observed to decrease with decreasing PO2 down to ~10−5 atm but to increase at lower PO2, while the [Stot]/[Fe] ratio decreased with PO2 over the whole examined range. A numerical model that considers the reactions among dissolved Fe and S species, pyrite surfaces and oxygen along with pyrite dissolution was developed to theoretically calculate stoichiometry. Comparison of the theoretical and experimental stoichiometry indicated that Fe3+ and thiosulfate are the dominantly released Fe and S species, respectively, upon pyrite dissolution over a wide range of PO2. Additionally, Fe2+, polythionates and polysulfides are likely released, but less significantly. These dissolution mechanisms suggest that pyrite dissolution can consume acids under low O2 conditions. The rate enhancement by intermediate O2 species, predominant release of intermediate S species and acid consumption are likely to have played significant roles in pyrite weathering and the associated sulfur and trace-metal cycles in the Precambrian.