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 O 2 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 O 2 ( P O 2 ) 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 P O 2 except for those at 0.21 atm which were conducted in ambient air. The rates of pyrite dissolution were determined as a function of P O 2 based on the concentrations of total dissolved Fe. The rates at −4  atm of P O 2 were found to be represented by 10 –10.07 [O 2 ] 0.16 (mol m −2  s −1 ) and to deviate from and be larger than those expected from the rate law at higher P O 2 levels, 10 –7.62 [O 2 ] 0.55 . The higher rates at −4  atm of P O 2 were caused by the reactions of pyrite with intermediate O 2 species, HO 2 • , H 2 O 2 and OH • , additionally produced from O 2 and/or water in the presence of pyrite. The stoichiometry of pyrite dissolution was examined based on the concentration ratios among SO 4 2− and total dissolved S and Fe, namely, [SO 4 2− ]/[Fe], [S tot ]/[Fe] and [SO 4 2− ]/[S tot ]. The ratios of [SO 4 2− ]/[Fe] and [SO 4 2− ]/[S tot ] were observed to decrease with decreasing P O 2 down to ~10 −5  atm but to increase at lower P O 2 , while the [S tot ]/[Fe] ratio decreased with P O 2 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 Fe 3+ and thiosulfate are the dominantly released Fe and S species, respectively, upon pyrite dissolution over a wide range of P O 2 . Additionally, Fe 2+ , polythionates and polysulfides are likely released, but less significantly. These dissolution mechanisms suggest that pyrite dissolution can consume acids under low O 2 conditions. The rate enhancement by intermediate O 2 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.