Sulfur biogeochemistry: Kinetics of intermediate sulfur species reactions in the environment

A geochemical model describing sequential mineral precipitation has been developed based on a thermodynamic description of metal sulfide stability, coupled with consideration of fluid flow through a biofilm, sulfide precipitation kinetics, and sulfate reducing bacteria (SRB) activity. This model accurately predicts metal sulfide precipitation as a result of SRB activity in engineered bioreactors, wetland remediation systems, and some low-temperature metal sulfide deposits.; Consideration of known mechanisms and pathways of pyrite oxidation showed that the oxidation kinetics of tetrathionate (S₄O₆ 2−) and elemental sulfur required definition in order identify intermediate sulfur species potentially bioavailable in low pH environments. Results show that the oxidation kinetics of tetrathionate in acidic solutions with ferric iron are quite slow, defined by the rate law at 70°C and pH 1.5: r=10^-6.61±0.3S4 O ^2-6^0.3±0.08 Fe3+^{0.15± 0.09} ; (r = mol L−1 sec−1). The apparent activation energy (E_A) for tetrathionate oxidation at pH 1.5 is 105 ± 4 KJ/mol. Polythionates (SO₆2−) are not reactive with H₂O₂, but oxidize very quickly (second order rate constant >108 M−1 sec −1, 25°C) in the presence of the hydroxyl radical (OH*).; Elemental sulfur oxidizes very slowly at low pH (first order rate constant of 5 × 10−10 mol m−2 sec −1 at pH 1.5 and 1 × 10−8 mol M −2 sec−1 at pH 0.5 and 25°C), and is not reactive with H₂O₂ or OH*. Ferric iron approximately doubles the reaction rate, but that effect is independent of ferric iron concentration. Additional experiments indicate flocculation limits reactive surface area and that oxidation is not controlled by solubilization of S₈ rings from the crystalline α-S₈ form.; Results of these experiments coupled with field observations at the Richmond 5-way site in the Iron Mountain Mine near Redding, CA suggest that pyrite oxidation at low pH likely proceeds through multiple parallel pathways. Detachment of thiosulfate is not a viable mechanism under these conditions, though the reactivity of a similar surface-bound group is likely very important in understanding pyrite oxidation. Microbial populations reflect a predominant pathway in which Fe3+ is regenerated as the principle oxidant of surface-bound sulfur species at pyrite surfaces.