Bacterial Disproportionation of Elemental Sulfur Inferred from a Field Study of Stable-Isotope Fractionations between Elemental Sulfur and Pyrite

Elemental sulfur (ES) is a common product of pyrite oxidation during acid mine drainage (AMD), but bacterial disproportionation of ES has not previously been inferred in acidic environments. Pore water profiles were collected seasonally within a coal-mine waste deposit, Minnehaha, in Southwest Indiana that has been abandoned for over 30 years. Geochemical characterization and modeling were used to assess how the interactions between the sulfur and iron cycle are affected by seasonally dynamic hydrologic conditions. Pore waters were collected seasonally and concentrations of Fe-species and sulfur isotopic compositions of sulfate were determined. Additionally, a sediment core was collected and used for sequential extraction and isotopic characterization of solid-phase sulfur species including elemental sulfur (δ³⁴S_es), pyrite (δ³⁴S_py), acid-volatile sulfides, water-soluble sulfates, and acid-soluble sulfates. The dominant disulfide phase was found to be pyrite through x-ray diffraction of an additional sediment core. Sulfur isotope fractionations between δ³⁴S_py and δ³⁴S_es (Δ³⁴S_es-py) of up to -33% are inferred to indicate bacterial disproportionation of ES in the presence of a non-limiting sulfide 'scrub' Fe(III). The initial isotopic composition, following formation from pyrite oxidation, is inferred from δ³⁴S_py, found to be ≈ 8.75% at the study site. Although ES has previously been found to accumulate in acidic Fe(III)-rich pore waters, ES is typically assumed to account for less than 1% of the oxidized sulfur pool and measurements of the ES isotopic composition are often neglected during field studies of acid AMD. The pore waters at Minnehaha were seasonally suboxic with sharp transitions from Fe(III)- to Fe(II)- dominated conditions near the phreatic surface. It is hypothesized that the sulfide product of ES disproportionation, fractionated by up to -8.6%, is immediately re-oxidized to ES near the redox gradient via reaction with Fe(III). Sulfide re-oxidation allows for the accumulation of isotopically light ES that can then become subject to further sulfur disproportionation. A mass-balance model for ES, incorporating seasonally varying rates for pyrite oxidation, ES disproportionation, and ES oxidation, was developed in order to determine the potential and conditions necessary for extensive recycling of ES by disproportionating bacteria to produce ES enriched in ³²S compared to the pyrite source. Simulations run for 32 seasonal cycles resulted in a Δ³⁴S_es-py of -16.4.% and an ES concentration of 170 ppm, which is consistent with average values obtained from the sediment core. The findings suggest that ES disproportionation is likely an important microbial process in AMD that should be considered at similar mining waste deposits experiencing seasonally varying hydrologic conditions and that Δ³⁴S_es-py can be used to estimate multiple cycles of ES disproportionation in oxic settings where the original source of ES is pyrite oxidation.