Sulfammox driving sulfate reduction and thioarsenate formation in groundwater: evidence from multiple isotopes

Abstract: The mechanisms governing sulfur cycling and its impacts on arsenic speciation and enrichment in groundwater remain poorly understood. This gap was investigated by using multi-sulfur isotopes (including δ34S-SO4 and δ34S-H2S), nitrogen isotopes (δ15N-NH4), and molecular characteristics of dissolved organic matter (DOM) in high-arsenic groundwater from the Datong Basin, China. Results show a concurrent increase in total arsenic concentrations (from 0.14 to 700 μg/L) and thioarsenate proportions (up to 92%) from the alluvial fan (Zone I), through the transition area (Zone II), and to the flat plain (Zone III). This trend was accompanied by increased H2S concentrations (from <1 μg/L to 2920 μg/L) and δ34S-SO4 (from 2.2‰ to 64.5‰), but decreasing DOC/NH4+ molar ratios. Higher H2S concentrations and δ34S-SO4 values in Zone III, together with relatively lower δ34S-H2S values in Zone III (from -45.2‰ to 25.4‰; median -11.1‰) than those in Zone II (from -18.04‰ to 0.2‰; median -8.92‰), supported greater extent of SO42- bioreduction in Zone III. DOM and NH4+ acted as competing electron donors for SO42- bioreduction. The significantly negative correlations between DOC/NH4+ molar ratios and H2S concentrations and δ34S-SO4 values supported sulfate-driven anaerobic ammonium oxidation (Sulfammox) as a key SO42- bioreduction pathway. Under conditions of limited DOM biodegradability, low DOC/NH4+ molar ratio, and high NH4+/SO42- molar ratios, NH4+ predominated over DOM as the electron donor, driving Sulfammox. This process was particularly prominent in Zone III, as evidenced by positive correlations between δ15N-NH4 values and H2S concentrations (r = 0.717, p < 0.001) and δ34S-SO4 values (r = 0.51, p = 0.09) in this zone. The produced H2S concentrations were significantly positively correlated with both total arsenic concentrations and thioarsenates proportions (r > 0.75, p < 0.001), indicating that H2S was responsible for thioarsenate formation and consequently for elevated arsenic concentrations and high thioarsenate proportions in groundwater. These findings provided the first geochemical and isotopic evidence to highlight the critical role of NH4+ acting as electron donors in promoting Sulfammox processes in groundwater, elucidating a novel mechanism of groundwater arsenic mobility.
Datasets on groundwater geochemistry, thioarsenate species, multi-sulfur and nitrogen isotope values, and FT-ICR MS results.