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.摘要：调控硫循环的机制及其对地下水中砷形态与富集过程的影响，目前仍未得到充分阐明。本研究以中国大同盆地高砷地下水为研究对象，采用多硫同位素（包括δ34S-SO4与δ34S-H2S）、氮同位素（δ15N-NH4）以及溶解性有机质（dissolved organic matter, DOM）的分子特征，对这一研究空白展开探究。研究结果显示，从冲积扇（I区）、过渡带（II区）至冲积平原（III区），总砷浓度（0.14~700 μg/L）与硫代砷酸盐占比（最高可达92%）均呈现同步上升趋势。该趋势伴随硫化氢（H2S）浓度（<1 μg/L~2920 μg/L）与δ34S-SO4值（2.2‰~64.5‰）的升高，以及DOC/NH4+摩尔比的降低。III区的硫化氢浓度与δ34S-SO4值更高，且其δ34S-H2S值（范围：-45.2‰~25.4‰，中位数：-11.1‰）显著低于II区（范围：-18.04‰~0.2‰，中位数：-8.92‰），这表明III区的硫酸根（SO42-）生物还原程度更高。溶解性有机质（DOM）与铵根离子（NH4+）均可作为硫酸根生物还原过程的竞争性电子供体。DOC/NH4+摩尔比与硫化氢浓度、δ34S-SO4值之间显著的负相关关系，表明硫酸盐驱动的厌氧氨氧化（Sulfammox）是硫酸根生物还原的关键途径。在溶解性有机质生物可利用性有限、DOC/NH4+摩尔比较低且NH4+/SO42-摩尔比较高的条件下，铵根离子相较于溶解性有机质更占优势，作为电子供体驱动硫驱动厌氧氨氧化（Sulfammox）过程。该过程在III区尤为显著，该区域内δ15N-NH4值与硫化氢浓度（r=0.717，p<0.001）、δ34S-SO4值（r=0.51，p=0.09）均呈显著正相关，可为这一结论提供佐证。生成的硫化氢浓度与总砷浓度及硫代砷酸盐占比均呈显著正相关（r>0.75，p<0.001），表明硫化氢是硫代砷酸盐形成的关键因素，进而导致地下水中砷浓度升高与硫代砷酸盐占比偏高。本研究的发现首次提供了地球化学与同位素证据，阐明了铵根离子作为电子供体在促进地下水中硫驱动厌氧氨氧化过程中的关键作用，揭示了地下水砷迁移的全新机制。本数据集包含地下水地球化学数据、硫代砷酸盐形态数据、多硫与氮同位素值数据以及傅里叶变换离子回旋共振质谱（FT-ICR MS）测试结果。