Oxidation of Organosulfur-Coordinated Arsenic and Realgar in Peat: Implications for the Fate of Arsenic

Organosulfur-coordinated As­(III) and realgar (α-As₄S₄) have been identified as the dominant As species in the naturally As-enriched minerotrophic peatland <i>Gola di Lago</i>, Switzerland. In this study, we explored their oxidation kinetics in peat exposed to atmospheric O₂ for up to 180 days under sterile and nonsterile conditions (25 °C, ∼100% relative humidity). Anoxic peat samples were collected from a near-surface (0–38 cm) and a deep peat layer (200–250 cm) and studied by bulk As, Fe, and S <i>K</i>-edge X-ray absorption spectroscopy as well as selective extractions as a function of time. Over 180 days, only up to 33% of organosulfur-coordinated As­(III) and 44% of realgar were oxidized, corresponding to half-life times, <i>t</i>_1/2, of 312 and 215 days, respectively. The oxidation of both As species was mainly controlled by abiotic processes. Realgar was oxidized orders of magnitude slower than predicted from published mixed-flow reactor experiments, indicating that mass-transfer processes were rate-limiting. Most of the As released (>97%) was sequestered by Fe­(III)–(hydr)­oxides. However, water-extractable As reached concentrations of 0.7–19 μmol As L^–1, exceeding the WHO drinking water limit by up to 145 times. Only a fraction (20–36%) of reduced S­(-II to I) was sensitive to oxidation and was oxidized faster (<i>t</i>_1/2 = 50–173 days) than organosulfur-coordinated As­(III) and realgar, suggesting a rapid loss of reactive As-sequestering S species following a drop in the water table. Our results imply that wetlands like <i>Gola di Lago</i> can serve as long-term sources for As under prolonged oxidizing conditions. The maintenance of reducing conditions is thus regarded as the primary strategy in the management of this and other As-rich peatlands