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

Abstract

Organosulfur-coordinated As(III) and realgar (α-As4S4) have been identified as the dominant As species in the naturally As-enriched minerotrophic peatland Gola di Lago, 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 K-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, t1/2, 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 (t1/2 = 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 Gola di Lago 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.