Arsenic Fate in Peat Controlled by the pH-Dependent Role of Reduced Sulfur

Reduced sulfur (S) has a contrasting role in the fate of arsenic (As) in peatlands. Sulfur bridges provide efficient binding of As to organic carbon (C), but the formation of aqueous As–S species, so-called thioarsenates, leads to a low to no sorption tendency to organic C functional groups. Here, we studied how pH changes the role of reduced S in desorption and retention of presorbed As in model peat. Control desorption experiments without S addition revealed that As was mobilized, predominantly as arsenite, in all treatments with relative mobilization increasing with pH (4.5 < 7.0 < 8.5). Addition of sulfide or polysulfide caused substantial As retention at acidic conditions but significantly enhanced As desorption compared to controls at neutral to alkaline pH. Thioarsenates dominated As speciation at pH 7.0 and 8.5 (maximum, 79%) and remained in solution without (re)sorption to peat. Predominance of arsenite in control experiments and no evidence of surface-bound thioarsenates at pH 7.0 suggest mobilization to proceed via arsenite desorption, reaction with dissolved or surface-bound reduced S, and formation of thioarsenates. Our results suggest that natural or management-related increases in pH or increases in reduced S in near-neutral pH environments can turn organic matter from an As sink into a source.