Arsenic sequestration by organic sulphur in peat

Wetlands cover more than 6% of the global ice-free land area, and have been recognized as important sinks for arsenic. Wetland soils and sediments are subject to frequent changes in redox conditions, driven by fluctuations in the water table and shifts in biological activity. Under oxic conditions, natural organic matter promotes arsenic release from metal-(hydr)oxides, thereby enhancing arsenic mobility. Under strongly reducing conditions, however, organic matter triggers the formation of arsenic-sequestering sulphides, leading to a reduction in arsenic mobility. Furthermore, the sorption of arsenic to natural organic matter is increasingly thought to suppress arsenic mobility, but the binding mechanisms have remained elusive. Here we use X-ray absorption spectroscopy to analyse the speciation of solid-phase arsenic in peat samples collected from a naturally arsenic-enriched peatland in Switzerland. We show that natural organic matter can completely sequester arsenic through the formation of covalent bonds between trivalent arsenic and organic sulphur groups, which have an average arsenic-sulphur bond distance of 2.26 angstrom. We suggest that by binding arsenic in this way, natural organic matter plays an active role in arsenic immobilization in sulphur-enriched, anoxic wetlands.