Synchrotron X-ray absorption spectroscopy reveals antimony sequestration by reduced sulfur in a freshwater wetland sediment

The biogeochemistry of antimony (Sb) in wetland sediments is poorly characterised, despite their importance as contaminant sinks. The organic-rich, reducing nature of wetland sediments may facilitate sequestration mechanisms that are not typically present in oxic soils, where the majority of research to date has taken place. Using X-ray absorption spectroscopy (XAS), we present evidence of antimony speciation being dominated by secondary antimony-sulfur phases in a wetland sediment. Our results demonstrate that, by incorporating a newly developed Sb-III-organic sulfur reference standard, linear combination fitting analysis of antimony K-edge XAS spectra and robust statistical assessment of fit quality allows the reliable discrimination of Sb-III coordination environments. We found that a contaminated wetland sediment in New South Wales, Australia, contained 57% of the total antimony as Sb-III-phases, with 44% present as a highly-disordered antimony phase, likely consisting of Sb-III complexed by organic sulfur (e.g. thiols) or an amorphous Sb-III sulfide (e.g. SbS3). The methodological approach outlined in this study and our identification of the importance of reduced sulfur in sequestering antimony has implications for future research in the area of antimony biogeochemistry, and for the management of both natural and artificial wetlands contaminated with antimony.