Molecular-scale mechanisms of antimony binding to soil organic matter

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Antimony (Sb) is a toxic metalloid which is increasingly released into the environment owing to its use in a vast range of industrial and technological products. Antimony compounds are therefore considered as priority pollutants by environmental protection agencies. Despite high Sb inputs to soils (ca. 26,000 t/a) only few studies have examined the speciation of Sb in soils and factors controlling its environmental fate. The low mobility of Sb observed in peat soils suggests that Sb complexation by soil organic matter (OM) represents a key element in the soil Sb cycle. Yet, information on molecular-scale interactions of Sb with inherently complex dissolved and particulate soil OM is missing. Therefore, this project aims at the investigation of molecular-scale mechanisms of Sb binding to soil OM. In the first part of the project we use synchrotron-based Sb K-edge X-ray absorption spectroscopy (XAS) to explore the mechanisms of Sb binding to soil OM under both oxic and anoxic conditions, focusing on Sb binding by carboxyl/hydroxyl and thiol groups as well as organically complexed Fe (ternary Sb complexes). We also elucidate the kinetic stability of the Sb-OM complexes using an enriched stable Sb isotope tracer approach. In the second part of this project, we study the speciation and distribution of Sb in peat soils of the Peak District, U.K. These organic soils have been impacted by long-term atmospheric deposition of Sb and are thus extensively contaminated with Sb. With the help of XAS we explore major Sb sequestration mechanisms operating in these soils. These measurements are complemented by scanning electron microscopy – energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS) analyses to study the micro-scale distribution and speciation of Sb and its association with other elements potentially controlling its speciation (e.g., Fe and S). In summary, this project will improve our mechanistic understanding of Sb-OM interactions and clarify the role of OM as a sorbent for Sb in organic soils impacted by anthropogenically released and atmospherically deposited Sb.