Speciation and mobility of antimony and arsenic in a highly contaminated freshwater system and the influence of extreme drought conditions

<p>Aqueous and solid-state antimony (Sb) and arsenic (As) speciation is assessed in an Australian freshwater system contaminated by mining of primary sulfide minerals. The study aims to understand metalloid transformation and mobilisation in the system, and coincides with a severe drought providing the opportunity to examine the influence of extreme low-flow conditions. X-ray absorption spectra identified only Sb^V in <2 mm sediments, despite boulder size stibnite evident in the creek. Roméite-group minerals were detected by X-ray diffraction in oxidation rims of creek-bed stibnite, which potentially limit the contribution of dissolved Sb^III to the waterway. Arsenic in <2 mm sediments was dominated by As^V (17-91 %) and orpiment (16-93 %), while the co-occurrence of As^III (11-36 %) with orpiment suggests that primary As minerals are an important ongoing source of As^III to the system. Dissolved metalloids (<45 µm filtered) dominated total water column concentrations and comprised mainly pentavalent species. Arsenic(III) was however identified in most water samples (up to 6.6 µg L^-1), while dissolved Sb^III was only detected in one sample (3.4 µg L^-1) collected during the drought period. Dissolved As^V increased significantly in samples collected in low-flow conditions, considered a result of reductive dissolution of sediment Fe-oxyhydroxide host phases, but a similar increase in dissolved Sb was not observed. This study highlights a greater risk from As in this system, and the likelihood of increased As mobility under the warmer and drier environmental conditions predicted with climate change, especially during first-flush events.</p>