Speciation and mobility of Antimony and Arsenic in mine waste and the aqueous environment in the region of the Giant mine, Yellowknife, Canada
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Antimony is a potential human and ecosystem health risk, yet large gaps in the existing knowledge of the geochemical behaviour of Sb persist. At the Giant mine, an abandoned gold mine about to undergo remediation, Sb is elevated in mine waste and in downstream pond and stream sediments that have been impacted by mine waste. Gold at the Giant Mine, Yellowknife NT, is refractory, hosted in arsenopyrite (FeAsS) and pyrite (FeS2), and associated with stibnite (Sb2S3) and Sb-sulfosalts. The gold was liberated by roasting the ore which produced two arsenic (As) and antimony (Sb)-rich waste streams: calcine and electrostatic precipitator (ESP) residue. The roaster-derived As and Sb host phases, maghemite and hematite, are found in sediment, and are undergoing post-depositional transformations. Bulk and micro- X-ray Absorption Near Edge Spectroscopy (XANES) methods, and synchrotron-based micro-X-ray fluorescence (μXRF) and micro-X-ray diffraction (μXRD), were employed to characterize the Sb-host phases and determine the solid-phase speciation of Sb in mine waste and sediment. Antimony in mine waste is largely associated with roaster-derived maghemite and is hosted as Sb(III) and Sb(V). The bulk and μXANES analyses indicate a more prominent Sb(III) composition in the ESP residue, compared to the calcine. In the surficial and deeper sediment Sb(III) and Sb(V) bound to oxygen are present, as well as Sb(III) bound to sulfur in the deeper sediment. The presence of the Sb(III) bound to sulfur phase in the deeper sediment implies that Sb associated with the roaster oxides is destabilized, but subsequently precipitates as, or adsorbs onto, a sulfide. Furthermore, the preferential attenuation of the Sb(III) species likely accounts for the dominance of Sb(V) in pore-water at all three sites, and at all depths. Co-existing with the sulfur-bound Sb phases in the deeper, relatively reducing sediment, is a compositionally significant, and finely disseminated Sb(V) bound to oxygen phase. Antimony and As are often assumed to exhibit similar geochemical behaviour. This study offered an opportunity to compare the mobility of As and Sb since both are elevated in concentration in the environment and in mine waste. Arsenic and Sb exhibit similar geochemical behaviour in the upper 2cm, but the higher affinity of As for sorption sites results in dissimilar distribution in pore-water. Both elements undergo post-depositional reduction resulting in an increase in the mobility of As, and attenuation of Sb. Aquatic horsetails (Equisetum fluviatile) present in Baker Creek significantly reduces the mobility of both elements.