Antimony isotopic composition in stream waters impacted by acid mine drainage

International audienceAcid mine drainage (AMD) generated by the oxidativedissolution of sulfidic ores is a global environmental issuecausing long-term contaminations of surface waters andsediments with metals and metalloids. Metal isotopes arevaluable geochemical tracers to better understand theprocesses involved in the transport of these contaminantsin AMD-impacted streams and possibly identifycontamination sources. Recent analytical developmentsallowed the precise measurement of antimony isotopiccomposition in natural samples. However, to date onlyfew studies investigated the potential of Sb isotopes astracers in mining-impacted environments.In this study, we examined the variations of Sbisotopic composition in a AMD-impacted stream at theSan José mine (Oruro, Bolivia). Samples of water andsediments were collected from the mine discharge inOruro city down to the lake Uru-Uru located ~15 kmdownstream. Metal and metalloid concentrations weredetermined and Sb isotopic composition was measuredby HG-MC-ICP-MS after Sb separation from the samplematrix [1].The mine effluent exhibited a low pH (~1-2), highdissolved Fe concentration (3 g/L) and extreme Sbconcentrations both in water (7824 µg/L) and insediments (14.5 mg/g). Sb isotopic composition washigher in the dissolved phase (ὀ123Sb=0.46‰) than insediments (ὀ123Sb=-0.09‰) suggesting that the oxidativedissolution of sulfides preferentially releases heavy Sbisotope. A decrease in dissolved Sb concentration downto 10 µg/L occurred along the stream as a result of naturalattenuation processes and was accompanied by a slightincrease in isotopic composition (ὀ123Sb=0.60‰). Anapparent fractionation of +0.35‰ was observed betweenthe solution and sediments which is similar to thefractionation factor determined during SbV adsorptiononto ferrihydrite in laboratory experiment (ongoing work)suggesting that adsorption may be an important processleading to an enrichment in heavy Sb isotope in AMDwaters.These results indicate that Sb isotopes may be usefulto investigate the mechanisms controlling Sb fate inmining-impacted rivers. However, further laboratoryexperiments are required to better characterize Sbisotopic fractionation associated with independentprocesses including sulfide dissolution, biotic and abioticredox reactions and Sb adsorption onto newly formedsolid phases