Antimony isotopic composition in stream waters impacted by acid mine drainage
Résumé
Acid mine drainage (AMD) generated by the oxidative
dissolution of sulfidic ores is a global environmental issue
causing long-term contaminations of surface waters and
sediments with metals and metalloids. Metal isotopes are
valuable geochemical tracers to better understand the
processes involved in the transport of these contaminants
in AMD-impacted streams and possibly identify
contamination sources. Recent analytical developments
allowed the precise measurement of antimony isotopic
composition in natural samples. However, to date only
few studies investigated the potential of Sb isotopes as
tracers in mining-impacted environments.
In this study, we examined the variations of Sb
isotopic composition in a AMD-impacted stream at the
San José mine (Oruro, Bolivia). Samples of water and
sediments were collected from the mine discharge in
Oruro city down to the lake Uru-Uru located ~15 km
downstream. Metal and metalloid concentrations were
determined and Sb isotopic composition was measured
by HG-MC-ICP-MS after Sb separation from the sample
matrix [1].
The mine effluent exhibited a low pH (~1-2), high
dissolved Fe concentration (3 g/L) and extreme Sb
concentrations both in water (7824 µg/L) and in
sediments (14.5 mg/g). Sb isotopic composition was
higher in the dissolved phase (ὀ123Sb=0.46‰) than in
sediments (ὀ123Sb=-0.09‰) suggesting that the oxidative
dissolution of sulfides preferentially releases heavy Sb
isotope. A decrease in dissolved Sb concentration down
to 10 µg/L occurred along the stream as a result of natural
attenuation processes and was accompanied by a slight
increase in isotopic composition (ὀ123Sb=0.60‰). An
apparent fractionation of +0.35‰ was observed between
the solution and sediments which is similar to the
fractionation factor determined during SbV adsorption
onto ferrihydrite in laboratory experiment (ongoing work)
suggesting that adsorption may be an important process
leading to an enrichment in heavy Sb isotope in AMD
waters.
These results indicate that Sb isotopes may be useful
to investigate the mechanisms controlling Sb fate in
mining-impacted rivers. However, further laboratory
experiments are required to better characterize Sb
isotopic fractionation associated with independent
processes including sulfide dissolution, biotic and abiotic
redox reactions and Sb adsorption onto newly formed
solid phases.