Abstract
The present study investigated lake and river sediments affected by metals from an acid mine drainage (AMD) from a former uranium mine. The role of bacterial sulfate reduction in the immobilization of contaminants was evaluated, and the analyses of acid volatile sulfide (AVS) and sequential extraction were performed. Consequently, the potential mobility and bioavailability of contaminants were established. Sulfur isotopic fractionation (δ34S), AVS, and sequential extraction procedure were used to assess the sulfate bacterial reduction and the availability of contaminants in the environment at six sampling stations. The δ34S indicated that bacterial reduction is a key process in the natural attenuation of contamination in the Aguas Claras reservoir, precipitating metal sulfides. According to the USEPA criteria, adverse biological effects are expected for sample S1 (inside the reservoir) which is likely to be toxic, while for sediment S4 (in the river), the toxicity is uncertain. The other samples were classified as non-toxic, likely because of the decreased solubility of zinc sulfide. A decrease in the concentration of the contaminants downstream of the reservoir was observed. The predominance of U (0.4 %) in the labile fraction and the elevated concentrations of Zn (0.5 %) and Mn (0.7 %) in the sediments inside the reservoir raises concerns regarding the availability of these contaminants in the environment. The main environmental impact appears to be concentrated in the Aguas Claras reservoir, whereas the Antas creek does not seem to be affected by the AMD process. Although the bacterial sulfate reduction is effective in its production of sulfides capable of immobilizing the contaminants, the presence of Zn and U in the labile and reducible fraction is a matter of concern due to its long-term bioavailability. Thus, continuous monitoring of the redox potential of the waters and sediments, mainly in the reservoir, is recommended in order to assess and possibly prevent later dissolution of sequestered contaminants.
Highlights
Acid mine drainage (AMD) is a critical environmental issue caused by chemical and bacterial oxidation of pyrite (FeS2) and other sulfide bearing minerals exposed to atmospheric conditions during the mining of metal ores and coal (Taylor et al 1984; Balci et al 2007)
Purpose The present study investigated lake and river sediments affected by metals from an acid mine drainage (AMD) from a former uranium mine
Considering that the SO4 formed via oxidation of pyrite presents small or negligible sulfur isotope fractionation (Taylor et al 1984; Balci et al 2007), it may be concluded that the pyrite oxidation is the main source of sulfate
Summary
Acid mine drainage (AMD) is a critical environmental issue caused by chemical and bacterial oxidation of pyrite (FeS2) and other sulfide bearing minerals exposed to atmospheric conditions during the mining of metal ores and coal (Taylor et al 1984; Balci et al 2007). Sulfate from the AMD can be reduced to sulfide due to bacterial action and can be assessed trough isotopic analyses. The assessment of sulfur isotopic fractionation, the quantification of the sulfides, and the determination of the solid phases along with their association with the contaminants are essential for the following reasons: firstly they provide information about distribution, mobility, and bioavailability of the contaminants, and secondly, they indicate the occurrence of natural attenuation processes (Morillo et al 2008; Sarmiento et al 2009; Byrne et al 2010; Charriau et al 2011)
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