Release and bioavailability of heavy metals in three typical mafic tailings under the action of Bacillus mucilaginosus and Thiobacillus ferrooxidans

Abstract

Mafic tailings are the common tailings which appear around iron and magnesium mines in China. These tailings can release heavy metals to the local environment, and can enter into soils and plants by physicochemical and biological processes. This paper concerns the release and bioavailability of Cr, Cu, Mn, Ni, and Zn in three typical mafic tailings under the action of Bacillus mucilaginosus (B. mucilaginosus) and Thiobacillus ferrooxidans (T. ferrooxidans). The research results indicate that the release efficiencies of heavy metals with T. ferrooxidans are significantly higher than those with B. mucilaginosus for the same tailing. The bioavailable forms of heavy metals in residual tailings markedly decrease with the bioleaching of T. ferrooxidans. On the contrary, the bioavailable forms of heavy metals increase because of the action of B. mucilaginosus in most cases. The total concentrations of Cr and Ni and their fractions show a little variation with the microbial action. However, concentrations of Cu and Zn and their oxidable fractions decrease significantly after bioleaching. The element Mn decreases significantly in reducible form with the microbial action due to its special mineral form. The release ratios of heavy metals in different tailings follow the order of Serpentine-type tailing > Olivine-type tailing > Amphibole-type tailing. Compared to the sulfide and oxide tailings, the mafic tailings show relative low release ratios of heavy metals. Based on the percentage of acid-soluble fraction, the ecological risks of heavy metals decline with Olivine-type tailing > Amphibole-type tailing > Serpentine-type tailing after bioleaching.