Effect of bacterial activity on trace metals release from oxidation of sphalerite at low pH (<3) and implications for AMD environment

A series of experiments was conducted to better understand the bacterial influence on the release of trace metals during oxidation of sphalerite mineral and element cycles in acid mine drainage (AMD) systems. Batch experiments were carried out as biotic and abiotic control at pH 3. Acidithiobacillus ferrooxidans, sulfur and Fe(II) oxidizer, was used in the biotic sphalerite experiment. The abiotic control experiment was run without adding the bacteria. The release behavior of six trace metals (As, Cd, Co, Pb, Cu and Mn), Fe and Zn were determined during the period of 54 days. Compared to the abiotic experiments, enhanced oxidation of sphalerite by bacteria produced high sulfate (~2,000 mg/L) and Fe_tot (139 mg/L) along with the low pH (<2.3). Consistent with this, the concentration of trace metals (As, Cd, Co, Pb, Cu and Mn) was significantly higher in the biotic experiments than those in the abiotic experiments. Results indicate that the distributions of Co and Cd in both biotic and abiotic experiments are directly related to the sphalerite dissolution whereas Pb, Cu distribution shows no strong relation to sphalerite dissolution especially in the abiotic experiments. Pb distribution in the solution appears to be controlled by pH-dependent solubility. Approximately 80% of the trace metals were removed from the solution at the end of the biotic experiments along with biologically induced Fe precipitation. Experimental results showed that bacteria play major role not only in the release of trace metal from sphalerite but also in controlling concentration of the metals in the solution by producing Fe-oxyhydroxides. The study suggest that in order to develop an effective rehabilitation strategy for AMD, it is necessary to understand bio/geochemical processes governing mobilization and deposition of trace metals in the environment.