Solubilization of heavy metals from a fluvial AMD generating tailings sediment by heterotrophic microorganisms. Part I: Influence of pH and solid content

The study demonstrates that a heterotrophic mixed culture is able to mobilize considerable amounts of heavy metals (Fe, Zn, Mn, Pb, Cu, Cd, Cr) from a sulfidic AMD generating fluvial tailings material even under moderately acidic conditions as they exist at real sites. The experiments were carried out in shaking flasks by inoculation with a mixed culture of heterotrophic bacteria. The solid content and a decrease in initial pH of the suspensions were investigated as important factors of the viability of the heterotrophic microorganisms. With the mixed culture, up to 17% Pb, 27% Cd, 100% Cr, 25% Fe, 35% Cu, 79% Mn, and 28% Zn were mobilized in one batch run. Compared to the autotrophic processes, especially Pb seems to be mobilized by heterotrophic microbial action. Even under increasingly unfavorable conditions (increase in solid content, decrease in initial pH) the mixed culture demonstrated a high diversity and a good viability. As a result, it was clearly shown that heterotrophic processes also play an important role in the mobilization of metals in such acid-generating materials. Heterotrophic microorganisms improve the solubility of metals primarily released by biooxidation, and enhance the transport of these environmentally relevant metals by excretion of complexing agents. The synergistic action of both autotrophic and heterotrophic bioleaching processes generates at such sites a long-range contamination of ground- and surface-water.