METAL REMOVAL FROM MINE PIT LAKE WATERS THROUGH THE INDUCTION OF BIOLOGICAL SULFIDE GENERATION: A LIMNOCORRAL STUDY

The reactions involved in the generation of acid rock drainage (ARD) are reversible and mine pit lakes may provide the ideal reaction chamber. ARD produces protons (acid), sulfate and metal cations, all of which may be toxic to biota. Biological sulfate reduction consumes all of these products and converts them back to metal sulfide minerals. In mine pit lakes, the in-situ generation and subsequent precipitation of metal sulfides to the sediments provides a "permanent" repository, away from atmospheric oxygen.

Several limnocorral experiments were undertaken at the former Equity Silver Mine, BC, Canada (Waterline Pit), to assess the viability of the sulfide generation strategy. This pit lake is permanently anoxic below 8m, maintains near-neutral pH, is ultra-oligotrophic and contains elevated concentrations of many metals including As, Zn and Fe in deep waters (1.2, 0.8 and 45 ppm respectively). Biological production of sulfide, and subsequent metal removal was attempted by addition of nutrients (N, P) and ethanol . Addition of nutrients to surface waters induced eutrophic levels of phytoplankton, reducing surface water metal concentrations substantially (analogous to Crusius et al., this session); however, insignificant sulfide generation occurred at depth, presumably because of limited export of algal matter to deep waters. By contrast, additions of ethanol to the anoxic water (at 7 m depth) at concentrations of ~1 mM generated phototrophic bacterial blooms with sulphate reducing bacteria, hydrogen sulfide and metal sulfides being detected. Significant removal of metals from the dissolved phase was observed in the anoxic waters where sulfides were generated (~ two orders of magnitude reduction for Zn, however no reduction for As). These results demonstrate that the in-situ, bacterially mediated generation of sulfides is viable in water heavily polluted with metals via relatively minor additions of a labile carbon source.