Acid mine drainage (AMD) is a major environmental challenge for the mining industry in northern climates. Laboratory-scale experiments were conducted to test various simple and complex carbon sources available in the Yukon as electron donors for sulfate reduction to allow the subsequent removal of Cd and Zn from a Yukon AMD. The 1 L capacity bioreactors were monitored biweekly at 5 °C. After 162 days, a diminution of both total organic carbon and sulfate concentrations was observed in all bioreactors. A long adaptation period was necessary before consumption of the carbon source started, which might be due to the cold temperature. Simple organic sources of carbon (methanol and ethylene glycol) and complex organic sources of carbon (potato oil, brewery residue, peat, and straw) were used to support SRB growth. Methanol and ethylene glycol led to a diminution of sulfate concentrations of 71.2 and 36.9%, respectively, while the decrease of sulfate concentrations was limited to 13.8 and 5.3% when using peat and straw, respectively.
Passive treatment is a promising, green technology that is increasingly being used for mine drainage treatment. However, several challenges remain concerning its implementation in locations where the temperature of the water remains cold year round and bacterial growth is limited by the low temperatures. The impacts of cold on the activity of sulfate-reducing bacteria (SRB) and the subsequent removal of Cd and Zn from acid mine drainage were studied by conducting static tests at 4.5 °C over a 90 day period. Different sources of carbon were tested to support native SRB: molasses, methanol, and a mix of molasses/methanol at different concentrations. The reactors were monitored biweekly, and the pH, oxido-reduction potential, and residual concentrations of Zn, Cd, sulfate, and total organic carbon were measured. The use of carbon sources clearly improved bacterial activity and consequently, the removal of Cd and Zn by precipitation as sulfide. Up to 94.8% of the Zn and up to 99.4% of the Cd were removed after 90 days, reducing metal concentrations below the discharge limits ([Cd] < 50 µg/L and [Zn] < 500 µg/L). The molasses + methanol mix was slightly more efficient than either. These findings indicated that native SRB might be used successfully to treat metal-contaminated mine water.
Mine drainage contaminated with metals is a major environmental threat since it is a source of water pollution with devastating effects on aquatic ecosystems. Conventional active treatment technologies are prohibitively expensive and so there is increasing demand to develop reliable, cost-effective and sustainable passive or semi-passive treatment. These are promising alternatives since they leverage the metabolism of microorganisms native to the disturbed site at in situ or close to in situ conditions. Since this is a biological approach, it is not clear if semi-passive treatment would be effective in remote locations with extremely cold weather such as at mines in the subarctic. In this study we tested the hypothesis that sulfate-reducing bacteria, which are microorganisms that promote metal precipitation, exist in subarctic mine environments and their activity can be stimulated by adding a readily available carbon source. An experiment was setup at a closed mine in the Yukon Territory, Canada, where leaching of Zn and Cd occurs. To test if semi-passive treatment could precipitate these metals and prevent further leaching from waste rock, molasses as a carbon source was added to anaerobic bioreactors mimicking the belowground in-situ conditions. Microbial community analysis confirmed that sulfate-reducing bacteria became enriched in the bioreactors upon addition of molasses. The population composition remained fairly stable over the 14 month operating period despite temperature shifts from 17 to 5 °C. Sulfate reduction functionality was confirmed by quantification of the gene for dissimilatory sulfite reductase. Metals were removed from underground mine drainage fed into the bioreactors with Zn removal efficiency varying between 20.9% in winter and 89.3% in summer, and Cd removal efficiency between 39% in winter and 90.5% in summer. This study demonstrated that stimulation of native SRB in MIW was possible and that in situ semi-passive treatment can be effective in removing metals despite the cold climate.