Performance of Sulfate-reducing Passive Bioreactors for the Removal of Cd and Zn from Mine Drainage in a Cold Climate

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.