Over 300 wetlands have been constructed to treat acidic coal mine drainage. In general, they improve water quality, although supplementary chemical treatment is usually required to meet effluent limitations. The principal reaction mechanism is believed to be microbially catalyzed oxidation of dissolved iron. Uptake of metals by plants and algae and adsorption and complexation by the organic substrate can also contribute, but available biomass typically limits the significance of these processes. Because oxidation through bacterial activity will not remove metal mine drainage contaminants such as zinc, nickel, lead, and cadmium, recent research by the U.S. Bureau of Mines has focused on the bacterial conversion of sulfate to hydrogen sulfide (an acid- consuming reaction), since many metals react rapidly with hydrogen sulfide to form virtually insoluble precipitates. Bacterial sulfate reduction and the formation of metal sulfides have been confirmed in constructed wetlands. Research is continuing on how best to route the drainage water through this anaerobic zone and optimize the desired biological processes.
(Proceeds, Int'l Symp. Tailings & Effluent Mgmt., Halifax, 8/20- 24/89. Pergamon, PP. 173-179