Microbiological treatment of uranium mine waters

Some uranium mines in New Mexico had to be continuously drained to remain operational. The drainage water could contain selenium, uranium, molybdenum, and sulphate in concentrations exceeding environmental standards. Ion exchange could be used to recover most of the uranium, but had no effect on the other ions. The feasibility of using soil bacteria of the genus Clostridium to remove selenium from the drainage water was studied in the laboratory. The bacteria were capable of reducing the selenium content from 1.6 mg per litre to less than 0.05 mg per litre by reduction of soluble leachate and selenite to insoluble selenium metal. The efficiency of the process was affected by temperature, flow rate, and nutrient concentration. Sulphate-reducing bacteria in the soil, such as Desulfovibrio, metabolized sulphate in the drainage water to hydrogen sulphide, which reacted with uranyl and molybdate ions to form insoluble uranium and molybdenum species. The results of the laboratory tests were confirmed by a 7-month field test in which mine drainage water was percolated through 2 soil columns.