Electrobiochemical neutralization of acid mine water

When iron sulfides in a coal mine are ex posed to water containing oxygen and sulfur oxidizing bacteria, oxidation takes place, which results in the formation of sulfuric acid. The acid mine water so formed constitutes a formid able water pollution problem that seriously affects thousands of kilometers of streams and surface acreage in the U. S. In 1972, more than 3.5 billion kilograms (4 million tons) per year of sulfur acids were estimated to have drained from active and abandoned coal mines.1 The additional strip mining for coal brought about by the energy crisis could result in even more serious pollution problems. To cope with these problems, some water treatment methods have been suggested by Koehrsen and Gandt2 whereby acids are neutralized and iron is partially removed by adding lime or limestone to the water. This technique does not remove the S042from the system; it produces a chemical sludge that must be removed periodically and it is costly to maintain. Other techniques such as reverse osmosis,3 electrocoagulation,4 and electrodi alysis and distillation 5' & have been suggested but again the costs are high. Campbell and Lind 7 have noted that it takes acid strip-mine lakes a long time to recover naturally. Randies 8 has summarized the early experiments on the use of sulfate-reducing bac teria to neutralize acid mine water, and Buttle et al.9> 10 have called attention to their im portance in the natural recovery of acid strip mine lakes. They recognized the need of an energy-source material for these bacteria in order to hasten lake recovery and to reestablish essentially neutral water. The addition of wastewater was suggested as an energy source for the microbes. The lack of a carbon energy source in acid strip-mine lakes led King X1 to suggest this deficiency as a cause for the low algal productivity. Brochway et al.12 have since shown that carbon as organic or inorganic compounds must be considered as a regulating nutrient for both heterotrophic and autotrophic populations. Decker and King,13 assuming that the slow recovery of acid strip-mine lakes might be caused by the low flow of organic matter into the water, have shown that intro duction of an organic energy source (waste water sludge) into an acid strip-mine lake will substantially hasten the recovery of the lake. The bacteria removed the S042_ and produced S2~ and C02. Some sulfur was removed as H2S gas from the system at low pH, but as the pH increased, more of the sulfur was present as S2and HS~. At higher pH, the sulfide ion was precipitated as FeS and thus reduced the sulfur concentration in the water. In this paper we have combined the basic technique of Decker and King with the bio chemical fuel-cell principle (described below) and have shown that by adding electrodes and a simple external circuit to the system, one can further increase the rate of acid neutralization and water quality. In addition, it may be possible to obtain a modest amount of electric power for plant operation, a practical means of disposal of sludge, and the recovery of some free sulfur.