The microbiology of acid mine drainage: genesis and biotreatment Review

摘要

Waters draining abandoned mines, spoil heaps and mineral tailings are often acidic (sometimes extremely so) and rich in metals. A primary cause of acid mine drain age (AMD) formation is thought to be the oxidative dissolution of pyrite and other sulphidic minerals within mineral-rich strata and wastes, which is accelerated by chemolithotrophic acidophilic bacteria such as Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans. However, microbial life in AMD waters is not restricted to iron-oxidizing bacteria, but includes sulphur-oxidizing bacteria and archaea, heterotrophic microorganisms and some lower eukaryotic life-forms. Among the acidophilic heterotrophic bacteria that have been isolated from AMD are facultatively anaerobic Acidiphilium species which catalyse the dissimilatory reduction of ferric iron, and obligate anaerobes that catalyse the dissimilatory reduction of sulphate to sulphide. In a survey of AMD from seven mine sites in the U.K. and mainland Europe, iron-oxidizing bacteria were found in varying numbers (10(2) to 5.7 x 10(6) ml(-1)) in waters draining metal mine sites. Smaller numbers (<10(2) ml(-1)) were found in AMD at two abandoned coal mine sites, though iron-oxidizing bacteria could be readily isolated from enrichment cultures of these waters. Acid-tolerant/acidophilic sulphate-reducing bacteria were also isolated successfully from AMD stream sediment, using acidic overlay media. The importance of microbial oxidation of ferrous iron in AMD was demonstrated by comparing rates of oxidation in native and filter-sterilized mine water samples. Packed-bed bioreactor columns using porous glass beads as a support matrix for microbial biofilm development were set up, and different populations of acidophilic microorganisms isolated from AMD were introduced, as follows: (i) an iron-oxidation bioreactor, using a mixed population of iron-oxidizing bacteria, obtained by enrichment of coal mine AMD; (ii) iron-reduction bioreactors, using Acidiphilium species and (ill) sulphate-reduction bioreactors, using a mixed population of acidophilic and acid-tolerant sulphate-reducing bacteria isolated from mine waters and related environments. The performance of the bioreactors was assessed over several months in the laboratory. The results showed that indigenous acidophilic bacteria can be used successfully in packed-bed bioreactors to promote targeted changes in AMD water chemistry, and thereby facilitate bioremediation of these polluting waste waters.