Micro-organisms involved in iron oxidation and acid mine drainage formation in KwaZulu-Natal and their control by soil covers on coal waste dumps

[摘要] ENGLISH ABSTRACT:The biologically catalysed oxidation of pyrite in the outer layers of coal waste dumps leads to theformation of acid mine drainage. The oxidation of pyrite to ferric iron and sulphate is a complexprocess involving various abiotic and biologically catalysed reactions. Pyrite is abioticallyoxidized by ferric iron, with the formation of thiosulphate and ferrous iron. Thiosulphatedecomposes to form various inorganic sulphur compounds. Bacterial catalysis of pyrite oxidationis achieved by iron-oxidizing bacteria oxidizing ferrous iron to ferric iron. Bacteria that oxidizesulphur compounds assist the catalysis by oxidizing thiosulphate and its decomposition products.Heterotrophic organisms may play a role by consuming organic substances inhibitory to thelithotrophic bacteria.Abiotic ecological factors, acid formation and populations of iron-oxidizing bacterial groups werestudied in 10 differently constructed pilot scale coal waste dumps, as the second phase of a studywhich started in September 1993. Gas samples were withdrawn weekly from coal waste throughpermanently buried stainless steel probes, for analysis in the field using a portable oxygen/carbondioxide meter. Samples of coal waste were extracted by auger for analysis of moisture, pH andmicrobial populations. The analyses of oxygen and pH can be recommended for the routinemonitoring of rehabilitated waste dumps.Covers of Avalon soil 0.3 or 0.5 m thick, were not adequate to prevent acidification. Coal wastecovered with 0.7 m compacted beneath 0.3 m uncompacted Avalon soil, showed a slow pH decline,but reached approximately pH 3 in 1997. Covers of compacted Estcourt soil beneath tmcompactedAvalon soil to a cover depth of 1 m were effective in preventing acidification and generally kept thecoal waste anaerobic. However, all covers developed cracks during drought conditions in 1995,allowing aeration. Low pH of some samples from these dumps during 1995/1996 may haveindicated the start of acidification.Bacteria oxidizing high concentrations of ferrous iron and considered to be Thiobacillusferrooxidans, were monitored routinely, but may not have been the dominant iron-oxidizer, aspopulation counts using media with a lower ferrous iron concentration were higher. The majority ofthe latter organisms could also not oxidize sulphur, hence were not T. ferrooxidans. The populations of the high ferrous iron-oxidizing bacteria were affected by pH, tending to be high inacidified and low in non-acidified coal waste.Investigations of microbial populations forming iron-oxidizing consortia in enrichment culturesfrom coal waste and acid drainage samples showed the presence of T. ferrooxidans, theheterotrophic bacterial genus Acidiphilium, fungi of the genus Penicillium, unidentified filamentousfungi, including Cladophialophora-like morphological types, and a yeast of the genus Dipodascus.In interaction studies, the Penicillium isolate had an inhibitory effect on T. ferrooxidans (subjectedto organic compound stress), but the Cladophialophora-like fungi reduced inhibition by organics.Fungi have not previously been studied in detail as components of iron-oxidizing consortia, but thebacterial isolations agree with those elsewhere, indicating that appropriate conclusions from acidmine drainage research in other parts of the world can be applied in KwaZulu-Natal.

[发布机构] Stellenbosch University