1989-11-22_REVISION - M1988112 - Laserfiche WebLink

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- 3 - bacterium was able to establish acidic semi-isolated micro-environments and then spread once the local pH was lowered to about 4. As such, direct oxidation of pyrite by Thiobacillus ferrooxidans probably initiated the mechanism in their experiments. Nordstrom (4) in his review of acid sulphate weathering also concludes that inorganic oxidation by molecular oxygen is too slow and therefore, the initial step must be microbielogically catalyzed although he concedes the initial processes are still subject to controversy. Van Breeman (5), however suggested that the role of Thio~baccilli in enhancing pyrite oxidation and causing the pH of a pyrite-water system to fall from near neutral to pH 4 has probably been over emphasized. Experiments by Arkesteyn (6) on pyrite oxidation in acid sulphate salts showed that microorganisms were not involved in the initial drop of pH. Part of the explanation for this controversy may lie in the comparable rates at near neutral pN of the chemical and microbiologically catalyzed processes (l). Halbert et. al. (7) found the sulphate generation rate of pyrite at pH 7 and 21°C for non-innoculated substrate to be 3.81 x 10-2 moleslkg•month and for innoculated substrate 6.70 x 10-2 moles/kg•month. The overriding principle in the generation of acid mine water is that if the alkalinity released by minerals or already present in the con- tacting water exceeds the acidity produced then the system remains neutral or basic and the process does not enter Stage II. In this case, the t bacteria responsible for catalyzing the acid generation reactions are inhibited and the solubility of ferrous and ferric iron are reduced - both factors inhibit acid production. The neutralization of the acid produced by pyrite oxidation may be represented as follows (33): CaC03(s) + 2H+ = Ca++ + H2O + CO2 CaC03(s) + SOq + 2H+ = CaSOq(s) + H2O + CO2 CaC03(s) + 2Fe++ = 3Ca++ + 2Fe(OH)3(s)