The microorganisms involved in the bioleaching of sulphidic mineral ores are acidophilic.Generally, a pH of less than pH 2.5 and preferably more than pH 1 is applied for optimal growth inthese bioleaching systems. In operational heaps, perturbation of conditions could result inchanges in the pH outside this “safe” window, so understanding is needed of the effect ofchanges in pH and associated hydrogen ion (H+) concentration on growth and activity ofbioleaching microbes. Previous work has shown that some microorganisms e.g. Acidithiobacillusthiooxidans, Leptospirillum ferriphilum and L. ferrooxidans are able to adapt to low pHenvironments. However, most studies on the response of micro-organisms implicated in mineralbioleaching to pH have been conducted under submerged, aerated culture conditions, with limitedperformance-based studies conducted under conditions mimicking a heap environment. In thisstudy, the effect of acid stress on thermophilic microbial colonisation in a low grade ore heapleach environment is considered. Five columns loaded with a low grade chalcopyrite ore were operated with an irrigation feed pH of1.7 and temperatures sequentially increased from 25°C to 50°C, resulting in an Eh above 850 mVacross all columns. The irrigation feed pH was then varied across the range pH 1.0 to 1.7 at50°C. Eh values greater than 800 mV could be attain ed in the columns with feed pH valuesbetween pH 1.2 and pH 1.7 at 50°C. The Eh of the c olumn receiving feed solution at a pH of 1.0at 50°C dropped to below 700 mV and did not recover . The temperature was then increasedgradually to 60°C. All the columns with feed pH of 1.2 and higher achieved Eh values above 800mV. Quantitative analyses of the microbial community on selected PLS and ore samplesindicated that lower pH values affected microbial colonisation and persistence, especially fromthe Archaeal domain. The microbial population detached from the ore sample after 120 daysdecreased by a factor of 5-15 and 25-100 fold on decreasing the operating pH from 1.5-1.7 to 1.4and 1.2 respectively. Poor microbial activity was found at pH 1.0, suggesting ineffectivecolonisation or persistence. The dominant microorganisms on ore at the end of the experimentalrun were At. caldus, L. ferriphilum and Sulfobacillus thermosulfidooxidans. Successfulcolonisation by extreme thermophiles was not achieved in this experiment at 60°C.
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