Browsing by Author "Dew, D"

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    Determining the effect of acid stress on the persistence and growth of thermophilic microbial species after mesophilic colonisation of low grade ore in a heap leach environment
    (Elsevier, 2013) Tupikina, O V; Minnaar, S H; van Hille, R P; van Wyk, N; Dew, D; Harrison S T L; Rautenbach, G F
    The microorganisms involved in the bioleaching of sulphidic mineral ores are acidophilic. Generally, a pH in the range of pH 1–2.5 is applied for optimal growth in these systems. In operating heaps, perturbation of conditions could result in changes in the pH outside this “safe” window, so an understanding of the effect of changes in pH on growth and activity of bioleaching microbes is needed. Previous work has shown that some microorganisms e.g. Acidithiobacillus thiooxidans, Leptospirillum ferriphilum and Leptospirillum ferrooxidans are able to adapt to low pH environments (∼pH 0.9). However, most studies on the response of micro-organisms implicated in mineral bioleaching to pH have been conducted under submerged, aerated culture conditions, with limited performance-based studies conducted under conditions mimicking a heap environment. In this study, the effect of acid stress on the persistence of the thermophilic micro-organisms in the ore bed inoculated at mesophilic conditions and their subsequent growth on reaching thermophilic conditions is considered. Following inoculation, five columns loaded with a low grade chalcopyrite ore were irrigated at a feed pH of 1.7 at 25 °C. After a few days, the temperature was sequentially increased from 25 °C through 37 °C to 50 °C, resulting in an Eh above 850 mV across all columns. The irrigation feed pH was then varied across the range pH 1.0 to 1.7 at 50 °C. Eh values greater than 800 mV could be attained in the columns with feed pH values between pH 1.2 and pH 1.7 at 50 °C. The Eh of the column receiving feed solution at a pH of 1.0 at 50 °C dropped to below 700 mV and did not recover. The temperature was then increased gradually to 60 °C. All the columns with feed pH of 1.2 and higher achieved Eh values above 800 mV. Quantitative analyses of the microbial community on selected PLS and ore samples indicated that lower pH affected the persistence of the thermophilic micro-organisms in the ore bed and their subsequent growth on reaching thermophilic conditions. The microbial population detached from the ore sample after 120 days decreased by a factor of 5–15 and 25–100 fold on decreasing the operating pH from 1.5–1.7 to 1.4 and 1.2 respectively. Poor microbial activity was found at pH 1.0, suggesting ineffective growth or persistence of the archaea.
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    The Effect of CO2 Availability on the Growth, Iron Oxidation and CO2-Fixation Rates of Pure Cultures of Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans
    (Wiley, 2012) Bryan, C G; Davis-Belmar, C S; van Wyk, N; Fraser, M K; Dew, D; Rautenbach, G F; Harrison, S T L
    Understanding how bioleaching systems respond to the availability of CO2 is essential to developing operating conditions that select for optimum microbial performance. Therefore, the effect of inlet gas and associated dissolved CO2 concentration on the growth, iron oxidation and CO2-fixation rates of pure cultures of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum was investigated in a batch stirred tank system. The minimum inlet CO2 concentrations required to promote the growth of At. ferrooxidans and L. ferriphilum were 25 and 70 ppm, respectively, and corresponded to dissolved CO2 concentrations of 0.71 and 1.57 mM (at 308C and 378C, respectively). An actively growing culture of L. ferriphilum was able to maintain growth at inlet CO2 concentrations less than 30 ppm (0.31–0.45 mM in solution). The highest total new cell production and maximum specific growth rates from the stationary phase inocula were observed with CO2 inlet concentrations less than that of air. In contrast, the amount of CO2 fixed per new cell produced increased with increasing inlet CO2 concentrations above 100 ppm. Where inlet gas CO2 concentrations were increased above that of air the additional CO2 was consumed by the organisms but did not lead to increased cell production or significantly increase performance in terms of iron oxidation. It is proposed that At. ferrooxidans has two CO2 uptake mechanisms, a high affinity system operating at low available CO2 concentrations, which is subject to substrate inhibition and a low affinity system operating at higher available CO2 concentrations. L. ferriphilum has a single uptake system characterised by a moderate CO2 affinity. At. ferrooxidans performed better than L. ferriphilum at lower CO2 availabilities, and was less affected by CO2 starvation. Finally, the results demonstrate the limitations of using CO2 uptake or ferrous iron oxidation data as indirect measures of cell growth and performance across varying physiological conditions. Biotechnol. Bioeng. 2012;109: 1693–1703