Some aspects of the effect of pH and acid stress in heap bioleaching

The chemical and physical conditions in sulphide heaps provide a complex environment for micro-organisms, with differences in redox potential, acidity, temperature, oxygen and solution chemistry conditions being experienced both temporally and spatially. One of the most important parameters for successful microbial colonisation and active microbial metabolism is suitable pH conditions in the heap. Typically heaps reach tens of metres high and the pH of irrigation solution travelling through heap changes significantly.In this study, we investigated the effect of pH and acid stress for moderately thermophilic and thermophilic mixed cultures, operating at 50-60 °C in a heap bioleaching environment. Results collected from laboratory scale column reactors packed with the low grade whole ore and irrigated with different pH solutions during a temperature shift from moderately thermophilic conditions to thermophilic conditions are discussed.     

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

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.     

The effect of As(III) and As(V) on the batch bioleaching of a pyrite-arsenopyrite concentrate

The bioleaching of arsenical gold-bearing sulphide ores and concentrates solubilises iron, arsenic and sulphur. Previous work has shown that high concentrations of iron and arsenic in solution inhibit bacterial growth, with As(III) reported to inhibit bacteria to a greater degree than As(V).Batch bioleaching experiments were carried out over periods of one month. Varying quantities of either 0.020–0.040 M As(III) or 0.107–0.220 M As(V), were added to a slurry, consisting of a pyrite-arsenopyrite concentrate (20% solids (m.v⁻¹)) in a nutrient solution. The slurry was inoculated with a culture, consisting primarily of Leptospirillum ferrooxidans and Thiobacillus thiooxidans. The culture was obtained from a continuous bioleaching mini-plant treating the same concentrate. The results obtained were compared with those of a culture to which no arsenic was added. The effect of the added arsenic was determined by monitoring three parameters: the oxygen utilisation rate, r_O2, of the culture, the rate at which the arsenic in the concentrate was solubilised and the speciation of the dissolved arsenic.The results suggest that the nature of the As(III) and As(V) toxicity is different. The addition of the culture to a slurry containing As(III) resulted in a reduced rate of bacterial oxidation. However, the addition of the culture to a slurry containing As(V) resulted in both a lag phase and a reduced rate of bacterial oxidation. At sufficiently high dosages of As(III) and As(V) the maximum oxygen utilisation rate, r_O2^max, of the culture was also affected. The results indicate that As(V) toxicity, and the relative toxicity of As(III) and As(V) to a mixed culture, appear to be affected by the availability of an energy source. Hence the toxicity of As(III) is not necessarily in the region of three times that of As(V). Furthermore, the results suggest that the mechanism of arsenic resistance may be attributed to the Pst⁺ Pit⁻ mutations and an energy dependent efflux pump.