pH Threshold Impacts Chalcopyrite Bioleaching Dynamics for the Extreme Thermoacidophile Sulfurisphaera ohwakuensis

The extremely thermoacidophilic archaeon Sulfurisphaera ohwakuensis served as the basis for probing how initial pH (pH_initial) affects copper mobilization from chalcopyrite. Screening of small-scale cultures (75 mL) at 75°C revealed that ~pH 3.0 was a maximal threshold for bioleaching onset. Subsequently, chalcopyrite at 10 g/L in 750 mL culture media, containing small amounts of ferric ion, adjusted to pH 2.5 with sulfuric acid and incubated for 24 h at 75°C before inoculation, brought the pH to approximately 3.0 through abiotic chemical reactions. However, the resulting subtle differences in pH_initial (3.0 ± 0.15) in bioleaching cultures, while not affecting microbial growth, were critical to bioleaching onset and progress. Initial iron levels were less important than pH_initial in starting the bioleaching process. X-Ray Diffraction (XRD) surface analysis informed bioleaching trajectories over 21 days and reinforced the impact of pH_initial. The subtle differences in pH_initial markedly affected S. ohwakuensis onset and outcomes, as it presumably would for other bioleaching thermoacidophilic archaea. Furthermore, the findings here highlight the challenges faced in replicating bioleaching experiments across, and even within, laboratories as well as in achieving consistent results in bioleaching processes.