Mineralogical Features of Copper-Bearing Shale Observed After an Incubation Experiment with Pseudomonas fluorescens and Acidithiobacillus thiooxidans

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The alteration of copper-rich black shale was evaluated under various leaching conditions (Acidithiobacillus thiooxidans and Pseudomonas fluorescens, their abiotic controls, and nutrient-limited conditions). Polished sections were used instead of ground shale to enable comparable before-and-after leaching observations using reflected light and scanning electron microscopy–energy-dispersive X-ray spectroscopy. Large-scale reflected-light imaging indicated that Acidithiobacillus thiooxidans activity stimulated the occurrence of secondary, copper-depleted sulfides relative to primary minerals, whereas changes associated with Pseudomonas fluorescens activity indicated a lesser degree of alteration. The highest efficiency of copper extraction was achieved with Acidithiobacillus thiooxidans, which is the opposite trend to that observed for copper extraction from ground shale. This could be attributed to differences in surface area available to leaching agents, namely fine particles versus polished surfaces. Under similar leaching conditions (i.e., bacteria, abiotic medium, and nutrient-depleted medium), the optical features and Cu:S ratios (ranges of 1.00–1.75 and 1.50–2.00 for Acidithiobacillus thiooxidans and Pseudomonas fluorescens, respectively) of post-leaching surfaces were similar, while copper extraction differed (e.g., 21.51 mg/L, 41.39 mg/L, and 27.25 mg/L for the bacteria, nutrient-limited condition, and abiotic control of Acidithiobacillus thiooxidans, respectively). Our results suggest that pH played the most significant role in altering the shale surface, while the chemical composition of growth media and the presence or absence of microorganisms were secondary factors, even if metal yields were significantly affected.