Contribution of Sessile Acidophiles on Chalcopyrite Bioleaching Under Controlled Redox Potentials

Abstract

Although the bioleaching of secondary copper sulfides has been industrialized for decades, the application of chalcopyrite bioleaching remains under development because of its low leaching rate. The effect of contact microbes on chalcopyrite leaching is still unclear due to the technical challenges in separating the contact (sessile micro-organisms) and the non-contact (planktonic micro-organisms) processes. Chalcopyrite bioleaching experiments were conducted using a novel device that stabilizes the redox potential and distinguishs between the microbial contact and non-contact effects. The contribution of the microbial “contact mechanism” in chalcopyrite leaching was quantified considering different redox potentials, compared to the “non-contact mechanism”. Based on the copper leaching kinetics and morphology of the leaching residue, it was demonstrated that the leaching rate of chalcopyrite was significantly influenced by the redox potential (850 mV > 650 mV > 750 mV), from 6.30% to 14.02% in 8 days leaching time. At each redox potential, the chalcopyrite leaching rate was 9.3%–30.6% higher with the presence of sessile microbes than without sessile microbes. Analysis of the leached chalcopyrite surface using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectrometry (XPS) revealed the formation of polysulfide and elemental sulfur at the surface. While the contacted sulfur oxidized the microbes, here, the Acidithiobacillus caldus preferred sessile at the chalcopyrite surface rather than Leptospirillum ferriphilum. Sulfur-oxidizing bacteria reduced the elemental sulfur content at the leach residue surface, thus playing an important role in degrading the sulfur passivation layer. In chalcopyrite bioleaching, the “contact mechanism” was primarily explained by sulfur-oxidizing bacteria promoting chalcopyrite oxidation through the removal of sulfur intermediates, while the “non-contact mechanism” was explained by ferrous-oxidizing microbes influencing the redox potential.