Exploring microbial adaptation of immobilised acidophilic cultures to improve microbial oxidation rates and copper tolerance in e-waste bioleaching

Abstract

The rate of ferric iron regeneration by microbial oxidation is the rate determining step in the bioleaching of printed circuit boards (PCBs). Therefore, improvement of the volumetric rate of ferric iron formation is key for the overall bioleaching efficiency. The volumetric rate is impacted by both the specific rate and the biomass concentration. Limiting the exposure of the bioleaching microorganisms to accumulated toxic metal ions in solution, as well as the presence of inhibiting material components in PCBs through biofilm formation and immobilisation, as well as their adaptation to these inhibitors, is proposed to mitigate against decrease in specific rate while biomass retention by immobilisation is expected to positively impact volumetric rate. In this study, a mixed mesophilic culture, consisting of Leptospirillum (L.) ferriphilum, Acidithiobacillus (At.) caldus, Acidiplasma (Ap.) cupricumulans as dominant species was immobilised on polyurethane foam (PUF) and adapted to 6.0 g/L Cu2+. The rate of microbial ferrous iron oxidation by the immobilised culture was compared to that of planktonic cultures. Further, the performance of both the planktonic and immobilised Cu-adapted cultures (6.0 g/L of Cu2+) were compared with non-adapted planktonic and immobilised cells in the presence of increasing Cu2+ concentrations (0–50 g/L Cu2+).Both the Cu-adapted planktonic cells and the non-adapted immobilised cells demonstrated improved tolerance to Cu2+ stress relative to non-adapted planktonic cells. The Cu-adapted planktonic cells showed higher volumetric ferrous iron oxidation rates (0.0359 ± 0.0023 g Fe2+.L−1.h−1 at 22 g/L Cu2+) than the non-adapted immobilised cells (0.0331 ± 0.0021 g Fe2+.L−1.h−1) in the presence of Cu2+ concentrations exceeding 6 g/L. The Cu-adapted immobilised cells exhibited higher tolerance to Cu2+ stress and higher oxidation kinetics (0.0622 ± 0.0064 g Fe2+.L−1.h−1) than the Cu-adapted planktonic and non-adapted immobilised cells. This study provides a novel approach for improving culture performance and minimising the inhibitory effect of Cu2+ ion on L. ferriphilum dominated cultures. High microbial activity and metal tolerance of Cu-adapted immobilised cells presents an opportunity for immobilised microbial systems to be used both in bioleaching of e-waste streams, such as PCBs, and in bioleaching of mineral concentrates, ores and wastes with Cu2+-rich leachates.