Microbial immobilisation and adaptation to Cu2+ enhances microbial Fe2+ oxidation for bioleaching of printed circuit boards in the presence of mixed metal ions

Abstract

A circular economy requires effective re-use of finite resources, such as metals from waste electrical and electronic equipment (WEEE). Bioleaching for extraction and recovery of base metals from printed circuit boards (PCBs) before recovering precious metals has potential to increase metal circularity. However, inhibition by base metals released from the PCBs and accumulated in PCB leachates on microbial Fe²⁺ oxidation, a critical bioleaching sub-process for Fe³⁺ regeneration, can limit this approach. Here, we explore the potential of microbial immobilisation on polyurethane foam (PUF) and adaptation to cupric ions to minimise inhibition by mixed metals released from PCBs, particularly zinc, nickel, and tin, and enhancing Fe²⁺ oxidation rates in PCB bioleaching systems. A mixed mesophilic culture dominant in Leptospirillum ferriphilum, Acidiplasma cupricumulans and Acidithiobacillus caldus was immobilised on PUF and adapted to 6 g/L Cu²⁺. Tolerance of Cu-adapted immobilised cells to the inhibitory metal ions Zn²⁺, Ni²⁺, and Sn²⁺, as individual (0–10 g/L) and mixed metal ions at concentrations typically leached from PCBs at solids loadings of 0–20% (mass/volume) was compared to that of non-adapted immobilised cells. Further, the impact of solutes from PCB leachates was evaluated. Inhibition by individual metal ions decreased in the order Sn²⁺ > Ni²⁺ > Zn²⁺. Inhibition of ferrous iron oxidation by mixed metal ions was synergistic with respect to individual metal ions. PCB leachates were more inhibitory than both mixed and individual metal ions even where metal concentration was low. Cu-adapted immobilised cells exhibited higher tolerance to increasing concentrations of inhibitory metal ions than non-adapted cells. These results are promising for the application of Cu-adapted cells in the bioleaching of PCBs and multi-metal concentrates.