Bioleaching of printed circuit boards in a two-stage reactor system with enhanced ferric iron regeneration in a re-circulating packed-bed reactor from PCB leaching

We report the bio-assisted leaching of PCBs in a continuous two-stage reactor system, comprised of a stirred tank reactor for chemical leaching of PCBs through the reduction of ferric iron, coupled to a packed-bed column bioreactor in which biological ferrous iron oxidation is optimised to regenerate the ferric leach agent. The bioreactor was packed with polyurethane foam biomass support particles colonised with the moderately thermophilic mixed culture, dominated by Leptospirillum ferriphilum. On chemical leaching of the PCBs with acidic ferric iron in the stirred tank reactor, a ferrous-rich solution was generated and circulated through the packed bed bioreactor to re-oxidise the ferrous iron to ferric iron. A one-stage reactor system, where both PCB leaching and re-oxidation of the resultant ferrous iron occur in one pot, was run concurrently. PCB loading was varied from 3 % to 18 % (w/v) and the regeneration of the required ferric iron for leaching was studied. In both one- and two-stage systems, high bioleaching efficiency was achieved, with the two-stage system achieving 98 % Al, 58 % Ca, 93 % Cr, 96 % Cu, >100 % Mg, 79 % Ni, 80 % Pb, >100 % Sn, >100 % Zn solubilisation from 18 % PCB solid loading, whereas low release of 7 % Co and 11 % Sr was achieved. Despite the high PCB solid loading, microbial cultures maintained their oxidative activity post exposure to the leached metals, with more rapid Fe³⁺ regeneration occurring in the two-stage system compared to the one-stage system. The two-stage reactor system exhibited an improvement of about 60 % in the ratio of the Fe³⁺ reduction and regeneration rates relative to the one-stage reactor system. These findings present a promising approach to maximising metal recovery while maintaining and improving microbial activity in the bioleaching of PCBs. Such insight into the bioleaching of the elemental form of base metals from PCBs at the reactor level contributes to process design and optimisation, to enable upscaling and subsequent commercialisation of the process.