Recovery of rare earth elements (REE) from waste printed circuit boards (WPCBs) through heterotrophic bioleaching

Recovering rare earth elements (REE) from secondary sources, such as electronic waste (e-waste), is increasingly gaining attention as these elements are at high supply risk, and the environmental impacts of mining them are of concern. Global REE recycling rates remain below 1%. Waste printed circuit boards (WPCBs) represent a valuable source of critical materials (e.g., Cu, Au, REE). However, the recovery of REE from WPCBs has received comparatively less attention until now. The overall aim of this research was to investigate the extraction of REE from WPCBs through bioleaching and provide insights into the microbial mechanisms involved in the REE bioleaching process. Industrially comminuted WPCBs were provided by e-waste recycling companies from the UK. The chemical composition of the WPCBs material was analysed. The distribution of REE and other metals in different size fractions was determined, and the correlation between elemental concentrations and particle size was investigated. The material was subsequently used for bioleaching studies. Bioprospecting of microbial strains exhibiting bioleaching capabilities involved screening strains isolated from a contaminated site. The optimization of the bioleaching process was carried out using a Response Surface Methodology (RSM) approach, which included the analysis of microbial organic acid production. The study explored the kinetics aspect of the bioleaching process, examining both biotic and abiotic settings to identify distinctions between these approaches and provide an understanding of the microbial mechanisms driving the bioleaching of REE.

Characterization of the WPCBs material revealed a remarkable inverse correlation between REE concentrations and particle size. Concentrations of Y, La, and Gd were found up to a thousand times higher in the smaller particle size compared with coarser particles. Base metals including Cu, Sn, Pb, and Zn did not show this trend. A newly isolated Penicillium expansum strain outperformed established reference strains (e.g. Aspergillus niger) in the heterotrophic bioleaching of REE from WPCBs. Adjusting initial pH (7.5), providing low phosphate concentrations (0.1 mM), and excluding the use of a buffering agent in the bioleaching medium were identified as the most effective factors for maximizing bioleaching of REE. Pulp density and contact time with WPCBs were also critical variables. Significant improvements were achieved for extraction of La and Tb, with increases from 20 to 40%, and 26 to 50%, respectively. Bioleaching efficiencies of Pr, Nd, and Gd improved by more than 25%, approaching 70% recovery. Gluconic acid was identified as the primary produced organic acid and main leaching agent. The dissolution of REE from WPCBs was dominated by the proton-promoted acidolysis mechanism, with challenges associated with REE entrapment within complex compounds (e.g. mixed oxides and silicates). Furthermore, complexolysis of metals with the deprotonated organic acid was significant, particularly for Cu extraction. REE solubilization occurred within 24 hours, a considerably faster pace compared to published studies on fungal-mediated bioleaching. The microbial mechanisms involved in the bioleaching of REE were related to the regulation of fungal extracellular pH and enhanced biolixiviant production in response to the phosphate-limited growth conditions. The gluconic acid-rich biolixiviant was more effective than abiotic experiments containing similar concentrations of organic acids, indicating the involvement of other metabolic products. The presence of plasma membrane H+-ATPase (proton pumps) in the fungal isolate was proposed as a potential contributor to the bioleaching process. Exploring fungal genomics and underlying mechanisms of pH homeostasis is a promising avenue to enhance understanding of the process and improve extraction rates.