Waste Printed Circuit Boards (WPCBs) as a potential source for the recovery of rare earth elements through bioleaching.
Rare earth elements (e.g., neodymium, dysprosium and praseodymium) have contributed to the miniaturization, energy efficiency, durability, and high speed of many technology gadgets. Due to their electric conductivity, magnetic, luminescence and optical properties, rare earth elements (REE) are crucial and potentially irreplaceable in current and future technologies, especially for the world’s transition towards low-carbon economies. The recovery of REE from secondary sources, such as waste electrical and electronic equipment (WEEE or e-waste), is gaining more attention as these elements are at high supply risk, and the environmental impacts of mining primary sources are of increasing concern. Waste printed circuit boards (WPCBs) represent a significant proportion of e-waste, they contain hazardous components but also valuable and critical materials (e.g., copper, gold, silver, rare earth elements), making this waste stream a suitable alternative for beneficiation purposes. The aim of this research is to investigate the recovery of REE from WPCBs using an environmentally friendly method, bioleaching, which exploits the ability of microorganisms to recover metal ions from the waste matrix. WPCBs were supplied by three local e-waste recycling companies following comminution process. First, physical characterization and elemental analysis of the material was performed, with emphasis on REE content. The distribution of REE and other metals in different size fractions of the WPCBs was determined. Furthermore, bioleaching was investigated for the extraction of REE from WPCBs. The most abundant REE found in the WPCBs samples include Nd, La and Pr, with concentrations up to 4500 g/ton, 2500 g/ton and 670 g/ton respectively. Spearman’s rank correlation analysis revealed strong correlations between REE and particle sizes (rs up to -0.98). Concentrations of REE were found up to a thousand times higher in the smaller particle size range (<0.25mm) compared with coarser particles (>2mm). Most of base metals including Cu, Sn, Pb and Zn did not show this trend. These findings are particularly important as the REE content in WPCBs and their potential recovery have scarcely been addressed in the scientific literature [1, 2]. Unlike base and precious metals (e.g. Cu, Al, Au, and Ag), REE are not considered for metal recovery purposes in current commercial recycling processes. The low yield of REE in e-waste has been a major drawback for their recycling. However, this study has shown that a cost-effective size separation could enrich the REE content for further recovery steps, preventing potential losses and enhancing the valorisation of WPCBs as an untapped resource for REE recovery. Particles below 0.5mm size, significantly rich in REE, were therefore used for bioleaching experiments. Following a two-step bioleaching process for 7 days at 28°C, with 1% (w/v) pulp density, two different fungal isolates were able to leach up to 40% of Nd, Gd and Pr, and 20% of Dy, almost doubled the leaching efficiency than the commercially available control strain. Although pyro- and hydro-metallurgical methods are the most common industrial practices for metal recovery, these processes still represent a risk to the environment due to their high chemical and energy requirements. While bioleaching of REE is still under development, many researchers have accomplished high leaching rates making use of different microbial strains; over 80% of REE have been recovered from WEEE shredding dust, and up to 100% from spent magnets [1, 3]. The present study evidenced the potential of WPCBs as a source of critical materials that have hardly been recovered from this waste stream, such as REE. Furthermore, the application of bioleaching enhances the green credentials of material extraction from WPCBs. The upcoming work in this research will aim to maximize the leaching efficiency and to provide a thorough understanding of the REE-microbial interactions in this complex e-waste matrix.