Abstract
Currently, the world is faced with a shortage of resources such as metals as the natural reserves are over-utilized, thus leading to their depletion. Secondary sources of these resources are sought such that they can be repurposed for various applications. The fastest-growing waste stream globally is electronic waste (e-waste) and this is due to the technological advancement and the advent of the Fourth Industrial Revolution (4IR). Although e-waste is an environmental hazard, these materials are regarded as good secondary sources of metals since these waste materials contain elevated concentrations of metals. This has provided an opportunity to recycle these metals for their usage in various applications. Extractive metallurgical techniques have been investigated in the past in the extraction of metals from e-waste. These techniques utilized various chemicals which are detrimental to the environment, and normally, these processes produce additional contaminants which would require treatment after the extraction process. Although these processes have their own shortcomings, they have proven to rapidly extract the metals of interest from e-waste. However, the biological extraction process, which is referred as bioleaching, has been proven to be a sustainable, eco-friendly, and robust approach for the extraction of metals from e-waste. Hence, this study employed the bioleaching approach for the extraction of metals of interest in e-waste.
This study examined the bioleaching of metals from printed circuit boards (PCBs) which were obtained from old television sets. This technique utilized a two-step bioleaching approach using acidophilic iron- and sulfur-oxidizing microorganisms. The PCBs were crushed manually, and the non-metal components were separated from the metal-containing components, manually. The metal-containing components were pulverized, and the resultant e-waste was categorized based on the particle sizes of 38-150 μm, 150-300 μm, 300-425 μm, 425-500 μm, and 500-710 μm. The different PCB particle sizes (PS) were characterized using X-ray fluorescence (XRF) to determine the elemental composition of the PCB at each PS. The study proceeded to source microbial cultures which were iron and sulfur-oxidizing acidophiles, which operated at different temperature regimes of 25 – 40 °C, 40 – 55 °C and 65 – 80 °C, normally referred to as mesophiles, moderate thermophiles, and thermophiles, respectively. These organisms were identified using a
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Quantitative Polymerase Chain Reaction (qPCR) from the genomic DNA which was extracted from the mixed cultures using species-specific PCR primers.
The qPCR data revealed that the microbial composition of the mesophilic microorganisms was dominated by Leptospirillum ferriphilum, Acidithiobacillus thiooxidans, Acidithiobacillus ferrooxidans at 67.25%, 9.53% and 23.22% respectively while the moderate thermophiles was dominated by Acidithiobacillus caldus, Sulfobacillus thermosulfidooxidans, Acidiplasma cupricumulans, Ferroplasma acidiphilum and Acidithiomicrobium spp at 65,33%, 0,12%, 1,33%, 1,77% and 31,45% respectively. The thermophiles were dominated by Sulfolobus metallicus and Metallosphaera sedula at 2,88% and 98,22% respectively.
The XRF data revealed that copper (Cu), tin (Sn), and lead (Pb) were predominantly deposited in a coarser fraction, specifically within the range of 500-710 μm with metal content of 28.7 wt.%, 20.5 wt.%, and 11.1 wt.%, respectively. On the other hand, metals such as iron (Fe), zinc (Zn), nickel (Ni), manganese (Mn), and aluminum (Al) with metal content of 37.3 wt.%, 5.9 wt.%, 8.8 wt.%, 1.3 wt.% 4.2 wt.%, respectively, were primarily deposited on PS ranging from 38-150 μm. Secondly, the study investigated the influence of PS (38-150 μm, 150-300 μm, 300-425 μm, 425-500 μm, and 500-710 μm) on metal recovery efficiency (RE). The influence of PS findings revealed that the PS ranging from 38-150 μm was the optimum fraction, which achieved RE of 62.9%, 68.2%, 95.3%, 86.1%, 61.9%, 47.2%, 21.2%, and 63.6% Al, Cu, Fe, Mn, Ni, Pb, Sn, and Zn respectively. Lastly, the study investigated the influence of different operational temperatures (37 ℃, 45 ℃, and 70 ℃) on metal RE using the optimum PS fraction (38-150 μm). It was observed that the metal extraction at mesophilic conditions (37 ℃) achieved the lowest RE of 63.55%, 21%, 62.9%, 62.9%, 95%, 88.1%, 61.9%, and 47% for Zn, Sn, Al, Cu, Fe, Mn, Ni and Pb respectively. On the other hand, at 70 ℃ and 45 ℃, it was observed that different metals exhibited various responses to temperature changes. It was observed that at moderate thermophiles (45 ℃) achieved the highest RE for Cu, Zn, Fe, and Al, which were 98.2%, 98.5%, 98.3%, and 98.6%, respectively, while the thermophiles (70 ℃) had the highest RE for Ni, Pb, Mn, and Sn, which were 99.1%, 98.2%, 99.9%, and 99.95%, respectively. The microbial composition after the bioleaching process at mesophilic conditions were observed to be dominated by Leptospirillum ferriphilum, Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans at 1,45%, 6,22% and 92,33%
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respectively, while moderate thermophilic conditions was dominated by Acidithiobacillus caldus, Sulfobacillus thermosulfidooxidans, Acidiplasma cupricumulans, Ferroplasma acidiphilum and Acidithiomicrobium spp at 17,89%, 0,00%, 45,33%, 34,22% and 2,56% respectively. The microbial composition after the experiment where the temperature was evaluated showed that there was a drastic change in the dominance of the microbial species compared to the microbial composition of the inoculum. This change in microbial composition was attributed to the presence of high concentrations of metals and other toxic compounds within the PCBs, thus resulting in the change in the microbial composition.
The data generated in this study provides a comprehensive understanding of the bioleaching process for metal recovery from PCBs and offer practical insights for optimizing the bioleaching process to recover valuable metals from e-waste. This study managed to fill the gap in understanding the impact of PS, different temperature regimes, and the microbial population dynamics before and after bioleaching on metal recovery from the PCBs, which was knowledge that was missing in literature.
Keywords: Bioleaching; Printed Circuit Boards; Particle Size; Mesophiles; Moderate Thermophiles; Thermophiles.