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The transition to renewable energies and electric vehicles has triggered an unprecedented demand for metals. Sustainable development of these technologies relies on effectively managing the lifecycle of critical raw materials, including their responsible sourcing, efficient use, and recycling. Metal recycling from electronic waste (e-waste) is of paramount importance owing to ore-exceeding amounts of critical elements and high toxicity of heavy metals and organic pollutants in e-waste to the natural ecosystem and human body. Heterotrophic microbes secrete numerous metal-binding biomolecules such as organic acids, amino acids, cyanide, siderophores, peptides, and biosurfactants which can be utilized for eco-friendly and profitable metal recycling. In this review paper, we presented a critical review of heterotrophic organisms in biomining, and current barriers hampering the industrial application of organic acid bioleaching and biocyanide leaching. We also discussed how these challenges can be surmounted with simple methods (e.g., culture media optimization, separation of microbial growth and metal extraction process) and state-of-the-art biological approaches (e.g., artificial microbial community, synthetic biology, metabolic engineering, advanced fermentation strategies, and biofilm engineering). Lastly, we showcased emerging technologies (e.g., artificially synthesized peptides, siderophores, and biosurfactants) derived from heterotrophs with the potential for inexpensive, low-impact, selective and advanced metal recovery from bioleaching solutions.

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Waste liquid crystal displays (LCDs) are one of the most substantial and rapidly growing e-waste streams that contain a notable amount of critical, precious, and toxic elements. This study presented a novel thermal-biological hybrid method for resource recovery from waste LCDs. Through the design of a multistage thermal treatment process with the addition of optimized 20 wt% B2O3 to waste, the LCD's glass structure was separated into two interconnected phases, resulting in the transfer of metals from the LCD's glass phase to the B2O3 phase that can solubilize in the acid solution. Following the thermal treatment step, the biometabolites of Aspergillus niger were used for bioleaching of In, Sr, Al, and As from the obtained thermally treated product. The optimal bioleaching parameters were a pulp density of 10 g/L, temperature of 70 °C, and leaching time of 2 days, which led to the highest extraction of 82.6% Al, 70.8% As, 64.5% In, and 36.2% Sr from thermally treated LCD waste, representing a multifold increase in Al, As, and Sr extraction levels compared to untreated waste. This study demonstrated that the proposed hybrid method could successfully overcome waste complexities and ensure effective element extraction from discarded LCDs.

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Metals recovery from spent automotive catalytic converters (SACCs) has gained great attention due to high metal content of SACCs and their potential to pollute the environment. This study presented a novel green strategy for treating SACCs using oxalic acid-enriched spent culture medium from Aspergillus niger cultivations. To enhance oxalic acid production, the Central Composite Design (CCD) was applied, which demonstrated that glucose (27.06 g/L), NaNO3 (0.9 g/L), disodium oxalate (7.7 g/L), MnSO4·H2O (0.28 g/L), and ethanol (0.65%(v/v)) were the optimum values leading to production of 15.3 g/L oxalic acid. The results of metals biorecovery with the fungal metabolites showed that pulp density of 15 g/L, temperature of 60 °C, and leaching time of 6 h resulted in the highest extraction of 99.1% Al, 99.3% Si, 82.2% Mn, 91.9% Zn, 17.6% Ba, 99.5% Fe, 92.2% Sr, 35.7% Ti, 60.9% Pt, and 73.7% Pd, as well as maximum enrichment of rare earth elements (REEs) in the residual powder. The EDX-mapping analysis indicated that the concentration of ∑REEs was nearly 8% in the initial waste powder, while it reached around 81% in the residual powder after bioleaching. The bioleaching mechanism was further analyzed by characterizing the bioleaching residues through XRD, FTIR, and FESEM analyses.

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Environmentally appropriate economic recycling of spent automotive catalytic converters (SACCs) is difficult due to their complexity. The prominent reason is the lack of knowledge and comprehensive characterization of SACCs. This study focused on the characterization of SACCs in terms of their structural, morphological, physiochemical, surface, and thermal properties. The accurate determination of metals content, including 4975 mg/kg platinum group metals, 42,119 mg/kg rare earth elements, and other base metals, showed a great potential wealth in SACCs. Besides, the sequential extraction method was applied for metals fractionation, which represents a unique harsh recycling approach needed due to the stable structure of SACCs, metals embedded in silicate phases, and the presence of barely soluble metal phosphates. This waste was also examined for environmental criteria and leaching tests, including Toxicity Characteristic Leaching Procedure, Waste Extraction Test, and Synthetic Precipitation Leaching Procedure. The findings declare that Waste Extraction Test was the most aggressive procedure to assess mobility. The contamination indexes, such as risk assessment code, contamination factor, and global contamination factor, were also investigated, which show SACCs must be regarded as hazardous waste. As an example, the global contamination factor of 11.87 depicts SACCs have a moderate contamination degree.

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The technology for recycling the spent coin cells is pressing needed due to a large amount of generated spent coin cells. However, there is little information about the recycling technology of spent coin cells. In this work, a two-step bioleaching method for recovery of metals from spent coin cells by Acidithiobacillus thiooxidans is performed for the first time. In this regard, the growth characteristics of A. thiooxidans was investigated in pure culture and during the two-step bioleaching approach. The highest recovery of Li, Co and Mn was achieved at a pulp density of 30 g L-1, in values of 99%, 60%, and 20%, respectively. The structural analyzes confirmed the progress of bioleaching process. In addition, the kinetics models showed that the chemical reaction was the rate-controlling step of the two-step bioleaching of spent coin cells. The comparative results between bioleaching and chemical leaching showed that Acidithiobacillus thiooxidans can enhance the leaching of metals. Toxicity characteristic leaching procedure of the spent coin cells powder demonstrated that the bioleached residue met the environmental limitations for safe disposal. In fact, bioleaching is an effective and promising route to reduce the environmental hazard of spent coin cells.

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In the present study, spent medium bioleaching method was performed using organic acids produced by Aspergillus niger to dissolve Ni, Co, Mn, Li, Cu and Al from spent lithium-ion batteries (LIBs). Response surface methodology was used to investigate the effects and interactions between the effective factors of sucrose concentration, initial pH, and inoculum size to optimize organic acid production. Maximum citric acid, malic acid, and gluconic acid concentrations of 26,478, 1832.53 and 8433.76ppm, respectively, and a minimum oxalic acid concentration of 305.558ppm were obtained under optimal conditions of 116.90 (gl-1) sucrose concentration, 3.45% (vv-1) inoculum size, and a pH value of 5.44. Biogenically-produced organic acids are used for leaching of spent LIBs at different pulp densities. The highest metal recovery of 100% Cu, 100% Li, 77% Mn, and 75% Al occurred at 2% (wv-1) pulp density; 64% Co and 54% Ni recovery occurred at 1% (wv-1) pulp density. The bioleaching of metals from spent LIBs can decrease the environmental impact of this waste. The results of this study suggest that the process can be used for large scale industrial purposes.

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