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The number of metal-containing waste streams resulting from electronic end-of life products, metallurgical by-products, and mine tailings to name but a few, is increasing worldwide. In recent decades, the potential to exploit these waste streams as valuable secondary resources to meet the high demand of critical and economically important raw materials has become more prominent. In this review, fundamental principles of bio-based metal recovery technologies are discussed focusing on microbial metabolism-dependent and metabolism-independent mechanisms as sustainable alternatives to conventional chemical metal recovery methods. In contrast to previous reviews which have partially addressed this topic, a special focus will be given on how fundamental principles of bio-based recovery technologies can influence the selectivity and specificity of metal recovery. While conventional methods for metal recovery show benefits in terms of economic affordability, bio-based recovery technologies offer advantages in terms of efficiency and environmentally friendliness. Modifications and adaptations in the processes of biosorption, bioaccumulation and bioelectrochemical systems are highlighted, further emphasizing the application of metal-binding peptides and siderophores to increase selectivity in the recovery of metals. Single metal solutions or mixtures with a low complexity have been the focus of previous studies and reviews, but this does not reflect the nature of complex industrial effluents. Therefore, key challenges that arise when dealing with complex polymetallic solutions are addressed and the focus is set on optimizing bio-based technologies to recover metals efficiently and selectively from bio-leachates or liquid waste streams.

While the amount of electronic waste is increasing worldwide, the heterogeneity of electronic scrap makes the recycling very complicated. Hydrometallurgical methods are currently applied in e-waste recycling which tend to generate complex polymetallic solutions due to dissolution of all metal components. Although biosorption has previously been described as a viable option for metal recovery and removal from low-concentration or single-metal solutions, information about the application of selective metal biosorption from polymetallic solutions is missing. In this study, an environmentally friendly and selective biosorption approach, based on the pH-dependency of metal sorption processes is presented using spent brewer’s yeast to efficiently recover metals like aluminum, copper, zinc and nickel out of polymetallic solutions. Therefore, a design of experiment (DoE) approach was used to identify the effects of pH, metal, and biomass concentration, and optimize the biosorption efficiency for each individual metal. After process optimization with single-metal solutions, biosorption experiments with lyophilized waste yeast biomass were performed with synthetic polymetallic solutions where over 50% of aluminum at pH 3.5, over 40% of copper at pH 5.0 and over 70% of zinc at pH 7.5 could be removed. Moreover, more than 50% of copper at pH 3.5 and over 90% of zinc at pH 7.5 were recovered from a real polymetallic waste stream after leaching of printed-circuit boards. The reusability of yeast biomass was confirmed in five consecutive biosorption steps with little loss in metal recovery abilities. This proves that spent brewer’s yeast can be sustainably used to selectively recover metals from polymetallic waste streams different to previously reported studies.

Harnessing microbial capabilities for metal recovery from secondary waste sources is an eco-friendly and sustainable approach for the management of metal-containing wastes. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) are the two main groups of extracellular compounds produced by microorganisms in response to metal stress that are of great importance for remediation and recovery of metals. These include various high-, and low, molecular weight components, which serve various functional and structural roles. These compounds often contain functional groups with metal binding potential that can attenuate metal stress by sequestering metal ions, making them less bioavailable. Microorganisms can regulate the content and composition of EPS and SMP in response to metal stress in order to increase the compounds specificity and capacity for metal binding. Thus, EPS and SMP represent ideal candidates for developing technologies for selective metal recovery from complex wastes. To discover highly metal-sorptive compounds with specific metal binding affinity for metal recovery applications, it is necessary to investigate the metal binding affinity of these compounds, especially under metal stressed conditions. In this review we critically reviewed microbial EPS and SMP production as a response to metal stress with a particular emphasis on the metal binding properties of these compounds and their role in altering metal bioavailability. Furthermore, for the first time, we compiled the available data on potential application of these compounds for selective metal recovery from waste streams.

Recently, recycling lithium-ion batteries (LIBs) waste and the efficient recovery of valuable metals has become crucial for the circular economy. Deep eutectic solvents (DESs) have shown immense potential for metal extraction from end-of-life (EoL) LIBs. However, a complicated subsequent separation process is required due to the non-selectivity of the DES components for metal. Herein, we propose a tandem precipitant-free hydrometallurgical process for the selective recovery of Li and other valuable metals (Ni, Co, Mn) from EoL LIBs using a green DES. The process parameters, viz., reaction temperature (T), time (t), pulp density (S/L), and molar ratio (MR) of ethylene glycol to tartaric acid, were optimized via the statistical optimization technique of response surface methodology. The results revealed maximum extraction efficiencies of 99.2 ± 0.5%, 96.1 ± 1.4%, 95.2 ± 1.2%, and 97.8 ± 1.5% for Li, Ni, Co, and Mn, respectively, under the optimized process conditions of T = 118 °C, t = 17 min, S/L = 42 g/L, and MR = 3:1. The kinetic study showed that interfacial reaction governed the extraction process. A plausible mechanism depicting the role of DES constituents in synergistically facilitating the extraction reaction was elucidated. Moreover, Li and other metals (Ni, Co, and Mn) were selectively separated and recovered with an overall efficiency of ∼99%. The reusability of the recovered metal products and the DES were also assessed for a closed-loop recycling process. At the outset, the proposed process paves the way for a greener alternative for recycling valuable metals from EoL LIBs rather than conventional acid-based leaching processes. [Display omitted] • A tandem closed-loop process with an efficient metal recovery using a green DES. • High extraction of Li (99.2%), Ni (96.1%), Co (95.2%) and Mn (97.8%) from EoL LIBs. • Precipitant-free selective separation (99%) of Li and transition metals (Ni/Co/Mn). • Statistical optimization of operating process parameters using RSM. • Plausible extraction mechanism elucidating the role of DES components. [ABSTRACT FROM AUTHOR]

The flue dust generated during the processing of e-waste is valuable as a secondary metal resource. In this study, a process of selective metal recovery from flue dust through the magnetic separation and the leaching process were developed. All the experiments were carried out at atmospheric pressure below 60°C. A continuous four-step process was carried out for the selective recovery of Al, Cu, Au, and Pb, using NaOH, H2SO4, CH4N2S, and HNO3 as leaching reagents in the four steps, respectively. As a result of this continuous recovery process, leaching efficiencies of 57.3% for Al, 98.0% for Cu, 75.2% for Au, and 77.5% for Pb were obtained. [ABSTRACT FROM AUTHOR]

Transforming industrial waste into high-value-added products represents a new avenue in the field of material science. This paper puts forward a comprehensive example for future works which target fabricating low-carbon footprint electrodes for rechargeable batteries. Within this scope, first time in the open literature, an integrated hydrometallurgical process has been designed to fabricate metal oxide powders of different properties from electric arc furnace (EAF) dust. Leaching, cementation, distillation, chemical precipitation, and calcination are used to produce powders rich in zinc oxide (Sample 1), ferrite (Sample 2), and nanocomposite (Sample 3) from the electric arc furnace flue dust. While sample 1 (zinc oxide with 97.8% purity) may be used in optics and other engineering applications, sample 2 (rich in zinc ferrite) and sample 3 (made of nanocomposites) that are recovered from the EAF dust may be utilized as electrode active material in lithium-ion batteries: Sample 2 and Sample 3 retain 43% and 50% of their capacities after 125th cycle, respectively. To further improve the cycle performance of Sample 2, the powder rich in ferrite is mixed with nickel salt, then heat treated. The latter performs 65% capacity retention after 125th cycle. The fact that every fabricated electrode achieves 125 cycles with success demonstrates the viability of the proposed technique; thus, various procedures may be developed to further elaborate the use of various industrial wastes in electrode fabrication. [Display omitted] • A novel integrated hydrometallurgical process is designed to valorize EAF flue dust. • Accurate source deployment and functional process design lead to low-C footprint materials. • Structural, morphological, chemical, and electrochemical properties of each powder are studied. [ABSTRACT FROM AUTHOR]

Harnessing microbial capabilities for metal recovery from secondary waste sources is an eco-friendly and sustainable approach for the management of metal-containing wastes. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) are the two main groups of extracellular compounds produced by microorganisms in response to metal stress that are of great importance for remediation and recovery of metals. These include various high-, and low, molecular weight components, which serve various functional and structural roles. These compounds often contain functional groups with metal binding potential that can attenuate metal stress by sequestering metal ions, making them less bioavailable. Microorganisms can regulate the content and composition of EPS and SMP in response to metal stress in order to increase the compounds specificity and capacity for metal binding. Thus, EPS and SMP represent ideal candidates for developing technologies for selective metal recovery from complex wastes. To discover highly metal-sorptive compounds with specific metal binding affinity for metal recovery applications, it is necessary to investigate the metal binding affinity of these compounds, especially under metal stressed conditions. In this review we critically reviewed microbial EPS and SMP production as a response to metal stress with a particular emphasis on the metal binding properties of these compounds and their role in altering metal bioavailability. Furthermore, for the first time, we compiled the available data on potential application of these compounds for selective metal recovery from waste streams. • EPS and SMP have metal-binding affinity and thus can impact metal bioavailability. • Metal stress induce production of microbial EPS and SMP. • Proteins display the highest metal-binding capacity and selectivity. • Extracellular proteins could act as biosorbents for selective metal recovery. [ABSTRACT FROM AUTHOR]

Valuable metals including V, Mo, Ni, Co, and Al were selectively recovered from spent desulfurization catalyst by means of extractive separation. Prior to selective metal recovery the spent catalyst was roasted at temperatures of 400–800 °C, and BET, SEM, and XRD data showed that roasting the spent catalyst at 550 °C gave birth to the surface and pore structures accounting for good extractibility of the roasted spent catalyst. A sequential scheme consisting of leaching and chelate extraction was developed to recover the metals from the spent catalyst roasted at 550 °C and oxygen space velocity of ca. 1800 hr−1. [ABSTRACT FROM AUTHOR]