Comparative assessment of metallurgical recovery of metals from electronic waste with special emphasis on bioleaching

Comparative life cycle assessment of copper and gold recovery from waste printed circuit boards: Pyrometallurgy, chemical leaching and bioleaching

Printed circuit boards (PCBs) make up a substantial amount of electronic waste (e-waste) generated annually. Waste PCBs contain high quantities of copper and gold in comparison to natural ores. As such, "urban mining" of waste PCBs to recover these metals is of commercial interest. In this work, we used life cycle assessment to compare the environmental impact of four copper and gold recovery processes. We evaluated pyrometallurgy, chemical leaching, and bioleaching, as well as a hybrid leaching process that uses bioleaching to recover copper and chemical leaching to recover gold. Furthermore, we considered differences in environmental impact based on differences in electricity sources. If electricity comes from fossil fuels, the pyrometallurgical process results in the lowest environmental impact in all impact categories studied. If electricity comes from carbon-free sources, the pyrometallurgical process results in the lowest environmental impact in all categories studied except global warming, where the hybrid leaching process results in the lowest impact. In all cases, metal recovery from waste PCBs leads to lower environmental impact than primary metal production. Our goal is to guide e-waste recyclers towards more environmentally sustainable metal recovery processes and to provide knowledge gaps in the field to guide future research.

Copper recovery by solvent extraction for nanoparticle synthesis from waste motherboards

Printed circuit boards, which make up part of waste from electrical and electronic equipment, contain elements that can be economically reused, such as copper, silver, gold, and nickel, as well as metals that are harmful to the environment and health, such as lead, mercury, and cadmium. Thus, through recycling this scrap, materials that would otherwise be discarded can be reinserted as secondary raw materials to produce new consumer goods through urban mining. In this context, the synthesis of nanoparticles shows promise as it allows the reinsertion of these materials in the manufacture of new products. Therefore, this study used obsolete computer motherboards as a secondary material to obtain copper to produce nanoparticles of this metal. From a solution based on the leach liquor of this scrap, a purification route using solvent extraction was defined and applied to the real leach liquor. Applying the hydroxyoxime extractant at a dilution of 20% (v/v) in kerosene, A/O of 1/1, 298 K, and 0.25 h of contact during extraction, and stripping in H2SO4 (2 M), 298 K, 0.25 h, W/O ratio of 3/1, and two theoretical countercurrent stages, a solution containing more than 95% of the copper in the leach liquor could be obtained with less than 1% of contaminants. From this purified liquor, nanoparticles containing copper and metallic copper oxides and hydroxides were produced, with an average size of 84 nm, at pH 11, 3 h of hot stirring, volume of 0.015 L of ascorbic acid (0.50 M) and 0.015 L of precursor solution (0.03 M Cu), and temperature (343 K).

Can e-waste recycling provide a solution to the scarcity of rare earth metals? An overview of e-waste recycling methods

Recycling e-waste is seen as a sustainable alternative to compensate for the limited natural rare earth elements (REEs) resources and the difficulty of accessing these resources. Recycling facilitates the recovery of valuable products and minimizes emissions during their transportation. Numerous studies have been reported on e-waste recycling using various techniques, including thermo-, hydro- and biometallurgical approaches. However, each approach still has technical, economic, social, or environmental limitations. This review highlights the potential of recycling e-waste, including outlining the current unutilized potential of REE recycling from different e-waste components. An in-depth analysis of e-waste generation on a global scale and Australian scenario, along with various hazardous impacts on ecosystem and human health, is reported. In addition, a comprehensive summary of various metal recovery processes and their merits and demerits is also presented. Lifecycle analysis for recovering REEs from e-waste indicate a positive environmental impact when compared to REEs produced from virgin sources. In addition, recovering REEs form secondary sources eliminated ca. 1.5 times radioactive waste, as seen in production from primary sources scenario. The review outcome demonstrates the increasing potential of REE recycling to overcome critical challenges, including issues over supply security and localized dependency.

Carbide lime as substrates to boost energy recuperation and dyestuff removal in constructed wetland-microbial fuel cell integrated with copper oxide/carbon cloth cathode

Methylene blue (MB) was regarded as a highly toxic and hazardous substance owing to its irreparable hazard and deplorable damage on the ecosystem and the human body. The treatment of this colorant wastewater appeared to be one of the towering challenges in wastewater treatment. In this study, a microbial fuel cell coupled with constructed wetland (CW-MFC) with effective MB elimination and its energy recuperation concurrently based on the incorporation of carbide lime as a substrate in a new copper oxide-loaded on carbon cloth (CuO/CC) cathode system was studied. The crucial influencing parameters were also delved, and the MB degradation and chemical oxygen demand (COD) removal efficiencies were correspondingly incremented by 97.3% and 89.1% with maximum power output up to 74.1 mW m-2 at optimal conditions (0.2 g L-1 carbide lime loading and 500 Ω external resistance). The carbide lime with high calcium ion content was greatly conducive for the enrichment of critical microorganism and metabolic activities. The relative abundances of functional bacteria including Proteobacteria and Actinobacteriota were vividly increased. Moreover, the impressive results obtained in printed ink wastewater treatment with a COD removal efficiency of 81.3% and a maximum power density of 58.2 mW m-2, which showcased the potential application of CW-MFC.

Scalable and Consolidated Microbial Platform for Rare Earth Element Leaching and Recovery from Waste Sources

Chemical methods for the extraction and refinement of technologically critical rare earth elements (REEs) are energy-intensive, hazardous, and environmentally destructive. Current biobased extraction systems rely on extremophilic organisms and generate many of the same detrimental effects as chemical methodologies. The mesophilic methylotrophic bacterium Methylobacterium extorquens AM1 was previously shown to grow using electronic waste by naturally acquiring REEs to power methanol metabolism. Here we show that growth using electronic waste as a sole REE source is scalable up to 10 L with consistent metal yields without the use of harsh acids or high temperatures. The addition of organic acids increases REE leaching in a nonspecific manner. REE-specific bioleaching can be engineered through the overproduction of REE-binding ligands (called lanthanophores) and pyrroloquinoline quinone. REE bioaccumulation increases with the leachate concentration and is highly specific. REEs are stored intracellularly in polyphosphate granules, and genetic engineering to eliminate exopolyphosphatase activity increases metal accumulation, confirming the link between phosphate metabolism and biological REE use. Finally, we report the innate ability of M. extorquens to grow using other complex REE sources, including pulverized smartphones, demonstrating the flexibility and potential for use as a recovery platform for these critical metals.

Fungal bioleaching of metals from WPCBs of mobile phones employing mixed Aspergillus spp.: Optimization and predictive modelling by RSM and AI models

In the present study, optimization and prediction models for fungal bioleaching for effective metal extraction from waste printed circuit boards (WPCBs) of mobile phones were developed employing central composite design (CCD) of response surface methodology (RSM), and two artificial intelligence (AI) models, i.e., artificial neural network (ANN) and, support vector machine (SVM), respectively. Two continuous process parameters, such as pH (4-9) and pulp density (1-10 g/L), and the bioleaching approaches, viz., one-step and two-step, were experimentally optimized for the extraction of targeted metals, i.e., Cu, Ni, and Zn from WPCBs by mixed cultures of Aspergillus niger and Aspergillus tubingensis. Datasets were then used for predictive modelling using AI tools. Results showed that the highest simultaneous bioleaching of Cu, Ni, and Zn, with an extraction efficacy of about 86%, 51%, and 100%, respectively, achieved at an optimal condition of pH 5.7 and pulp density of 3 g/L following the two-step bioleaching approach. Effective metal extraction in the two-step approach could be attributed to the abundant production of organic acids with a content of about 16.3 g/L, 8.4 g/L, and 0.5 g/L of citric acid, oxalic acid, and malic acid, respectively. Further, the predictive modelling revealed that the ANN model was found to predict the fungal bioleaching responses more accurately as compared to the SVM model with R2 values exceeding 0.96 for all targeted metals. This research demonstrates the applicability of the optimization and prediction models for efficient metal extraction from WPCBs using mixed Aspergillus spp. following the two-step approach.

Recovery of precious metals from waste printed circuit boards though bioleaching route: A review of the recent progress and perspective

The rapid proliferation of electronic waste (e-waste), including waste printed circuit boards (WPCBs), has exerted immense pressure on the environment. The recovery of precious metals from WPCBs not only serves as an effective means of alleviating this environmental burden but also generates economic value. This review focuses on bioleaching, an environmentally friendly method for extracting precious metals from WPCBs. Under various conditions, this method has achieved leaching rates of 30%-73% for Au and 33.8%-90% for Ag. However, there is a relative scarcity of studies on the bioleaching of precious metals from WPCBs. In this paper, we provide an overview of the current status of bioleaching for precious metals from WPCBs and describe the underlying mechanisms. We also briefly outline the influence of various process factors on leaching efficiency. While this review underscores the considerable potential of bioleaching in WPCBs applications, certain limitations hinder the engineering-scale application of the technology. Consequently, this paper describes the current enhanced processes for enhancing leaching efficiency. Overall, this review can serve as a valuable reference for future research endeavors, ultimately promoting the widespread utilization of bioleaching for the recovery of precious metals from WPCBs.

Recycling and reutilization of smelting dust as a secondary resource: A review

Smelting dust is a toxic waste produced in metal-mineral pyrometallurgical processes. To eliminate or reduce the adverse environmental impacts of smelting dust, valuable components need to be selectively separated from the toxic components present in the waste. This paper reviews the chemical composition, phase composition and particle size distribution characteristics of smelting dust, and the results show that smelting dust has excellent physicochemical characteristics for recovering valuable metals. The process flow, critical factors, development status, advantages and disadvantages of traditional technologies such as pyrometallurgy, hydrometallurgy and biometallurgy were discussed in depth. Conventional treatment methods typically prioritize separating and reclaiming specific elements with high concentrations. However, these methods face challenges such as excessive chemical usage and limited selectivity, which can hinder the sustainable utilization of smelting dust. With the increasing scarcity of resources and strict environmental requirements, a single treatment process can hardly fulfil the demand, and a physical field-enhanced technology for releasing and separating valuable metals is proposed. Through analysing the effect of electric field, microwave and ultrasound on recovering valuable metals from smelting dust, the enhancement mechanism of physical field on the extraction process was clarified. This paper aimed to provide reference for the resource utilization of smelting dust.

Resource Recycling, Recovery, and Xenobiotic Remediation from E-wastes Through Biofilm Technology: A Review

Around 50 million tonnes of electronic waste has been generated globally per year, causing an environmental hazard and negative effects on human health, such as infertility and thyroid disorders in adults, endocrine and neurological damage in both animals and humans, and impaired mental and physical development in children. Out of that, only 15% is recycled each year and the remaining is disposed of in a landfill, illegally traded or burned, and treated in a sub-standard way. The processes of recycling are challenged by the presence of brominated flame retardants. The different recycling technologies such as the chemical and mechanical methods have been well studied, while the most promising approach is the biological method. The process of utilizing microbes to decontaminate and degrade a wide range of pollutants into harmless products is known as bioremediation and it is an eco-friendly, cost-effective, and sustainable method. The bioremediation process is significantly aided by biofilm communities attached to electronic waste because they promote substrate bioavailability, metabolite transfer, and cell viability, all of which accelerate bioleaching and biodegradation. Microbes existing in biofilm mode relatable to free-floating planktonic cells are advantageous of bioremediation due to their tolerant ability to environmental stress and pollutants through diverse catabolic pathways. This article discusses the harmful effects of electronic waste and its management using biological strategies especially biofilm-forming communities for resource recovery.

Lanmodulin-Functionalized Magnetic Nanoparticles as a Highly Selective Biosorbent for Recovery of Rare Earth Elements

Recovering rare earth elements (REEs) from waste streams represents a sustainable approach to diversify REE supply while alleviating the environmental burden. However, it remains a critical challenge to selectively separate and concentrate REEs from low-grade waste streams. In this study, we developed a new type of biosorbent by immobilizing Lanmodulin-SpyCatcher (LanM-Spycatcher) on the surface of SpyTag-functionalized magnetic nanoparticles (MNPs) for selective separation and recovery of REEs from waste streams. The biosorbent, referred to as MNP-LanM, had an adsorption activity of 6.01 ± 0.11 μmol-terbium/g-sorbent and fast adsorption kinetics. The adsorbed REEs could be desorbed with >90% efficiency. The MNP-LanM selectively adsorbed REEs in the presence of a broad range of non-REEs. The protein storage stability of the MNP-LanM increased by two-fold compared to free LanM-SpyCatcher. The MNP-LanM could be efficiently separated using a magnet and reused with high stability as it retained ∼95% of the initial activity after eight adsorption-desorption cycles. Furthermore, the MNP-LanM selectively adsorbed and concentrated REEs from the leachate of coal fly ash and geothermal brine, resulting in 967-fold increase of REE purity. This study provides a scientific basis for developing innovative biosorptive materials for selective and efficient separation and recovery of REEs from low-grade feedstocks.

A review on recovery processes of metals from E-waste: A green perspective

E-waste management has become a global concern because of the enormous rise in the rate of end-of-life electrical and electronic equipment's (EEEs). Disposal of waste EEE directly into the environment leads to adverse effects on the environment as well as on human health. For the management of E-waste, numerous studies have been carried out for extracting metals (base, precious, and rare earth) following pyrometallurgy, hydrometallurgy, and biometallurgy. Irrespective of the advantages of these processes, certain limitations still exist with each of these options in terms of their adoption as treatment techniques. Several journal publications regarding the different processes have been made which aids in future research in the field of E-waste management. This review provides a comprehensive summary of the various metal recovery processes (pyrometallurgy, hydrometallurgy, and biometallurgy) from E-waste, along with their advantages and limitations. A bibliometric study based on the published articles using different keywords in Scopus has been provided for a complete idea about E-waste with green technology perspective like bioleaching, biosorption, etc. The present study also focussed on the circular economic approach towards sustainable E-waste management along with its socio-economic aspects and the economic growth of the country. The present study would provide valuable knowledge in understanding E-waste and its different treatment processes to the students, researchers, industrialists, and policymakers of the country.

Metal bioleaching from printed circuit boards by bio-Fenton process: Optimization and prediction by response surface methodology and artificial intelligence models

Recycling printed circuit boards (PCBs) in the e-waste stream is essential for ecological protection and metal recycling for a circular economy. Efficient metal recovery from PCBs is highly dependent on the determination of the optimum combination of inputs in the recycling process. In this study, optimization and predictive modelling of the bio-Fenton process were performed employing the response surface methodology (RSM) and the artificial intelligence (AI) models for efficient enzymatic metal bioleaching from discarded cellphone PCBs. The Box-Behnken design (BBD) of RSM was chosen as the design matrix. Further, two AI models, i.e., support vector machine (SVM) and artificial neural network (ANN) were employed to predict complex metal bioleaching process. Experiments were performed based on variations of four input process parameters, namely, glucose oxidase (GOx) content (100-1000 U/L), Fe²⁺ content (10-50 mM), PCB pulp density (1-10 g/L), and shaking speed (150-450 rpm). Results revealed that the maximum simultaneous enzymatic metal extraction of 100% Cu, 70% Ni, 40% Pb, and 100% Zn was attained at the optimized conditions: GOx content: 300 U/L, Fe²⁺ content: 10 mM, pulp density: 1 g/L, and shaking speed: 335 rpm. A comparative analysis of the AI models suggested that the ANN-based model predicting more accurate results than the SVM-based model with coefficient of determination values > 0.99 for all the targeted metals. The FTIR analysis confirmed the partial disintegration of PCB polymeric base by OH radicals (OH•), which helped in liberating and exposing the embedded metals to the bio-Fenton solution. Further, the oxidation of metals by ferric ions produced from GOx-mediated oxidation of ferrous ions ensued efficient enzymatic metal bioleaching. Selective metal recovery of >99% was obtained by the chemical precipitation of bioleachate.

Alkaliphiles for comprehensive utilization of red mud (bauxite residue)-an alkaline waste from the alumina refinery

The mining industry has powered the human endeavor to make life more innovative, flexible, and comfortable. However, it has also led to concerns due to the increasing amount of mining and associated industrial waste. Special attention is highly desired for its proper management and safe disposal in the environment. The problem has only augmented with the increase in the mining costs because of the investments needed for ecological remediation after the mining operation. It is pertinent that the targeted technologies need to be developed to utilize mining and associated industrial waste as a secondary resource to ensure sustainable mining operations. Every perceived waste is a valuable resource that is needed to be utilized to create additional value. In this review, the case of alkaline bauxite residue (red mud)-alumina refinery waste has been discussed at length. The highlight of the proposed work is to understand the importance of alkaliphile-assisted biomining-a sustainable alternative to conventional metal recovery processes. Along with the recovery of metals, pH reduction of red mud is possible through biomining, which ultimately paves the way for its complete utilization. The unique adaptation strategies of alkaliphiles make them more suitable for biomining of red mud through bioleaching, biosorption, and bioaccumulation, which have been discussed here. Furthermore, we have focused on the potential of the indigenous microflora of red mud for metal recovery in addition to its neutralization. The study of indigenous alkaliphiles from red mud, including its isolation and propagation, is crucial for the industrial-scale application of alkaliphile-based technology and has been emphasized.

Fungal biotechnology for urban mining of metals from waste printed circuit boards: A review

Rapid surge in electronic waste (e-waste) and its unscientific handling has an adverse impact on humans and the environment. Waste printed circuit board (WPCB), an integrated component of e-waste, has a high metallic content that includes both toxic and precious metals. Therefore, metal recovery is essential not just to avoid environmental degradation but also for economic growth. The current literature analysis focuses on one such eco-friendly approach, known as fungal biotechnology, for extracting metals from WPCBs. Among diverse bioleaching agents, fungi have shown promising metal extraction efficiency (Al: 65-96%; Co: 45-90%; Cu: 34-100%; Ni: 8-95%; Mn: 70-95%; Pb: 27-95%; Zn: 54-99%) and the ability to work in a wide pH range. However, in terms of metal recovery from WPCBs, fungal bioleaching has been less explored. This review, thus, assesses the fungal biotechnology for metal extraction from WPCBs and discusses the associated mechanism and kinetics involved. Different process parameters affecting the fungal bioleaching have also been discussed briefly. The review highlights that, while this process has enough potential, some associated drawbacks hinder its practical applicability on an industrial scale. Lastly, some suggestions for scaling up and reducing the cost of the process have been made, which need to be addressed.

Safety Concerns for the Management of End-of-Life Lithium-Ion Batteries

Lithium-ion battery (LIB) usage is growing dramatically worldwide. Relatedly, there is a need for the management of end-of-life (EOL) LIBs. EOL requires closed-loop systems and supply chains. Although many studies related to managing EOL in closed-loop supply chains exist, one especially pernicious issue is overlooked-safety. This study seeks to address this major safety oversight for EOL LIBs using closed-loop supply chains that are critical to a larger circular economy environment. The evaluation is completed along a technology-organization-environment (TOE) framework; potential research directions for mitigating safety issues are part of the analysis of this study. Specific and general research questions pertaining to secure management of EOL LIBs are put forward to help advance academic research. Practical concerns are also described for policymakers and organizations. This study reveals implications of these questions for the intersection of materials science, supply chain management, and fire-protection engineering.

Environmental behavior, human health effect, and pollution control of heavy metal(loid)s toward full life cycle processes

Heavy metal(loid)s (HMs) have caused serious environmental pollution and health risks. Although the past few years have witnessed the achievements of studies on environmental behavior of HMs, the related toxicity mechanisms, and pollution control, their relationship remains a mystery. Researchers generally focused on one topic independently without comprehensive considerations due to the knowledge gap between environmental science and human health. Indeed, the full life cycle control of HMs is crucial and should be reconsidered with the combination of the occurrence, transport, and fate of HMs in the environment. Therefore, we started by reviewing the environmental behaviors of HMs which are affected by a variety of natural factors as well as their physicochemical properties. Furthermore, the related toxicity mechanisms were discussed according to exposure route, toxicity mechanism, and adverse consequences. In addition, the current state-of-the-art of available technologies for pollution control of HMs wastewater and solid wastes were summarized. Finally, based on the research trend, we proposed that advanced in-operando characterizations will help us better understand the fundamental reaction mechanisms, and big data analysis approaches will aid in establishing the prediction model for risk management.

Response of microbiomes with different abundances to removal of metal fractions by soil washing

Toxic metal contamination causes a great threat to soil ecosystem and human health. Soil washing is a fast practice for removing metals, but its influences on microbial diversity and the stability of soil ecosystem remain unknown. In this study, ethylenediaminetetraacetic acid (EDTA), citric acid (CA), and fermented pineapple peel residue (FPP) were used as representatives of chelates, low molecular organic acids and biological materials to wash Pb-polluted soils, and their impacts on microbial community were investigated. Washing with these agents effectively removed Pb, but altered microbial community structure. After washing with EDTA, CA, and FPP, 3-8 bacterial phyla and 1 fungal phylum greatly increased, while 7-20 bacterial and 0-6 fungal phyla severely decreased or even disappeared. The alterations of different microbiomes were closely related to soil metal fractions. The labile metal fraction had negative effects on most bacteria and fungi, but also showed positive influences on Actinobacteria, Patescibacteria, and Fusobacteria. The moderately stable and stable fractions were nontoxic to the most microbes, but still harmful to Patescibacteria and Deinococcus-Thermus. These findings provide new insights for the effects of soil washing remediation and toxicity of metal fractions on the microbiomes with different abundance.

Potential and current practices of recycling waste printed circuit boards: A review of the recent progress in pyrometallurgy

Over the last few decades, a substantial amount of e-waste including waste printed circuit boards (WPCBs) has been produced and is accumulating worldwide. More recently, the rate of production has increased significantly, and this trend has raised some serious concerns regarding the need to develop viable recycling methods. The presence of other materials in the WPCBs, such as ceramics and polymers, and the multi-metal nature of WPCBs all contribute to the increased complexity of any recycling process. Among the viable techniques, pyrometallurgy, with the inherent ability to process the waste independent of its composition, is a promising candidate for both rapid and large-scale treatment. In the present study, firstly, the principles of the pyrometallurgical methods for WPCB recycling are discussed. Secondly, the different unit operations of thermochemical pretreatment including incineration, pyrolysis, and molten salt processing are reviewed. Thirdly, the smelting processes for the recovery of metals from WPCBs, as well as the issues surrounding slag formation and subsequent treatment are explained. Fourthly, alternative methods for the recovery of polymers and ceramics, in addition to metal recycling, are elucidated. Fifthly, emission control techniques and the potential for energy recovery are evaluated.

Morphology, Phase and Chemical Analysis of Leachate after Bioleaching Metals from Printed Circuit Boards

The article presents the assessment of solutions and dried residues precipitated from solutions after the bioleaching process of Printed Circuit Boards (PCB) utilizing the Acidithiobacillus ferrooxidans. The obtained dried residues precipitated from bioleaching solution (leachate) and control solution were tested using morphology, phase, and chemical composition analysis, with particular emphasis on the assessment of crystalline and amorphous components. The analysis of the dried residues from leachate after bioleaching as well as those from the sterile control solution demonstrated a difference in the component oxidation—the leachate consisted of mainly amorphous spherical particles in diameter up to 200 nm, forming lacy aggregates. In the specimenform control solution larger particles (up to 500 nm) were observed with a hollow in the middle and crystalline outer part (probably Fe2O3, CuFeS2, and Cu2O). The X-ray diffraction phase analysis revealed that specimen obtained from leachate after bioleaching consisted mainly of an amorphous component and some content of Fe2O3 crystalline phase, while the dried residue from control solution showed more crystalline components. The share of the crystalline and amorphous components can be related to efficiency in dissolving metals during bioleaching. Obtained results of the investigation confirm the activity and participation of the A. ferrooxidans bacteria in the solubilization process of electro-waste components, with their visible degradation–acceleration of the reaction owing to a continuous regeneration of the leaching medium. The performed investigations allowed to characterize the specimen from leachate and showed that the application of complementary cross-check of the micro (SEM and S/TEM) and macro (ICP-OES and XRD) methods are of immense use for complete guidance assessment and obtained valuable data for the next stages of PCBs recycling.

Green Recycling Methods to Treat Lithium-Ion Batteries E-Waste: A Circular Approach to Sustainability

E-waste generated from end-of-life spent lithium-ion batteries (LIBs) is increasing at a rapid rate owing to the increasing consumption of these batteries in portable electronics, electric vehicles, and renewable energy storage worldwide. On the one hand, landfilling and incinerating LIBs e-waste poses environmental and safety concerns owing to their constituent materials. On the other hand, scarcity of metal resources used in manufacturing LIBs and potential value creation through the recovery of these metal resources from spent LIBs has triggered increased interest in recycling spent LIBs from e-waste. State of the art recycling of spent LIBs involving pyrometallurgy and hydrometallurgy processes generates considerable unwanted environmental concerns. Hence, alternative innovative approaches toward the green recycling process of spent LIBs are essential to tackle large volumes of spent LIBs in an environmentally friendly way. Such evolving techniques for spent LIBs recycling based on green approaches, including bioleaching, waste for waste approach, and electrodeposition, are discussed here. Furthermore, the ways to regenerate strategic metals post leaching, efficiently reprocess extracted high-value materials, and reuse them in applications including electrode materials for new LIBs. The concept of "circular economy" is highlighted through closed-loop recycling of spent LIBs achieved through green-sustainable approaches.

Recovering metal(loids) and rare earth elements from closed landfill sites without excavation: Leachate recirculation opportunities and challenges

Metal (loids) and Rare Earth Elements (REE) ('metals') are used in a wide range of products, and therefore, the improvement of expectations for everyday comforts with demand continues to grow. Metal-bearing wastes are a secondary source of raw material that can meet this demand by providing a previously unconsidered low impact supply source. Total annual leachate production is 1,056,716 m³. Therefore, landfill leachate emerges as a significant potential resource as it contains high concentrations of metals. However, realising a profitable return on investment for leachate processing is a challenge due to relatively low recovery rates of approximately 0.02% of total heavy metals in a landfill being leached out in 30 years. Variation within the multi-element value and the effect of other chemicals in these complex mixtures. There is a need to better understand the mechanisms and potential applicability of extraction methods for optimising metals recovery from leachate. This paper addresses this need by providing a systematic review of the critical factors and environmental conditions that influence the behaviour of metals within the landfilled waste. The paper provides a synthesis of how the factors and conditions may affect leachate recirculation efficiency for recovery in the context of a range of opportunities and challenges facing circular economy practitioners. To approach feasibility metal recovery economically from landfill leachate without energy-intensive and environmentally destructive, future research actions need to be initiated in lab-based and later on semi-pilot to pilot studies, which the review can help achieve the challenges.

A review on recent advancements in recovery of valuable and toxic metals from e-waste using bioleaching approach

This review is intent on the environmental pollution generated from printed circuit boards and the methods employed to retrieve valuable and hazardous metals present in the e-wastes. Printed circuit boards are the key components in the electronic devices and considered as huge e-pollutants in polluting our surroundings and the environment as a whole. Composing of toxic heavy metals, it causes serious health effects to the plants, animals and humans in the environment. A number of chemical, biological and physical approaches were carried out to recover the precious metals and to remove the hazardous metals from the environment. Chemical leaching is one of the conventional PCBs recycling methods which was carried out by using different organic solvents and chemicals. Need of high cost for execution, generation of secondary wastes in the conventional methods, forces to discover the advanced recycling methods such as hydrometallurgical, bio-metallurgical and bioleaching processes to retrieve the valuable metals generate through e-wastes. Among them, bioleaching process gain extra priority due to its higher efficiency of metal recovery from printed circuit boards. There are different classes of microorganisms have been utilized for precious metal recovery from the PCBs through bioleaching process such as chemolithoautotrophy, heterotrophy and different fungal species including Aspergillus sp. and Penicillium sp. The current status and scope for further studies in printed circuit boards recycling are discussed in this review.

Microbial recovery of critical metals from secondary sources

The continuous development of technologies involving critical metals, both in Europe and over the world, and geopolitical challenges in areas rich in critical metal sources, imposed increased research efforts to recover them from secondary sources, by eco-efficient processes. Yet, microbes-metal interactions are not sufficiently exploited to recover metals from secondary sources, although they are already used in ore extraction. This review examines and compare strategies and processes involving microorganisms for critical metals recovery, since conventional physico-chemical methods are energy-intensive and often polluting. Two groups of microbial assisted recovery processes are discussed: metal mobilization from metal bearing waste, and selective metal separation from leaching solutions by immobilization on microbial biomass. Because most of the identified microbial technologies are developed on laboratory scale, the increase of biorecovery efficiency is compulsory for enhancing scaling-up potential. Future developments focused on novel microorganisms and high-performance strategies for critical metal recovery by microbial processes are considered.

Cobalt Recovery from Li-Ion Battery Recycling: A Critical Review

The increasing demand for Li-ion batteries for electric vehicles sheds light upon the Co supply chain. The metal is crucial to the cathode of these batteries, and the leading global producer is the D.R. Congo (70%). For this reason, it is considered critical/strategic due to the risk of interruption of supply in the short and medium term. Due to the increasing consumption for the transportation market, the batteries might be considered a secondary source of Co. The outstanding amount of spent batteries makes them to a core of urban mining warranting special attention. Greener technologies for Co recovery are necessary to achieve sustainable development. As a result of these sourcing challenges, this study is devoted to reviewing the techniques for Co recovery, such as acid leaching (inorganic and organic), separation (solvent extraction, ion exchange resins, and precipitation), and emerging technologies—ionic liquids, deep eutectic solvent, supercritical fluids, nanotechnology, and biohydrometallurgy. A dearth of research in emerging technologies for Co recovery from Li-ion batteries is discussed throughout the manuscript within a broader overview. The study is strictly connected to the Sustainability Development Goals (SDG) number 7, 8, 9, and 12.

Pretreatment of low-grade shredded dust e-waste to enhance silver recovery through biocyanidation by Pseudomonas balearica SAE1

Shredded dust originated during mechanical dismantling of waste electrical and electronic equipments (WEEEs) is enriched source of some valuable metals which might be lost as unworthy waste. Composition analysis of shredded dust printed circuit boards (PCBs) revealed the presence of 12.75 mg g^-1  copper (Cu) and 10.34 mg g^-1 silver (Ag) along with some other metals (Fe, Ni, and Au). Low concentration of precious metal, such as gold (Au i.e., 0.04 mg g^-1) classified this shredded dust as low-grade scrap. Despite low concentration of Au this e-waste can be considered as potential "secondary ore" to recover other valuable metals like Ag. To improve the efficiency of Ag bioleaching using Pseudomonas balearica SAE1, pretreatment of e-waste was done using cost-effective ferric chloride (FeCl₃) chemical lixiviant. The concentration of FeCl₃ lixiviant was optimized to recover Cu metal prior to bioleaching process. Bioleaching of Ag was done under optimized conditions by Pseudomonas balearica SAE1 using 100 mL Luria Broth (LB) medium, 5 g L^-1 glycine, pH 9, temperature 30 °C and 150 rpm. 95% Cu was recovered with 1% FeCl₃ prior to bioleaching. Ag solubilization was increased for treated e-waste (36%) as compared to untreated e-waste PCBs (25%). Prerecovery of Cu enhanced Ag bioleaching, as available cyanide was utilized by Ag metal. Therefore, this study provides an economical hybrid method to enhance retrieval of precious metal (Ag) by Pseudomonas balearica SAE1 with economic and ecofriendly redox lixiviant even from low-grade e-scrap.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02977-4.

Electronic Waste Low-Temperature Processing: An Alternative Thermochemical Pretreatment to Improve Component Separation

The production of electronic waste due to technological development, economic growth and increasing population has been rising fast, pushing for solutions before the environmental pressure achieves unprecedented levels. Recently, it was observed that many extractive metallurgy alternatives had been considered to recover value from this type of waste. Regarding pyrometallurgy, little is known about the low-temperature processing applied before fragmentation and subsequent component separation. Therefore, the present manuscript studies such alternative based on scanning electron microscopy characterization. The sample used in the study was supplied by a local recycling center in Rio de Janeiro, Brazil. The mass loss was constant at around 30% for temperatures higher than 300 °C. Based on this fact, the waste material was then submitted to low-temperature processing at 350 °C followed by attrition disassembling, size classification, and magnetic concentration steps. In the end, this first report of the project shows that 15% of the sample was recovered with metallic components with high economic value, such as Cu, Ni, and Au, indicating that such methods could be an interesting alternative to be explored in the future for the development of alternative electronic waste extraction routes.

Bioleaching metals from waste electrical and electronic equipment (WEEE) by Aspergillus niger: a review

In the twenty-first century, the increasing demand for electrical and electronic equipment (EEE) has caused its quick update and the shortening of its service life span. As a consequence, a large number of waste electrical and electronic equipment (WEEE) needs to be processed and recycled. As an environmentally friendly method, biometallurgy has received extensive attention in the disposal of WEEE in recent years. Aspergillus niger is an acid-producing fungus with a potential applicability to improve metals' recycling efficiency. This review article describes the latest statistical status of WEEE and presents the latest progress of various metallurgical methods involved in WEEE recycling for metal recovery. Moreover, based on the summary and comparison towards studies have been reported for bioleaching metals from WEEE by A. niger, the bioleaching mechanisms and the bioleaching methods are explained, as well as the effects of process parameters on the performance of the bioleaching process are also discussed. Some insights and perspectives are provided for A. niger to be applied to industrial processing scale.

Urban mining of obsolete computers by manual dismantling and waste printed circuit boards by chemical leaching and toxicity assessment of its waste residues

Waste residues and acidic effluents (post-processing of E-waste) released into the local surroundings cause perilous environmental threats and potential risks to human health. Only limited research and information are available toward the sustainable management of waste residues generated post resource recovery of E-waste components. In the present study, the manual processing of obsolete computer (keyboard, monitor, CPU, and mouse) and chemical leaching of waste printed circuit boards (WPCBs) (motherboard, hard drive, DVD drive, and power supply) were performed for urban mining. The toxicity characteristics of typical pollutants in the residues of the WPCBs (post chemical leaching) were studied by toxicity characteristics leaching procedure (TCLP) test. Manual dismantling techniques resulted in an efficient urban mining concept with an overall average profit estimation of INR 2513.73/US$ 34.59. The chemical leaching of WPCBs showed a high concentration of metal leaching like Cu (229662 ± 575.3 mg/kg) and Pb (36785.67 ± 13.07 mg/kg) in the motherboard after stripping epoxy coating. The toxicity test revealed that the concentration of Cu (245.746 ± 0.016 mg/l) in the treated waste residue and Cu (430.746 ± 0.0015 mg/l) and Pb (182.09 ± 0.0035 mg/l) in the non-treated waste residue exceeded the threshold limit. The concentrations of other elements As, Cd, Co, Cr, Ag, Mn, Zn, Ni, Fe, Se, and In were within the permissible limit. Hence, the waste residue stands non-hazardous except Cu and Pb. Stripping out the epoxy coating of WPCBs enhances the metal leaching concentrations. The study highlighted that efficient and appropriate E-waste urban mining has immense potential in tracing the waste scrap into secondary resources. This study also emphasized that the final processed waste residue (left unattended or discarded due to lack of appropriate skill and technology) can be taken into consideration and exploited for value-added materials.

Lithium bioleaching: An emerging approach for the recovery of Li from spent lithium ion batteries

The rapidly growing demand for lithium has resulted in a sharp increase in its price. This is due to the ubiquitous use of lithium-ion batteries (LIBs) in large-scale energy and transportation sectors as well as portable devices. Recycling of the LIBs for being the supply of critical metals hence becomes environmentally and economically viable. The presently used approaches for the recovery of spent LIBs like pyrometallurgical process can effectively recover nickel, cobalt, and copper, while lithium is usually lost in slag. Bioleaching process as an alternative method of extraction and recovery of valuable metals from the primary and secondary resources has been attracting a large pool of attraction. This method can provide higher recovery yield even for low concentration of metals which makes it viable among conventional methods. The bioleaching process can work with lower operating cost and consumed water and energy along with a simple condition, which produces less hazardous by-products ultimately. Here, we comprehensively review the biological and chemical mechanisms of the bioleaching process with a conclusive discussion to help how to extend the use of bioleaching for lithium extraction and recovery from the spent LIBs with a focus on recovery yields improvement. We elaborate on the three main types of the reported bioleaching with considering effective parameters including temperature, initial pH, pulp density, aeration, and medium and cell nutrients to sustain microorganism activity. Finally, practical challenges and future opportunities of lithium are discussed to inspire future research trends and pilot studies to realize the full potential of lithium recovery using sustainable bioleaching processes to extend a clean energy future.

Bioleaching of metals from waste printed circuit boards using bacterial isolates native to abandoned gold mine

In the present study, native bacterial strains isolated from abandoned gold mine and Chromobacterium violaceum (MTCC-2656) were applied for bioleaching of metals from waste printed circuit boards (WPCBs). Toxicity assessment and dose-response analysis of WPCBs showed EC₅₀ values of 128.9, 98.7, and 90.8 g/L for Bacillus sp. SAG3, Bacillus megaterium SAG1 and Lysinibacillus sphaericus SAG2, respectively, whereas, for C. violaceum EC₅₀ was 83.70 g/L. This indicates the viable operation range and technological feasibility of metals bioleaching from WPCBs using mine isolates. The influencing factors such as pH, pulp density, temperature, and precursor molecule (glycine) were optimized by one-factor at a time method (OFAT). The maximum metal recovery occurred at an initial pH of 9.0, a pulp density of 10 g/L, a temperature of 30 °C and a glycine concentration of 5 g/L, except for L. sphaericus which showed optimum activity at initial pH of 8.0. Under optimal conditions the metals recovery of Cu and Au from WPCBs were recorded as 87.5 ± 8% and 73.6 ± 3% for C. violaceum and 72.7 ± 5% and 66.6 ± 6% for B. megaterium, respectively. Kinetic modeling results showed that the data was best described by first order reaction kinetics, where the rate of metal solubilization from WPCBs depended upon microbial lixiviant production. This is the first report on bioleaching of metals from e-waste using bacterial isolates from the gold mine of Solan, HP. Our study demonstrated the potential of bioleaching for resource recovery from WPCBs dust, aimed to be disposed at landfills, and its effectiveness in extraction of elements those are at high supply risk and demand.

Bioleaching: urban mining option to curb the menace of E-waste challenge

Resource Recovery from Waste Electronics has emerged as one of the most imperative processes due to its pressing challenges all over the world. The Printed Circuit Board (PCB) is one of the typical E-waste components that comprise large varieties of metals and nonmetals. Urban Mining of these metals has received major attention all over the world. The existing treatment procedures used extensively for the resource extraction are hydrometallurgy and pyro-metallurgy and crude recycling practices in the informal sector. However, these methods are prone to cause secondary pollutants with certain drawbacks. Also, the existing informal recycling procedures resulted in insignificant occupational health hazards and severe environmental threats. The application of biotechnology is extensively exploited for metal extraction and emerged as one of the sustainable and eco-friendly tools. However, a limited field-scale study is prevailing in the realm of resource recovery from E-waste using bioleaching method. Hence, the application of bioleaching requires more attention and technical know-how in developing countries to curtail crude practices. The application of bioleaching in E-waste, including its available methods, kinetics mechanism associated opportunities, and barriers, have been discussed in this paper. A glance of E-waste management in India and the menace of 95% crude E-waste recycling are also elaborated. The incentives toward profit, socio-economic, and environmentally sustainable approaches have been delineated based on critical analysis of the available literature.

Food Waste-Assisted Metal Extraction from Printed Circuit Boards: The Aspergillus niger Route

A low-energy paradigm was adopted for sustainable, affordable, and effective urban waste valorization. Here a new, eco-designed, solid-state fermentation process is presented to obtain some useful bio-products by recycling of different wastes. Urban food waste and scraps from trimmings were used as a substrate for the production of citric acid (CA) by solid state fermentation of Aspergillus niger NRRL 334, with a yield of 20.50 mg of CA per gram of substrate. The acid solution was used to extract metals from waste printed circuit boards (WPCBs), one of the most common electronic waste. The leaching activity of the biological solution is comparable to a commercial CA one. Sn and Fe were the most leached metals (404.09 and 67.99 mg/L, respectively), followed by Ni and Zn (4.55 and 1.92 mg/L) without any pre-treatments as usually performed. Commercial CA extracted Fe more efficiently than the organic one (123.46 vs. 67.99 mg/L); vice versa, biological organic CA recovered Ni better than commercial CA (4.55 vs. 1.54 mg/L). This is the first approach that allows the extraction of metals from WPCBs through CA produced by A. niger directly grown on waste material without any sugar supplement. This “green” process could be an alternative for the recovery of valuable metals such as Fe, Pb, and Ni from electronic waste.

Studies on leaching characteristics of electronic waste for metal recovery using inorganic and organic acids and base

In this paper, we report leaching of precious and scattered metals such as gold (Au), copper (Cu), nickel (Ni), zinc (Zn), iron (Fe), and lead (Pb) from printed circuit boards of scrap mobile phones by hydrometallurgical process using inorganic acid, organic acid and base. The amount of metals leached by different leachants are quantified using atomic absorption spectroscopy. Among various inorganic acids, aqua regia (mixture of nitric acid (HNO₃) and hydrochloric acid) is found to be the strongest leachant for most of the metals such as Zn (2.04 wt %), Fe (17.90 wt %), Ni (0.66 wt %), Pb (5.86 wt %) and Au (0.04 wt %). The basic leachant, ammonium thiosulphate is found to be very effective in leaching of Au (0.03125 wt %). The dissolution of Cu in HNO₃ gives the highest amount of Cu in the solvent, that is, ∼ 7.52 wt %. The metallic phases present in the electronic waste before and after leaching are identified by X-ray diffraction analysis. The microscopic structure has been studied using a scanning electron microscope which depicts erosion of the structure after leaching.

Biological, biomolecular, and bio-inspired strategies for detection, extraction, and separations of lanthanides and actinides

Lanthanides and actinides are elements of ever-increasing technological importance in the modern world. However, the similar chemical and physical properties within these groups make purification of individual elements a challenge. Current industrial standards for the extraction, separation, and purification of these metals from natural sources, recycled materials, and industrial waste are inefficient, relying upon harsh conditions, repetitive steps, and ligands with only modest selectivity. Biological, biomolecular, and bio-inspired strategies towards improving these separations and making them more environmentally sustainable have been researched for many years; however, these methods often have insufficient selectivity for practical application. Recent developments in the understanding of how lanthanides are selectively acquired and used by certain bacteria offer the opportunity for a newer, more efficient take on these designs, as well as the possibility for fundamentally new designs and strategies. Herein, we review current cell-based and biomolecular (primarily small-molecule and protein-based) methods for detection, extraction, and separations of f-block elements. We discuss how the increasing knowledge regarding the selective recognition, uptake, trafficking, and storage of these elements in biological systems has informed and will continue to promote development of novel approaches to achieve these ends.

Magnetic separation of ferrous fractions linked to improved bioleaching of metals from waste-to-energy incinerator bottom ash (IBA): a green approach

Ferrous fractions in incinerated bottom ash (IBA) are linked to lower metal dissolution. In the present study, a novel eco-friendly biotechnological approach has been tested for multi-metal leaching using meso-acidophilic Fe²⁺/S° oxidizing bacterial consortium from magnetically separated IBA, owing to the inherent property of IBA to release Fe²⁺. Comprehensive lab-scale studies, first-of-its-kind, considered all the potential elements to understand targeted metal dissolutions from the sample under differential conditions. Concentrations of metals, Al > Ti > Ni > Zn > Cu, as analyzed by ICP-OES, were targeted to be bioleached. XRD analysis indicated the sample to be amorphous with magnetite (Fe₃O₄) and iron (Fe) forming major phases in the magnetic part (IBAM) and titano-magnetite (Fe_3-x. TixO₄) and iron (Fe) for the nonmagnetic part (IBAN). The study indicated that 73.98% Cu, 98.68% Ni, 59.09% Zn, 58.84% Al, and 92.85% Ti could be leached from IBAM when the bioleaching system operates at pH 1.5, 5% pulp density for 8 days. Under similar conditions, within 6 days, 37.55% Cu, 87.99% Ni, 45.03% Zn, 40.72% Al, and 63.97% Ti could be leached from IBAN. Two routes were identified and the mechanism of action has been proposed for the leaching of metals.

Oxidative Stress Induced by Metal Ions in Bioleaching of LiCoO2 by an Acidophilic Microbial Consortium

An acidophilic microbial consortium (AMC) was used to investigate the fundamental mechanism behind the adverse effects of pulp density increase in the bioleaching of waste lithium ion batteries (WLIBs). Results showed that there existed the effect of metal-ion stress on the bio-oxidative activity of AMC. The Li+ and Co2+ accumulated in the leachate were the direct cause for the decrease in lithium and cobalt recovery yields under a high pulp density. In a simulated bioleaching system with 4.0% (w ⋅v-1) LiCoO2, the intracellular reactive oxygen species (ROS) content in AMC increased from 0.82 to 6.02 within 24 h, which was almost three times higher than that of the control (2.04). After the supplementation of 0.30 g⋅L-1 of exogenous glutathione (GSH), the bacterial intracellular ROS content decreased by 40% within 24 h and the activities of intracellular ROS scavenging enzymes, including glutathione peroxidase (GSH-Px) and catalase (CAT), were 1.4- and 2.0-folds higher in comparison with the control within 24 h. In the biofilms formed on pyrite in the bioleaching of WLIBs, it was found that metal-ion stress had a great influence on the 3-D structure and the amount of biomass of the biofilms. After the exogenous addition of GSH, the structure and the amount of biomass of the biofilms were restored to some extent. Eventually, through ROS regulation by the exogenous addition of GSH, very high metal recovery yields of 98.1% Li and 96.3% Co were obtained at 5.0% pulp density.

Bench scale microbial catalysed leaching of mobile phone PCBs with an increasing pulp density

The study reports the effect of increasing pulp density on the bio-catalyzed leaching of metals from waste mobile phone printed circuit boards. Mixed microbial consortia of iron and sulfur-oxidizing microorganisms were used for batch bioleaching at varying pulp density of 7%, 10% and 15% (w/v). The copper content in the feed material was 26.3% (w/w) and the prime focus was to recover maximum copper along with other minor metals, such as zinc, aluminum, and nickel. All the bioleaching experiments resulted with 98-99% of copper recovery together with reasonable extraction Zn, Al, and Ni. The optimum copper recovery from bioleaching experiments demonstrates the possibility of scaling up with high pulp density, which could be economical as well as eco-friendly.

Bioleaching of heavy metals from printed circuit board (PCB) by Streptomyces albidoflavus TN10 isolated from insect nest

Background

E-waste management is extremely difficult to exercise owing to its complexity and hazardous nature. Printed circuit boards (PCBs) are the core components of electrical and electronic equipment, which generally consist of polymers, ceramics, and heavy metals.

Results

The present study has been attempted for removal of heavy metals from printed circuit board by metal-resistant actinobacterium Streptomyces albidoflavus TN10 isolated from the termite nest. This bacterium was found to recover different heavy metals (Al 66%, Ca 74%, Cu 68%, Cd 65%, Fe 42%, Ni 81%, Zn 82%, Ag 56%, Pb 46%) within 72 h under laboratory conditions. The metal content of PCB after bioleaching was analyzed by ICP-MS. The crude PCB and bioleaching residue were characterized by FT-IR, XRD, SEM for the determination of structural and functional group changes for confirmation of bioleaching.

Conclusion

The findings of the present study concluded that Streptomyces albidoflavus TN10 is a promising candidate for bioleaching of heavy metals from the printed circuit board as an eco-friendly and cost-effective process.

Wastes generated by automotive industry – Spent automotive catalysts

Rhodium, ruthenium, palladium, and platinum are classified as platinum group metals (PGM). A demand for PGM has increased in recent years. Their natural sources are limited, therefore it is important, and both from economical and environmental point of view, to develop effective process to recover PGM from waste/secondary sources, such as spent automotive catalysts. Pyrometallurgical methods have always been used for separation of PGM from various materials. However, recently, an increasing interest in hydrometallurgical techniques for the removal of precious metals from secondary sources has been noted. Among them, liquid-liquid extraction by contacting two liquid phases: aqueous solution of metal ions and organic solution of extractant is considered an efficient technique to separate valuable metal ions from solutions after leaching from spent catalysts.

Fungal bioleaching of WPCBs using Aspergillus niger: Observation, optimization and kinetics

In this study, Aspergillus niger (A. niger) as an environmentally friendly agent for fungal bioleaching of waste printed circuit boards (WPCBs) was employed. D-optimal response surface methodology (RSM) was utilized for optimization of the bioleaching parameters including bioleaching method (one step, two step and spent medium) and pulp densities (0.5 g L^-1 to 20 g L^-1) to maximize the recovery of Zn, Ni and Cu from WPCBs. According to the high performance liquid chromatography analysis, citric, oxalic, malic and gluconic acids were the most abundant organic acids produced by A.niger in 21 days experiments. Maximum recoveries of 98.57% of Zn, 43.95% of Ni and 64.03% of Cu were achieved based on acidolysis and complexolysis dissolution mechanisms of organic acids. Based on the kinetic studies, the rate controlling mechanism for Zn dissolution at one step approach was found to be diffusion through liquid film, while it was found to be mixed control for both two step and spent medium. Furthermore, rate of Cu dissolution which is controlled by diffusion in one step and two step approaches, detected to be controlled by chemical reaction at spent medium. It was shown that for Ni, the rate is controlled by chemical reaction for all the methods studied. Eventually, it was understood that A. niger is capable of leaching 100% of Zn, 80.39% of Ni and 85.88% of Cu in 30 days.

Electronic wastes

E-waste amount is growing at about 4% annually, and has become the fastest growing waste stream in the industrialized world. Over 50 million tons of e-waste are produced globally each year, and some of them end up in landfills causing danger of toxic chemicals leakage over time. E-waste is also sent to developing countries where informal processing of waste electrical and electronic equipment (WEEE) causes serious health and pollution problems. A huge interest in recovery of valuable metals from WEEE is clearly visible in a great number of scientific, popular scientific publications or government and industrial reports.

Comprehensive characterization of printed circuit boards of various end-of-life electrical and electronic equipment for beneficiation investigation

Comprehensive characterization of printed circuit board (PCB) of end-of-life electrical and electronic equipment (EEE) is obligatory for prospective profitable beneficiation. In this study, beneficiation oriented comprehensive characterization of two brands of PCBs each of 16 end-of-life EEE was conducted in terms of their physicochemical characteristics with special emphasis on the content of 16 general elements, 2 precious metals and 15 rare earth elements (REEs). General elements and their highest weight percent composition found in different PCBs of the EEEs were Cu (23% in laptop), Al (6% in computer), Pb (15% in DVD player) and Ba (7% in TV). The high abundant of precious metals such as Au (316 g/ton) and Ag (636 g/ton) in mobile phone and laptop, respectively coupled with rapid obsolescence age makes waste PCBs of information technology and telecommunication equipment the most potent resource reservoir. Additionally, most of the waste PCBs were observed to contain REEs in considerable quantity with Sc up to 31 g/ton and Ce up to 13 g/ton being the major constituents. Comprehensive characterization of waste PCBs therefore will systematically help towards better understanding of e-waste recycling processes for beneficiation purpose and sustainable resource circulation and conservation.

Bioleaching of metals from WEEE shredding dust

A bioleaching process developed in two separate steps was investigated for the recovery of base metals, precious metals and rare earth elements from dusts generated by Waste Electrical and Electronic Equipment (WEEE) shredding. In the first step, base metals were almost completely leached from the dust in 8 days by Acidithiobacillus thiooxidans (DSM 9463) that lowered the pH of the leaching solution from 3.5 to 1.0. During this step, cerium, europium and neodymium were mobilized at high percentages (>99%), whereas lanthanum and yttrium reached an extraction yield of 80%. In the second step, the cyanide producing Pseudomonas putida WSC361 mobilized 48% of gold within 3 h from the A. thiooxidans leached shredding dust. This work demonstrated the potential application of biohydrometallurgy for resource recovery from WEEE shredding dust, destined to landfill disposal, and its effectiveness in the extraction of valuable substances, including elements at high supply risk as rare earths.

Enhancement of gold and silver recovery from discarded computer printed circuit boards by Pseudomonas balearica SAE1 using response surface methodology (RSM)

Two-step bioleaching was applied using a cyanogenic bacterium Pseudomonas balearica SAE1 to recover gold (Au) and silver (Ag) from the computer printed circuit boards (CPCBs) via central composite design of a response surface methodology (CCD-RSM). To enhance Au and Ag recovery, factors like pH level, pulp density, temperature and glycine concentration were optimized and their interactions were studied. CCD-RSM optimization resulted in 73.9 and 41.6% dissolution of Au and Ag, respectively, at initial pH 8.6, pulp density 5 g/L, temperature 31.2 °C, and glycine concentration 6.8 g/L, respectively. Two quadratic models were proposed by RSM which can be utilized as an efficient tool to predict Au and Ag recovery through bioleaching. The experimental results are in line with the predicted results, indicating reliability of RSM model in enhancing the Au and Ag recovery from CPCBs. The increased bioleaching yield of Au and Ag from discarded CPCBs has its importance in industrial e-waste recycling and safe disposal.

Effect of temperature on copper, iron and lead leaching from e-waste using citrate solutions

E-waste is a potential source of large quantities of metals. The ability of citrate solutions to recover base metals from these materials has been demonstrated. In the present study, the effect of the temperature on base metal leaching capacity by the citrate solutions is determined. The material employed consisted of a mechanically prepared, gravity concentrated e-waste, with a metallic content greater than 90%. The leaching conditions were selected based on previous research performed by the authors (0.5 M sodium citrate, pH 4.5 and 20 g per liter e-waste concentrate). Leaching tests were performed at temperatures between 0° and 70 °C. The initial leaching rates for the three metals increased with temperature. However, these tapered off with time for temperatures above 30 °C, which can be associated to citrate destruction.

Qualitative and quantitative metals liberation assessment for characterization of various waste printed circuit boards for recycling

Metals liberation and composition are decisive attributes in characterization of e-waste for metal recycling. Though end-of-life printed circuit board (PCB) is an integral part of e-waste as secondary resource reservoir, yet no standardized procedure exists for metals liberation and dissolution for its characterization. Thus, the paper aims at assessment of metals liberation upon comminution employing scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) followed by comparative assessment of the existing United States Environmental Protection Agency (USEPA) digestion procedures, viz., USEPA 3050B, USEPA 3051A, and USEPA 3052, in effective dissolution of metals from comminuted particles of waste PCBs of computer, laptop, mobile phone, and television. Effect of comminution and digestion conditions was assessed to have significant role in metal liberation and dissolution from PCBs. The SEM-EDS analysis demonstrated partial release of metals from the silica matrix of PCBs. The USEPA digestion methods showed statistically significant (P < 0.05) difference with greater dissolution of metals complexed to PCB matrix by the USEPA 3052 method owing to use of strong acid like hydrofluoric acid. Base metals like Cu and Zn and toxic metals such as Pb and Cd were present in abundance in PCBs and in general exceeded the total threshold limit concentration (TTLC). The maximum contents of Cu (20.13 ± 0.04 wt.%) and Zn (1.89 ± 0.05 wt.%) in laptop PCBs, Pb (2.26 ± 0.08 wt.%) in TV PCBs, and Cd (0.0812 ± 0.0008 wt.%) in computer PCBs were observed.