Catalysing electrowinning of copper from E-waste: A critical review

Biotechnological Approaches for Metal Recovery from Electronic Wastes

The disposal of electronic waste (EW) in open landfills has caused several toxic environmental effects. The harmful metallic components released in the environment due to deposition of EW act as hazards for living systems. EW management has been widely studied in recent days across the world. Though, several processes are implemented in extraction, recycling and recovery of heavy metals from the EW, most of them are not effective in recovering the precious metals. Various chemical processes are executed for efficient extraction of precious metals from e-wastes. Though the techniques are easy to process and rapid, however, the chemical leaching also has detrimental environmental consequences. Biological approaches, on the other hand, solves the purpose for efficient and environmentally friendly recovery of precious metals. Thus, both resource recovery as well as remediation can be targeted simultaneously. Biotechnological methods offer sustainable and efficient solutions for metal recovery from electronic wastes, presenting a viable alternative to traditional methods. Continued advancements in this field hold significant promise for addressing the growing e-waste challenge.

Phytoremediation: Sustainable Approach for Heavy Metal Pollution

Rapid industrialization, mining, and other anthropogenic activities have poisoned our environment with heavy metals, negatively impacting all forms of life. Heavy metal pollution causes physiological and neurological disorders, as heavy metals are endocrine disrupters, carcinogenic, and teratogenic. Therefore, it becomes mandatory to address the challenge of heavy metal contamination on a global scale. Physical and chemical approaches have been employed for pollutant removal and detoxification, but these methods cannot be adopted universally due to high cost, labor intensiveness, and possible negative impact on natural microflora. Phytoremediation is one of the preferred and safest approaches for environmental management due to its high efficiency and low cost of investment. The plant can uptake the pollutants and heavy metals from water and soil through an intense root network via rhizofiltration and process via phytostabilization, phytovolatilization, and accumulation. At a cellular level, the phytoremediation process relies on natural mechanisms of plant cells, e.g., absorption, transpiration, intracellular storage, and accumulation to counter the detrimental effects of pollutants. It is widely accepted because of its novelty, low cost, and high efficiency; however, the process is comparatively slower. In addition, plants can store pollutants for a long time but again become a challenge at the end of the life cycle. The current review summarizes phytoremediation as a potential cure for heavy metal pollutants, released from natural as well as anthropogenic sources. It will provide insight into the advancement and evolution of advanced techniques like nanoremediation that can improve the rate of phytoremediation, along with making it sustainable, cost-effective, and economically viable.

Improving Scalability of copper recovery in saline microbial fuel cells with microtubular polypyrrole-based cathodic electrocatalysts

Microbial fuel cells (MFC) are emerging energy-efficient systems for copper (Cu) electrowinning from waste streams by coupling it with anodic oxidation of organics in wastewater. However, there is a lack of research examining scalable electrocatalysts for Cu electrowinning at low cathodic overpotentials in highly saline catholytes often found in e-waste leachates. The challenge of developing resilient anodic biofilms that withstand the antagonistic effects of ions migrating from catholytes in saline MFC also needs to be addressed. In this study, polypyrrole (PPy) cathodic electrocatalysts were developed and coupled with a robust halophilic anodic biofilm in MFC to improve the kinetics of Cu electrowinning from acidic chloride-based catholytes. Electrochemical characterisation of these cathodes revealed shuttling of electrons by redox-active PPy via the formation of intermediate Cu+-complexes as an energy-efficient pathway for producing metallic Cu. High power densities ranging from 0.63 ± 0.17 to 0.73 ± 0.05 W m-2 were achieved with undoped-PPy and phytic acid doped-PPy cathodes with simultaneous recovery of ∼97% Cu. These electrocatalysts also exhibited low charge transfer resistance (3-8 mΩ m2) that met the requisites for scalable cathodes in MFC. However, a decrease in the efficiency of PPy cathodes was observed over 5 d due to competing reactions at their interfaces, including re-oxidation of deposited Cu and cathodic corrosion, with further studies suggested to enhance their corrosion resistance. Nonetheless, integrating PPy electrocatalysts for Cu electrowinning in saline MFC has advanced its outlooks as an energy-efficient downstream process for urban mining of Cu from e-waste.

Physicochemical reactions in e-waste recycling

Electronic waste (e-waste) recycling is becoming a global concern owing to its immense quantity, hazardous character and the potential loss of valuable metals. The many processes involved in e-waste recycling stem from a mixture of physicochemical reactions, and understanding the principles of these reactions can lead to more efficient recycling methods. In this Review, we discuss the principles behind photochemistry, thermochemistry, mechanochemistry, electrochemistry and sonochemistry for metal recovery, polymer decomposition and pollutant elimination from e-waste. We also discuss how these processes induce or improve reaction rates, selectivity and controllability of e-waste recycling based on thermodynamics and kinetics, free radicals, chemical bond energy, electrical potential regulation and more. Lastly, key factors, limitations and suggestions for improvements of these physicochemical reactions for e-waste recycling are highlighted, wherein we also indicate possible research directions for the future.

Negative-carbon recycling of copper from waste as secondary resources using deep eutectic solvents

Copper plays a crucial role in the low-carbon transformation of global communities with prevalent use of electric vehicles. This study proposed an environmentally friendly approach that utilizes a deep eutectic solvent (DES), choline chloride-ethylene glycol (ChCl-EG), as green solvent for the selective extraction of copper from scrap materials. With hydrogen peroxide as an oxidizing agent, the copper species from the printed circuit boards (PCBs) scraps were efficiently leached by the DES through oxidation-complexation reactions (conditions: 25 min, 20 °C, and 5 wt% H2O2). Molecular dynamics and density functional theory were performed to simulate the intricate cascade of interactions between copper species and hydrogen bond donors/acceptors of DES, providing insights into the mechanistic processes involved. Copper was selectively recovered from the DES leachate containing impurities (e.g., Pb2+, Sn2+, and Al3+) through electrodeposition via a diffusion-controlled reaction under a constant potential mode. A comprehensive life cycle assessment of the process demonstrated that the utilisation of DES in the extraction of copper from waste PCBs could result in significant reduction in carbon dioxide emissions (-93.6 kg CO2 eq of 1000 kg waste PCBs), thus mitigating the carbon footprint of global copper use through the proposed solvometallurgical recycling process of secondary resources.

Scaling up of dual-chamber microbial electrochemical systems - An appraisal using systems design approach

Impetus to minimise the energy and carbon footprints of evolving wastewater resource recovery facilities has promoted the development of microbial electrochemical systems (MES) as an emerging energy-neutral and sustainable platform technology. Using separators in dual-chamber MES to isolate anodic and cathodic environments creates endless opportunities for its myriad applications. Nevertheless, the high internal resistance and the complex interdependencies among various system factors have challenged its scale-up. This critical review employed a systems approach to examine the complex interdependencies and practical issues surrounding the implementation and scalability of dual-chamber MES, where the anodic and cathodic reactions are mutually appraised to improve the overall system efficiency. The robustness and stability of anodic biofilms in large-volume MES is dependent on its inoculum source, antecedent history and enrichment strategies. The composition and anode-respiring activity of these biofilms are modulated by the anolyte composition, while their performance demands a delicate balance between the electrode size, macrostructure and the availability of substrates, buffers and nutrients when using real wastewater as anolyte. Additionally, the catholyte governed the reduction environment and associated energy consumption of MES with scalable electrocatalysts needed to enhance the sluggish reaction kinetics for energy-efficient resource recovery. A comprehensive assessment of the dual-chamber reactor configuration revealed that the tubular, spiral-wound, or plug-in modular MES configurations are suitable for pilot-scale, where it could be designed more effectively using efficient electrode macrostructure, suitable membranes and bespoke strategies for continuous operation to maximise their performance. It is anticipated that the critical and analytical understanding gained through this review will support the continuous development and scaling-up of dual-chamber MES for prospective energy-neutral treatment of wastewater and simultaneous circular management of highly relevant environmental resources.