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

Current recycling innovations to utilize e-waste in sustainable green metal manufacturing

The ever-increasing market demand and the rapid uptake of the technologies of electronics create an unavoidable generation of high-volume electronic waste (e-waste). E-waste is embedded with valuable metals, alloys, precious metals and rare earth elements. A substantial portion of e-waste ends up in landfills and is incinerated due to its complex multi-material structure, creating loss of resources and often leading to environmental contamination from the release of landfill leachates and combustion gases. Conversely, due to the ongoing demand for valuable metals, global industrial and manufacturing supply chains are experiencing enormous pressure. To address this issue, researchers have put multifaceted efforts into developing viable technologies and emphasized right-scaling for e-waste reclamation. Several conventional and emerging recycling technologies have been recognized to be efficient in recovering metal alloys, precious and rare earth metals from e-waste. The recovery of valuable metals from e-waste will create an alternative source of value-added raw materials, which could become part of supply chains for manufacturing. This review discusses the urgency of metal recycling from e-waste for sustainability and economic benefit, up-to-date recycling technologies with an emphasis on their potential role in creating a circular economy in e-waste management.This article is part of the discussion meeting issue 'Sustainable metals: science and systems'.

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.

Reprogramming Filamentous fd Viruses to Capture Copper Ions

C-terminal truncated variants (A, VA, NVA, ANVA, FANVA and GFANVA) of our recently identified Cu(II) specific peptide "HGFANVA" were displayed on filamentous fd phages. Wild type fd-tet and engineered virus variants were treated with 100 mM Cu(II) solution at a final phage concentration of 1011 vir/ml and 1012 vir/ml. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging before Cu(II) exposure showed ≈6-8 nm thick filamentous virus layer formation. Cu(II) treatment resulted in aggregated bundle-like assemblies with mineral deposition. HGFANVA phage formed aggregates with an excessive mineral coverage. As the virus concentration was 10-fold decreased, nanowire-like assemblies were observed for shorter peptide variants A, NVA and ANVA. Wild type fd phages did not show any mineral formation. Energy dispersive X-ray spectroscopy (EDX) analyses revealed the presence of C and N peaks on phage organic material. Cu peak was only detected for engineered viruses. Metal ion binding of viruses was next investigated by enzyme-linked immunosorbent assay (ELISA) analyses. Engineered viruses were able to bind Cu(II) forming mineralized intertwined structures although no His (H) unit was displayed. Such genetically reprogrammed virus based biological materials can be further applied for bioremediation studies to achieve a circular economy.

From waste management to circular economy: Leveraging thermophiles for sustainable growth and global resource optimization

Waste of any origin is one of the most serious global and man-made concerns of our day. It causes climate change, environmental degradation, and human health problems. Proper waste management practices, including waste reduction, safe handling, and appropriate treatment, are essential to mitigate these consequences. It is thus essential to implement effective waste management strategies that reduce waste at the source, promote recycling and reuse, and safely dispose of waste. Transitioning to a circular economy with policies involving governments, industries, and individuals is essential for sustainable growth and waste management. The review focuses on diverse kinds of environmental waste sources around the world, such as residential, industrial, commercial, municipal services, electronic wastes, wastewater sewerage, and agricultural wastes, and their challenges in efficiently valorizing them into useful products. It highlights the need for rational waste management, circularity, and sustainable growth, and the potential of a circular economy to address these challenges. The article has explored the role of thermophilic microbes in the bioremediation of waste. Thermophiles known for their thermostability and thermostable enzymes, have emerged to have diverse applications in biotechnology and various industrial processes. Several approaches have been explored to unlock the potential of thermophiles in achieving the objective of establishing a zero-carbon sustainable bio-economy and minimizing waste generation. Various thermophiles have demonstrated substantial potential in addressing different waste challenges. The review findings affirm that thermophilic microbes have emerged as pivotal and indispensable candidates for harnessing and valorizing a range of environmental wastes into valuable products, thereby fostering the bio-circular economy.

Molding the future: Optimization of bioleaching of rare earth elements from electronic waste by Penicillium expansum and insights into its mechanism

The recovery of rare earth elements (REE) from electronic waste is crucial for ensuring future demand security, as there is a high supply risk for this group of elements, and mitigating the environmental impacts of conventional mining. This research focuses on extracting REE from waste printed circuit boards through bioleaching, addressing the limited attention given to this source. A strain of Penicillium expansum demonstrated efficient bioleaching under optimal conditions of 7.5 initial pH, 0.1 mM phosphate concentration, and excluding a buffering agent. The study achieved significant improvements in La and Tb extraction and enhancements in Pr, Nd, and Gd recovery, approaching 70 % within 24 h. Fungal mechanisms involved in REE extraction included fungal pH control, organic acid biosynthesis, phosphate bioavailability, and potential fungal proton pump involvement. This approach offers a promising solution for sustainable REE recovery from e-waste, contributing to resource security and circular economy.

Effective oxidation decomplexation of Cu-EDTA and Cu2+ electrodeposition from PCB manufacturing wastewater by persulfate-based electrochemical oxidation: Performance and mechanisms

Complex wastewater matrices such as printed circuit board (PCB) manufacturing wastewater present a major environmental concern. In this work, simultaneous decomplexation of metal complex Cu-EDTA and reduction/electrodeposition of Cu2+ was conducted in a persulfate-based electrochemical oxidation system. Oxidizing/reductive species were simultaneously produced in this system, which realized 99.8% of Cu-EDTA decomplexation, 94.5% of Cu2+ reduction/electrodeposition under the conditions of original solution pH = 3.2, electrode distance = 3 cm, [Na2S2O8]0 = 5 mM, current density = 12 mA/cm2, and reaction time = 180 min. The total treatment cost is as low as 0.80 USD/mol Cu-EDTA. Effective mineralization (74.1% total organic carbon removal) of the solution was obtained after 3 h of treatment. •OH and SO4•- drove the Cu-EDTA decomplexation, destroying the chelating sites and finally it was effectively mineralized to CO2, H2O and Cu2+. The mechanisms of copper electrodeposition on the stainless steel cathode and persulfate activation by the BDD anode were proposed based on the electrochemical measurements. The electrodes exhibited excellent reusability and low metal (total iron and Ni2+) leaching during 20 cycles of application. This study provide an effective and sustainable method for the application of the electro-persulfate process in treating complex wastewater matrices.

Artificial intelligence and machine learning algorithms in the detection of heavy metals in water and wastewater: Methodological and ethical challenges

Heavy metals (HMs) enter waterbodies through various means, which, when exceeding a threshold limit, cause toxic effects both on the environment and in humans upon entering their systems. Recent times have seen an increase in such HM influx incident rates. This requires an instant response in this regard to review the challenges in the available classical methods for HM detection and removal. As well as provide an opportunity to explore the applications of artificial intelligence (AI) and machine learning (ML) for the identification and further redemption of water and wastewater from the HMs. This review of research focuses on such applications in conjunction with the available in-silico models producing worldwide data for HM levels. Furthermore, the effect of HMs on various disease progressions has been provided, along with a brief account of prediction models analysing the health impact of HM intoxication. Also discussing the ethical and other challenges associated with the use of AI and ML in this field is the futuristic approach intended to follow, opening a wide scope of possibilities for improvement in wastewater treatment methodologies.

Highly efficient recovery of Zn2+/Cu2+ from water by using hydrotalcite as crystal seeds

The efficient and waste-free recovery of heavy metals is critical for heavy metal wastewater treatment. In this work, we explored how heavy metals can be recovered as valuable chemicals in the presence of crystal seeds. Hydrotalcite (one kind of layered double hydroxides (LDHs)) was used as crystal seeds to recover Zn2+ in the presence of Al3+ from water (i.e., seed-Zn2+-Al3+ system), which was compared with the monometallic heterogeneous system (seed-Zn2+) and direct coprecipitation (Zn2+-Al3+) system. Our results demonstrated that the seed-Zn2+-Al3+ system possessed a recovery rate of 2.6-2.8 times and a recovery kinetic rate of 2.7-5.9 times higher than those of the other two systems. Differing from the latter two systems, hydrotalcite seeds could induce Zn2+ and Al3+ to form ZnAl-LDH in seed-Zn2+-Al3+. Interestingly, the ZnAl-LDH presents a compositional divalent/trivalent cation molar ratio of ca. 3, which is comparable with the value in the hydrotalcite. It was demonstrated that the hydrotalcite seeds could act as a template to significantly induce the formation of ZnAl-LDH complying with the seed's structure and compositional ratio. Similar induction effect of seeds as the Zn2+ system was further verified in Cu2+ systems. This work provides a novel strategy for efficient recovery of heavy metals with product selectivity.

Driving sustainable circular economy in electronics: A comprehensive review on environmental life cycle assessment of e-waste recycling

E-waste, encompassing discarded materials from outdated electronic equipment, often ends up intermixed with municipal solid waste, leading to improper disposal through burial and incineration. This improper handling releases hazardous substances into water, soil, and air, posing significant risks to ecosystems and human health, ultimately entering the food chain and water supply. Formal e-waste recycling, guided by circular economy models and zero-discharge principles, offers potential solutions to this critical challenge. However, implementing a circular economy for e-waste management due to chemical and energy consumption may cause environmental impacts. Consequently, advanced sustainability assessment tools, such as Life Cycle Assessment (LCA), have been applied to investigate e-waste management strategies. While LCA is a standardized methodology, researchers have employed various routes for environmental assessment of different e-waste management methods. However, to the authors' knowledge, there lacks a comprehensive study focusing on LCA studies to discern the opportunities and limitations of this method in formal e-waste management strategies. Hence, this review aims to survey the existing literature on the LCA of e-waste management under a circular economy, shedding light on the current state of research, identifying research gaps, and proposing future research directions. It first explains various methods of managing e-waste in the circular economy. This review then evaluates and scrutinizes the LCA approach in implementing the circular bioeconomy for e-waste management. Finally, it proposes frameworks and procedures to enhance the applicability of the LCA method to future e-waste management research. The literature on the LCA of e-waste management reveals a wide variation in implementing LCA in formal e-waste management, resulting in diverse results and findings in this field. This paper underscores that LCA can pinpoint the environmental hotspots for various pathways of formal e-waste recycling, particularly focusing on metals. It can help address these concerns and achieve greater sustainability in e-waste recycling, especially in pyrometallurgical and hydrometallurgical pathways. The recovery of high-value metals is more environmentally justified compared to other metals. However, biometallurgical pathways remain limited in terms of environmental studies. Despite the potential for recycling e-waste into plastic or glass, there is a dearth of robust background in LCA studies within this sector. This review concludes that LCA can offer valuable insights for decision-making and policy processes on e-waste management, promoting environmentally sound e-waste recycling practices. However, the accuracy of LCA results in e-waste recycling, owing to data requirements, subjectivity, impact category weighting, and other factors, remains debatable, emphasizing the need for more uncertainty analysis in this field.

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.

From E-Waste to High-Value Materials: Sustainable Synthesis of Metal, Metal Oxide, and MOF Nanoparticles from Waste Printed Circuit Boards

The exponential growth of electronic waste (e-waste) has raised significant environmental concerns, with projections indicating a surge to 74.7 million metric tons of e-waste generated by 2030. Waste printed circuit boards (WPCBs), constituting approximately 10% of all e-waste, are particularly intriguing due to their high content of valuable metals and rare earth elements. However, the presence of hazardous elements necessitates sustainable recycling strategies. This review explores innovative approaches to sustainable metal nanoparticle synthesis from WPCBs. Efficient metal recovery from WPCBs begins with disassembly and the utilization of advanced equipment for optimal separation. Various pretreatment techniques, including selective leaching and magnetic separation, enhance metal recovery efficiency. Green recovery systems such as biohydrometallurgy offer eco-friendly alternatives, with high selectivity. Converting metal ions into nanoparticles involves concentration and transformation methods like chemical precipitation, electrowinning, and dialysis. These methods are vital for transforming recovered metal ions into valuable nanoparticles, promoting sustainable resource utilization and eco-friendly e-waste recycling. Sustainable green synthesis methods utilizing natural sources, including microorganisms and plants, are discussed, with a focus on their applications in producing well-defined nanoparticles. Nanoparticles derived from WPCBs find valuable applications in drug delivery, microelectronics, antimicrobial materials, environmental remediation, diagnostics, catalysis, agriculture, etc. They contribute to eco-friendly wastewater treatment, photocatalysis, protective coatings, and biomedicine. The important implications of this review lie in its identification of sustainable metal nanoparticle synthesis from WPCBs as a pivotal solution to e-waste environmental concerns, paving the way for eco-friendly recycling practices and the supply of valuable materials for diverse industrial applications.

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.

Review on the gentle hydrometallurgical treatment of WPCBs: Sustainable and selective gradient process for multiple valuable metals recovery

The metal resource crisis and the inherent need for a low-carbon circular economy have driven the rapid development of e-waste recycling technology. High-value waste printed circuit boards (WPCBs) are an essential component of e-waste. However, WPCBs are considered hazardous to the ecosystem due to the presence of heavy metals and brominated organic polymers. Therefore, achieving the recycling of metals in WPCBs is not only a strategic requirement for building a green ecological civilization but also an essential guarantee for achieving a safe supply of mineral resources. This review systematically analyzes the hydrometallurgical technology of metals in WPCBs in recent years. Firstly, the different unit operations of pretreatment in the hydrometallurgical process, which contain disassembly, crushing, and pre-enrichment, were analyzed. Secondly, environmentally friendly hydrometallurgical leaching systems and high-value product regeneration technologies used in recent years to recover metals from WPCBs were evaluated. The leaching techniques, including cyanidation, halide, thiourea, and thiosulfate for precious metals, and inorganic acid, organic acid, and other leaching methods for base metals such as copper and nickel in WPCBs, were outlined, and the leaching performance and greenness of each leaching system were summarized and analyzed. Eventually, based on the advantages of each leaching system and the differences in chemical properties of metals in WPCBs, an integrated and multi-gradient green process for the recovery of WPCBs was proposed, which provides a sustainable pathway for the recovery of metals in WPCBs. This paper provides a reference for realizing the gradient hydrometallurgical recovery of metals from WPCBs to promote the recycling metal resources.

Pectinases as promising green biocatalysts having broad-spectrum applications: Recent trends, scope, and relevance

Pectinases are a collection of multiple enzymes that have a common substrate, that is, pectin. They can act on different parts of pectin due to the structural heterogeneity of pectin. Therefore, they have been placed in different groups, such as protopectinases, polygalacturonases, polymethylesterases, pectin lyases, and pectate lyases. They are naturally present both in multicellular organisms such as higher plants and in unicellular organisms such as microbes. In past decade, it has been witnessed that chemical and mechanical methods employed in industrial processes have led to environmental hazards and serious health disorders, thus increasing the search for eco-friendly approaches with minimal health risks. Hence, microbial enzymes have been extensively used as safer alternative for these environmentally unsafe methods. Among these microbial enzymes, pectinases hold great significance and is one of the principal enzymes that have been used commercially. It is predominantly used as a green biocatalyst for fruit, fiber, oil, textile, beverage, pulp, and paper industry. Thus, this review focuses on the structure of pectin, microbial sources of pectin, and principle industrial applications of pectinases.

Material Design Strategies for Recovery of Critical Resources from Water

Population growth, urbanization, and decarbonization efforts are collectively straining the supply of limited resources that are necessary to produce batteries, electronics, chemicals, fertilizers, and other important products. Securing the supply chains of these critical resources via the development of separation technologies for their recovery represents a major global challenge to ensure stability and security. Surface water, groundwater, and wastewater are emerging as potential new sources to bolster these supply chains. Recently, a variety of material-based technologies have been developed and employed for separations and resource recovery in water. Judicious selection and design of these materials to tune their properties for targeting specific solutes is central to realizing the potential of water as a source for critical resources. Here, the materials that are developed for membranes, sorbents, catalysts, electrodes, and interfacial solar steam generators that demonstrate promise for applications in critical resource recovery are reviewed. In addition, a critical perspective is offered on the grand challenges and key research directions that need to be addressed to improve their practical viability.

Heavy metal pollution in the aquatic environment: efficient and low-cost removal approaches to eliminate their toxicity: a review

Heavy metal contamination of water sources has emerged as a major global environmental concern, threatening both aquatic ecosystems and human health. Heavy metal pollution in the aquatic environment is on the rise due to industrialization, climate change, and urbanization. Sources of pollution include mining waste, landfill leachates, municipal and industrial wastewater, urban runoff, and natural phenomena such as volcanic eruptions, weathering, and rock abrasion. Heavy metal ions are toxic, potentially carcinogenic, and can bioaccumulate in biological systems. Heavy metals can cause harm to various organs, including the neurological system, liver, lungs, kidneys, stomach, skin, and reproductive systems, even at low exposure levels. Efforts to find efficient methods to remove heavy metals from wastewater have increased in recent years. Although some approaches can effectively remove heavy metal contaminants, their high preparation and usage costs may limit their practical applications. Many review articles have been published on the toxicity and treatment methods for removing heavy metals from wastewater. This review focuses on the main sources of heavy metal pollution, their biological and chemical transformation, toxicological impacts on the environment, and harmful effects on the ecosystem. It also examines recent advances in cost-effective and efficient techniques for removing heavy metals from wastewater, such as physicochemical adsorption using biochar and natural zeolite ion exchangers, as well as decomposition of heavy metal complexes through advanced oxidation processes (AOPs). Finally, the advantages, practical applications, and future potential of these techniques are discussed, along with any challenges and limitations that must be considered.

Recovery of copper and silver from industrial e-waste leached solutions using sustainable liquid membrane technology: a review

Waste electrical and electronic equipment or e-waste has recently emerged as a significant global concern. This waste contains various valuable metals, and via recycling, it could become a sustainable resource of metals (viz. copper, silver, gold, and others) while reducing reliance on virgin mining. Copper and silver with their superior electrical and thermal conductivity have been reviewed due to their high demand. Recovering these metals will be beneficial to attain the current needs. Liquid membrane technology has appeared as a viable option for treating e-waste from various industries as a simultaneous extraction and stripping process. It also includes extensive research on biotechnology, chemical and pharmaceutical, environmental engineering, pulp and paper, textile, food processing, and wastewater treatment. The success of this process depends more on the selection of organic and stripping phases. In this review, the use of liquid membrane technology in treating/recovering copper and silver from industrial e-waste leached solutions was highlighted. It also assembles critical information on the organic phase (carrier and diluent) and stripping phase in liquid membrane formulation for selective copper and silver. In addition, the utilization of green diluent, ionic liquids, and synergist carrier was also included since it gained prominence attention latterly. The future prospects and challenges of this technology were also discussed to ensure the industrialization of technology. Herein, a potential process flowchart for the valorization of e-waste is also proposed.

Biolixiviation of Metals from Computer Printed Circuit Boards by Acidithiobacillus ferrooxidans and Bioremoval of Metals by Mixed Culture Subjected to a Magnetic Field

Crushed and ground printed circuit board (PCB) samples were characterized to evaluate copper, lead, and aluminum using X-ray fluorescence spectroscopy (XRF) and the morphology was done by scanning electron microscopy (SEM). The XRF characterizations showed 0.12% lead, 3.72% copper, and 12.73% aluminum in the PCBs. The metal solubilization experiments using Acidithiobacillus ferrooxidans indicated higher values of total metal solubilization when the initial pH of the inoculum was adjusted. However, these experiments did not show higher metal solubilization by bioleaching. The sequential bioremoval experiments using mixed culture after bioleaching assays with A. ferrooxidans with initial adjustment of inoculum pH and without applying a magnetic field removed 100% of Al, 27.34% of Cu, and 96.43% of Pb from the lixiviate medium; with magnetic field application, 100% of Al, 83.82% of Cu, and 98.27% of Pb were removed. A similar bioleaching experiment without inoculum pH adjustment and without field application achieved 99.74% removal for Cu and 91.92% for Pb. When the magnetic field was applied, 100% of Cu and 95.76% of Pb were removed. Bioreactors with a magnetic field do not show significantly better removal of any of the metals analyzed.

The Impact of Heavy Metal Accumulation on Some Physiological Parameters in Silphium perfoliatum L. Plants Grown in Hydroponic Systems

Heavy metals like cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn), resulting from anthropogenic activities, are elements with high persistence in nature, being able to accumulate in soils, water, and plants with significant impact to human and animal health. This study investigates the phytoremediation capacity of Silphium perfoliatum L. as a specific heavy metal hyperaccumulator and the effects of Cu, Zn, Cd, and Pb on some physiological and biochemical indices by growing plants under floating hydroponic systems in nutrient solutions under the presence of heavy metals. One-year-old plants of S. perfoliatum grown for 20 days in Hoagland solution with the addition of (ppm) Cu-400, Zn-1200, Cd-20, Pb-400, and Cu+Zn+Cd+Pb (400/1200/20/400) were investigated with respect to the control. The level of phytoremediation, manifested by the ability of heavy metal absorption and accumulation, was assessed. In addition, the impact of stress on the proline content, photosynthetic pigments, and enzymatic activity, as being key components of metabolism, was determined. The obtained results revealed a good absorption and selective accumulation capacity of S. perfoliatum plants for the studied heavy metals. Therefore, Cu and Zn mainly accumulate in the stems, Cd in the roots and stems, while Pb mainly accumulates in the roots. The proline tended to increase under stress conditions, depending on the pollutant and its concentration, with higher values in leaves and stems under the associated stress of the four metals and individually for Pb and Cd. In addition, the enzymatic activity recorded different values depending on the plant organ, its type, and the metal concentration on its substrate. The obtained results indicate a strong correlation between the metal type, concentration, and the mechanisms of absorption/accumulation of S. perfoliatum species, as well as the specific reactions of metabolic response.

Heavy metals accumulation in bivalve mollusks collected from coastal areas of southeast China

The distribution of six heavy metal and metalloids (As, Cd, Cr, Hg, Ni and Pb) was analyzed in 597 bivalve mollusks (8 species) collected from coastal areas of southeast China. Target hazard quotient, total hazard index, and target cancer risk were calculated to evaluate potential human health risks from bivalve consumption. The mean concentrations of As, Cd, Cr, Hg, Ni and Pb were 1.83, 0.581, 0.111, 0.0117, 0.268 and 0.137 mg kg^-1 wet weight in bivalves. The average estimated daily intakes for As, Cd, Cr, Hg, Ni and Pb were 1.156, 0.367, 0.07, 0.007, 0.167 and 0.087 μg kg^-1 body weight/day. Health risk assessment showed that there was no non-carcinogenic health risk to general residents to these metals from consumption of bivalves. Cd intake through mollusks posed a potential cancer risk. Accordingly, regular monitoring for heavy metals, especially Cd is recommended with respect to potential contaminant on marine ecosystems.

Environmentally sustainable and cost-effective recycling of Mn-rich Li-ion cells waste: Effect of carbon sources on the leaching efficiency of metals using fungal metabolites

Metals recovery from spent lithium coin cells (SCCs) is enjoying great attention due to environmental problems and metal-rich contents such as Mn and Li. Fungi can generate many organic acids, and metals can be dissolved, but sucrose is not an economical medium. The main objective of this study is to find a suitable carbon substrate in place of sucrose for fungal bioleaching. We have developed an environmentally friendly, cost-effective, and green method for recycling and detoxifying Mn and Li from SCCs using the spent culture medium fromPenicillium citrinumcultivation. Sugar cane molasses and sucrose were selected as carbon sources. Based on the extracted fungal metabolites, the effects of pulp density, temperature, and leaching time were assessed on metal dissolution. The most suitable conditions were 30 g/L of pulp density, a temperature of 40 °C, and 4 days of leaching time in spent molasses medium, which led to a high extraction of 87% Mn and 100% Li. Based on EDX-mapping analyses, it was found that the initial concentration of ∑ (Mn + C) in the SCCs powder was almost 100% while reaching nearly 6.4% after bioleaching. After bioleaching, an analysis of residual powder confirmed that metal dissolution from SCCs was effective owing to fungal metabolites. The economic study showed that the bioleaching method is more valuable for the dissolution of metals than the chemical method; In addition to improving bioleaching efficiency, molasses carbon sources can be used for industrial purposes.

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.

Bioleaching and immobilizing of copper and zinc using endophytes coupled with biochar-hydroxyapatite: Bipolar remediation for heavy metals contaminated mining soils

Copper and zinc are toxic heavy metals in soils that require development of feasible strategies for remediation of contaminated soils around the mine areas. In this study, the processing conditions and mechanisms of immobilization and bioleaching for remediation of highly contaminated soils with heavy metals are investigated. Soil remediation is carried out using a bioleaching-immobilization bipolar method. The results show that LSE03 bacteria provide efficient leaching result and immobilization on Cu²⁺ and Zn²⁺. Among the bacterial metabolites, cis, cis-muconic acid and isovaleric acid play major roles in the bioleaching process. The bacterial extracellular polymeric substances are rich in a variety of organic acids that show a significant decrease in content after the adsorption process, indicating that all of these substances are involved in the binding of heavy metals. Characterization of the endophytes and immobilizing agents with FTIR, TEM-mapping, and XPS techniques reveal the ability of both bacteria and composites to adsorb Cu-Zn as well as the main functional groups of -OH, -COOH, -PO₄^3-, and -NH. According to the heavy metals species analyses, competitive adsorption experiments, and bioleaching desorption experiments, it is planned to carry out the bipolar remediation of contaminated soil through immobilization followed by bioleaching process. After bipolar remediation processing, 97.923% and 96.387% of available Cu and Zn are respectively removed. Soils fertility significantly increases in all cases. Our study provides a green, practical, and environmentally friendly treatment method for soils contaminated with high concentrations of heavy metals.

Olive-tree polyphenols and urban mining. A greener alternative for the recovery of valuable metals from scrap printed circuit boards

Recycling printed circuit boards (PCBs) is becoming a source of precious metals and an alternative to conventional mining. This phenomenon is now known as "urban mining." In this work, a polyphenols-rich plant extract has been obtained from olive-tree leaves, and its ability to contribute to reducing four metals, namely, Ag, Cu, Cr, and Sn, that are present in scrap PCBs has been studied. Three reductants (NaBH₄, Fe°, and the olive-tree leaves extract) have been used to recover these valuable metals. An attempt has been made to minimize the concentration of the first two, replacing them with a natural, cheaper, and less toxic reductant. To achieve this goal, a computer-assisted factorial, composed, centered, orthogonal, and rotatable statistical design of experiments (FCCORD) has been used to build the experimental matrix to be carried out in the laboratory and, next, for the statistical treatment of the results. The results show that it is possible to achieve only a partial recovery of the four metals (silver, copper, chromium, and tin) from PCBs leachates by using sodium borohydride, iron, and the extract separately. In other words, none of these three reductants alone can completely remove any of the four metals in the leachate. Nevertheless, using the statistical design of experiments, the total recovery of the four metals has been achieved by combining the three reductants in the appropriate concentrations. Hence, polyphenols-rich plant extracts in general and olive-tree leaves extract in particular can be regarded as promising coadjuvants in the rising field of urban mining.

Facile path for copper recovery from waste printed circuit boards via mechanochemical approach

Recycling copper (Cu⁰) from waste printed circuit boards (PCBs) is a prevalent challenge. Here, we propose a new pathway and reveal mechanisms for recovering Cu⁰ from waste PCBs via a mechanochemical approach. The successful application of mechanical force avoids using inorganic acid in the Cu⁰ recovery process. Our work demonstrates that ferric chloride (FeCl₃) was superior to ferric sulfate and ferric nitrate as a solid-phase reagent for Cu⁰ recovery due to chloride complexation. Under the induction of mechanical force, the Cu⁰ in the waste PCBs was oxidized by Fe³⁺ and complexed by Cl^¯ to form a meta-stable cuprous chloride, which was susceptible to leaching in an acidic liquid-phase system constructed by hydrolysis of ferric salt. Further mechanism analysis reveals that in the mechanochemical solid-phase reaction, Cu⁰, metallic impurities, metal oxides, and carbon materials from waste PCBs could also reduce Fe³⁺ to Fe²⁺. The optimum conditions for Cu⁰ recovery from waste PCB powder with FeCl₃ as a solid-phase reagent were: rotational speed of 500 rpm, Cu⁰:Fe³⁺ molar ratio of 1:20, and reaction time of 120 min, achieving the highest recovery of 99.6 wt%. This study presents a facile path for Cu⁰ recovery from waste PCBs for resource circulation.

A systematic review of gold extraction: Fundamentals, advancements, and challenges toward alternative lixiviants

Since the birth of cyanidation, it has been dominant in the gold extraction industry. Recently, with the increasing awareness of environmental hazards and potential risks posed by the severe toxicity of cyanide, attempts to seek alternative lixiviants have arisen. Over the past three decades, a significant amount of literature has examined alternative lixiviants to cyanide for recovering gold, while few industrial applications have been reported due to various obstacles, such as toxicity, excessive consumption, or low leaching efficiency. These obstacles are progressively overcome in multiple ways, including process improvement, system optimization, use of co-intensifying systems, and development of additives. In this paper, related studies about alternative lixiviants and methods such as cyanide, thiosulfate, thiourea, thiocyanate, polysulfides, halides, and microbial leaching are summarized. The history, fundamentals, advancements, and challenges of alternative lixiviants are fully concluded to provide a reference for cleaner gold production. In addition, the comprehensive performance of lixiviants was evaluated according to a novel evaluation criterion proposed in terms of economy, efficiency, and environment.

Sustainable approach for valorization of solid wastes as a secondary resource through urban mining

The incessant population has increased the production and consumption of plastics, paper, metals, and organic materials, which are discarded as solid waste after their end of life. The accumulation of these wastes has created growing concerns all over the world. However, conventional methods of solid waste management i.e., direct combustion and landfilling have caused several negative impacts on the environment (releasing toxic chemicals and greenhouse gases, huge land use) besides affecting human health. Therefore, it is requisite to determine sustainable alternative technologies that not only help in mitigating environmental issues but also increase the economic value of the discarded solid wastes. This process is known as urban mining where waste is converted into secondary resources and thereby conserves the natural primary resources. Thus, this review highlights the technological advancements in the valorization process of discarded wastes and their sustainable utilization. We also discussed several limitations of the existing urban mining processes and further the feasibility of valorization techniques was critically analyzed from a techno-economical perspective. This paper recommends a novel sustainable model based on the circular economy concept, where waste is urban mined and recovered as a secondary resource to support the united nations sustainable development goals (SDGs). The implementation of this model will ultimately help the developing countries to achieve the target of SDGs 11, 12, and 14.

Review on biopolymers and composites - Evolving material as adsorbents in removal of environmental pollutants

The presence of pollutants and toxic contaminants in water sources makes it unfit to run through. Though various conventional techniques are on deck, development of new technologies are vital for wastewater treatment and recycling. Polymers have been intensively utilized recently in many industries owing to their unique characteristics. Biopolymers resembles natural alternative to synthetic polymers that can be prepared by linking the monomeric units covalently. Despite the obvious advantages of biopolymers, few reviews have been conducted. This review focuses on biopolymers and composites as suitable adsorbent material for removing pollutants present in environment. The classification of biopolymers and their composites based on the sources, methods of preparation and their potential applications are discussed in detail. Biopolymers have the potentiality of substituting conventional adsorbents due to its unique characteristics. Biopolymer based membranes and effective methods of utilization of biopolymers as suitable adsorbent materials are also briefly elaborated. The mechanism of biopolymers and their membrane-based adsorption has been briefly reviewed. In addition, the methods of regeneration and reuse of used biopolymer based adsorbents are highlighted. The comprehensive content on fate of biopolymer after adsorption is given in brief. Finally, this review concludes the future investigations in recent trends in application of biopolymer in various fields in view of eco-friendly and economic perspectives.

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.

Co-inoculation with beneficial microorganisms enhances tannery sludge bioleaching with Acidithiobacillus thiooxidans

Bioleaching of tannery sludge is an efficient and environmentally friendly way for chromium (Cr) removal, which supports the sustainable development of the leather industry. Acidithiobacillus thiooxidans has been reported effective in Cr bioleaching of tannery sludge. However, little is known about whether the presence of other benefiting species could further improve the Cr leaching efficiency of A. thiooxidans. Here, we studied the enhancing roles of four species namely Acidiphilium cryptum, Sulfobacillus acidophilus, Alicyclobacillus cycloheptanicus, and Rhodotorula mucilaginosa in chromium bioleaching of tannery sludge with A. thiooxidans by batch bioleaching experiments. We found that each of the four species facilitated the quick dominance of A. thiooxidans in the bioleaching process and significantly improved the bioleaching performance including bioleaching rate and efficiency. The bioleaching efficiency of Cr in the tannery sludge could reach 100% on the sixth day by co-inoculating A. thiooxidans and four auxiliary species. The achievements shed a light on the role of the community-level interactions on bioleaching and may also serve as guidance for managing bioleaching consortiums for better outcomes.

Synthesis and Phase Behavior of a Platform of CO2-Soluble Functional Gradient Copolymers Bearing Metal-Complexing Units

The synthesis and characterization of a platform of novel functional fluorinated gradient copolymers soluble in liquid and supercritical CO₂ is reported. These functional copolymers are bearing different types of complexing units (pyridine, triphenylphosphine, acetylacetate, thioacetate, and thiol) which are well-known ligands for various metals. They have been prepared by reversible addition–fragmentation chain-transfer (RAFT) polymerization in order to obtain well-defined gradient copolymers. The copolymers have been characterized by proton nuclear magnetic resonance (¹H-NMR) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, thermal gravimetric analysis (TGA), dynamical scanning calorimetry (DSC) and cloud point measurements in dense CO₂. All the investigated metal-complexing copolymers are soluble in dense CO₂ under mild conditions (pressure lower than 30 MPa up to 65 °C), confirming their potential applications in processes such as metal-catalyzed reactions in dense CO₂, metal impregnation, (e.g., preparation of supported catalysts) or metal extraction from various substrates (solid or liquid effluents). Particularly, it opens the door to greener and less energy-demanding processes for the recovery of metals from spent catalysts compared to more conventional pyro- and hydro-metallurgical methods.

Resource Recycling with the Aim of Achieving Zero-Waste Manufacturing

The management of the huge amounts of waste generated from domestic and industrial activities has continued to be a source of concern for humanity globally because of its impact on the ecosystem and human health. Millions of tons of such used materials, substances, and products are therefore discarded, rejected, and abandoned, because they have no further usefulness or application. Additionally, owing to the dearth of affordable materials for various applications, the environmental impact of waste, and the high cost of procuring virgin materials, there have been intensive efforts directed towards achieving the reduction, minimization, and eradication of waste in human activities. The current review investigates zero-waste (ZW) manufacturing and the various techniques for achieving zero waste by means of resource recycling. The benefits and challenges of applying innovative technologies and waste recycling techniques in order to achieve ZW are investigated. Techniques for the conversion of waste glass, paper, metals, textiles, plastic, tire, and wastewater into various products are highlighted, along with their applications. Although waste conversion and recycling have several drawbacks, the benefits of ZW to the economy, community, and environment are numerous and cannot be overlooked. More investigations are desirable in order to unravel more innovative manufacturing techniques and innovative technologies for attaining ZW with the aim of pollution mitigation, waste reduction, cost-effective resource recovery, energy security, and environmental sustainability.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.