Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants--A review

A review on e-waste contamination, toxicity, and sustainable clean-up approaches for its management

Ecosystems and human health are being negatively impacted by the growing problem of electrical waste, especially in developing countries. E-waste poses a significant risk to ecological systems because it can release a variety of hazardous substances into the environment, containing polybrominated diphenyl ethers and heavy metals, brominated flame retardants, polychlorinated dibenzofurans and polycyclic aromatic hydrocarbons, and dioxins. This review article provides a critical assessment of the toxicological consequences of e-waste on ecosystems and human health and data analyses from scientific journals and grey literature on metals, BFRs, PBDEs, PCDFs, and PAHs in several environmental compartments of commercial significance in informal electronic trash recycling. The currently available techniques and tools employed for treating e-waste are sustainable techniques such as bioremediation, chemical leaching, biological leaching, and pyrometallurgy have been also discussed along with the necessity of implementing strong legislation to address the issue of unregulated exports of electronic trash in recycling practices. Despite the ongoing hurdles, implementing environmentally sustainable recycling methods have the potential to address the detrimental impacts of e-waste and foster positive economic development.

Pathway to industrial application of heterotrophic organisms in critical metals recycling from e-waste

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

Highly Selective Recovery of Gold by In Situ Magnetic Field-Assisted Fe/Co-MOF@PDA/NdFeB Double Network Gel

There are enormous economic benefits to conveniently increasing the selective recovery capacity of gold. Fe/Co-MOF@PDA/NdFeB double-network organogel (Fe/Co-MOF@PDA NH) is synthesized by aggregation assembly strategy. The package of PDA provides a large number of nitrogen-containing functional groups that can serve as adsorption sites for gold ions, resulting in a 21.8% increase in the ability of the material to recover gold. Fe/Co-MOF@PDA NH possesses high gold recovery capacity (1478.87 mg g-1) and excellent gold selectivity (Kd = 5.71 mL g-1). With the assistance of an in situ magnetic field, the gold recovery capacity of Fe/Co-MOF@PDA NH is increased from 1217.93 to 1478.87 mg g-1, and the recovery rate increased by 24.7%. The above excellent performance is attributed to the efficient reduction of gold by FDC/FC+, Co2+/Co3+ double reducing couple, and the optimization of the reduction reaction by the magnetic field. After the samples are calcined, high-purity gold (95.6%, 22K gold) is recovered by magnetic separation. This study proposes a forward-looking in situ energy field-assisted strategy to enhance precious metal recovery, which has a guiding role in the development of low-carbon industries.

Effect of sodium acetate additive on gold leaching with cyanide solution: Laboratory and semi-pilot leaching tests

Heap leaching with a cyanide solution is used for processing low-grade ores; however, owing to the chemical, mineralogical, and physical characteristics of ores and their particle size distribution, extraction efficiencies are often low. This study investigated the effects of sodium acetate addition on gold extraction from Akshoky deposit ores under laboratory and semi-pilot laboratory test conditions. The gold-bearing ore used in this study had average gold and silver contents of 1.32 and 3.27 g/t, respectively. The chemical composition of the ore was as follows (wt%): copper: 0.0185, nickel: 0.0090, cobalt: 0.0025, zinc: 0.0470, lead: 0.0095, total iron: 4.10, calcium oxide: 3.08, magnesium oxide: 1.10, sodium oxide: 1.40, potassium oxide: 0.82, silicon oxide: 64.22, aluminum oxide: 13.37, arsenic: 0.023, antimony: 0.0024, total sulfur: 0.24, sulfate sulfur: 0.040, and sulfide sulfur: 0.20. Gold in the ore occurs in different forms: free/native form grains (82 %), covered with films (3.28 %), associated with sulfides (6.56 %), and in fine-grained form (8.20 %). Laboratory tests showed that gold dissolution from ground ore by a cyanide solution without sodium acetate addition was 83.08 %. However, with sodium acetate addition (0.1; 0.5; 1.0 kg/t), it increased to 84.38-86.61 %. Semi-pilot laboratory tests under heap-leaching conditions confirmed the positive effects of sodium acetate. The increase in gold extraction was 7.6 % (62.9 %) compared with that in the experiments without reagent addition (55.30 %).

Drivers and Pathways for the Recovery of Critical Metals from Waste-Printed Circuit Boards

The ever-increasing importance of critical metals (CMs) in modern society underscores their resource security and circularity. Waste-printed circuit boards (WPCBs) are particularly attractive reservoirs of CMs due to their gamut CM embedding and ubiquitous presence. However, the recovery of most CMs is out of reach from current metal-centric recycling industries, resulting in a flood loss of refined CMs. Here, 41 types of such spent CMs are identified. To deliver a higher level of CM sustainability, this work provides an insightful overview of paradigm-shifting pathways for CM recovery from WPCBs that have been developed in recent years. As a crucial starting entropy-decreasing step, various strategies of metal enrichment are compared, and the deployment of artificial intelligence (AI) and hyperspectral sensing is highlighted. Then, tailored metal recycling schemes are presented for the platinum group, rare earth, and refractory metals, with emphasis on greener metallurgical methods contributing to transforming CMs into marketable products. In addition, due to the vital nexus of CMs between the environment and energy sectors, the upcycling of CMs into electro-/photo-chemical catalysts for green fuel synthesis is proposed to extend the recycling chain. Finally, the challenges and outlook on this all-round upgrading of WPCB recycling are outlined.

Recent Advances in Thiourea Based Colorimetric and Fluorescent Chemosensors for Detection of Anions and Neutral Analytes: A Review

Thioureas and their derivatives are organosulfur compounds having excellent biological and non-biological applications. These compounds contain S- and N-, which are nucleophilic and allow for establishing inter-and intramolecular hydrogen bonding. These characteristics make thiourea moiety a very important chemosensor to detect various environmental pollutants. This article covers a broad range of thioureas and their derivatives that are used for highly sensitive, selective, and simple fluorimetric (turn-off and turn-on), and colorimetric chemosensors for the detection and determination of different types of anions, such as CN-, AcO-, F-, ClO- and citrate ions, etc., and neutral analytes such as ATP, DCP, and Amlodipine, etc., in biological, environmental, and agriculture samples. Further, the sensing performances of thioureas-based chemosensors have been compared and discussed, which could help the readers for the future design of organic fluorescent and colorimetric sensors to detect anions and neutral analytes. We hope this study will support the new thoughts to design highly efficient, selective, and sensitive chemosensors to detect different analytes in biological, environmental, and agricultural samples.

Screening of variables affecting the selective leaching of valuable metals from waste motherboards' PCBs

The presence of valuable and hazardous metals in waste printed circuit boards, especially, motherboards, makes their recovery necessary as implies great economic and environmental advantages and develops urban mining processes. Hence, this research is focused on the selective leaching of Cu, Pb, and Sn as base metals using nitric acid and hydrochloric acid and Au, Ag, and Pd as precious metals using thiourea and sodium thiosulfate from waste motherboards' PCBs in a sequential eco-friendly two-stage process. Previously, thiourea and sodium thiosulfate were used as leaching agents to investigate their applicability for the leaching of metals from PCBs in a single-stage process. Screening experimental design was applied to screen the variables affecting the leaching process in order to evaluate their impact on the recovery of metals and select the significant factors. The results demonstrated that base and precious metals can be leached appropriately in two consecutive stages compared to a single-stage process. Nitric acid was found to be a much more efficient agent to leach Cu and Pb in comparison with hydrochloric acid which was more suitable for the leaching of Sn. In the case of precious metals, higher amounts of Au were leached using thiourea, whereas sodium thiosulfate was able to leach more Pd. Roughly similar results were obtained for the leaching of Ag using these leaching agents. Nitric acid concentration, average particle size, temperature, and leaching time were found to be significant to maximize the leaching of Cu and Pb and minimize that for Au, Ag, and Pd in the first stage. Initial pH was the only variable influencing the second stage, in particular, Au leaching by thiourea.

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.

Advances in natural polysaccharides for gold recovery from e-waste: Recent developments in preparation with structural features

The increasing demand for gold because of its high market price and its wide use in the electronic industry has attracted interest in gold recovery from electronic waste (e-waste). Gold is being dumped as solid e-waste which contains gold concentrations ten times higher than gold ores. Adsorption is a widely used approach for extracting gold from e-waste due to its simplicity, low cost, high efficiency, and reusability of adsorbent material. Natural polysaccharides received increased attention due to their natural abundance, multi-functionality, biodegradability, and nontoxicity. In this review, a brief history, and advancements in this technology were evaluated with recent developments in the preparation and mechanism advancements of natural polysaccharides for efficient gold recovery. Moreover, we have discussed some bifunctional modified polysaccharides with detailed gold adsorption mechanisms. The modified adsorbent materials developed from polysaccharides coupled with inorganic/organic functional groups would demonstrate an efficient technology for the development of new bio-based materials for efficient gold recovery from e-waste. Also, future views are recommended for highlighting the direction to achieve fast and effective gold recovery from e-waste in a friendly and sustainable manner.

Process intensification for sustainable extraction of metals from e-waste: challenges and opportunities

The electrical and electronic waste is expected to increase up to 74.7 million metric tons by 2030 due to the unparalleled replacement rate of electronic devices, depleting the conventional sources of valuable metals such as rare earth elements, platinum group metals, Co, Sb, Mo, Li, Ni, Cu, Ag, Sn, Au, and Cr. Most of the current techniques for recycling, recovering, and disposing of e-waste are inappropriate and therefore contaminate the land, air, and water due to the release of hazardous compounds into the environment. Hydrometallurgy and pyrometallurgy are two such conventional methods used extensively for metal recovery from waste electrical and electronic equipment (WEEE). However, environmental repercussions and higher energy requirements are the key drawbacks that prevent their widespread application. Thus, to ensure the environment and elemental sustainability, novel processes and technologies must be developed for e-waste management with enhanced recovery and reuse of the valued elements. Therefore, the goal of the current work is to examine the batch and continuous processes of metal extraction from e-waste. In addition to the conventional devices, microfluidic devices have been also analyzed for microflow metal extraction. In microfluidic devices, it has been observed that the large specific surface area and short diffusion distance of microfluidic devices are advantageous for the efficient extraction of metals. Additionally, cutting-edge technologies have been proposed to enhance the recovery, reusability, and recycling of e-waste. The current study may support decision-making by researchers in deciding the direction of future research and moving toward sustainable development.

Amine-Based MOF for Precious Metal Remediation

Due to the continuous growth rate of the electronic industry, hi-tech companies depend on mining and extracting precious metals to meet the public demand. The high turnover of modern devices generates an alarming amount of electronic waste (e-waste), which contains more precious metals than mined ores and therefore needs efficient recovery procedures. A highly stable homopiperazine-derived Cd-MOF, poly-[Cd(H2L)]·9H2O, with a protonated amine ligand core, exists as a twofold interpenetrated 3D framework with 1D channels into which the N+-H bond is directed. The geometry of these channels appears to be suitable to host square planar metalate complexes. Under acidic conditions, [MCl4]x- anions containing Au, Cu, Ni, and Pt, representing common components of e-waste under extraction conditions, were tested for capture and recovery. Cd-MOF exhibits remarkable selectivity and uptake performance toward Au with an adsorbent capacity of 25 mg g-1ads and shows a marked selectivity for Au over Cu in competitive experiments. The adsorption mechanism of Au appears to be predominantly physical adsorption at the surface of the material.

Green Deep Eutectic Solvents (DESs) for Sustainable Metal Recovery from Thermally Treated PCBs: A Greener Alternative to Conventional Methods

Waste PCBs the core of e-waste is rich in copper, tin, zinc, iron, and nickel. Leaching base metals from PCB used to be done in toxic, corrosive acidic/alkali mediums. In this work, an environmentally friendly method for leaching metals from thermally treated PCBs (TPCBs) of mobile phones was proposed using choline chloride based deep eutectic solvents (DES). DES selectivity and solubility of metals from metal oxides were the main screening criteria. FA-ChCl had the maximum solubility of Cu, Fe, and Ni, while Urea-ChCl had high Zn selectivity and solubility. Oxalic acid has high selectivity for Sn. FA-ChCl extracted Cu and Fe best at 16 h, 100 °C, and 1/30 g/mL. Urea-ChCl extracted Zn (90.4±2.9 %) from TPCBs at 100 °C, 21 h, 1/20 g/mL, and 400 rpm. Oxalic acid (1 M) removed 92.3±2.1 % Sn from TPCBs in 1 h at 80 °C and 1/20 g/mL. The shrinking core model-based kinetic investigation of FA-ChCl for Cu extraction showed a diffusion-controlled process. The proposed method is greener than mineral acids utilized for metal extraction.

Switchable Metal-Ion Selectivity in Sulfur-Functionalised Pillar[5]arenes and Their Host-Guest Complexes

Nucleophilic substitution of pertosylated pillar[5]arene (P-OTs) with commercially available sulfur containing nucleophiles (KSCN, KSAc, and thiophenol), yields a series of sulfur-functionalised pillar[5]arenes. DLS results and SEM images imply that these pillararene macrocycles self-assemble in acetonitrile solution, while X-ray crystallographic evidence suggests solvent-dependent assembly in the solid state. The nature of the sulfur substituents decorating the rim of the pillararene controls binding affinities towards organic guest encapsulations within the cavity and dictates metal-ion binding properties through the formation of favorable S-M2+ coordination bonds outside the cavity, as determined by 1 H NMR and fluorescence spectroscopic experiments. Addition of a dinitrile guest containing a bis-triazole benzene spacer (btn) induced formation of pseudorotaxane host-guest complexes. Fluorescence emission signals from these discrete macrocycles were significantly attenuated in the presence of either Hg2+ or Cu2+ in solution. Analogous titrations utilizing the corresponding pseudorotaxanes alter the binding selectivity and improve fluorescence sensing sensitivity. In addition, preliminary liquid-liquid extraction studies indicate that the macrocycles facilitate the transfer of Cu2+ from the aqueous to the organic phase in comparison to extraction without pillar[5]arene ligands.

Selective gold extraction from electronic waste using high-temperature-synthesized reagents

For over two hundred years, cyanide has served as the primary reagent for gold extraction. However, due to its high toxicity, the use of cyanide poses significant risks. Traditional low-toxicity leaching reagents have limitations that restrict their widespread industrial application, leading to the necessity for the development of new, efficient, and low-toxic gold leaching reagents to support sustainable gold production. In this study, a novel, efficient, and low-toxicity gold extraction reagent was synthesized at high temperatures by combining urea, sodium carbonate, and a specific iron salt. The research delved into the leaching ability of the reagent under different synthesis conditions and examined the generation of free cyanide content as a by-product. Findings indicated that reagents synthesized with either potassium ferrocyanide or potassium ferricyanide displayed comparable leaching capabilities. Reagents synthesized at 800 °C exhibited lower levels of free cyanide ions and reduced toxicity. Additionally, this reagent demonstrated exceptional selectivity for gold, while in minimal dissolution of copper, iron, nickel, lead, and iron from computer central processing unit (CPU) pins. Under optimal conditions, the efficiency of gold extraction from CPU pins reached 94.65%. Hence, this reagent holds significant potential for the low-toxicity extraction of gold from electronic waste or auriferous concentrates.

Intensifying separation of Pb and Sn from waste Pb-Sn alloy by ultrasound-assisted acid leaching: Selective dissolution and sonochemistry mechanism

Clean and efficient extraction and separation of precious metals from discarded Pb-Sn alloy is critical to the sustainable utilization of solid waste resources. Dense oxide layer and compact alloy texture in the waste Pb-Sn alloy pose challenges to the effective leaching process. Ultrasonic waves are demonstrated to improve separation efficiency via the favorable physical and chemical effects in solution system. In this study, ultrasound-assisted leaching technology is attempted to rapidly and selectively extract Pb from the waste Pb-Sn alloy, and gives emphasis on ultrasonic electrochemical behaviors. The Eh-pH diagrams of Sn-H2O and Pb-H2O systems were firstly analyzed to lay the selective dissolution foundation. It's indicated that oxidizing HNO3 lixiviant is suitable to realize the selective separation of Pb. Both Sn and Pb can be dissolved to ionic Sn2+ and Pb2+ in the HNO3 solution. However, Sn2+ rapidly oxidizes to Sn4+ and Sn4+ further hydrolyzes to insoluble SnO2, which will agglomerate on unreacted materials to limit internal metal leaching in conventional leaching process. Due to the vibratory stripping of oxide layer by physical effect of ultrasound, the conventional acid leaching time for Pb extraction can be halved with the ultrasound assistance. About 99.12 % Pb and only 0.1 % Sn are dissolved in ultrasound-assisted leaching under the following optimal parameters: 0.5 mol/L HNO3, leaching temperature of 80 °C, time of 30 min, liquid-to-solid ratio of 20 mL/g, and ultrasound intensity of 0.52 W/cm2. Leaching kinetics of Pb, phase transition, microstructure evolution, Pb-Sn galvanic corrosion and dissolution polarization curve were studied to determine the ultrasonic enhanced dissolution mechanism. Notably, Pb and Sn form a microcorrosion galvanic cell in which Sn acts as a cathode and is protected while the Pb undergoes intensifying corrosion as the anode giving rise to the higher Pb dissolution efficiency. Eventually, it's suggested that Pb can be rapidly extracted and separated from the waste Pb-Sn alloy during the ultrasound-assisted HNO3 leaching process via the ultrasound physical and chemical effects, especially the sonochemistry aspect of intensified spot corrosion and galvanic corrosion. The proposed ultrasonic electrochemical corrosion in this work were applicable to the extraction of valuable metals from various waste alloys through leaching method.

Two-Stage Leaching of PCBs Using Sulfuric and Nitric Acid with the Addition of Hydrogen Peroxide and Ozone

The paper presents the possibility of recovering metals from printed circuit boards (PCBs) of spent mobile phones using the hydrometallurgical method. Two-stage leaching of Cu(II), Fe(III), Sn(IV), Zn(II), Ni(II) and Pb(II) with H2SO4 (2 and 5 M) and HNO3 (2 M) with the addition of H2O2 (10 and 30%) and O3 (9 or 15 g/h) was conducted at various process conditions (temperature-313, 333 and 353 K, time-60, 120, 240, 300 min, type and concentration of leaching agent, type and concentration of oxidant, solid-liquid ratio (S/L)), allowing for a high or total metals leaching rate. The use of two leaching stages allows for the preservation of selectivity, separation and recovery of metals: in the first stage of Fe(III), Sn(IV) and in the second stage of the remaining tested metal ions, i.e., Cu(II), Zn(II), Ni(II) and Pb(II). Removing Fe from the tested PCBs' material at the beginning of the process eliminates the need to use magnetic methods, the purpose of which is to separate magnetic metal particles (ferrous) from non-magnetic (non-ferrous) particles; these procedures involve high operating costs. Since the leaching of Cu(II) ions with sulfuric(VI) acid practically does not occur (less than 1%), this allows for almost complete transfer of these ions into the solution in the second stage of leaching. Moreover, to speed up the process and not generate too many waste solutions, oxidants in the form of hydrogen peroxide and ozone were used. The best degree of leaching of all tested metal ions was obtained when 2 M sulfuric(VI) acid at 353 K was used in the 1st research stage, and 2 M nitric(V) acid and 9 g/h O3 at 298 K in the 2nd stage of leaching, which allowed it to be totally leached 100% of Fe(III), Cu(II), Sn(IV), Zn(II), Ni(II) and 90% Pb(II).

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.

Hydrometallurgical recovery of silver and gold from waste printed circuit boards and treatment of the wastewater in a biofilm reactor: An integrated pilot application

A hydrometallurgical process for the recovery of gold and silver from waste printed circuit boards (PCBs) was experimentally verified and tested at pilot scale. The process comprises four sequential leaching stages; the first two based on HCl, correspond to base metals (e.g. Sn, Cu) removal, while the third is based on HNO3 for Ag leaching and the final on aqua regia for Au leaching. After base metals leaching, the solid residue, enriched in silver and gold about 5 times, contained silver almost quantitively as insoluble AgCl and significant losses (Ag loss <8%) were avoided. The necessary reduction of Ag in the solid phase was achieved with a solution of 0.5 M N2H4 and 3 M NaOH, at 80 °C and S/L ratio 10%. Leaching of silver by 4 M HNO3 was followed by its recovery from nitrate solution by 0.08 Μ N2H4 at ambient temperature with an efficiency of 83%. Gold was leached by aqua regia and quantitively recovered by 0.13 M N2H4 at ambient temperature. Wastewater resulting from the process, rich in nitrate (5 g/L) and chloride (50 g/L), was treated by an effective and novel biological denitrification system tolerating metals at ppm level, to comply with zero nitrate and residual metals discharge guidelines. The overall process requires low reagents and energy input and has zero discharge for liquid effluents. The scheme is appropriate to be applied at local small to medium industrial units, complying with decentralized circular economy principles for metal recovery from electronic waste.

High Value-Added Reutilization of Waste-Printed Circuit Boards Non-Metallic Components in Sustainable Polymer Composites

The reutilization non-metallic components from a waste-printed circuit board (WPCB) has become one of the most significant bottlenecks in the comprehensive reuse of electronic wastes due to its low value and complex compositions, and it has received great attention from scientific and industrial researchers. To effectively address the environmental pollution caused by inappropriate recycling methods, such as incineration and landfill, extensive efforts have been dedicated to achieving the high value-added reutilization of WPCB non-metals in sustainable polymer composites. In this review, recent progress in developing sustainable polymer composites based on WPCB non-metallic components was systematically summarized. It has been demonstrated that the WPCB non-metals can serve as a promising reinforcing and functional fillers to significantly ameliorate some of the physical and chemical properties of polymer composites, such as excellent mechanical properties, enhanced thermal stability, and flame retardancy. The recovery strategies and composition of WPCB non-metals were also briefly discussed. Finally, the future potentials and remaining challenges regarding the reutilization of WPCB non-metallic components are outlined. This work provides readers with a comprehensive understanding of the preparation, structure, and properties of the polymer composites based on WPCB non-metals, providing significant insights regarding the high value-added reutilization of WPCB non-metals of electronic wastes.

Thermally Reduced Graphene Oxide Membranes Revealed Selective Adsorption of Gold Ions from Mixed Ionic Solutions

The recovery of gold from water is an important research area. Recent reports have highlighted the ultrahigh capacity and selective extraction of gold from electronic waste using reduced graphene oxide (rGO). Here, we made a further attempt with the thermal rGO membranes and found that the thermal rGO membranes also had a similarly high adsorption efficiency (1.79 g gold per gram of rGO membranes at 1000 ppm). Furthermore, we paid special attention to the detailed selectivity between Au3+ and other ions by rGO membranes. The maximum adsorption capacity for Au3+ ions was about 16 times that of Cu2+ ions and 10 times that of Fe3+ ions in a mixture solution with equal proportions of Au3+/Cu2+ and Au3+/Fe3+. In a mixed-ion solution containing Au3+:Cu2+:Na+:Fe3+:Mg2+ of printed circuit board (PCB), the mass of Au3+:Cu2+:Na+:Fe3+:Mg2+ in rGO membranes is four orders of magnitude higher than the initial mass ratio. A theoretical analysis indicates that this selectivity may be attributed to the difference in the adsorption energy between the metal ions and the rGO membrane. The results are conducive to the usage of rGO membranes as adsorbents for Au capture from secondary metal resources in the industrial sector.

Characteristics and health risk assessment of heavy metals in dust of a waste printed circuit board recycling workshop, China

Physical separation is the most widely used technology concerning waste printed circuit board (WPCB) recycling in practical terms. The dust generated from the process poses a significant environmental and human health risk. Amounts of heavy metals in dust present in each processing zone of the workshop showed differences. However, to date, few studies have reported this. The mean metal concentrations in workshop dust from different processing zones were investigated in this study and it was found that Zn, Pb, and Sn appeared in higher levels than other metals, followed by Mn > Cr > Ni > V > As > Cd. The enrichment factors (EFs) ranged from 1.15 to 207.4, and decreased in the order of Cu > Sn > Pb > Zn > Cd > Cr > Ni > V > As, which was exactly consistent with the geo-accumulation index values. The comparison of the EF values of workshop dust in and outside showed that the EFs in workshop dust were mostly smaller. Metals in the dust of the crushing zone (CrZ) showed significant and strong enrichment. The non-carcinogenic risk for different processing zones was all less than 1, which is recognized safety for people's health. The total carcinogenic risk from Cr, and Ni in all zones and As in the CrZ exposure was not negligible. The carcinogenic and non-carcinogenic risks in the CrZ were significantly higher than in the other zones. Masks to filter dust, a ventilation system, daily work hours reduction, and automation improvement was proposed for reducing workers' exposure to heavy metal.

Algal sorbents and prospects for their application in the sustainable recovery of rare earth elements from E-waste

Efficient and sustainable secondary sourcing of Rare-Earth Elements (REE) is essential to counter supply bottlenecks and the impacts associated with primary mining. Recycled electronic waste (E-waste) is considered a promising REE source and hydrometallurgical methods followed by chemical separation techniques (usually solvent extraction) have been successfully applied to these wastes with high REE yields. However, the generation of acidic and organic waste streams is considered unsustainable and has led to the search for "greener" approaches. Sorption-based technologies using biomass such as bacteria, fungi and algae have been developed to sustainably recover REE from e-waste. Algae sorbents in particular have experienced growing research interest in recent years. Despite its high potential, sorption efficiency is strongly influenced by sorbent-specific parameters such as biomass type and state (fresh/dried, pre-treatment, functionalization) as well as solution parameters such as pH, REE concentration, and matrix complexity (ionic strength and competing ions). This review highlights differences in experimental conditions among published algal-based REE sorption studies and their impact on sorption efficiency. Since research into algal sorbents for REE recovery from real wastes is still in its infancy, aspects such as the economic viability of a realistic application are still unexplored. However, it has been proposed to integrate REE recovery into an algal biorefinery concept to increase the economics of the process (by providing a range of additional products), but also in the prospect of achieving carbon neutrality (as large-scale algae cultivation can act as a CO₂ sink).

Evaluating glycine as an alternative lixiviant for copper recovery from waste printed circuit boards

Current hydrometallurgical processing routes for copper recovery from waste printed circuit boards (PCBs) utilise strong mineral acids, which pose environmental hazards. Glycine has been proposed as an alternate lixiviant with a lower environmental impact. This study aimed to investigate the effectiveness of glycine as lixiviant for copper dissolution from waste PCBs. Bench scale laboratory leaching tests were performed to investigate the effect of key process variables such as temperature, oxidant type and lixiviant concentration on the rate, extent and selectivity of copper leaching. In the presence of oxygen as oxidant, the glycine concentration did not have a significant effect on the rate or extent of copper leaching in the range 1 M to 2 M. Increasing the temperature from 25 °C to 60 °C increased copper dissolution after 22 h from 29.6% to 81.2% when using a glycine concentration of 1 M. When air instead of oxygen was used as oxidant, the copper dissolution achieved with 1 M glycine after 22 h at 60 °C decreased by 43.9 percentage points to 37.3%, due to the lower concentration of dissolved oxygen in the system. Using hydrogen peroxide instead of oxygen as oxidant, did not improve the overall extent of copper leaching achieved. Leaching with 1 M glycine and oxygen as oxidant at 60 °C is proposed as the most feasible operating conditions within the ranges investigated, because these conditions yielded the highest copper dissolution (81.2%) with relatively low (1.3%) gold co-extraction.

Facile preparation of flame-retardant cellulose composite with biodegradable and water resistant properties for electronic device applications

The aim of the present study is to produce flexible, flame-retardant, water-resistant and biodegradable composite materials. The ultimate goal of this research is to develop simple processes for the production of bio-based materials capable of replacing non-degradable substrates in printed circuit board. Cellulose was chosen as a renewable resource, and dissolved in 1-ethyl-3-methylimidazolium acetate ionic liquid to prepare a cellulosic continuous film. Since flame retardancy is an important criterion for electronic device applications and cellulose is naturally flammable, we incorporated ammonium polyphosphate (APP) as a flame-retardant filler to increase the flame retardancy of the produced materials. The developed material achieved a UL-94 HB rating in the flammability test, while the cellulose sample without APP failed the test. Two hydrophobic agents, ethyl 2-cyanoacrylate and trichloro(octadecyl)silane were applied by a simple dip-coating technique to impart hydrophobicity to the cellulose-APP composites. Dynamic mechanical analysis indicated that the mechanical properties of the cellulosic materials were not significantly affected by the addition of APP or the hydrophobic agents. Moreover, the biodegradability of the cellulosic materials containing APP increased owing to the presence of the cellulase enzyme. The hydrophobic coating slightly decreased the biodegradability of cellulose-APP, but it was still higher than that of pure cellulose film.

Interfacial Coordination Bonding-Assisted Redox Mechanism-Driven Highly Selective Precious Metal Recovery on Covalent-Functionalized Ultrathin 1T-MoS2

Rational design of functional material interfaces with well-defined physico-chemical-driven forces is crucial for achieving highly efficient interfacial chemical reaction dynamics for resource recovery. Herein, via an interfacial structure engineering strategy, precious metal (PM) coordination-active pyridine groups have been successfully covalently integrated into ultrathin 1T-MoS2 (Py-MoS2). The constructed Py-MoS2 shows highly selective interfacial coordination bonding-assisted redox (ICBAR) functionality toward PM recycling. Py-MoS2 shows state-of-the-art high recovery selectivity toward Au3+ and Pd4+ within 13 metal cation mixture solutions. The related recycling capacity reaches up to 3343.00 and 2330.74 mg/g for Au3+ and Pd4+, respectively. More importantly, above 90% recovery efficiencies have been achieved in representative PMs containing electronic solid waste leachate, such as computer processing units (CPU) and spent catalysts. The ICBAR mechanism developed here paves the way for interface engineering of the well-documented functional materials toward highly efficient PM recovery.

Selective Recovery of Gold from Electronic Waste by New Efficient Type of Sorbent

Modular connectors are applied by computer users, and they can be metallic secondary sources containing metals such as gold and copper. Because gold is a micro-component, the solution obtained after the pin digestion contains a low concentration of gold(III) ions, and efficient and selective sorbent should be used for gold(III) ion recovery. The selective removal of small amounts of gold(III) from 0.001-6 M hydrochloric acid solutions using pure and solvent-impregnated macroporous polystyrene crosslinked with divinylbenzene sorbents (Purolite MN 202 and Cyanex 272) is presented. Gold(III) ions were recovered effectively from the chloride solution after the digestion of the modular connector RJ 45 (8P8C) using Purolite MN 202 after the impregnation process. The dependence of the recovery percentage (R%) of gold(III) on the contact time was determined. The highest value of gold(III) ion sorption capacity (259.45 mg·g^-1) was obtained in 0.001 M HCl for Purolite MN202 after the Cyanex 272 impregnation. The results can be applied to gold recovery from e-waste. The presented method of gold recovery does not generate nitrogen oxides and does not require the use of cyanides.

The Use of PCB Scrap in the Reduction in Metallurgical Copper Slags

The article presents the results of a study on metallurgical sludge reduction using electronic waste such as Printed Circuit Boards (PCBs). Two aspects were taken into account when selecting such a reducer, namely the environmental aspect and the technological aspect. The research was an attempt to use waste metal-bearing material of which the effective management causes many problems from an environmental point of view. In the technological aspect, the specific chemical composition of this waste was taken into account. Its gasification yields significant amounts of hydrocarbons, which are excellent reducing agents in such process. The separation of these compounds may additionally cause the mixing of the molten slag, which should result in faster separation of the formed metal droplets and the molten slag. In the case of the fragmented PCB (Printed Circuit Board) reducer used in this study, a significant degree of copper removal was achieved, as much as 92%. As the reduction-process time increased, the degree of copper removal also increased. For the 1 h process, the average value of copper removal was 60%, and for the 4.5 h process it was over 70%. The case was the same with the addition of reductant: as the amount of reductant added to the process increased, an increase in copper removal was observed. With the addition of 30 g of the reducing agent (per 65 g of slag), the degree of copper removal was over 90%.

Global research into the relationship between electronic waste and health over the last 10 years: A scientometric analysis

Introduction

The aims of this research were to conduct the first holistic and deep scientometric analysis of electronic waste and health and provide with the prediction of research trends and hot topics.

Method

A comprehensive literature search was conducted via the Web of Science Core collection databases on 26 August 2022 to identify all articles related to electronic waste and health. A total of 652 records have been extracted from the Web of Science after applying inclusion and exclusion criteria and were analyzed using bibliometrix software of R-package, VOSviewer, and CiteSpace, visualized by tables and diagrams.

Result

The number of publications and total citations had shown a general growth trend from 2012 to 2021, with an average annual growth rate of 23.74%. Mainland China was the significant nation with the greatest number of publications, citations, and international links. The journal publishing the most was "Science of the Total Environment" (n = 56). Huo X and Hu XJ were the top two author contributing to this field with the highest h-index (23). Over time, the focus in this field shifted to exposure to heavy metal, polychlorinated biphenyls, polybrominated biphenyl ethers, and poly- and perfluorinated alkyl substances from electronic waste, and managements, such as hydrometallurgy.

Discussion

By this scientometric analysis, we found that the most active country, journal, organization and author contributing to this filed, as well as high impact documents and references and research hotspots. Also, we found that the hotspots might be exposure to toxic substances from electronic waste procession, its impact on human health and relevant managements. And evironmentally friendly materials to replace heavy metal mate rials, and environmentally friendly and effective recycling methods of electronic waste need to be further studied.

Homogeneous Gold Catalysis Using Complexes Recovered from Waste Electronic Equipment

Despite the greater awareness of elemental sustainability and the benefits of the circular economy concept, much waste electrical and electronic equipment (WEEE) is still destined for landfill. Effective methods for valorizing this waste within our society are therefore imperative. In this contribution, two gold(III) complexes obtained as recovery products from WEEE and their anion metathesis products were investigated as homogenous catalysts. These four recovery products were successfully applied as catalysts for the cyclization of propargylic amides and the condensation of acetylacetone with o-iodoaniline. Impressive activity was also observed in the gold-catalyzed reaction between electron-rich arenes (2-methylfuran, 1,3-dimethoxybenzene, and azulene) and α,β-unsaturated carbonyl compounds (methyl vinyl ketone and cyclohexenone). These recovered compounds were also shown to be effective catalysts for the oxidative cross-coupling reaction of aryl silanes and arenes. When employed as Lewis acid catalysts for carbonyl-containing substrates, the WEEE-derived gold complexes could also be recovered at the end of the reaction and reused without loss in catalytic activity, enhancing still further the sustainability of the process. This is the first direct application in homogeneous catalysis of gold recovery products sourced from e-waste.

A sequential leaching procedure for efficient recovery of gold and silver from waste mobile phone printed circuit boards

The present study reports a sequential, non-acid process for effective recovery of copper and precious metals from mobile phone printed circuit boards. In this process, gold and silver were first enriched during the synthesis process of cuprous chloride and then leached by thiosulfate method. Results indicated that the distribution of gold and silver in the liquid and solid phases during the synthesis of cuprous chloride process was affected by the [Cu]/[Cu²⁺] ratio. Enrichment of gold and silver in the residue after the cuprous chloride synthesis could be achieved by the adjusting the [Cu]/[Cu²⁺] ratio. The silver and gold leaching rates of the residue after cuprous chloride synthesis (93.8 % silver and 99 % gold) were much higher than those of the raw PCB sample (27.0 % silver and 14.2 % gold) under the same conditions. This process has the advantages of high leaching efficiency, high leaching rate and avoiding the use of HNO₃ or aqua regia commonly used for copper, gold and silver recovery. Thus, this study offers a promising and environmentally friendly method for recovering valuable metals from e-waste.

Selective extraction of precious metals in the polar aprotic solvent system: Experiment and simulation

The traditional hydrometallurgical process is the mainstream technology to recover precious metals from e-waste, which usually adopts strong acid/base and strong oxide with high environmental cost and energy consumption. In the present study, the selective extraction of precious metals was simulated and experimented with DMF as the solvent and Cl^- ions provided by CaCl₂ and CuCl₂ (oxidizing agent). The leaching and precipitation rates of precious metals (Au, Ag, Pd) can reach more than 98% under optimization conditions. Kinetic data shows that the control model of the leaching process on precious metals was determined by linear fitting of the shrinkage model. The complex trace precious metals were extracted selectively using dimethylglyoxime and deionized water as precipitators by the leaching-precipitation-cycle method. Meanwhile, the waste liquid produced by this reaction process could be cyclically utilized. Furthermore, the leaching mechanism of precious metals was proposed. DMF could be complexed with the metals as well as coordination ions (Cl^-), which can reduce the redox potentials. Cu(II) could be easily reduced to Cu(I) in the DMF system due to the higher second ionization energy of copper, which is not influenced by the hydration effect, thus shifting the equilibrium to the metal leach side. Oppositely, the addition of water promoted the conversion of Cu(I) to Cu(II) since the higher hydration energy of Cu(II) compensates for the second ionization energy. This research opens up a new path of sustainable development and provides basic theory and practical experience for environmentally friendly recovery of precious metals from e-waste.

A critical review of the pre-processing and metals recovery methods from e-wastes

E-wastes being potential sources of numerous valuable metals are promoted to undergo recycling and recovery under the umbrella of urban mining and circular economy. Thus, the present study provides a critical review of the technological details of different metal recycling processes, pre-treatment methods, and the advancements made in these techniques. Critical evaluation of different metal recovery techniques has also been presented based on the available life cycle assessment (LCA), techno-economic, and industrial-scale studies. The study revealed that the integrated metal recovery techniques serve better in terms of recovery efficiency and environmental performance than any single recovery technique. Also, scaling up of biometallurgical, electrochemical, and super critical fluid extraction methods needs to be promoted due to their better environmental performances.

Enhanced metal bioleaching mechanisms of extracellular polymeric substance for obsolete LiNixCoyMn1-x-yO2 at high pulp density

Harmful chemicals present in electric vehicle Li-ion batteries (EV LIBs) can limit the pulp density of bioleaching processes using Acidithiobacillus sp. to 1.0% (w/v) or lower. The strong enhancing mechanisms of extracellular polymeric substances (EPS) on the bioleaching of metals from spent EV LIBs at high pulp density (4% w/v) were studied using bio-chemical, spectroscopic, surface structure imaging and bioleaching kinetic methods. Results demonstrated that the added EPS significantly improved bioleaching efficiency of Ni, Co and Mn improved by 42%, 40% and 44%, respectively. EPS addition boosted the growth of cells under adverse conditions to produce more biogenic H⁺ while Fe³⁺ and Fe²⁺ were adsorbed by the biopolymer. This increased Li extraction by acid dissolution and concentrated the Fe³⁺/Fe²⁺ cycle via non-contact mechanisms for the subsequent contact bioleaching of Ni, CO and Mn at the EV LIB-bacteria interface. During the leaching process, added EPS improved adhesion of the bacterial cells to the EV LIBs, and the resultant strong interfacial reactions promoted bioleaching of the target metals. Hence, a combination of non-contact and contact mechanisms initiated by the addition of EPS enhanced the bioleaching of spent EV LIBs at high pulp density.

Structural study and metal speciation assessments of waste PCBs and environmental implications: Outlooks for choosing efficient recycling routes

Environmental protection from risks and disposal management of discarded mobile phone printed circuit boards (MPhPCBs) is a global issue. Although recycling is proposed as a solution, it is challenging to choose a sustainable method due to the insufficient recognition from extreme structural heterogeneity of these wastes based on their types. To this end, a thorough study on the structural characterization of PCBs using different analyses and metal speciation by sequential extraction procedure were performed. Understanding these information is an essential step in order to choose efficient methods to maximize selective recycling of metals and minimize environmental implications. PCBs were found to be potent metallic reservoirs after all metal content of PCBs were precisely measured. The structural analysis results of the sample included identification of different phases, functional groups, 45.1 % of the crystalline and 54.9 % of amorphous, the mesoporous nature (pore diameter mean ∼ 7.24 nm), hydrophobic property (contact angle ∼93.4°), the positive ζ-potential of particles at pH < (isoelectric point ∼5.4) and vice versa, and the particle size that were not oversized. The metal speciation outcome indicated over 80 % of the total content of elements such as Si, Sn, Ag, Au, Sr, Al, Cr, Nd, Ca, Ba, and P was in a solid crystal structure/residual fraction, which were hard recycled. The assessment of contamination levels of waste indicated the considerable contamination for the environment at global contamination factor ∼27.7, the moderate ecological risk at potential ecological risk assessment ∼82.9, and threats to public health. In addition, the metals of Pr, Mn, and Zn pose high risks because of their risk assessment code values of 42.7 %, 36.7 %, and 33.1 %, respectively. Leaching tests proved Waste Extraction Test was an aggressive method. ANC4 proposed high level of H⁺ consumption are required for metal leaching in future works.

Highly efficient and selective extraction of gold by reduced graphene oxide

Materials capable of extracting gold from complex sources, especially electronic waste (e-waste), are needed for gold resource sustainability and effective e-waste recycling. However, it remains challenging to achieve high extraction capacity and precise selectivity if only a trace amount of gold is present along with other metallic elements . Here we report an approach based on reduced graphene oxide (rGO) which provides an ultrahigh capacity and selective extraction of gold ions present in ppm concentrations (>1000 mg of gold per gram of rGO at 1 ppm). The excellent gold extraction performance is accounted to the graphene areas and oxidized regions of rGO. The graphene areas spontaneously reduce gold ions to metallic gold, and the oxidized regions allow good dispersibility of the rGO material so that efficient adsorption and reduction of gold ions at the graphene areas can be realized. By controlling the protonation of the oxidized regions of rGO, gold can be extracted exclusively, without contamination by the other 14 co-existing elements typically present in e-waste. These findings are further exploited to demonstrate recycling gold from real-world e-waste with good scalability and economic viability, as exemplified by using rGO membranes in a continuous flow-through process.

Biofibrous nanomaterials for extracting strategic metal ions from water

Strategic metals play an indispensable role in the related industries. Their extraction and recovery from water are of great significance due to both their rapid consumption and environmental concern. Biofibrous nanomaterials have shown great advantages in capturing metal ions from water. Recent progress in extraction of typical strategic metal ions such as noble metal ions, nuclear metal ions, and Li-battery related metal ions is reviewed here using typical biological nanofibrils like cellulose nanofibrils, chitin nanofibrils, and protein nanofibrils, as well as their assembly forms like fibers, aerogels/hydrogels, and membranes. An overview of advances in material design and preparation, extraction mechanism, dynamics/thermodynamics, and performance improvement in the last decade is provided. And at last, we propose the current challenges and future perspectives for promoting biological nanofibrous materials toward extracting strategic metal ions in practical conditions of natural seawater, brine, and wastewater.

Managing e-waste from a closed-loop lifecycle perspective: China's challenges and fund policy redesign

E-waste is one of the fastest growing streams of solid waste globally, and its effective management has become a focused issue, which requires a deep understanding of the core guiding theory of extended producer responsibility (EPR). Over the past 20 years, China, one of the world's largest producers of electrical and electronic equipment (EEE), has made great efforts to improve e-waste management along with the massive generation of e-waste. In 2012, China implemented a unique EPR-based e-waste fund policy. However, the fund policy is unsustainable due to the challenges of non-closed resource use, informal recycling, and fund imbalance. Beginning with an overview of these challenges, this paper focuses on redesigning the fund policy from a closed-loop lifecycle perspective in order to maintain a balanced development of the resource use loop and the fund system in China's ten-year plan. In doing so, two EPR instruments, recycling content standards and consumer-oriented deposits, are added to the current fund policy. Subsequently, three extension scenarios alternately changed a critical parameter of the model to test the impact on sustainable capabilities. In this way, the sustainable supply of funds and secondary resources for the e-waste industry can be established in China and effectively demonstrate solid waste management in developing countries.

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.

Iodine-Catalysed Dissolution of Elemental Gold in Ethanol

Gold is a scarce element in the Earth's crust but indispensable in modern electronic devices. New, sustainable methods of gold recycling are essential to meet the growing eco-social demand of gold. Here, we describe a simple, inexpensive, and environmentally benign dissolution of gold under mild conditions. Gold dissolves quantitatively in ethanol using 2-mercaptobenzimidazole as a ligand in the presence of a catalytic amount of iodine. Mechanistically, the dissolution of gold begins when I2 oxidizes Au0 and forms a [AuI I2 ]- species, which undergoes subsequent ligand-exchange reactions and forms a stable bis-ligand AuI complex. H2 O2 oxidizes free iodide and regenerated I2 returns back to the catalytic cycle. Addition of a reductant to the reaction mixture precipitates gold quantitatively and partially regenerates the ligand. We anticipate our work will open a new pathway to more sustainable metal recycling with the utilization of just catalytic amounts of reagents and green solvents.

Leaching of Copper Contained in Waste Printed Circuit Boards, Using the Thiosulfate-Oxygen System: A Kinetic Approach

The present work is related to the treatment of crushed waste of printed circuit boards (WPCBs) from electrical and electronic devices (WEEE), carrying out the recovery of copper in solution. In the first stage, the studied material was characterized by AAS, SEM-EDS, and XRD. The results revealed significantly high amounts of copper (744.42 mg/g), compared with the rest of the metals present in the sample, mainly iron and zinc. In the second stage of the work, alkali dynamic leaching experiments were carried out in the S2O3−2− O2 medium, evaluating important kinetic variables in order to verify the controlling step of the system and adjust the data to a kinetic model. According to the results obtained from the various experimental tests executed, it was found that in the studied system of S2O3−2− O2, the leaching of copper was preferably adjusted to the model of spherical particles with a shrinking core finding a mixed chemical−diffusive control, with values of Ea = 25.78 kJ/mol and n = 0.22 (for the leaching reagent), indicating that the reaction was controlled by the oxygen transport to the solid−liquid interface and also by the chemical reaction in the surface of particles, obtaining up to 99.82% copper in solution.

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.

Optimization of elemental recovery from electronic wastes using a mild oxidizer

In this work, metals were recovered from electronic wastes under optimized conditions. The columnar extraction was used to increase the contact between the leachate solution and solid-state wastes. Industrial metals were recovered by an electrochemical process using a regenerated mild oxidizer under optimized operating parameters to enrich the metal concentrations and reduce waste generation. The maximum recovery rate (1.135 mg·min^-1) was recorded under the optimized conditions (160 A·m^-2 current density, 7 mL·min^-1 leachate flow rate, and 0.8 mol·L^-1 ferric concentration). The selective columnar extraction process was employed to extract gold, wherein the highest extraction efficiency (69.39%) was obtained under optimized conditions of 0.7 mol·L^-1 thiourea, 0.6 mol·L^-1 hydrochloric acid, 0.8 mol·L^-1 ferric chloride, 120 min circulation time, and 6 mL·min^-1 leachate flow rate. The adsorption process was used for the recovery of gold, which was investigated under the kinetic as well as equilibrium adsorption processes. The adsorption curves conformed to the Langmuir model and followed the first-order kinetics. The adsorption rate decreased with the increasing values of pH, temperature, adsorbent size, while the rate increased with the stirring speed and adsorbent quantity. Finally, acidic extraction under anaerobic and optimal conditions was performed to extract and selectively recover rare-earth elements. The rare-earth elements were initially precipitated in their sulfate forms and subsequently transformed into corresponding hydroxides and oxides. The total recovery efficiencies for cerium and neodymium were found to be 91.7% and 86.7%, respectively.

Characterization of top 100 researches on e-waste based on bibliometric analysis

With rapid development of energy, information, and communication technology, e-waste problem has become one of the global issues to be settled urgently. The main features on publication years, journals, countries and institutions, authors, keywords, and content types of the 100 most-cited articles on e-waste had been unfolded in this research. The direction and way forward were illustrated, the trends to date were demonstrated, and the terrain and pathways were evaluated on the research of e-waste. Bibliometric analysis method was applied to analyze various attributes on the 100 most-cited articles which were retrieved from WoSCC on May 25, 2021, by utilizing the software tools Microsoft Excel 2016 and VOS viewer 1.6.9. The publication year and citation number of the 100 articles ranged between 2003 and 2017 and from 83 to 925, respectively. Environmental Science & Technology (n=17) published the maximum articles. Waste Management, Journal of Hazardous Materials, and Environmental Science & Technology were the core journals on e-waste. One hundred twenty-three institutions and 25 countries were involved in publishing the 100 articles. Three hundred seventy authors contributed to the 100 articles in total. A total of 267 keywords occurred in the 100 articles. The keywords "e-waste" and "recycling" held the highest occurrences. The study content of the 100 articles could be classified into four types including the characteristic-and-property type, the environment-and-health type, the management-and-economic type, and the technique-and-processing type. Overall completeness and applicability of the evidence found in this study were verified sufficiently; the potential biases in the review process were also considered. The innovations of the research from the past bibliometric analysis work on e-waste were stated, and the implication for practice and research of this study were explained as well. 2007, 2008, and 2009 were a peak of the researches on e-waste, while the recent years were experiencing a valley. China and its institutions were most influential in this field on e-waste. India was becoming more and more influential on e-waste research in the world. Nigeria was the research center in Africa, and Brazil was the research center in Latin America. Wong Minghung was the most important expert on e-waste. The impact on environment and human being's health was the hot topic of researches on e-waste; the characteristic and property of e-waste were studied not enough. The researches of technique and processing would be the direction and way forward in the study field on e-waste. The characteristic and property on e-waste would need more attention to be researched. The researchers could develop new pathways based on and beyond the four types of content evaluated in this research.

Renewable redox couple system for sustainable precious metal recycling from e-waste via halide-regulated potential inversion

Precious metal (PM) retrievement from e-waste is of great significance for reducing virgin mining activity and promoting rare resource sustainability. However, current PM recycling methods rely mainly on caustic aqua regia or unstable sulfur-based ligand, which has caused severe environmental damage and process inefficiency. Here, we propose an environmentally friendly halide-regulated strategy, utilizing milder and renewable oxidant-cupric/ferric ion for facile PM dissolution. This is realized by the synergistic effect of enhanced oxidizing ability of Cu(II) and reduced oxidation potential of PM with halide addition. Electrochemical tests and leaching experiment results show that Cu(II)/Cu(I) redox potential experiences great change with bromide, increasing from 0.4 to 0.75 V. Fast corrosion feature was observed for Au in Cu(II)/Fe(III)-Br^- and Pd in Cu(II)/Fe(III)-Cl^-, and it can be accelerated by increasing oxidant and halide concentration. Our proposed strategy outperforms traditional methods with stable and fast dissolution, where 2.5 mol/L Br^- is appropriate for Au dissolution. Moreover, selective dissolution of base metal, Pd/Ag, and Au can be achieved via ligand alteration and be further combined with electrodeposition technique for multi metal recovery and oxidant regeneration. This halide-regulated strategy can lead PM recycling from pollutive status towards environmentally friendly road.