Microbial Bioleaching of Ag, Au and Cu from Printed Circuit Boards of Mobile Phones

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.

Composition and recycling of smartphones: A mini-review on gaps and opportunities

After more than a decade since smartphones became consolidated in the market, many recycling solutions have been proposed to deal with them. To continue developing useful solutions and enable adjustment of routes, this mini-review aims to analyse the current research scenario, presenting relevant gaps, trends and opportunities. From a structured searching and screening procedure, a vast source of data was arranged and is available to extract useful information (43 studies on composition and 93 studies on recycling). The study provides discussions about the history of smartphone development, constituent materials and recycling methods for different components, comparisons between feature phones and smartphones and others. Among some conclusions, the authors highlight the lack of studies on pre-extractive methods, green chemistry, recovery of critical and precious metals, determination of priority materials for recovery and solutions for entire devices. In the end, a list containing six research gaps for composition studies and seven research gaps for recycling studies is provided and may be seen as opportunities for future research.

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.

Microstructure and properties analysis of the brazing alloy prepared from recycled E-waste

In order to realize the efficient and comprehensive utilization of e-waste resources and short process preparation of alloy brazing materials, this study has analyzed the microstructure and properties of e-waste recycled brazing alloys by the analysis methods of inductively coupled plasma emission spectrometer, differential scanning calorimeter, scanning electron microscope, metalloscope, X-ray diffractometer, micro-hardness tester. Experimental results showed that phase compositions are significant differences between the alloys prepared by the recycled e-waste and the pure metals. The circuit board recycling alloy mainly consisted of α-Fe dendrites, (Cu, Sn) phases, Sn-rich phases and Cu matrix, while the alloy obtained by pure metals is composed of (Cu, Sn) phase, Sn-rich phase and Cu matrix. The melting temperature of alloy obtained by melting the circuit board is in the range of 985.3°C–1,053.0°C, which was wider and higher than that of alloy obtained by pure metal smelting. The shear strengths of the joints brazed by the brazing alloys prepared by the recycle e-waste and pure metals are 182.21 MPa and 277.02 MPa, respectively. There is little difference in hardness between the two types of brazed joints. In addition, there are a large number of precipitated phases in alloy obtained by the recycled circuit board, owing to the precipitation strengthening mechanism. The main strengthening mechanism of alloy obtained by pure metals is solid-solution strengthening. The paper focused primarily on alloy obtained by melting the circuit board and studying the specific composition, melting temperature, structure, and properties of alloys formed by melting the circuit board and pure metals. Meawhile, the size, morphology and other microstructure evolution of the second phase of brazing alloy were investigated to provide theoretical guidance for the brazing alloy in the subsequent actual production process.

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.

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.

Electrochemical oxidation of copper-clad laminate for manufacturing printed circuit boards via bioleaching by the fungus Phanerochaete chrysosporium

The leaching and electrochemical oxidation of the copper-clad laminate for manufacturing printed circuit boards were investigated in systems with and without the fungus P. chrysosporium, which yielded the copper-leaching efficiencies of 54% and 7.0%, respectively. In particular, the formation of a biofilm on the electrode surface reduced the open-circuit potential and increased the corrosion level, and the degree of increase and the rate of change of the current density in the fungal leaching system were higher than those of the sterile system. In addition, the cyclic voltammetry curves showed oxidation peaks that correspond to the oxidation of Cu to Cu2+. Further, for the fungal leaching system, the peak potential was highly negative and the curve area and peak current density were relatively high. Moreover, the electrochemical polarization parameters and the impedance characteristics were affected by the fungus, and the leaching systems were controlled by charge transfer and diffusion. In summary, P. chrysosporium can accelerate the leaching of copper as a result of the formation of extracellular electron transfer-induced microbiologically influenced corrosion (EET-MIC) and metabolite-induced microbiologically influenced corrosion (M-MIC). The enzymes and organic acids, which act as fungal metabolites, participate in the leaching of copper.

Bio-Based Processes for Material and Energy Production from Waste Streams under Acidic Conditions

The revolutionary transformation from petrol-based production to bio-based production is becoming urgent in line with the rapid industrialization, depleting resources, and deterioration of the ecosystem. Bio-based production from waste-streams is offering a sustainable and environmentally friendly solution. It offers several advantages, such as a longer operation period, less competition for microorganisms, higher efficiency, and finally, lower process costs. In the current study, several bio-based products (organic acids, biomethane, biohydrogen, and metal leachates) produced under acidic conditions are reviewed regarding their microbial pathways, processes, and operational conditions. Furthermore, the limitations both in the production process and in the scale-up are evaluated with future recommendations.

Photocatalytic Performance of Carbon-Containing CuMo-Based Catalysts under Sunlight Illumination

Carbon-doped nanostructured CuMo-based photocatalysts were prepared by solvothermal synthesis. Two thermal treatments—oxidative and inert atmosphere—were used for the synthesis of the catalysts, and the influence of spherical carbon structures upon the crystalline phases on the photocatalytic activity and stability was studied. XRD showed the catalysts are nanostructured and composed by a mixture of copper (Cu, Cu2O, and CuO) and molybdenum (MoO2 and MoO3) crystalline phases. The catalysts were used for the degradation of yellow 5 under solar light. A remarkable leaching of Mo both in dark and under solar irradiation was observed and quantified. This phenomenon was responsible for the loss of photocatalytic activity for the degradation of the dye on the Mo-containing series. Conversely, the Cu-based photocatalysts were stable, with no leaching observed after 6 h irradiation and with a higher conversion of yellow 5 compared with the Mo- and CuMo series. The stability of Cu-based catalysts was attributed to a protective effect of spherical carbon structures formed during the solvothermal synthesis. Regarding the catalysts’ composition, sample Cu4-800-N2 prepared by pyrolysis exhibited up to 4.4 times higher photoactivity than that of the pristine material, which is attributed to a combined effect of an enhanced surface area and micropore volume generated during the pyrolytic treatment due to the presence of the carbon component in the catalyst. Scavenger tests have revealed that the mechanism for tartrazine degradation on irradiated Cu-based catalysts involves successive attacks of •OH radicals.

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

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

Extraction of valuable metals from discarded AMOLED displays in smartphones using Bacillus foraminis as an alkali-tolerant strain

With the alarming rate of e-waste generation, resource recovery from secondary metal sources is essential for sustainable resource utilization and to prevent the release of potentially toxic elements into the environment. In the current study, the first-time extraction of Ag, Mo, and Cu from active-matrix organic light-emitting diode (AMOLED) screens of discarded smartphones have been achieved using organic acids produced by Bacillus foraminis cultured on a modified Horikoshi medium. The influences of initial pH, inoculation size, and pulp density on the bioleaching process were evaluated over six-day experiment. Maximum extraction of Ag, Mo, and Cu (100, 56.8, and 41.4%) at optimal values of three investigated factors was obtained over a 12-day bioleaching experiment. A diverse assemblage of organic acid was produced in the optimized bioleaching condition, including tartaric (12.1 mM), formic (49.8 mM), acetic (21.5 mM), lactic (78.5 mM), citric (2.7 mM), and propionic (69.6 mM) acid. The contact angle analysis highlighted more hydrophobicity of powder after the bioleaching. FTIR and CHNO data also confirmed the role of bioleaching in the powder wettability alteration. The sequential extraction method revealed high mobility of In, Fe, Co, Cu, Cr, and Mo and low mobility of Ag. The results exhibited high tolerance of alkali-tolerant bacteria to potentially toxic elements and its superior performance in the bioleaching of discarded mobile screens at high pulp density.

Fungal Tolerance: An Alternative for the Selection of Fungi with Potential for the Biological Recovery of Precious Metals

The behavior of various filamentous fungi in the presence of metals such as Cu, Zn, Ni, Fe, Mn, and V has been widely reported. However, there is little information regarding metals such as Au, Ag and Pt that are not in the form of nanoparticles. The growth of eight filamentous fungi was evaluated at increasing doses of Au, Ag and Pt. The fungi were reactivated in Petri dishes with potato dextrose agar. Subsequently, individual mycelial disks from each strain were inoculated in PDA plates with the following doses of AuCl3, Ag2SO4 and PtCl4: 0, 50, 150 and 300 mg L−1, respectively. The plates were then incubated for 20 days—a period in which the diameter of the colony was measured every 24 h. Au showed the highest toxicity for the tested fungi. All silver doses decreased the growth of most of the fungi, while platinum did not cause any inhibitory effect on the growth of the eight tested fungi. With a simple test, it was possible to observe the effect of precious metals (PMs) on the growth of filamentous fungi and consider their possible biotechnological applications in the recovery of PMs from primary or secondary sources.

Microorganisms and Plants in the Recovery of Metals from the Printed Circuit Boards of Computers and Cell Phones: A Mini Review

Most electrical and electronic equipment contain a printed circuit board (PCB), which is the board on which microelectronic components are mounted. The PCBs of obsolete and discarded electrical and electronic equipment are a material of great value due to their high metal content that is of commercial importance (i.e., Au, Ag, Pd, Pt, Ir, Ti, Ge, Si, Al, Cu, Ni, Zn, Fe, Sn, As, and Pb). Hydrometallurgical and pyrometallurgical methods have been used to extract metals from PCBs; however, these methods have energy and environmental disadvantages, which is why in recent years sustainable alternatives have been sought. Among these alternatives are the biological methods that contemplate the use of microorganisms and plants to recover metals from PCBs. In this review, only studies specifying the use of bacteria, fungi, and plants in the recovery of metals from the PCBs of computers and cell phones were considered, since the metallic composition of these plates varies according to the electronic equipment. In addition, the challenges and recommendations for these biotechnological processes to be improved and implemented at the industrial level in the coming years are discussed.

Sorption Behaviors of Light Lanthanides(III) (La(III), Ce(III), Pr(III), Nd(III)) and Cr(III) Using Nitrolite

The sorption of light lanthanides(III) (La(III), Ce(III), Pr(III), Nd(III)) and chromium(III) ions from acidic solutions on Nitrolite was studied at varying ions concentrations, pH, contact time and temperatures. The sorption capacity of lanthanides(III) and chromium(III) ions were examined in the ranges 2–9 and 2–5, respectively. The adsorption capacities of all metals are increase with the increasing pH (up to initial pH 9), despite the potential precipitation of metals at higher pH values. Therefore, an initial pH 9 of lanthanides gives the highest adsorption capacities. The kinetics of sorption chromium(III) and light lanthanides(III) were investigated. The experimental data were analyzed using the pseudo-first-order, pseudo-second-order forms, Elovich, and intra-particle diffusion models. The sorption kinetics of investigated ions was described by pseudo-second-order model the best. The results indicate the endothermic process of Cr(III), La(III), Ce(III), Pr(III) and Nd(III) ions sorption. The sorption capacities of La(III) 4.77 mg/g, Ce(III) 4.45 mg/g, Pr(III) 4.30 mg/g, Nd(III) 4.13 mg/g and Cr(III) 2.39 mg/g were calculated from the Langmiur model, which describes adsorption better than Freundlich and Dubinin–Radushkevich.

Recent advances in the recovery of metals from waste through biological processes

Wastes containing critical metals are generated in various fields, such as energy and computer manufacturing. Metal-bearing wastes are considered as secondary sources of critical metals. The conventional physicochemical methods of metals recovery are energy-intensive and cause further pollution. Low-cost and eco-friendly technologies including biosorbents, bioelectrochemical systems (BESs), bioleaching, and biomineralization, have become alternatives in the recovery of critical metals. However, a relatively low recovery rate and selectivity severely hinder their large-scale applications. Researchers have expanded their focus to exploit novel strain resources and strategies to improve the biorecovery efficiency. The mechanisms and potential applicability of modified biological techniques for improving the recovery of critical metals need more attention. Hence, this review summarize and compare the strategies that have been developed for critical metals recovery, and provides useful insights for energy-efficient recovery of critical metals in future industrial applications.

Adaptive Evolution of Sphingobium hydrophobicum C1T in Electronic Waste Contaminated River Sediment

Electronic waste (e-waste) has caused a severe worldwide pollution problem. Despite increasing isolation of degradative microorganisms from e-waste contaminated environments, the mechanisms underlying their adaptive evolution in such habitats remain unclear. Sphingomonads generally have xenobiotic-degrading ability and may play important roles in bioremediation. Sphingobium hydrophobicum C1^T, characterized with superior cell surface hydrophobicity, was recently isolated from e-waste contaminated river sediment. To dissect the mechanisms driving its adaptive evolution, we evaluated its stress resistance, sequenced its genome and performed comparative genomic analysis with 19 other Sphingobium strains. Strain C1^T can feed on several kinds of e-waste-derived xenobiotics, exhibits a great resistance to heavy metals and possesses a high colonization ability. It harbors abundant genes involved in environmental adaptation, some of which are intrinsic prior to experiencing e-waste contamination. The extensive genomic variations between strain C1^T and other Sphingobium strains, numerous C1^T-unique genes, massive mobile elements and frequent genome rearrangements reflect a high genome plasticity. Positive selection, gene duplication, and especially horizontal gene transfer drive the adaptive evolution of strain C1^T. Moreover, presence of type IV secretion systems may allow strain C1^T to be a source of beneficial genes for surrounding microorganisms. This study provides new insights into the adaptive evolution of sphingomonads, and potentially guides bioremediation strategies.