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

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

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

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

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

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.

Eco-friendly approach for enhancing the floatability of non-metallic components in waste printed circuit boards: Adding gutter oil during dry grinding

Waste printed circuit boards (WPCBs) are an attractive secondary resource that is challenging to dispose of due to its complexity. Reverse flotation is an effective method to remove non-metallic particles (NMPs) to obtain metals from WPCBs. Nevertheless, the removal of NMPs is usually inadequate in the present flotation practice. Thus, to provide a clean approach to improve the removal efficiency of NMPs, the method of adding gutter oil during dry grinding process was adopted to enhance the hydrophobic sites on the surface of NMPs to improve the floatability. The surface morphology of NMPs was analyzed by SEM, the results show that the rough morphology inhibited the adhesion of bubbles, while water occupied the cracks and pores, making it challenging for collector adsorption, which result in unstable particle-bubble adhesion. The results of FTIR indicate that both NMPs and gutter oil have -CH3, -CH2, -C = O, -C-O functional groups, which promotes the adsorption of gutter oil on the surface of NMPs. The contact angle (CA) results show that the adsorption of gutter oil on the particle surface is conducive to the formation of enhanced CA. Furthermore, the flotation enhancement effect was verified by flotation kinetic experiments. The accumulated floats yield of NMPs conditioned by gutter oil during grinding is increased from 67.05% (NMPs without conditioning) to 95.02%, and the resin recovery is increased by 31.10%. It is demonstrated that dry grinding with gutter oil can strengthen the floatability of NMPs, which provides a potential approach to increase the flotation efficiency of WPCBs.

Urban mining from biomass, brine, sewage sludge, phosphogypsum and e-waste for reducing the environmental pollution: Current status of availability, potential, and technologies with a focus on LCA and TEA

Rapid industrialization, improved standards of living, growing economies and ever-increasing population has led to the unprecedented exploitation of the finite and non-renewable resources of minerals in past years. It was observed that out of 100 BMT of raw materials processed annually only 10% is recycled back. This has resulted in a strenuous burden on natural or primary resources of minerals (such as ores) having limited availability. Moreover, severe environmental concerns have been raised by the huge piles of waste generated at landfill sites. To resolve these issues, 'Urban Mining' from waste or secondary resources in a Circular Economy' concept is the only sustainable solution. The objective of this review is to critically examine the availability, elemental composition, and the market potential of the selected secondary resources such as lignocellulosic/algal biomass, desalination water, sewage sludge, phosphogypsum, and e-waste for minerals sequestration. This review showed that, secondary resources have potential to partially replace the minerals required in different sectors such as macro and microelements in agriculture, rare earth elements (REEs) in electrical and electronics industry, metals in manufacturing sector and precious elements such as gold and platinum in ornamental industry. Further, inputs from the selected life cycle analysis (LCA) & techno economic analysis (TEA) were discussed which showed that although, urban mining has a potential to reduce the greenhouse gaseous (GHG) emissions in a sustainable manner however, process improvements through innovative, novel and cost-effective pathways are essentially required for its large-scale deployment at industrial scale in future.

Integration of electronic waste management: a review of current global generation, health impact, and technologies for value recovery and its pertinent management technique

The fast evolution of waste electrical and electronic equipment (WEEE) has developed into a prime environmental perturb in recent days. Today, electrical and electronic products merely become a needed part of people life's and professional lives. The whole process of e-waste contains an organized collection system, appropriate dismantling, and its treatment of recycling. The unparalleled hastening of e-waste and unceremonious discarding lead to an adverse impact on a country's development. Currently, challenges in e-waste have a lack of practical aid, poor structure, and insufficient economic support. Several legislations have been imposed which aim to enhance the handling of e-waste. Operative management of e-waste is now essential for the protective atmosphere and human beings as well. This article provides the systemic flow of the e-waste definition, global information, and generation of e-waste and composition of e-waste which were discussed. The study categorized the hazardous effect of e-waste on human beings, and the content analysis of e-waste in recent LCA applications was highlighted. Further different metal extraction and recovery techniques from e-waste have been reviewed. A few sets of current practices and some recommendations on a global scale level were provided. Finally, based on analysis, some approaches to e-waste was accomplished, and equitable environmental management was taken into account to identify the future outlook areas.