Recycling and recovery of metals and other components present in electronic and electrical waste or E-waste has become a major concern in terms of environment and socio-economic aspects. Approximately, 53.6 million metric tons (Mt) of E-waste was generated in 2019 and is expected to reach 74 Mt. in 2030 (Al-Salem et al., 2022). About 9.3 Mt. of E-waste has been reported as collected and recycled formally which is 17.4 % of the total amount of E-waste generated (Forti et al., 2020; Singh and Ogunseitan, 2022). Management of E-waste has become the most significant challenge across the world as it is made up of >100 various ‘hazardous’ or ‘non-hazardous compounds which include organics materials (glass-fibre, flame retardants, polymers), non-organics materials (ferrous metals and non-ferrous metals), and ceramics (Kaya, 2016; Shittu et al., 2021). Therefore, disposal of E-waste directly in the environment causes environmental and human health risks (Mohammed et al., 2013; Ankit et al., 2021; Parvez et al., 2021; Dutta et al., 2022). Improper E-waste recycling practices in the developing countries resulted in the generation of toxic gases, such as dioxin and furan (polychlorinated dibenzo-p-dioxins, and dibenzofurans (PCDD/Fs), and hazardous components comprising heavy metals, such as lead (Pb), cadmium (Cd), chromium (Cr), mercury (Hg), polychlorinated biphenyls (PCBs), polychlorinated diphenyl ethers (PCDEs), etc. (Song and Li, 2014; Awasthi et al., 2019). In view of the overall demand for valuable metals (base, precious, and rare earth metals (REMs)) for the production of new EEEs, E-waste has been considered as a secondary source (Vats and Singh, 2015; Maneesuwannarat et al., 2016; Debnath et al., 2018; Pourhossein and Mousavi, 2018). The lower recycling rates also show that significant amounts of REMs precious and base metals, and other high-value recoverable critical raw materials that were present in E-waste, valued at about US $57 billion, were either landfilled, burned, or processed using crude recycling techniques (Sengupta et al., 2022).
E-waste has been recycled mostly using mechanical and chemical processes for the last few decades (Pant and Singh, 2013; Kaya, 2016). For proper E-waste treatment, several studies are carried out which include physical, chemical, and biological processes. Presently, the hybrid of biological with the chemical process for metal recovery from E-waste is in high demand as it has been considered as a green technology (Kim et al., 2016). Each of these recycling processes has limitations like high capital costs in mechanical processes, and chemical process which leads to the generation of toxic gases and liquid waste (Arshadi and Mousavi, 2014; Hong and Valix, 2014; Baniasadi et al., 2019; Rozas et al., 2019). Thus, it is the utmost requirement at present to emphasize cost-effective methodology for the management of E-waste in a scientific way ensuring environmental protection, safe human health finally leading to the economic growth of the country.
Although several research and review papers are published regarding E-waste recycling technologies, yet there are only few publications comprising all the E-waste technologies (pyrometallurgy, hydrometallurgy, bioleaching and/or biometallurgy), their advantages, disadvantages, socio and techno-economic aspects. This review has summarized the major publications in last 5 years which provided an idea about the most promising technology for E-waste recycling. On comparing all the technologies, biometallurgy has been considered as a green technology and hence different processes involved in biometallurgy have been explained comprehensively. Along with the description of the processes, the circular economic framework has also been incorporated with the socio-economic and techno-economic analyses. The gaps and limitation of different technologies have been identified and potential future perspectives are discussed.
E-wastes comprise a heterogenous mixture of metals, glass, plastics, and ceramics. It contains precious metals (gold (Au), silver (Ag), palladium (Pd), and platinum (Pt)), and; hazardous materials (Pd, Cd, Cr, Hg, arsenic (As), nickel (Ni), copper (Cu), cobalt (Co), lithium (Li), etc.); non-hazardous materials (iron and steel) as well as REMs (neodymium, praseodymium, tantalum, and indium) (Wath et al., 2010; Dasgupta et al., 2014). The presence of various valuable metals in E-waste have been summarized in Table 1.
The plethora of toxic metals and other materials present in E-waste causes adverse effects (directly or indirectly) on the entire biota. Release of acids, heavy metals, lethal chemicals and compounds can be explained under the direct impacts, whereas, indirect impact comprises the heavy metal biomagnification concept. In the E-waste informal sector, the workers including women and children are exposed to the toxic metals and these contaminants get stored in fatty acids, resulting in health complications such as risk of causing cancer, DNA damage, etc. (Li and Achal, 2020; Dutta and Goel, 2021). Direct disposal of E-waste and improper recycling activities leads to air, soil, surface water, and ground water contamination as shown in Fig. 1. Studies are present which reported the adverse effect of E-waste on human health and on the environment (Sankhla et al., 2016; Fu et al., 2018; Li and Achal, 2020; Rautela et al., 2021; Dutta and Goel, 2021; Dutta et al., 2021a, Dutta et al., 2022).
This review consolidated the methodologies adapted for recovering metals and managing E-waste. In view of the limitations of different processes, an environmentally sound E-waste management practice has been identified which can provide potential solutions, and highlight the importance of advanced E-waste recycling technologies.
A bibliometric study was carried out in VOSviewer software version 1.6.18 with help of scopus database using different keywords such as i) E-waste, bioleaching, ii) E-waste, bioremediation, and iii) E-waste, circular economy. The authors, abstract, keywords, document title, and references for the data analysis, synthesis, and interpretation are all included in the information of the publications that were taken from the SCOPUS platform. A cluster analysis of the literature is particularly possible when all the data from the bibliometric database are analysed and the co-occurrence of text data is investigated (Bhattacharyya et al., 2022). The data obtained have been shown in Fig. 2. (a), (b) and (c), it can be seen that the minimum number of documents of authors (2 clusters) using keywords (a), (b), and (c) were 97, 88, and 109 respectively. Also, the density map of countries showed that the maximum number of documents were published in countries like India, China, Australia, etc. Moreover, the density map of keywords (E-waste, circular economy) was generated with a co-occurrence >2 cluster, including 806 keywords in the map. Whereas, the density map of keywords (E-waste, bioremediation) and (E-waste, bioleaching) were generated with a co-occurrence >2 cluster, including 649 and 526 keywords respectively, in the map.
Based on the bibliometric study, a holistic picture of the publications and their authors, countries and different keywords related to E-waste have been represented in the present study which would help in identifying the suitable processes for future research studies on recovery of metals from E-waste.
