A review of the recent development, challenges, and opportunities of electronic waste (e-waste)

This study reviews recent developments, challenges, and the prospect of electronic waste (e-waste). Various aspects of e-waste, including collection, pre-treatment, and recycling, are discussed briefly. It is found that Europe is the leading collector of e-waste, followed by Asia, America, Oceania, and Africa. The monetary worth of e-waste raw materials is estimated to be $57.0 billion. However, only $10.0 billion worth of e-waste is recycled and recovered sustainably, offsetting 15.0 million tonnes (Mt) of CO2. The major challenges of e-waste treatment include collection, sorting and inhomogeneity of waste, low energy density, prevention of further waste, emission, and cost-effective recycling. Only 78 countries in the world now have e-waste related legislation. Such legislation is not effectively implemented in most regions. Developing countries like south-eastern Asia and Northern Africa have limited or no e-waste legislation. Therefore, country-specific standards and legislation, public awareness, effective implementation, and government incentives for developing cost-effective technologies are sought to manage e-waste, which will play an important role in the circular economy.


Introduction
New products of information and communications technology (ICT) and other e-products are being introduced continuously into the current market, while previous products are swiftly becoming obsolete (Awasthi and Li 2017). The amount of e-waste produced is, therefore, rapidly increasing every year. E-waste is primarily described as the waste generated by all parts and items of electronic and electrical equipment (EEE) that have been discarded without the intention of being reused (Baldé et al. 2016). It is also known as waste electrical and electronic equipment (WEEE) and e-scrap in different parts of the world (Mary and Meenambal 2016). E-waste encompasses a diverse range of electronic devices, such as telecommunications and information technology equipment, large household items, lighting equipment, automatic dispensers, medical devices, monitoring and control devices, as well as consumer electronics, including electronic and electrical tools, sports and leisure equipment, toys, mobile phones, and computers (Huang et al. 2014;Chen et al. 2015a).
Advanced lifestyle, technological advancement, and worldwide economic progress have led to the ever-increasing generation of e-waste, leading to a significant environmental Editorial responsibility: S. Mirkia. and health concern (Matsakas et al. 2017;Bhuiya et al. 2020). Over two billion metric tonnes of waste are produced globally, with e-waste accounting for 53.6 million tonnes (Statista 2010). Global e-waste has been rising very sharply, with a total generation of 53.6 Mt in 2019, increasing by 21% since 2015 (Tiseo 2021). Around 83.0% of the total e-waste generated in 2019 was not documented and hence likely to be burnt openly or dumped illegally (Statista 2010;Balde et al. 2017), which causes a serious threat to human health and the environment. The remaining 17% of e-waste was collected and properly recycled in 2019. In a continent wise comparison of generating e-waste, Asia was the highest generator (46.4% in the world) in 2019, followed by America (24.4%), Europe (22.4%), Africa (5.4%), and Oceania (1.3%) . Although Asia was the highest e-waste generator among the continents, it generated far less waste per inhabitant (5.6 kg/inh) compared to Europe (16.2 kg/inh), Oceania (16.1 kg/inh), and the Americas (13.3 kg/inh), due to a large number of inhabitants (4.40 billion) in the region. However, the lowest e-waste generator was in Africa with 2.5 kg/inh .
Recycling e-waste is a valuable source of raw material as it comprises a wide range of materials like metals, glass, and plastics ( Fig. 1). However, only 17.4% of global e-waste is collected and recycled due to the complexity of collection and recycling . Recycling e-waste is a difficult task because e-waste consists of metals with different physicochemical properties, including hazardous halogens metals (Ilankoon et al. 2018a). If the substances containing these hazardous compounds are not properly disposed of, they may cause an environmental threat (Chen et al. 2015b;Lu et al. 2015;Rucevska et al. 2015). Proper waste management techniques and the safe disposal of e-waste have become a global priority to mitigate human health risks and environmental degradation caused by landfilling (Song and Li 2015).
E-waste recycling processes such as open burning, metal acid stripping, acid baths, and incineration can produce byproducts like heavy metals, furans, and dioxins (Dai et al. 2020). While resource recovery through e-waste can provide employment and commercial prospects, its management may be hindered by inadequate infrastructure and efficient scientific technologies (Lakshmi and Raj 2017). As a result, e-waste management in the twenty-first century is regarded as one of the biggest global challenges, posing serious threats to the environment and human health (Bhutta et al. 2011;Moletsane and Venter 2018;Magalini 2016).
Many researchers have reviewed the literature on e-waste, with some authors focusing on a specific location; for example, Andeobu et al. (Andeobu et al. 2021) investigated the generation of e-waste and environmental management in Asia Pacific countries, while (Meem et al. 2021) studied the environmental and health consequences of electronic waste disposal in Bangladesh. In contrast, (Jaibee 2015) provided an overview of e-waste generation and management in Malaysia. Some authors focused on e-waste management techniques, such as Murthy and Ramakrishna (Murthy and Ramakrishna 2022), highlighted the global best practices in E-waste management, emphasising the importance of policy implementation, technology requirements, and social awareness to achieve a sustainable and circular economy. In contrast, (Abdelbasir et al. 2018) reviewed various techniques for recycling the most important metals from the metallic fractions of e-waste. Ahirwar and Tripathi (2021) and (Doan et al. 2019) proposed some potential solutions for improving e-waste management, reporting the recent global trend in e-waste generation, recycling, and the impact on human health. Gupta  (Ilankoon et al. 2018a) et al. (Gupta et al. 2014) briefly discussed the negative consequences of hazardous components found in e-waste or created during the recycling process. Pérez-Belis et al. (2015) reviewed the literature from 1992 to 2014, focussing on the genesis, composition, management, and final treatment of e-waste.
Most of the reviews on e-waste in the literature have focused on different aspects, which do not represent the actual picture of e-waste management. Therefore, this review briefly outlined e-waste recycling technologies, including collection, pre-treatment, challenges, and prospects. In this review, the most recent data related to e-waste recycling and management have been reported, which will help researchers and policymakers to make informed decisions.

Types of electronic waste
E-waste can be categorised depending on the origin and purpose of electronic items. There are ten different types of e-waste ( Fig. 2) recognised globally (Health and E. Safety 2013;Quinet et al. 2005;Kumar et al. 2018;Faramarzi et al. 2004;European 2012). The average percentage of each category of e-waste can vary depending on many factors, including socio-economic conditions, consumer behaviours, populations, and the dependency of businesses and households on electronic and electrical equipment (EEE) (Balde et al. 2017). Nonetheless, large household items are the leading contributor (42.1%) to e-waste, followed by IT and telecommunication (33.9%), consumer devices (13.7%), and small household equipment (4.7%) (European 2012). Other categories have lesser contributions (< 2% for medical, lighting, electrical, and electronic tools while < 1% for automatic dispensers, toys, sports, monitoring, and control devices) (European 2012) for better readability. In developing countries, e-waste is mainly dominated by TVs, computers, and mobile phones (Needhidasan et al. 2014;Kalia et al. 2021;Nnorom and Osibanjo 2008).

Recycle and recovery of e-waste
Recycling and recovering e-waste are critical aspects of e-waste management that involve economic, environmental, and health-related benefits. E-waste is a resource only if it is recycled and precious materials are recovered. The following sections describe the collection and treatment of e-waste.

Collection
As the first step of e-waste waste management, waste generated by households and businesses is collected (Wath et al. 2010). The second stage of waste management is the pretreatment before they are sent to treatment centres or dumping sites. The collection processes can be one of the four ways: (a) the official take-back system, (b) mixed residual waste, (c) collection outside the official take-back system for countries with developed waste management, and (d) collection outside the official take-back system for countries without developed waste management (Balde et al. 2017;Nnorom and Osibanjo 2008;Alam et al. 2022;Tanskanen 2013;Ilankoon et al. 2018b;Khanna et al. 2020). In the first category, e-waste is collected by the designated collectors or government bodies such as councils or producers under the national e-waste laws (Balde et al. 2017;EPA-NSW 2022;Sthiannopkao and Wong 2013). This is the ideal way to collect e-waste where pre-treatment processes are applied before sending it to state-of-the-art treatment facilities for recovering useful materials.
The second collection category refers to mixed residual waste, where consumers dispose of e-waste in bins with other household or business waste (Balde et al. 2017;Kumar et al. 2017;Otto et al. 2018). The e-waste collected in this process has a low chance of separation and is likely to be burnt in incineration or dumped with other waste. In the third category, e-waste is collected by registered individual waste dealers or companies and then sell the e-waste to recycling companies (Balde et al. 2017). This system is known as the "collection outside the official take-back system". Finally, countries with no e-waste legislation, such as Bangladesh, manage their electronic waste in unorganised ways, such as informal collectors and recyclers with no central planning and control of e-waste (Islam et al. 2016). Table 1 shows e-waste collection, estimated values of useful materials in the waste, and their corresponding collection methods in different regions of the world. In 2019, Europe was the leading collector of e-waste (5.1 Mt, 42.3% of its total e-waste) with an estimated value of $5.48 billion. The second-best collector was Asia, which collected 2.9 Mt (11.7%), followed by Americas 1.2 Mt (9.4%), Oceania  ). The collection method in different regions is primarily evolved by their legislation, e-waste management infrastructures, economic conditions, and public awareness . Most developing countries like south-eastern Asia and Northern Africa, where it is limited or no e-waste legislation, have no record of their e-waste statistics (Balde et al. 2017). Governments in these regions rely on unprivileged and self-employed people to collect and recycle e-waste. These people collect the e-waste from consumers' homes through door-to-door activities, separate the items themselves, and finally sell the separated items to city vendors to be refurbished or recycled (Balde et al. 2017;Islam et al. 2016). Although this option is the worst choice due to its inherent risks to human health and low collection and management, still these types of informal collection activities are being used by many unskilled individuals to earn and lead a basic life. For instance, Bangladesh's informal collection of e-waste was estimated to be around 0.95 Mt/year (Islam et al. 2016). It was also estimated that many smallscale recycling plants in Dhaka city were able to earn up to $3,410.0/month using informal channels (Islam et al. 2016;Masud et al. 2019).

Pre-treatment and recovery treatment methods
After collecting e-waste, it goes through a pre-treatment process before the final processing in the treatment facilities (Wang et al. 2012). The objective of the pre-treatment process is to separate different useable materials from the bulk of the e-waste mix so that they can be transferred to appropriate recovery treatment plants. The pre-treatment process can be three types: (a) manual dismantling, (b) mechanical dismantling and separation, and (c) a combination of both manual and mechanical processes (Batinic et al. 2018). Manual dismantling often separates hazardous and valuable materials such as PCBs, casing, monitors, and batteries, sometimes labour intensive and costly (Zhang and Forssberg 1998). Mechanical pre-treatment comprises separating metals and non-metals, size reduction, shredding, and crushing (Batinic et al. 2018). It is worth noting that if the e-waste is a complex mix of various equipment and items, it is time and cost-effective to use the combined manual and mechanical pre-treatment processes. In developed countries, pre-treatment mostly comprises semi-automatic (manual and mechanical) separation followed by metal recovery in stateof-the-art units (Kaya 2016).
Manual separation is the dominant method in developing countries, followed by metals recovery in small workshops (Oliveira et al. 2012). Low-intensity magnet drums are commonly used to separate ferrous metals from e-waste, while non-ferrous metals are separated from non-metals using electric conductivity base separation tools (Batinic et al. 2018). Other separation techniques, including the gravity method using airflow or water flow tables and sifting, are also commonly used (Oliveira et al. 2012). Once metals are separated from other materials, they are then transferred to the final recovery stage, where two recovering methods are widely used, namely pyrometallurgy and hydrometallurgy (Sahajwalla and Hossain 2020). The pyrometallurgy technique uses high temperatures to purify and extract metals. Some examples of pyrometallurgy are refining, smelting, combustion and incineration (Kang and Schoenung 2005). On the other hand, hydrometallurgy is concerned with extracting metals from concentrated mixtures or a mixture of different materials using aqueous solutions. In a broader sense, pyrometallurgy is commonly used to extract ferrous metals from e-waste, whereas hydrometallurgy is used for recovering non-ferrous metals, e.g. copper, lead, and zinc (Xia et al. 2017).

Challenges and opportunities
There are many challenges and opportunities regarding e-waste and its management. One of the key challenges is the lack of e-waste legislation. Apart from developed nations, most of developing countries do not have e-waste legislation yet. Until 2014, 61 countries covering 44% of the world population had e-waste legislation (Balde et al. 2017). In 2019, 78 countries had their e-waste legislation covering 71% of the world population. Although the most populous countries in Asia, China and India, have now e-waste legislation, other populous countries (population of > 160 million), such as Pakistan and Bangladesh, do not have any e-waste law at this moment (Balde et al. 2017). However, Pakistan has recently banned importing e-waste from other countries, even though there is a substantial doubt whether the prohibition is fully effective (Imran et al. 2017). On the other hand, Bangladesh, the 8th largest country globally by population (Bank 2020), has been aiming to consider such a law in recent times only. Bangladesh's Department of Environment (DoE) has published draft rules (Hazardous waste management rules, 2019) for restricting the use of 15 hazardous substances in some selected EEE products (Wto 2020).
Even though all necessary legislation is in place for e-waste collection and recycling, it is still challenging to handle and process e-waste due to its complex mix of hazardous, precious, base, and other materials. In general, e-waste comprises 40% metal, 30% plastic polymers, and 30% oxides of different materials (Sahajwalla and Hossain 2020). However, these quantities can vary significantly from country to country due to an uneven mix of EEE in the e-waste. E-waste contains precious materials (Ag, Au, and Pd), base materials (Cu, Al, Ni, Sn, Zn, Fe, Bi, Sb, and In), hazardous materials (Hg, Be, Pb, Cd, and As), halogen materials (Br, F, Cl), and plastics, glass, and ceramics (Sahle-Demessie et al. 2018).
The particular challenge of e-waste management is recovering precious, rare earth, and useable materials while the hazardous materials are taken care of. The hazards in waste management include handling hazardous chemicals such as CFC fluids, polychlorinated biphenyls (PCBs), mercury, machinery safety, manual handling of large items, electrical safety, cuts and abrasion risk, and fire and explosion risk (Defra 2006). Other challenges of e-waste management are the limited number of chemical liquids (eco-friendly) that are approved to be used in e-waste management, lack of infrastructures facilities, thermodynamic limits of separating complex mix of materials leading to cost ineffective recovery challenge, financial and policy supports especially for developing countries, and uneven legislations that vary significantly from country to country (Balde et al. 2017;Sahle-Demessie et al. 2018).
On the other hand, the prospect and opportunities of e-waste are paramount if they are recycled properly. As mentioned earlier, e-waste contains precious metals that can be recovered using the urban mining of e-waste. A typical example of such urban mining is that one metric ton of circuit board can produce up to 1.5 kg of gold and 210 kg of copper (Bazargan et al. 2012). The concentration of precious metals is far better than that of the primary mining from ores. For instance, conventional mining of gold from ore contains 5 g/t (Sahle-Demessie et al. 2018), and copper contains 5.25 kg/t (Bazargan et al. 2012). These numbers demonstrate that gold and copper concentrations in urban mining are 300 and 40 times higher than in ores. Recovering these precious metals can generate significant profits if appropriate business models are applied. It was estimated that the money value of materials in e-waste generated worldwide is three times more than the total economic value of the world's silver mining and larger than the GDP of many countries (Forum 2019).
Another prospect of e-waste management is environmental and human health benefits (Rautela et al. 2021). In modern days, human health and environmental issues are considered a priority agenda (Omisore 2018; UN News 2013). In developing countries, however, e-waste management has never been seen as a serious issue, leading to poor and insecure health and the environment around the dumping sites (Masud et al. 2019). However, adopting and implementing e-waste management strategies, policies, and legislation can improve these conditions drastically.

Conclusion
E-waste comprises metal (40%), plastic polymers (30%), and oxides of other materials (30%). The collection, washing, sorting, pre-treatment, and treatment are the key steps for recycling and recovering e-waste. E-waste consists of three major categories: large household items, IT and telecommunication, and consumer and small household equipment. Currently, Europe is the leading collector of e-waste (5.10 Mt, 42.3% of its total e-waste), followed by Asia (2.9 Mt, 11.7%), America (1.2 Mt, 9.4%), Oceania (0.06 Mt 8.8%), and Africa (0.03 Mt, 0.9%). E-waste contains precious materials, base materials, hazardous materials, and other halogens, plastics, glass, and ceramics. Asia is the highest e-waste generator among the continents. However, Asia generates far less waste per inhabitant (5.6 kg/inh) than Europe, Oceania, and America. Unfortunately, 83% of the e-waste generated globally is not documented, meaning the fate of that e-waste is open burning or illegal dumping, leading to a serious threat to the environment and human health. A collective approach from national and international bodies, including public awareness, is crucial to managing e-waste, which will play an important role in the circular economy.
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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