# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 1 Introduction
Due to the scarcity of metal, as well as the environmental and health problems caused by metal pollution, it is imperative to implement comprehensive regulatory frameworks to manage EEW, as most of it is currently processed through unregulated and unsafe recycling practices. This prevalent practice results in a high percentage of EEW ending up in landfills, undergoing manual disposal, acid leaching, open burning, or illegal management and deposit [2; 9; 11]. These disposal methods can release harmful toxins and toxic gases into the environment, polluting the atmosphere, soil, and water resources. Workers in these recycling facilities are exposed to hazardous substances, particulate matter, high noise levels, and other health risks, with low-income countries being the most affected [2; 16; 17]. The transformation of EEW by natural processes is often hindered, leading to inappropriate disposal practices that pose significant environmental and health concerns. Therefore, developing sustainable management strategies for EEW is paramount [2; 16; 18]. Inadequ recycling practices result in the loss of valuable metals contained in EEW, contributing to environmental and human health impacts [9; 11]. Building the necessary infrastructure for effective EEW management presents a significant techno-economic challenge, requiringincentives to transform the processes [19]. The continued use of traditional technologies for metal disposal and recovery necessitates the development of more cost-effective and environmentally friendly management strategies [17]. In the literature, the use of process technology for metal waste management is usually coupled to the metal recovery within the frame of environmental applications and circular economy. This trend is driven by environmental concerns and the massive use of metals in electrical and electronic devices. Consequently, metal recovery serves not only to mitigate environmental issues but also as a valuable secondary resource, offering economic benefits. In the literature, a variety of techniques have been employed for the recovery of metals, including pyrometallurgy, hydrometallurgy, flotation, chemical precipitation, ion exchange, osmosis, and bioleaching, amongst others [10; 20; 21; 22]. Nowadays, the most established technique is pyrometallurgy, which relies on high-temperature processes to extract and refine metals. This approach typically involves smelting, roasting, and refining steps, where metal-containing ores or wastes are heated to temperatures that facilitate the physical and chemical separation of valuable metals from impurities. One of its key advantages is that it minimizes the need for extensive pre-treatment of the waste, as temperatures break down complex matrices and volatilize or oxidize non-metallic components. This makes pyrometallurgy a relatively fast and adaptable processing methodology. However, pyrometallurgy presents several drawbacks, including high energy consumption, low selectivity, loss of strategic metals, and the emission of polluting gases into the atmosphere [23]. During the last few years, hydrometallurgy has gained prominence due to its lower economic costs and reduced environmental impact compared with pyrometallurgy. It enables superior control of impurities, leading to high recovery rates and the obtention of high-purity metals. Hydrometallurgy is predominantly based on leaching, where metals are dissolved into solutions using inorganic or organic acids, followed by chemical precipitation, which selectively recovers the metals as solid compounds. Nevertheless, the extensive use of chemical reagents in these processes often leads to significant secondary pollution issues, e.g., the considerable volume of wastewater generated and the emission of pollutants such as chlorine gas [24; 25]. To address these challenges, alternative technologies have emerged, including bioelectrochemical systems (BESs).