Vol.:(0123456789) Journal of Material Cycles and Waste Management (2024) 26:1349–1368 https://doi.org/10.1007/s10163-024-01944-4 REVIEW Recent chemical methods for metals recovery from printed circuit boards: A review Emmanuel A. Oke1  · Herman Potgieter1,2 Received: 26 October 2023 / Accepted: 24 March 2024 / Published online: 3 April 2024 © The Author(s) 2024 Abstract As the volume of e-waste continues to rise, it is crucial to sustainably manage printed circuit boards (PCBs) and their valuable metal components. PCBs are ubiquitous in modern society, powering a variety of electronic devices. The metal resource crisis and the imperative for a low-carbon circular economy have accelerated the development of e-waste recycling technology. High-value discarded PCBs represent a vital component of e-waste. However, discarded PCBs are deemed hazardous to the ecosystem due to the presence of heavy metals and brominated organic polymers. Thus, recycling metals from discarded PCBs is not only a strategic necessity for fostering a green ecological civilisation but also a crucial guarantee for ensuring a safe supply of mineral resources. This comprehensive review gives the profound details of PCBs, and the performance of and advances in the latest chemical metal recovery methods. Reviewing the latest metal recovery processes, we explored the application of diverse leaching agents, including ionic liquids (ILs), deep eutectic solvents (DESs), organic acids and amino acids. These solvents were assessed in terms of their recovery efficiencies, and most of them demonstrated excellent leaching performance. The role of optimising leaching parameters such as concentration, oxidants, pH, particle size, solid-to- liquid ratios (S/L), temperature, and contact time is underscored, offering insights into achieving sustainable PCB recycling practices. Most of these recent leaching methods successfully extracted base metals (Cu, Fe, Zn, Sn, etc.), as well as precious metals (Au and Ag), achieving leaching efficiencies exceeding 90.0%. Interestingly, their effectiveness can compete with that of traditional hydrometallurgical methods. Keywords Electronic waste · Ionic liquids · Deep eutectic solvents · Organic acids · Amino acids Introduction In recent times, electrical and electronic companies across the globe have profoundly influenced the world, making their products an indispensable part of everyday life [1]. Electronic appliances encompass various household essen- tials, including refrigerators, televisions, washing machines, smartphones, laptops, photocopiers, iPads and so on [2, 3]. As the use of these items continues to grow among the populace, the inevitable consequence is the generation of associated waste. Electronic waste (e-waste) refers to dis- carded or non-functional electronic devices that have out- lived their utility [4, 5]. More precisely, e-waste can be defined as the entirety or components of electrical or elec- tronic equipment that users discard as waste, as well as the remnants from the manufacturing, repair, and refurbishment stages of the production process [3, 6, 7]. According to one of the previous reports, about 42.0% of e-waste originates from household appliances, 34.0% from communication devices, 14.0% from electronic gadgets, and the remaining 10.0% from accessories [8]. Moreover, about four years ago, the global production of e-waste reached a staggering 53.6 million metric tons (Mt) [9]. In addition, projections by the E-waste Statistics Partnership indicate that by 2030, the annual global accu- mulation of discarded electrical and electronic equipment (DEEE) will surge to an estimated 74.7 Mt [9]. Unfortu- nately, the issue of excessive DEEE generation has swiftly * Emmanuel A. Oke emmanuel.oke@wits.ac.za; okeemmanuela@gmail.com 1 Sustainable and Innovative Minerals and Metals Extraction Technology (SIMMET) Research Group, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag X3, PO Wits, Johannesburg 2050, South Africa 2 Department of Natural Science , Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK http://crossmark.crossref.org/dialog/?doi=10.1007/s10163-024-01944-4&domain=pdf http://orcid.org/0000-0001-5774-6215 1350 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 evolved into an urgent concern on a global scale [10]. DEEE comprises a multitude of components that can pose detri- mental effects on the environment as well as human health. Indiscriminate handling and informal recycling practices can escalate DEEE into a significant global concern for public health and the environment [11, 12]. On the other hand, owing to its physical attributes, DEEE is exception- ally well-suited for recycling, promising substantial benefits when subjected to sound environmentally benign recycling processes [13]. This dual nature of DEEE renders it a com- pelling secondary resource while simultaneously present- ing environmental challenges [14]. Furthermore, discarded printed circuit boards (DPCBs) represent a vital component within the scope of DEEE. They are characterised by their substantial inclusion of both high-value resources (metals) and hazardous substances, making them a crucial target for DEEE disposal and resource recycling [15–17]. In terms of their overall composition, DPCBs typically encompass a diverse array of bare boards intermixed with numerous electronic components (ECs). These ECs found on DPCBs often include capacitors, relays, resistors, and integrated circuits, among others. It is important to note that the com- position and concentration of these two categories of mate- rials exhibit significant variability, further intensifying the intricacy of the recycling process [18]. A typical recycling process for DPCBs is depicted in Fig. 1. Conventional methods for recovery of metals from DPCBs encompass mechanical, hydrometallurgical, and pyrometallurgical processes, as extensively explored in previous research [19–21]. However, these established methods are deemed unsustainable due to their high costs, the significant pollution they cause, and their substantial energy consumption, amidst other issues [22–24]. Biometallurgical processes harness microorganisms to extract metals from solid PCBs, converting them into soluble forms in an aqueous solution by generating lixiviants, which play a pivotal role in the metal extraction process [5, 25–27]. The use of such microorganisms holds significant promise because it is environmentally friendly, can operate at ambient temperatures and pressures, is cost-effective, and results in minimal secondary waste generation [28, 29]. However, the commercialisation of this approach faces challenges, primarily due to an insufficient scientific database, including optimal conditions, for scaling up the process and synchronising acidic and alkaline biometallurgical processes [5]. So, the biometallurgical process might not be able to replace traditional methods. Previous review articles have dealt with hydrometallurgy involving the use of traditional solvents such as H2SO4, HCl, HNO3, aqua regia, and alkaline reagents, among others, for the recovery of metals from DPCBs [30–32]. Other authors have also presented their review publications on pyrometallurgy/pyrolysis and biometallurgical methods [15, 21, 33, 34]. Notably absent from these discussions, however, is the utilisation of recent green solvents such as ionic liquids (ILs), deep eutectic solvents (DESs), organic acids, and amino acids in metal leaching from DPCBs. Therefore, this review aims to fill this gap, offering a comprehensive and up-to-date assessment of metal recovery from DPCBs using these innovative solvents. By focusing on ILs, DESs, organic acids, and amino acids, it strives to analyse the current state of research, evaluate efficiency, and propose future research directions. In other words, this article discusses specific chemical approaches for metal recovery, including the utilisation of ILs, DESs, organic acids, and amino acids. The factors influencing the recovery of metals from DPCBs are explored in a dedicated section, addressing key considerations in the process. The article concludes by summarising key findings and providing Fig. 1 Typical recycling flow chart for DPCBs in DEEE 1351Journal of Material Cycles and Waste Management (2024) 26:1349–1368 insights into potential future research directions in the field of metal recovery from DPCBs. Synthesising knowledge from past studies, this review seeks to enhance our understanding of the potential of these alternative solvents in DPCB recycling. In doing so, it aspires to contribute to sustainable practices in e-waste management and the circular economy of valuable metals. Recovery methods for metals from DPCBs Traditional approaches for recovering metals from DPBCs have historically relied upon energy-intensive pyrometallurgical processes or the use of highly oxidative and acidic conditions in hydro- and solvometallurgical methods. These methods involve the utilisation of potent acids such as HCl, H2SO4, and HNO3, as well as hazardous substances like aqua regia and piranha solution as earlier mentioned [35, 36]. Also, these methods may employ toxic oxidants like Cl2, or even resort to cyanide (CN−) and Hg, potentially resulting in the generation of substantial quantities of toxic and corrosive waste. The use of such “harsh” conditions can also contribute to reduced selectivity, thereby exacerbating the environmental impact [5, 37]. This approach contradicts the goals of sustainable development, which might appear paradoxical when considering recycling—an essential principle of the circular economy and green chemistry. The quest for safer, cost-effective, and environmentally friendly solvents, oxidants, reductants, ligands, additives, solid traps, and carriers is paramount to enhancing the sustainability of various metal recovery processes from DPCBs and other e-waste components. Therefore, in this section, we illuminated the use of ILs, DESs, organic acids and amino acids in the recovery of various metals from DPCBs. The summary of some of the research findings reviewed in this article is presented in Table 1. Recovery of metals mediated by ionic liquids ILs are a class of purely ionic, salt-like materials that are liquid at unusually low temperatures. The definition of ILs uses the boiling point of water as a point of reference: “ILs are ionic compounds which are liquid below 100.0 °C” [38, 39]. ILs have many potential applications owing to their ability to act as powerful solvents and electrolytes. Interestingly, ILs offer some great advantages over traditional leaching agents, such as acids or cyanides, for metal extraction from DPCBs and other components of e-waste or ores. ILs have high selectivity, low toxicity, low volatility, high stability, and an outstanding metal recovery ability [40–42]. Additionally, they can also be reused and recycled, thereby reducing waste generation and environmental impact. There are different methods of using ILs for metal extraction from e-waste. Such methods include leaching, liquid–liquid extraction, electrodeposition, and aqueous biphasic systems [41, 42]. However, our focus here is the leaching (i.e. solid–liquid extraction) of metals from DPCBs using ILs. Based on the available information in the literature, several ILs have been utilised solely for the recovery of both valuable and hazardous metals from DPCBs. For the first time, the research looked into the process of leaching Cu and other important metals from DPCBs using a Brønsted acidic IL known as 1-butyl-3-methyl-imidazolium hydrogen sulfate, [BMIM][HSO4] [43]. Interestingly, under the optimal conditions, the recovery of Cu surpassed 99.0%, approaching near-total recovery at almost 100.0%. So, after this pioneering work, the use of ILs in the recovery of metals from DPCBs has increased tremendously. For example, a study by Chen’s research group examined the leaching characteristics of Cu and Pb found in DPCBs by employing various acidic ILs as leaching agents [44]. In this case, after optimisation, a remarkable Cu leaching performance of almost 100.0% was achieved. However, Pb showed a lower leaching rate (<30.0%). Similarly, under specific optimal conditions involving a 30.0% concentration of H2O2, a solid-to-IL ratio (S/IL) of 1:20, a temperature of 80.0 °C, and a duration of 2.0 h, a study conducted by Zhang et al. successfully leached 98.3% of Cu using an acidic IL called 1-carboxymethyl-3-methylimidazolium hydrogen sulfate, [CM-MIM][HSO4] [45]. Also, the use of sulfonic acid- functionalized IL, N-sulfobutylpyridinium hydrosulfate, [BSO3HPy][OTf] has been reported and it resulted in leaching efficiency exceeding 99.7% Cu and 74.8% for Zn, albeit with low selectivity for Pb [46]. Barrueto et al.’s research demonstrated selective leach- ing of different valuable metals which can be influenced by the type of ILs used [47]. Acidic ILs such as [BMIM] [HSO4] and 1-H-3-methyl-imidazolium hydrogen sulfate ([HMIM][HSO4]) were effective at leaching Cu and Co. On the other hand, basic ILs like 1-butyl-3-methyl-imida- zolium bromide ([BMIM][Br]) and 1-butyl-3-methyl-imi- dazolium chloride ([BMIM][Cl]) were adept at leaching Au and Ag. Consequently, a multi-stage leaching approach can be employed to selectively extract specific metals from DPCBs. The authors observed that [BMIM]HSO4] acidic IL displayed the best recovery efficiencies for Cu (86.2%) and Co (96.7%), while [BMIM][Br] basic IL displayed the best recovery efficiencies for Au (40.8%) and Ag (44.7%). Similarly, under optimised conditions consisting of 30.0% [BMIM][HSO4], 10.0% H2O2, and 60.0% H2O at a tem- perature of 60.0 °C, Gomez et al. recently successfully extracted more than 99.0% of Cu from pre-treated DPCBs [19]. This extraction process was conducted with a S/L 1352 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 Table 1 Summary of the recovery of metals from DPCBs using ILs, DESs and organic acids at different experimental conditions Leaching solvents Concentration of the base leaching solvent Oxidant S/L Particle size T (°C) t (h) Stirring speed (rpm) Metal recovery (%) References [BMIM] [HSO4] & H2O2 80.00% IL 30.00% H2O2 1.00/25.00 0.10–0.25 mm 70.00 2.00 250.00 ≈100.00 (Cu) [43] [CM-MIM] [HSO4] & H2O2 90.00% IL 30.00% H2O2 1.00/20.00 >0.50 mm 80.00 2.00 – 98.30 (Cu) [45] [BSO3HPy] [OTf] 80.00% IL 25.00% H2O2 1.00/20.00 0.10 – 0.25 mm 50.00 2.00 250.00 99.70 (Cu) 74.80 (Zn) <10.00 (Pb) [46] [BMIM]HSO4], [BMIM][Br] & H2O2 60.00% IL 30.00% H2O2 1.00/15.00 2.00 × 2.00 cm 60.00 24.00 50.00 86.20 (Cu) 96.70 (Co) 40.80 (Au) 44.70 (Ag) [47] [BMIM] [HSO4] & H2O2 30.00% IL 10.00% H2O2 1.00/15.00 1.00 × 1.00  cm2 60.00 2.00 150.00 >99.00 (Cu) [19] [NNDMOA] [HSO4] & H2O2 0.50 M IL – 1.00/20.00 1.00 × 1.00 cm 75.00 3.00 500.00 550.00 mg (Cu) [48] ChCl:EG (1:2) & I2 – 0.10 M I2 – 1.00 × 1.00  cm2 85.00 72.00 150.00 >75.00 (Cu, Ni, & Sn) ≈45.00 (Zn) <40.00 (Fe) <10.00 (Al, Mg, & Pb) [59] ChCl:MA (1:1) H2O2 & DDACl – 30.00% H2O2 1.00/10.00 – 60.00 2.00 500.00 8.80 (Cu) 26.40 (Ag) 91.50 (Al) 19.70 (Fe) 2.30 (Zn) [56] ChCl:OA (1:2) & ChCl:GA (1:2) – 1 M OxA & 1 g of Fe – – 50.00 8.00 100.00 85.21(Cu) 75.3 (Fe) 83.1(Zn) 22.1 (Sn) 38.7 (Pb) [57] CA & H2O2 1.00 M CA 5.83% H2O2 – 4.00 × 4.00 cm 30.00 4.00 150.00 100.00 (Cu, Sn, Zn, Ni, Au, Pb, Al, Fe, Ag, & Pd) [61] CA & H2O2 0.50 M CA 5.83% H2O2 – 7.00 × 7.00 mm 30.00 <4.00 150.00 85.00 (Cu) 98.00 (Al) 94.00 (Pb) [62] CA, IA & OxA 50.00 mM CA, 30.00 mM IA & 20.00 mM OxA – – 60.00–80.00 mesh – 15.00 days – 80.3 (Cu) 75.70 (Zn) 73.60 (Ni) [60] CA, AA & H2O2 1.00 M CA, 5.00% AA & 5.00% H2O2 30.00% H2O2 – 3.00 × 3.00 cm 30.00 24.00 – 325.89 ppm (Cu) [63] Lemon juice & H2O2 74.00% Lemon juice 12.20% H2O2 1.41% 149.00– 177.00 μm 20.00 4.00 200.00 89.00 (Cu) 73.00 (Zn) [64] 1353Journal of Material Cycles and Waste Management (2024) 26:1349–1368 ratio of 1:15 for a duration of 2 h. The recovered Cu was then obtained through a direct electrowinning process using the [BMIM][HSO4]-leach solution as the electrolyte. This work employed minimal leaching agents, time, and energy compared to the previous works. Furthermore, the combination of N,N-dimethyloctylammonium hydrogen sulfate ([NNDMOA][HSO4]) and H2O2 proved effective in leaching 550.00 mg of Cu from DPCBs [48]. However, this particular IL did not demonstrate successful leaching of significant amounts of Au and Ag. This limitation can be attributed to the preference of Au and Ag for a basic environment over an acidic one [47]. In the process of recovering metals from DPCBs, conventional slurry electrolysis demands substantial amounts of strong acids or strong alkalis to form the electrolyte solution, leading to the discharge of pollutant waste acids or alkalis. Moreover, traditional slurry electrolysis involves complex operational processes and stringent conditions. Consequently, there is a pressing need to develop environmentally friendly electrolysis technologies for metal recovery that offer simplicity in operation, reduced reagent consumption, and minimal pollutant emissions [49]. The recovery of metals from DPCBs has advanced to the point whereby ILs are being employed in slurry elec- trolytic processes. The schematic diagram for this process is shown in Fig. 2(a). For example, N-butyl sulfonate pyri- dinium bisulfate ([BSO3HPy]HSO4]) IL, was used to replace H2SO4 in a slurry electrolytic system for one-step Cu recov- ery from DPCBs [50]. The findings reveal that substituting H2SO4 with [BSO3HPy]HSO4] can enhance Cu recovery rate, current efficiency, and purity. The optimal outcome is achieved when 10.0% H2SO4 is replaced, resulting in a 90.9% recovery, 70.7% current efficiency, 81.7% purity and Cu powder particle size of 2.3 μm. In a similar context of slurry electrolysis, employing an IL solution containing CuSO4, NaCl, H2SO4, and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6] as reported by He et al. in 2020, demonstrated improved performance with a 92.7% recovery rate and 80.0% current efficiency for Cu [49]. Moreover, 1-ethyl-3-methylimidazole chloride, [EMIM][Cl] and1-butyl-3-methylimidazole hexafluoroborate, [BMIM] [BF6] were identified as ideal ILs for achieving high recov- eries of Cu. The most recent work on the recovery of metals from DPCBs is the modification of adsorbent with ILs for the sole aim of enhancing their capacity. For instance, Lin et  al. developed an eco-friendly adsorbent using 1-(3- aminopropyl) imidazole (APIM) IL functionalised with chitosan fibres (CFs) to selectively recover Au from DPCBs bioleachate [51]. The APIM-CFs were created for Au (I) recovery under alkaline conditions, with uniform cross- sectional diameter. IL-CFs had a maximum Au adsorption capacity of 357.0 ± 13.2 mg/g, reaching equilibrium in 7.0  min. Pseudo-1st-order and pseudo-2nd-order rate constants were 0.89 ± 0.061/min and 0.0049 ± 0.0002 g/mg min. APIM-CFs exhibited high Au selectivity from DPCBs bioleachate, suggesting their effectiveness in recovering Au from urban mining bioleachates. The merits and demerits of employing ILs in the recovery of metals from DPCBs compared to conventional methods are highlighted in Fig. 2(b). Recovery of metals mediated by deep eutectic solvents In this subsection, we reviewed research publications that dealt with the leaching of metals from DPCBs using environmentally friendly solvents popularly known as DESs. In recent years, the use of DESs in separation technology has generated increasing research interest. DESs are mixtures of two or more compounds that form a liquid phase at a lower temperature than the melting point of each compound [52–54]. In this case, one component of the DES is known as a hydrogen bond donor (HBA) and the other is normally referred to as a hydrogen bond Table 1 (continued) Leaching solvents Concentration of the base leaching solvent Oxidant S/L Particle size T (°C) t (h) Stirring speed (rpm) Metal recovery (%) References MSA & H2O2 3.50 M MSA 0.50 M H2O2 – 7.00 × 7.00 mm 30.00 1.50 150.00 35.98 ppm (Cu) 1254.00 ppm (Pb) 822.30 ppm (Sn) [62] MSA & H2O2 1.00 M MSA 0.60 M H2O2 1:20 2.00 × 2.00 cm 50.00 2.00 500.00 100.00 (Cu) 100.00 (Zn) 90.00 (Ni) [70] 1354 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 acceptor (HBD). DESs have several advantages over conventional solvents, such as low toxicity, low cost, high biodegradability, and high solubility of metals [22, 55]. DESs can be used to leach metals from DPCBs by dissolving the metal oxides or salts present on the surface of the boards. The leached metals can then be recovered by various techniques, such as precipitation, electroplat- ing, or solvent extraction. Only a few research papers have reported the leaching of metals from DPCBs using DESs [56–59]. Fig. 2 a Experimental setup for the electrolytic slurry process, reprinted from [49], Copyright 2020, with permission from Elsevier, b Merits and demerits of using ILs for the recovery of metals from DPCBs, c Etch depth of a Au-coated PCB after oxidation in CaCl2·6H2O:EG DES with either 1 M FeCl3 or CuCl2, at 50 °C, d Etch depth in the first 2.0 h of etching; etch depth of Cu over time from the cross-section of a square- shaped block terminal with two different concentrations of e FeCl3 and f CuCl2 as oxidis- ing agents in the CaCl2·6H2O: EG eutectic system at 50 °C. (c–f) was reproduced from [58], Copyright 2022, with permis- sion from RSC 1355Journal of Material Cycles and Waste Management (2024) 26:1349–1368 The pioneering work involving the use of DESs in the leaching of metals from DPCBs was reported in 2021 by Kolliopoulos’ research team [59]. In this study, the researchers focused on extracting metals from DPCBs using choline chloride (ChCl) and ethylene glycol (EG) DES. Specifically, they successfully extracted Cu, Ni, Zn, and Sn among others from the DPCBs. The leaching efficiency was impressive, with over 75.0% recovery achieved after 72.0 h for Cu, Ni, and Sn, and approximately 45.0% recovery for Zn when I2 was employed as oxidizing agent. In the absence of I2 in ChCl:EG DES, the recovery of metals was generally observed to be low. In that case, less than 15.0% recovery was observed for most metals, except Sn, where leaching was about 100.0%. Recently, the recovery of metals from DPCBs using DES has also been carried out by Abbott’s research group [58]. In this study, researchers confirmed that FeCl3 and CuCl2 have high solubility in CaCl2·6H2O:EG DES and exhibit reversible redox behaviour. These properties make these inorganic salts suitable as strong and electrocatalytic oxidising agents. They used optical profilometry to measure the dissolution rates of Cu, Ni, and Au in a PCB block terminal. Cu showed the fastest dissolution rate, while Ni did not dissolve, likely due to the high water content in the eutectic mixture. CuCl2 was found to be a more effective oxidizing agent than FeCl3, requiring less time for etching. In other words, their study explores the use of catalytic dissolution in a eutectic solvent for selectively recovering metals from DPCB. FeCl3 and CuCl2 serve as effective oxidising agents, with CuCl2 demonstrating faster leaching kinetics compared to FeCl3 as illustrated in Fig. 2 (c–f). Furthermore, the possibility of extracting and separating metals from DPCBs, either through leaching or without the need for high-temperature acidic leaching was investigated using two commonly employed DESs: {ChCl + lactic acid [LA], 1:2} and {ChCl + malonic acid [MA], 1:1} [56]. The authors added H2O2 as an oxidant and didecyldimethylammonium chloride (DDACl) as a surfactant primarily to improve the leaching efficiency of the DESs utilised. Interestingly, much higher leaching efficiency was obtained for Al (91.5%) and other metals at pH 2.5 when ChCl:MA DES was used compared to when ChCl:LA DES was employed. Most recently, three different DESs were prepared and used in a two-step process for the recovery of various metals from DPCBs [57]. In this study, ChCl served as HBA, while EG, oleic acid (OA), and glycolic acid (GA) were employed as HBDs in separate eutectic mixtures. Zn, Cu, and Sn recovery was exclusively accomplished through the introduction of oxalic acid (OxA), water, and Fe powder into DESs. In other words, when oxalic acid solution was added to the ChCl:GA DES, a blue precipitate formed. This precipitate comprised ZnC2O4⋅2H2O, PbC2O4⋅2H2O, and CuC2O4⋅2H2O, with recovery yields of 90.4% Zn, 87.5% Pb, and 16.8% Cu. The residue was subjected to ChCl:OA DES leaching. In this case, Cu was successfully recovered by mixing the ChCl:OA DES leachate with water. Specifically, 74.9% of Cu was precipitated as CuC2O4⋅2H2O with a purity of 98.1 wt%. The remaining Sn in the aqueous solution was efficiently recovered by adding reduced Fe powder, resulting in a Sn recovery yield of 51.3%. Nevertheless, it is possible to enhance the recovery yields of the desired metals and improve the purity of CuC2O4⋅2H2O by fine-tuning the experimental conditions. Research into the utilisation of DESs for recovering met- als from DPCBs is relatively scarce as mentioned earlier. This scarcity of research could be attributed to the greater complexity in the compositions of DPCBs sourced from var- ious origins when compared to the other simpler waste types. Nevertheless, the solvometallurgical approach involving the use of DESs for the recovery of metals from DPCBs offers a promising alternative that reduces the need for excessive water and acidic/basic reagents in the recovery process. The results obtained so far by different research groups demon- strate its potential to compete with existing methods of metal leaching, pointing towards the achievement of a sustainable circular economy through zero-waste green urban mining. Fig. 2 (continued) 1356 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 Recovery of metals mediated by organic acids An alternative approach to the use of harmful inorganic acids during the recovery of metals from DPCBs also entails utilising natural acids such as citric acid (CA), itaconic acid (IA), and oxalic acid (OxA), among others. These solvents are biodegradable, less harmful, and exhibit greater selectivity compared to their inorganic counterparts. Organic acids can be produced by microorganisms, such as fungi or bacteria, through fermentation processes [60]. This renders organic acid leaching a more environmentally friendly and sustainable method for extracting metals from DPCBs. Interestingly, numerous research studies have investigated the utilisation of organic acids in metal recovery from DPCBs. CA stands as an eco-friendly and auspicious leaching substance for extracting valuable metals from PCBs. For instance, Jadhav et  al. in their recent study completely dissolved a range of metals, including Cu, Sn, Zn, Ni, Au, Pb, Al, Fe, Ag, and Pd using CA (1.0 M) and 5.8% H2O2 [61]. Furthermore, CA readily dissolves in water and can be readily broken down in both aerobic and anaerobic environments just like most organic acids. Also, Anwer et al. devised a systematic procedure for separating components and extracting metals from DPCBs using both organic and inorganic acids [62]. They discovered that CA could serve as an eco-friendly substitute for inorganic acids, yielding comparable or superior results as presented in Table 1. The outstanding ability demonstrated by the solution of CA and H2O2 during the recovery of metals from DPCBs can be attributed to strong oxidant formation (i.e., peroxyl carboxylic acid) during the leaching process. Similar to H2O2, these substances readily undergo decomposition in an aqueous environment, leading to the formation of the peroxy carboxyl group (–COOOH), which in turn oxidizes the metal. The reactions that took place during the leaching process which led to the formation of peroxyl carboxylic acid are shown in Eqs. (1 & 2) [61]. In the above equations, “R” represents the organic substituent group, and “M” represents the metal. According to the reaction shown in Eq.  (3), there is a potential alternative pathway for the generation of metal hydrogen citrate when both H2O2 and CA are present [62]. (1)R−COOH + H2O2 → R−COOOH − H2O (2)R−COOOH +M + 2H+ → R−COOH + H2O +M2+ (3) C6H8O7(CA) +M + H2O2 → M(C6H8O7) i.e. metal hydrogen citrate + H2O +M2+ In addition, Krishnamoorthy et al. harnessed organic acids (i.e., a mixture of 50.0  mM CA  +  30.0  mM IA + 20.0 mM OxA) produced by Aspergillus niveus for the leaching of various metals from DPCBs [60]. The authors achieved a maximum recovery of 75.7% for Zn, 73.6% for Ni, and 80.3% for Cu from DPCBs using a two-step leach- ing process with particle sizes ranging from 60.0 to 80.0 mesh. Recently, Nagarajan and Panchatcharam explored the combination of CA and acetic acid (AA) solution for Cu leaching from DPCBs in the presence of H2O2 [63]. Their observations revealed that when CA, AA, and H2O2 were applied individually for the leaching process, lower Cu concentrations were achieved compared to using a leaching solution composed of 1.0 M CA, 5.0% AA, and 5.0% H2O2. In specific terms, leaching with CA, AA, and H2O2 yielded Cu concentrations of 26.9, 22.3, and 6.3 ppm, whereas the combined solution of these three chemicals resulted in a Cu concentration of 325.9 ppm. Therefore, the combination of organic acids could be established as a standardised method for leaching Cu and other metals present in DPCBs. Moreover, natural organic acids obtained from agricultural fruits have also been utilised for the leaching of metals from DPCBs and are found to be effective. One such work is the one recently carried out by Rastegar’s research team [64]. In their study, they obtained natural acid from lemon juice, and this was used in combination with H2O2 to recover Cu and Zn from DPCBs successfully. The authors applied the response surface method to optimise the extraction which resulted in the maximum recoveries of 89.0 and 73.0%, respectively, for Cu and Zn. The use of methanesulfonic acid (MSA) is trending cur- rently as a leaching agent for metal recovery from DPCBs. MSA is a biodegradable and non-volatile organic acid that has been widely used in industrial applications, such as electroplating, metal cleaning, and organic synthesis [65, 66]. The minimal toxicity of MSA becomes apparent when examining its lethal dose 50 (LD50) value. The reported LD50 (oral, cat) for MSA is 1158 mg/kg, in stark contrast to the LD50 (oral, rat) of HCl, which falls within the range of 238–277 mg/kg [67]. The chemical makeup of MSA can be likened to that of H2SO4, with the exception that one of the two hydroxyl (OH) groups has been replaced by a methyl (CH3) group. MSA demonstrates remarkable acidity, as indicated by its low pKa value of −1.19 [68]. Consequently, MSA is significantly more acidic when compared to other organic acids such as formic acid, AA, or CA. Its level of acidity rivals that of potent mineral acids. Additionally, in a solution with a concentration of 0.1 M, MSA undergoes almost complete ionisation. The heat released during the formation of the methanesulfonate ion in an aqueous solu- tion is −163.79 ± 1.04 kcal/mol, and the heat of formation for liquid MSA is −152.39 ± 1.12 kcal/mol at 25 °C [69]. 1357Journal of Material Cycles and Waste Management (2024) 26:1349–1368 In summary, MSA has several advantages over conventional acids like H2SO4 or HCl in metal leaching. So, MSA can selectively dissolve the solder (Sn–Pb alloy) from the DPCBs without affecting the epoxy resin or the Cu interconnects [62]. The reaction for this process is shown in Eq. (4). This facilitates the separation of the metallic and non-metallic components of the DPCBs and reduces the consumption of acid and energy. MSA can leach Cu and other base metals from the DPCBs at low concentrations and temperatures. For example, a study by Anwer et al. showed that 3.5 M MSA and 0.5 M H2O2 can dissolve Cu, Pb and Sn from DPCBs in 1.5 h at 28.0 °C 62. Furthermore, Jadhao et al. investigated the recovery of base metals like Cu, Zn, and Ni from DPCBs using MSA and H2O2 [70]. After optimisation, the authors were able to suc- cessfully leach 100.0% Cu and Zn in addition to 90.0% Ni. They also found that the extraction of these metals is primar- ily controlled by diffusion in which the activation energies for Cu, Zn, and Ni extraction were estimated to be 9.4, 10.9, (4) 2MSA−H 2 O 2 + Sn + Pb + 4H + → Sn 2+ + Pb 2+ + 2MSA + 4H 2 O and 18.9 kJ/mol, respectively. By combining electrowinning and cementation, 99.9% purity was obtained for Cu and Zn. A typical overall method for recovering metals from DPCBs using MSA is depicted in Fig. 3. In summary, the use of organic acids, whether in isolation or as mixtures, coupled with optimisation techniques, presents a promising avenue for eco-conscious metal recovery from DPCBs. Additionally, the adoption of MSA, with its selective solder dissolution and low toxicity, shows great promise for reducing environmental harm and energy consumption while recovering base and precious metals from DPCBs. These innovative approaches can contribute to the sustainable management of e-waste and underscore the importance of eco-friendly alternatives in the field of metal recycling. Recovery of metals mediated by amino acids Proteins are made of organic molecules called amino acids. Amino acids have raised significant attention in research across various scientific domains within the fields of chemistry and biology. There are 20 common Fig. 3 A typical overall recovery methodology of metals from DPCBs using organic acids, reproduced from [70], Copyright 2023, with permis- sion from Elsevier 1358 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 types of amino acids, known as α-amino acids, that have a carbon atom (α-carbon) with three groups attached to it: a carboxyl group (–COOH), an amino group (–NH2), and a variable group (called the R-group) [71]. These amino acids are weak acids that can lose more than one hydrogen atom in water. The amount of hydrogen atoms that are lost depends on the pH of water [72]. The simplest amino acid is glycine (NH2CH2COOH), and it is the most widely used amino acid for the recovery of metals from DPCBs. Amino acids are usually employed as lixiviants in an alkaline setting to extract base and precious metals from DPCBs. NH2CH2COOH is industrially produced. It is cost- effective and possesses attributes such as non-toxicity, non- volatility, and non-combustibility [73]. In numerous research studies, NH2CH2COOH has been favoured among amino acids due to its lower cost and abundant availability [74]. It has demonstrated the ability to reduce the requirement for high cyanide concentrations in NH2CH2COOH-cyanide solutions [75]. Furthermore, NH2CH2COOH can dissolve Au independently, without the need for cyanide [76]. When common oxidants are introduced to alkaline-NH2CH2COOH solutions, it was observed that they can enhance the rate and efficiency of Au leaching [77]. Therefore, in this section, we elaborated on the recovery of metals from DPCBs using amino acids, most especially NH2CH2COOH as a lixiviant. Fascinatingly, many studies have explored the recovery of Cu and other metals from DPCBs by employing NH2CH2COOH as a lexiviant. It is worth noting that NH2CH2COOH tends to form complexes with Zn and Pb more readily than with precious metals [78]. This behaviour primarily arises from the standard reduction potential of base metals, which is typically higher than that of precious metals. This makes the oxidation of base metals more straightforward. For example, Li et al. achieved a maximum Cu recovery of 96.5% (the reaction mechanisms are shown in Eqs. 5 and 6), but this process resulted in significant co-extraction of base metals [79]. The concentration of NH2CH2COOH and the solid content had a notable impact on the extraction of base metals, while factors like H2O2, temperature, and particle size had minimal influence. Their kinetic study, conducted at room temperature with ambient O2 as the oxidant and particle sizes below 2 mm, suggested that the extraction of Cu from DPCBs is governed by internal diffusion. For example, Han et al. also investigated the effective parameters for Cu leaching from DPCBs using NH2CH2COOH [80]. Interestingly, their findings indicated that increasing the H2O2 content improved Cu leaching efficiency. In a similar study, Li et al. demonstrated that Cu could be efficiently precipitated as CuS with an optimised Cu/HS ratio of 1:1.2, achieving over 99.0% precipitation efficiency in less than 5 min [81]. Most recently, a complex four-stage process of Cu leaching from PCB waste powder using NH2CH2COOH was studied by Mokhlis et al. under ambient conditions [82]. During this process, NH2CH2COOH displays remarkable selectivity for Cu, enabling the dissolution of Cu from DPCB waste at a substantial rate and 92.8% recovery. Furthermore, Broeksma and Dorfling studied the optimal conditions for achieving the highest Cu dissolution and the lowest Ag dissolution recently from DPCBs [83]. They found that a temperature of 60.0  °C, a NH2CH2COOH concentration of 1.0 M, the utilisation of O2 as the oxidant, a pH level of 11.0, and a pulp density of 25.0  g/L are required. Under these conditions, a remarkable 81.2% Cu dissolution was achieved within 22.0 h, accompanied by minimal co-extraction of Au, which accounted for just 1.3%. A system composed of NH2CH2COOH  +  H2O2 proved effective for extracting Cu and base metals, with NH2CH2COOH concentration being the most influential factor, followed by the H2O2 concentration [84]. Higher pulp density negatively impacted the Cu leaching efficiency. Under optimal conditions (0.5 M NH2CH2COOH, 1.0% H2O2, and 20 g/L pulp density) at ambient temperature, 99.9% of Cu was extracted, and 95.2% of it precipitated as CuS. Also, Ag recovery from the residue was achieved using the NH2CH2COOH +  KMnO4 system, with 96.0% recovery at ambient temperature under optimal conditions. Monosodium glutamate (C5H8NO4Na), derived from glutamic acid and known as one of the most abundant non-essential amino acids in nature, has also recently found application in the recovery of metals from DPCBs. For instance, the leaching of Cu and Au from DPCBs was achieved recently under pH 7.0 (neutral pH) using C5H8NO4Na as the base lixiviant [85]. The highest Cu and Au extractions were achieved through a two-stage leaching process conducted under mild conditions (30.0 °C, 150.0 rpm, and an initial pH of 7.0). In the initial leaching stage, a significant 93.0% of Cu was dissolved with minimal contamination of other metals, using a combination of 0.75 M C5H8NO4Na and 0.5% H2O2 (as shown in Eqs. 7 & 8). In the subsequent leaching stage, Au (as presented in Eq. 9) extraction was maximised at 86.0% from the solid residue by employing 1.0 M C5H8NO4Na supplemented with 0.25% H2O2. Following this, a direct electrowinning process enabled the recovery of 98.0–100.0% of Cu from the initial leachate within 5.0–6.0 h. Additionally, Perea et al. explored how the choice and concentration of oxidising agents affect Cu leaching kinetics. They found that using 0.5 M C5H8NO4Na and 0.03 M H2O2 resulted in the highest Cu extraction of 92.0% after 2.0 h. (5)2Cu + O2 → 2CuO (6) CuO + H 2 O + 2NH 2 CH 2 COO −+ → Cu(NH 2 CH 2 COO 2 ) + 2OH − 1359Journal of Material Cycles and Waste Management (2024) 26:1349–1368 Moreover, C5H8NO4Na outperformed NH2CH2COOH in both dissolution kinetics and extraction rate, with maximum recovery values of 92.0 and 84.0%, respectively. Furthermore, a few of the recent studies deal with an integrated approach involving the use of amino acids and other substances. For example, in a recent study conducted by Li et al., an innovative approach was adopted to enhance both the solid content and the leaching efficiency in the NH2CH2COOH-oxidant system [86]. In this study, NH3, a well-known Cu lixiviant, was introduced into the process. It was discovered that NH3 served multiple roles as a pH modifier, an additional lixiviant, and a potential catalyst during the NH2CH2COOH-based leaching of DPCBs. This modification resulted in a significant reduction in leaching time, from 72.0 to 24.0 h, and an increase in solid content to as high as 15.0%, yielding more than 40.0 g/L of Cu in the pregnant leach solution. However, it is essential to implement additional safety precautions to mitigate the potential hazards associated with NH3, despite its reduced usage by 40.0–80.0% compared to traditional NH3–NH4 leaching methods [87]. Also, in another study by the same research group, NH2CH2COOH was employed alongside oxidants, either KMnO4 or potassium ferricyanide (K3[Fe(CN)6]), to extract precious metals, Au, Ag, and Pd, from the residue remaining after the initial NH2CH2COOH leaching of DPCBs [88]. Under optimised conditions, which include 0.5 M NH2CH2COOH, room temperature, a pH level of 11.0, and 2.0% solids content, the NH2CH2COOH-KMnO4 combination yielded higher extractions of these metals. These extraction efficiencies were found to be comparable to cyanide-based leaching methods. However, it is worth noting that the consumption of oxidants was relatively high, with 632.0 kg/t of KMnO4 and 610.0 kg/t K3[Fe(CN)6] used under the optimised conditions. Owing to this, further research is recommended to explore oxidant reduction or regeneration methods and to investigate leaching at higher solids content, such as levels exceeding 10.0%, which would complement the findings of this study. Recently, a novel leaching system was developed by uti- lising thiosulfate-cobalt-glycine for Au recovery from PCB of mobile phones [89]. This innovative system effectively addresses the challenge of high thiosulfate consumption that limits the commercial use of conventional thiosulfate leaching. Notably, the authors achieved an impressive 97.8% Au recovery using the thiosulfate-cobalt-glycine system. (7)Cu2+ + C5H8NO4Na → Cu(C5H8NO4) + + Na+ (8)Cu2+ + 2C5H8NO4Na → Cu(C5H8NO4)2 + 2Na+ (9)Au2+ + 2C5H8NO4Na → Au(C5H8NO4) − 2 + 2Na+ Furthermore, the research investigated key parameters, revealing that higher concentrations of Co(II), thiosulfate, or glycine enhance Au leaching, while elevated temperatures can lead to thiosulfate decomposition and reduced Co stabil- ity, which negatively impacts Au leaching efficiency. A simi- lar study conducted by Godigamuwa and Okibe highlighted the growing significance of thiosulfate and glycine systems for Au leaching from DPCBs [90]. The authors found that the glycine-thiosulfate system outperformed the histidine- thiosulfate system, achieving an impressive Au leaching effi- ciency of 93.7% at a pH of 9.3 and a temperature of 40 °C. In comparison, the stand-alone thiosulfate and glycine sys- tems achieved Au leaching percentages of 47.1 and 50.7%, respectively. Notably, in the dual system, Fe leaching was insignificant, interestingly, the Ag and Al leaching reached 95.3 and 27.0%, respectively. Furthermore, when compared to the thiosulfate system, the dual system demonstrated enhanced stability in leached Au. The optimal conditions for Au leaching from PCBs in the dual system were found to be 60.0 mM thiosulfate and 0.5 M glycine at 40 °C and a pH of 9.3. Kinetic analysis revealed that Au and Ag leaching in the dual system followed a diffusion-controlled model. Interestingly, the initial Au leaching rate in the dual system was observed to resemble that of the glycine-cyanide system. In most cases, using amino acids on their own cannot achieve a satisfactory extraction of metals. Additives, includ- ing oxidants/reductants, catalysts, synergists, pH modifiers, etc., are often used alongside amino acids, to mobilise met- als from different material matrixes to leaching solution. Figure 4(a) illustrates a typical amino acid leaching system used in hydrometallurgy. Oxidants or reductants are usually used to oxidise or reduce metal ions to their preferred oxida- tion state before complexing with amino acids. For example, when leaching DPCBs, where Cu presents as its native form, oxidant (ambient O2/H2O2 etc.) is indispensable in the sys- tem to oxide metallic Cu to cupric form that is to be domi- nantly complexed with amino acid [79]. The selection of the appropriate redox agent is important and system-dependent and care should be taken to not destroy the amino acid itself in the process of amino acid recycling and reuse. Catalysts and synergists are used to catalyse the reaction of amino acids with metals and to be synergistic with amino acids during the leaching process. A typical example is the use of starved cyanide in the amino acid leaching of Ag. Several studies have shown that the addition of starved levels of cyanide (no “free” cyanide in the reactor exit) to the amino acid solution could enhance the leaching kinetics of Ag sig- nificantly [86–88]. Additionally, as indicated in Fig. 4(b), different amino acid species will predominate at different pH of the leaching system. A pH modifier is hence required for an amino acid leaching system, such as NaOH, CaO, ammonia water (NH3⋅H2O) and dilute mineral acids, which 1360 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 is to make the preferred amino acid species predominant in the system for effective coordinative reaction. When dealing with the leaching of metals from DPCBs using amino acids or an amino acid-integrated approach, it is advisable to carry out leaching in two distinct stages. During the first stage, base metals, comprising primarily Cu and Zn (over 90.0%), can be efficiently extracted at room temperature and under normal environmental conditions. However, the second stage of leaching, which targets precious metals, necessitates the introduction of an additional oxidant, such as H2O2, and/or a synergist like starved cyanide. This stage also covers the extraction of the remaining base metals. It is important to note that because some metals are embedded within the polymer substrate of DPCBs, fine grinding or pulverising of materials could be required for the successful leaching of precious metals and the remaining base metals. This additional step may incur extra costs in the process. Factors affecting the recovery of metals from DPCBs The recovery of metals from DPCBs is a complex and highly controlled process, heavily influenced by a multitude of criti- cal factors. These factors encompass a range of physical and chemical parameters, including pH levels, temperature, par- ticle size, leaching agent concentrations, oxidant concentra- tions, reaction time, and many others. The interplay of these variables can significantly impact the efficiency and yield of metal recovery from DPCBs, making it a precise science that requires careful consideration and optimisation. Therefore, the discussion in this section focuses on the intricacies of these influencing factors and their crucial roles in determining the success of DPCB metal recovery processes. Understanding how pH, temperature, particle size, and other variables affect leaching and extraction processes is essential for developing efficient and sustainable methods for reclaiming valuable met- als from e-waste in general. By unravelling the complexities of these factors, researchers can work towards more effective and Fig. 4 a Basic illustration of the hydrometallurgical system employing amino acid leach- ing, b Fraction of amino acid species in water at 25 °C and different pH, adapted from [87], Copyright 2023, with permis- sion from Elsevier 1361Journal of Material Cycles and Waste Management (2024) 26:1349–1368 environmentally responsible solutions for recycling DPCBs and conserving precious resources. Effect of contact time and temperature In general, longer contact times between the solvent and PCBs tend to enhance metal recovery, as it allows for a more complete dissolution of metals from the complex e-waste [19, 45]. However, prolonged exposure may also lead to increased energy consumption and may not always result in a proportional increase in metal yields. Optimising the leach- ing time is, therefore, crucial, and recent studies empha- sise the need to strike a balance between contact time and resource efficiency. Temperature is another critical factor that affects metal recovery from PCBs. Higher temperatures generally accelerate the leaching process by increasing the solubility of metals in the chosen solvents [83]. For exam- ple, the leaching rate of Cu from the PCBs increased with time and temperature [19]. After 1 h, the leaching efficiency reached a stable level at different temperatures: 55.0 at 20.0, 66.0 at 40.0, 85.0 at 60.0 and 92.0% at 80.0 °C. However, at 60.0 °C, the leaching efficiency continued to rise until it reached 90.0% in 2.0 h. Thus, it is highly important to strike a balance between the energy consumption required and the yield obtained in the leaching reaction. The effect of contact time and temperature on the leaching efficiency of metals from DPCBs are shown in Fig. 5 (a & b), respectively. Also, similar observations were noticed for Cu and other metals by other researchers [48, 70]. This is because higher tem- perature makes the reactant particles move faster and collide more often, which helps them overcome the energy barrier for the reaction. This means that temperature acts like a cata- lyst and speeds up the reaction [49, 83]. Nevertheless, it is essential to carefully control the temperature to prevent ther- mal degradation of the solvent or undesirable side reactions. Effect of concentration of leaching agents The concentration of leaching agents is one of the important factors that affect the recovery of metals from PCBs. Different leaching agents have different mechanisms and efficiencies for dissolving metals from PCBs. In most cases, the recovery of metals from PCBs tends to increase as the concentration of the leaching agent increases [45, 83]. This is because a higher concentration provides a larger number of reactive ions or molecules, increasing the chances of contact with the target metals. However, leaching agents at higher concentrations after reaching the optimum could decrease the recovery of metals. For example, the work of Nagarajan and Panchatcharam investigated the influence of CA concentration on metal leaching [63]. The leaching process occurred over 24.0  h at 30.0  °C under static conditions, utilising 5.0% H2O2 and 5.0% AC. The CA concentration varied from 0.5 to 2.5 M. The researchers found that the most effective CA concentration for Cu leaching is 1.5 M. At this concentration, leaching performance is enhanced because metal-citrate complexes are formed, which are subsequently broken down by H2O2 to release free metal ions into the solution. However, when CA concentration surpasses 1.5 M, it leads to the formation of more metal complexes with citrate, but a deficiency of H2O2 prevents their conversion into metal ions, resulting in reduced leaching efficiency. Effect of presence/concentration/nature of oxidants Oxidants play a pivotal role in the recovery of metals from PCBs during recycling processes. Their primary function is to facilitate the dissolution of metals by promoting oxidation on the PCB surfaces. This oxidation makes the metals more amenable to dissolution by leaching agents, effectively breaking down metal oxides and sulfides [84, 91]. This, in turn, enables the release of valuable metal ions into the leach solution, a critical step in the metal recovery process. Oxidants such as H2O2, I2, KMnO4, (K3[Fe(CN)6] or O2 not only enhance metal dissolution but also accelerate the overall leaching reaction kinetics, reducing the time required for metal extraction [80, 87]. This accelerated reaction speed is particularly advantageous for large-scale recycling operations, improving process efficiency. In the presence of I2, the recovery rates for Cu, Sn, and Ni exceeded 75.0%, and Zn showed an extraction rate of approximately 45.0% [59]. Conversely, in the absence of I2 within the ChCl:EG DES solution, metal recovery was generally less than 15.0%, except for Sn, which exhibited an extraction rate of around 100.0% from DPCB [59]. This underscores the vital role played by the presence of I2 in augmenting the leaching and extraction processes, particularly for metals like Cu, Ni, and Zn, stressing its significance in optimising the recovery of metals from the solution. Furthermore, as depicted in Fig. 5(c), increasing KMnO4 (oxidant) concentration improved Cu leaching kinetics initially but caused decreased extraction after 3.0 h [92]. This behaviour could be due to overpotential and Cu precipitation [93]. The highest Cu extraction (77.6%) was achieved at 0.01 M KMnO4 after 6.0 h, while both assays reached nearly 72.0% extraction after 24.0 h at different KMnO4 concentrations. Low KMnO4 concentrations may suffice for Cu leaching, preventing glutamate degradation, but higher concentrations can be used for faster extraction, potentially reducing capital costs in industrial applications. Similar observations were noticed by Rezaee et  al., in 1362 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 their recent study where the most significant Au recovery occurred at a KMnO4 concentration of 2.0  g/L [84]. However, when the KMnO4 was increased to 4.0, 8.0, and 16.0 g/L, the final Au recovery rates decreased to 89.0, 69.0, and 32.0%, respectively. The nature of oxidants has also been reported to affect the recovery of metals. For instance, during the recovery of Cu, as illustrated in Fig. 5(d), it was reported that the recovery platform containing H2O2 consistently demonstrated a higher Cu dissolution rate compared to KMnO4 throughout the 24.0 h leaching study [92]. It achieved a maximum Cu extraction of 92.0% after just 2.0 h of leaching. This superior performance can be attributed to the increased reaction rate of the cathode and the presence of OH−, which has a high standard reduction potential of 2.38 V [79, 94]. A Fig. 5 Effect of various influencing parameters on the recov- ery of metals from DPCBs: a contact time, b temperature, reproduced from [70], Copyright 2023, with permission from Elsevier, c concentration of oxidants, d nature of oxidants, repro- duced from [92], Copyright 2021, with permission from Else- vier, e particle size (F1  <  0.071  mm; 0.071  <  F2  <  0.100  mm; 0.100  <  F3  <  0.250  mm; 0.250  <  F4  <  0.500  mm; and F5  > 0.500 mm), adapted from [45], Copyright 2018, with permis- sion from Elsevier 1363Journal of Material Cycles and Waste Management (2024) 26:1349–1368 decline was observed in the recovery of Cu when KMnO4 was employed as presented in Fig. 5(d). This decline can also be explained by the oxidation of glutamate due to the overpotential generated by the high concentration of KmnO4 as mentioned earlier. Effect of pH The pH level plays a crucial role in the recovery of metals from PCBs regardless of the leaching agent used. Generally, the choice of pH range depends on the specific metal to be recovered. In other words, metals like Cu, Zn, Al, Au, Pb, and Ag among others exhibit better solubility and recovery rates within specific pH ranges [78, 79, 87]. For instance, an increase in the initial pH from around 8.0 to 10.0 resulted in a significant 60.0% increase in Cu extraction within 72.0 h [79]. This enhancement in Cu extraction can be attributed to the predominance of the glycinate anion at higher pH levels. Similarly, according to another study by the same research group, higher Cu and Zn extractions were achieved at elevated alkalinity (pH 11.0) [78]. In contrast, Pb exhibits better solubility at lower pH levels (pH 7.0). This finding suggests the possibility of selectively leaching Pb over Zn by adjusting the leaching pH. Furthermore, the same authors confirm that Cu, Zn, and Pb are all soluble in NH2CH2COOH (the leaching agent) over a wide pH range, with optimal pH levels for each metal. However, Al does not dissolve at lower pH levels due to its inability to form an aluminate complex (and it does not form stable glycinate complexes either). Precious metals like Au and Ag, are better extracted in an alkaline medium with pH ≥ 11.0 as demonstrated by some studies [87, 95, 96]. In a 2023 study by Hao et al., Au leaching efficiency exhibited a pH-dependent behaviour [89]. Increasing the pH from 7.0 to 8.0 resulted in a significant rise in Au leaching efficiency from 76.5% to 85.9%. However, within the pH range of 8–10, Au leaching efficiency decreased notably from 85.9% to 49.7%. This pH sensitivity was attributed to changes in the stability of the Co(II)-glycine complex, a key factor in Au oxidation. Furthermore, higher pH levels promoted the dissolution of Ag and Cu, leading to increased thiosulfate consumption and a reduction in available free thiosulfate. These combined factors contributed to the decline in Au leaching efficiency at elevated pH values. Consequently, the study identified the optimal pH for Au leaching as 8. Effect of solid/liquid ratio The S/L ratio, which represents the amount of PCB material to the leaching solution, has a notable impact on the recovery of metals from PCBs. The choice of S/L ratio is crucial, as an excessive amount of PCB material may result in inefficient mass transfer and reduced metal recovery. Conversely, insufficient PCB material may lead to a diluted leaching solution, lowering metal concentration and recovery. In general, the leaching of metals shows a notable increase when the S/L ratio is reduced until a certain threshold is reached [19, 70, 97]. For instance, as the S/L ratio decreases from 1/2.5 to 1/15, the Cu leaching rate increases substantially from 11.9 to 87.2%, and the Zn leaching rate rises from 9.6 to 73.4% [46]. Similarly, there was a substantial increase in Cu leaching, rising significantly from 60.0% at an S/L of 1:5 to a complete 100.0% leaching efficiency at an S/L ratio of 1:15 [19]. Reducing the S/L ratio effectively increases the volume of the leaching solution, which improves contact between the DPCBs powder and the leaching solution. This enhanced contact facilitates more efficient mass transfer, thereby accelerating the leaching of metals from the DPCBs [43, 45]. Moreover, in a recent study, the S/L ratio varied from 1/10 to 1/30 and at an S/L ratio of 1/10, Cu, Zn, and Ni extraction efficiencies were 66.0, 70.0, and 60.0%, respectively [70]. Nevertheless, when the S/L ratio was changed to 1/20 improved metal extraction, achieving complete Cu and Zn extraction and approximately 80.0% Ni extraction was observed. However, a further decrease in the S/L ratio did not significantly impact Ni extraction. Consequently, the S/L ratio of 1/20 was observed to be the optimal condition for metal extraction from DPCBs. Similar studies indicated that above a certain S/L, additional changes do not enhance metal extraction [84, 97]. Effect of particle size Smaller PCB particle sizes tend to offer a larger surface area for interaction with leaching agents, potentially leading to more efficient metal recovery. This is due to the increased accessibility of metals within the particles. Nevertheless, there exists an optimal particle size range where metal recovery is maximised [43, 45]. This range varies depending on the specific leaching process and the targeted metals. In this range, the balance between increased surface area and manageable processing is achieved. However, extremely fine particle sizes may not always result in higher metal recovery [46]. Beyond the optimal range, diminishing returns can occur as fine particles may aggregate, leading to processing 1364 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 challenges and lower recovery efficiency. For example, when the particle size of PCB was increased from <0.075 mm to 0.1–0.25 mm, the Cu leaching performance increased significantly from 51.3 to 99.2% [43]. This increase can be attributed to the larger surface area per unit mass, facilitating better contact between the leaching solution and Cu in fine particles. However, when the particle size was further increased from 0.25–0.5 mm to >0.5 mm, the Cu leaching rate decreased from 73.5 to 63.5%. Similarly, the maximum Cu yield reached was 94.3% as depicted in Fig. 5 (e) when the particle size was within the range of 0.100–0.250 mm [45]. However, for particle sizes larger than 0.250 mm, the Cu leaching rate showed a decreasing trend. This decline occurred because reducing the particle size below a critical level could lead to increased particle–particle collision and severe attrition. Consequently, the leaching liquid struggled to permeate through the fine DPCB powder, hindering the leaching process [98]. Conclusion and future directions The review underscores the critical importance of responsible PCB management today. As we navigate the challenges of e-waste disposal and resource scarcity, the findings presented here emphasise the need for sustainable practices and innovative solutions in the field of PCB recycling. The integration of environmentally friendly approaches, coupled with the optimisation of leaching parameters, offers a promising pathway towards reducing the environmental burden of DPCBs and harnessing their valuable metal components. In the field of metal recovery from DPCBs, the utilisation of ILs, DESs, organic acids, and amino acids has emerged as a dynamic frontier. However, not much information is available yet on the leaching of metals from DPCBs using DESs. Nevertheless, each of these solvent systems exhibits distinct interactions with metals and PCB, emphasising their versatility in metal recovery processes. Most of these recent leaching methods were able to leach base metals (Cu, Fe, Zn, Sn, etc.), as well as precious metals (Au and Ag), successfully with leaching efficiencies reaching more than 90.0% in most cases. Hence, their effectiveness rivals that of traditional hydrometallurgical approaches using inorganic acids such as H2SO4, HNO3, HCl, etc., and pyrometallurgical methods that require high-temperature operations, as well as biometallurgical methods [30, 99]. In addition, most of the studies conducted so far reported that base metals demonstrated good recovery in an acidic medium (although some authors reported otherwise), while precious metals can be easily recovered in an alkaline medium. The recovery of metals from DPCBs is influenced by several key parameters. The choice of leaching agent, including acids, organic acids, amino acids, DESs, or ILs, is crucial, as each agent has unique interactions with metals and PCB substrates. Additionally, the concentration of the leaching agent plays a significant role, with optimal concentrations varying for different agents and metals. The presence of oxidants like H2O2, I2, KMnO4, K3[Fe(CN)6] or O2 can enhance metal recovery by promoting oxidation reactions, but careful control of oxidant type and concentration is essential. pH levels impact metal complex solubility and stability, with varying optimal pH conditions for different metals and leaching agents. Particle size affects metal recovery, with smaller sizes offering greater surface area for leaching, but excessive attrition must be avoided. The S/L ratio, temperature, and contact time also play significant roles in optimising metal recovery processes from DPCBs. Significant further research is needed to enhance our comprehension of the molecular interactions, molecular dynamics, and interfacial electrochemistry between diverse leaching agents (both alkaline and acidic) and metals. Also, as we look towards the future, the need for solvent regeneration becomes a pivotal aspect of sustainable metal recovery. Furthermore, conducting a comprehensive life cycle assessment study is essential to assess the environmental impacts and sustainability of the developed metal recovery technologies. Future research should aim to quantify the environmental benefits and drawbacks, allowing for informed decisions and improvements in the technology. Finally, to bridge the gap between laboratory research and practical application, pilot-scale studies are crucial. Researchers should conduct pilot-scale experiments to validate the scalability and feasibility of the developed metal recovery technologies, laying the foundation for potential industrial implementation. Acknowledgement(s) Emmanuel A. Oke acknowledges the Research Office of the University of the Witwatersrand for the Centennial Post- doctoral Fellowship granted to him between 2023 and 2025. Authors contributions EAO: Conceptualisation, Methodology, Data curation, Writing—Original Draft, Writing—Review & Editing, Validation, Visualisation. HP: Writing—Review & Editing, Supervision, Validation, Resources. Funding Open access funding provided by University of the Witwa- tersrand. This work did not receive any specific funding. Declarations Conflict of interest The authors declare that they have no known com- peting financial interests or personal relationships that could have ap- peared to influence the work reported in this paper. 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Oke EA, Potgieter H (2024) Discarded e-waste/printed circuit boards: a review of their recent methods of disassembly, sorting and environmental implications. J Mater Cycles Waste Manag. https:// doi. org/ 10. 1007/ s10163- 024- 01917-7 2. Kaya M (2016) Recovery of metals and nonmetals from electronic waste by physical and chemical recycling processes. Waste Man- age 57:64–90. https:// doi. org/ 10. 1016/j. wasman. 2016. 08. 004 3. Oke EA, Osibanjo O, Raheem SA, Oluyinka OA, Ibe KK (2021) Metals concentration levels in printed circuit boards of discarded cathode ray tube television: trends over the years. Int J Environ Anal Chem 103:1–12. https:// doi. org/ 10. 1080/ 03067 319. 2021. 19588 02 4. Nithya R, Sivasankari C, Thirunavukkarasu A (2021) Elec- tronic waste generation, regulation and metal recovery: a review. Environ Chem Lett 19:1347–1368. https:// doi. org/ 10. 1007/ s10311- 020- 01111-9 5. Magoda K, Mekuto L (2022) Biohydrometallurgical recovery of metals from waste electronic equipment: current status and proposed process. Recycling 7:67. https:// doi. org/ 10. 3390/ recyc ling7 050067 6. Kumar A, Holuszko M, Espinosa DCR (2017) E-waste: an over- view on generation, collection, legislation and recycling practices. Resour Conserv Recycl 122:32–42. https:// doi. org/ 10. 1016/j. resco nrec. 2017. 01. 018 7. Deng S, Xiao Z, Zhang W, Noble A, Das S, Yih Y, Sutherland JW (2023) Economic analysis of precious metal recovery from elec- tronic waste through gas-assisted microflow extraction. Resour Conserv Recycl 190. https:// doi. org/ 10. 1016/j. resco nrec. 2022. 106810. 8. Needhidasan S, Samuel M, Chidambaram R (2014) Elec- tronic waste—an emerging threat to the environment of urban India. J Environ Health Sci Eng 12:36. https:// doi. org/ 10. 1186/ 2052- 336X- 12- 36 9. Forti V, Balde CP, Kuehr R, Bel G (2020) The Global E-waste Monitor 2020: quantities, flows, and the circular economy poten- tial, Bonn, Geneva and Rotterdam 10. Roy H, Rahman TU, Suhan MdBK, Al-Mamun MdR, Haque S, Islam MdS (2022) A comprehensive review on hazardous aspects and management strategies of electronic waste: Bangladesh per- spectives. Heliyon 8:e09802. https:// doi. org/ 10. 1016/j. heliy on. 2022. e09802 11. Su M, Zhu Z, Li T, Jin J, Hu J (2022) Levels, profiles and potential human health risks of brominated and parent polycyclic aromatic hydrocarbons in soils around three different types of industrial areas in China. Sci Total Environ 846:157506. https:// doi. org/ 10. 1016/j. scito tenv. 2022. 157506 12. Twagirayezu G, Irumva O, Huang K, Xia H, Uwimana A, Nizeyi- mana JC, Manzi HP, Nambajemariya F, Itangishaka AC (2022) Environmental effects of electrical and electronic waste on water and soil: a review. Pol J Environ Stud 31:2507–2525. https:// doi. org/ 10. 15244/ pjoes/ 144194. 13. Zhao W, Xu J, Fei W, Liu Z, He W, Li G (2023) The reuse of electronic components from waste printed circuit boards: a critical review. Env Sci Adv 2:196–214. https:// doi. org/ 10. 1039/ D2VA0 0266C 14. Abalansa S, El Mahrad B, Icely J, Newton A (2021) Electronic waste, an environmental problem exported to developing coun- tries: the GOOD, the BAD and the UGLY. Sustainability 13:5302. https:// doi. org/ 10. 3390/ su130 95302 15. Faraji F, Golmohammadzadeh R, Pickles CA (2022) Potential and current practices of recycling waste printed circuit boards: a review of the recent progress in pyrometallurgy. J Environ Manage 316:115242. https:// doi. org/ 10. 1016/j. jenvm an. 2022. 115242 16. Huang T, Zhu J, Huang X, Ruan J, Xu Z (2022) Assessment of precious metals positioning in waste printed circuit boards and the economic benefits of recycling. Waste Manage 139:105–115. https:// doi. org/ 10. 1016/j. wasman. 2021. 12. 030 17. Niu B, Shanshan E, Xu Z, Guo J (2023) How to efficient and high-value recycling of electronic components mounted on waste printed circuit boards: recent progress, challenge, and future per- spectives. J Clean Prod 415:137815. https:// doi. org/ 10. 1016/j. jclep ro. 2023. 137815 18. Mir S, Dhawan N (2022) A comprehensive review on the recy- cling of discarded printed circuit boards for resource recovery. Resour Conserv Recycl 178. https:// doi. org/ 10. 1016/j. resco nrec. 2021. 106027 19. Gómez M, Grimes S, Fowler G (2023) Novel hydrometallurgi- cal process for the recovery of copper from end-of-life mobile phone printed circuit boards using ionic liquids. J Clean Prod 420:138379. https:// doi. org/ 10. 1016/j. jclep ro. 2023. 138379 20. Panda R, Jadhao PR, Pant KK, Naik SN, Bhaskar T (2020) Eco-friendly recovery of metals from waste mobile printed circuit boards using low temperature roasting. J Hazard Mater 395:122642. https:// doi. org/ 10. 1016/j. jhazm at. 2020. 122642 21. Zhu Y, Li B, Wei Y, Zhou S, Wang H (2023) Recycling potential of waste printed circuit boards using pyrolysis: status quo and perspectives. Process Saf Environ Prot 173:437–451. https:// doi. org/ 10. 1016/j. psep. 2023. 03. 018 22. Yuan Z, Liu H, Yong WF, She Q, Esteban J (2022) Status and advances of deep eutectic solvents for metal separation and recovery. Green Chem 24:1895–1929. https:// doi. org/ 10. 1039/ D1GC0 3851F 23. Rezaee M, Abdollahi H, Saneie R, Mohammadzadeh A, Rezaei A, Karimi Darvanjooghi MH, Brar SK, Magdouli S (2022) A cleaner approach for high-efficiency regeneration of base and precious metals from waste printed circuit boards through stepwise oxido-acidic and thiocyanate leaching. Chemosphere 298:134283. https:// doi. org/ 10. 1016/j. chemo sphere. 2022. 134283 24. Liu K, Wang M, Tsang DCW, Liu L, Tan Q, Li J (2022) Fac- ile path for copper recovery from waste printed circuit boards via mechanochemical approach. J Hazard Mater 440:129638. https:// doi. org/ 10. 1016/j. jhazm at. 2022. 129638 25. Arya S, Kumar S (2020) Bioleaching: urban mining option to curb the menace of e-waste challenge. Bioengineered 11:640– 660. https:// doi. org/ 10. 1080/ 21655 979. 2020. 17759 88 26. Arslan V (2021) Bacterial leaching of copper, zinc, nickel and aluminum from discarded printed circuit boards using acido- philic bacteria. J Mater Cycles Waste Manag 23:2005–2015. https:// doi. org/ 10. 1007/ s10163- 021- 01274-9 27. Arshadi M, Esmaeili A, Yaghmaei S, Arab B (2022) Evaluation of Aspergillus niger and Penicillium simplicissimum for their ability to leach Zn–Ni–Cu from waste mobile phone printed circuit boards. J Mater Cycles Waste Manag 24:83–96. https:// doi. org/ 10. 1007/ s10163- 021- 01299-0 28. Ji X, Yang M, Wan A, Yu S, Yao Z (2022) Bioleaching of typi- cal electronic waste—printed circuit boards (WPCBs): a short review. Int J Environ Res Public Health 19:7508. https:// doi. org/ 10. 3390/ ijerp h1912 7508 http://creativecommons.org/licenses/by/4.0/ https://doi.org/10.1007/s10163-024-01917-7 https://doi.org/10.1016/j.wasman.2016.08.004 https://doi.org/10.1080/03067319.2021.1958802 https://doi.org/10.1007/s10311-020-01111-9 https://doi.org/10.1007/s10311-020-01111-9 https://doi.org/10.3390/recycling7050067 https://doi.org/10.1016/j.resconrec.2017.01.018 https://doi.org/10.1016/j.resconrec.2017.01.018 https://doi.org/10.1016/j.resconrec.2022.106810 https://doi.org/10.1016/j.resconrec.2022.106810 https://doi.org/10.1186/2052-336X-12-36 https://doi.org/10.1186/2052-336X-12-36 https://doi.org/10.1016/j.heliyon.2022.e09802 https://doi.org/10.1016/j.heliyon.2022.e09802 https://doi.org/10.1016/j.scitotenv.2022.157506 https://doi.org/10.1016/j.scitotenv.2022.157506 https://doi.org/10.15244/pjoes/144194 https://doi.org/10.15244/pjoes/144194 https://doi.org/10.1039/D2VA00266C https://doi.org/10.1039/D2VA00266C https://doi.org/10.3390/su13095302 https://doi.org/10.1016/j.jenvman.2022.115242 https://doi.org/10.1016/j.wasman.2021.12.030 https://doi.org/10.1016/j.jclepro.2023.137815 https://doi.org/10.1016/j.jclepro.2023.137815 https://doi.org/10.1016/j.resconrec.2021.106027 https://doi.org/10.1016/j.resconrec.2021.106027 https://doi.org/10.1016/j.jclepro.2023.138379 https://doi.org/10.1016/j.jhazmat.2020.122642 https://doi.org/10.1016/j.psep.2023.03.018 https://doi.org/10.1016/j.psep.2023.03.018 https://doi.org/10.1039/D1GC03851F https://doi.org/10.1039/D1GC03851F https://doi.org/10.1016/j.chemosphere.2022.134283 https://doi.org/10.1016/j.chemosphere.2022.134283 https://doi.org/10.1016/j.jhazmat.2022.129638 https://doi.org/10.1080/21655979.2020.1775988 https://doi.org/10.1007/s10163-021-01274-9 https://doi.org/10.1007/s10163-021-01299-0 https://doi.org/10.1007/s10163-021-01299-0 https://doi.org/10.3390/ijerph19127508 https://doi.org/10.3390/ijerph19127508 1366 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 29. Ghulam ST, Abushammala H (2023) Challenges and opportu- nities in the management of electronic waste and its impact on human health and environment. Sustainability 15:1837. https:// doi. org/ 10. 3390/ su150 31837 30. Udayakumar S, Razak MIBA, Ismail S (2022) Recovering valu- able metals from Waste Printed Circuit Boards (WPCB): a short review. Mater Today Proc 66:3062–3070. https:// doi. org/ 10. 1016/j. matpr. 2022. 07. 364 31. Li H, Eksteen J, Oraby E (2018) Hydrometallurgical recovery of metals from waste printed circuit boards (WPCBs): current status and perspectives—A review. Resour Conserv Recycl 139:122–139. https:// doi. org/ 10. 1016/j. resco nrec. 2018. 08. 007 32. Mishra G, Jha R, Rao MD, Meshram A, Singh KK (2021) Recovery of silver from waste printed circuit boards (WPCBs) through hydrometallurgical route: a review. Environ Chall 4:100073. https:// doi. org/ 10. 1016/j. envc. 2021. 100073 33. Yaashikaa PR, Priyanka B, Senthil Kumar P, Karishma S, Jee- vanantham S, Indraganti S (2022) A review on recent advance- ments in recovery of valuable and toxic metals from e-waste using bioleaching approach. Chemosphere 287:132230. https:// doi. org/ 10. 1016/j. chemo sphere. 2021. 132230. 34. Trivedi A, Vishwakarma A, Saawarn B, Mahanty B, Hait S (2022) Fungal biotechnology for urban mining of metals from waste printed circuit boards: a review. J Environ Manage 323:116133. https:// doi. org/ 10. 1016/j. jenvm an. 2022. 116133 35. Zupanc A, Install J, Jereb M, Repo T (2023) Sustainable and selective modern methods of noble metal recycling. Angew Chem Int Ed 62. https:// doi. org/ 10. 1002/ anie. 20221 4453. 36. Khan A, Inamuddin, Asiri AM (2020) E-waste recycling and man- agement: present scenarios and environmetnal issues. Springer Cham. 37. Dave SR (2018) Microbial technology for metal recovery from e-waste printed circuit boards. J Bacteriol Mycol: Open Access 6. https:// doi. org/ 10. 15406/ jbmoa. 2018. 06. 00212. 38. Oke EA, Ijardar SP (2023) The roles of ionic liquids in separation of pollutants from Aqueous Solutions. In: Properties and applica- tions of ionic liquids, 1st edn. Nova Science Publishers. https:// doi. org/ 10. 52305/ HFEO4 188. 39. Oke EA, Oluyinka OA, Afolabi SD, Ibe KK, Raheem SA (2023) Latest insights on technologies for halides and halogenated com- pounds extraction/abatement from water and wastewater: chal- lenges and future perspectives. J Water Proc Eng 53. https:// doi. org/ 10. 1016/j. jwpe. 2023. 103724. 40. Teimouri S, Potgieter JH, Simate GS, van Dyk L, Dworzanowski M (2020) Oxidative leaching of refractory sulphidic gold tailings with an ionic liquid. Miner Eng 156:106484. https:// doi. org/ 10. 1016/j. mineng. 2020. 106484 41. AliAkbari R, Marfavi Y, Kowsari E, Ramakrishna S (2020) Recent studies on ionic liquids in metal recovery from e-waste and secondary sources by liquid-liquid extraction and electrodepo- sition: a review. Mater Circ Econ 2:10. https:// doi. org/ 10. 1007/ s42824- 020- 00010-2 42. Kaim V, Rintala J, He C (2023) Selective recovery of rare earth elements from e-waste via ionic liquid extraction: a review. Sep Purif Technol 306. https:// doi. org/ 10. 1016/j. seppur. 2022. 122699. 43. Huang J, Chen M, Chen H, Chen S, Sun Q (2014) Leaching behavior of copper from waste printed circuit boards with Brøn- sted acidic ionic liquid. Waste Manage 34:483–488. https:// doi. org/ 10. 1016/j. wasman. 2013. 10. 027 44. Chen M, Zhang S, Huang J, Chen H (2015) Lead during the leach- ing process of copper from waste printed circuit boards by five typical ionic liquid acids. J Clean Prod 95. https:// doi. org/ 10. 1016/j. jclep ro. 2015. 02. 045. 45. Zhang D, Dong L, Li Y, Wu Y, Ma Y, Yang B (2018) Copper leaching from waste printed circuit boards using typical acidic ionic liquids recovery of e-wastes’ surplus value. Waste Manage 78:191–197. https:// doi. org/ 10. 1016/j. wasman. 2018. 05. 036 46. Li F, Chen M (2017) Copper recovery from waste printed circuit boards and the correlation of CU, PB, ZN by ionic liquid. Environ Prot Eng 43:55–66. https:// doi. org/ 10. 37190/ epe17 0405. 47. Barrueto Y, Hernández P, Jiménez Y, Morales J (2021) Leaching of metals from printed circuit boards using ionic liquids. J Mater Cycles Waste Manag 23:2028–2036. https:// doi. org/ 10. 1007/ s10163- 021- 01275-8 48. Wstawski S, Emmons-Burzyńska M, Rzelewska-Piekut M, Skr- zypczak A, Regel-Rosocka M (2021) Studies on copper(II) leach- ing from e-waste with hydrogen sulfate ionic liquids: effect of hydrogen peroxide. Hydrometallurgy 205:105730. https:// doi. org/ 10. 1016/j. hydro met. 2021. 105730 49. He J, Yang J, Tariq SM, Duan C, Zhao Y (2020) Comparative investigation on copper leaching efficiency from waste mobile phones using various types of ionic liquids. J Clean Prod 256:120368. https:// doi. org/ 10. 1016/j. jclep ro. 2020. 120368 50. Zhang Y, Chen M, Tan Q, Wang B, Chen S (2018) Recovery of copper from WPCBs using slurry electrolysis with ionic liquid [BSO3HPy]∙HSO4. Hydrometallurgy 175:150–154. https:// doi. org/ 10. 1016/j. hydro met. 2017. 11. 004 51. Lin X, Tran DT, Choi J-W, Song M-H, Yun Y-S (2023) Ionic liq- uid-modified chitosan fibers for Au(I) recovery from waste printed circuit boards bioleachate: preparation, adsorption mechanism, and application. Sep Purif Technol 306:122533. https:// doi. org/ 10. 1016/j. seppur. 2022. 122533 52. Mokhodoeva O, Maksimova V, Shishov A, Shkinev V (2023) Separation of platinum group metals using deep eutectic sol- vents based on quaternary ammonium salts. Sep Purif Technol 305:122427. https:// doi. org/ 10. 1016/j. seppur. 2022. 122427 53. E.A. Oke, R. Sharma, N.I. Malek, S.P. Ijardar, Applications of deep eutectic solvents in remediation of emerging contaminants. In: Current developments in biotechnology and bioengineering, Elsevier, 2022: pp. 223–246. https:// doi. org/ 10. 1016/ B978-0- 323- 99905-2. 00004-2. 54. Oke EA, Ijardar SP (2023) Aqueous biphasic systems composed of alcohol-based deep eutectic solvents and inorganic salts: application in the extraction of dyes with varying hydropho- bicity. J Mol Liq 375:121372. https:// doi. org/ 10. 1016/j. molliq. 2023. 121372 55. Wang M, Liu K, Xu Z, Dutta S, Valix M, Alessi DS, Huang L, Zimmerman JB, Tsang DCW (2023) Selective extraction of criti- cal metals from spent lithium-ion batteries. Environ Sci Technol 57:3940–3950. https:// doi. org/ 10. 1021/ acs. est. 2c076 89 56. Łukomska A, Wiśniewska A, Dąbrowski Z, Lach J, Wróbel K, Kolasa D, Domańska U (2022) Recovery of metals from elec- tronic waste-printed circuit boards by ionic liquids DESs and organophosphorous-based acid extraction. Molecules 27:4984. https:// doi. org/ 10. 3390/ molec ules2 71549 84 57. Zhao Q, Ma S, Ho W, Wang Y, Ho JYT, Shih K (2023) Simple and environmentally friendly metal recovery from waste printed circuit boards by using deep eutectic solvents. J Clean Prod 421:138508. https:// doi. org/ 10. 1016/j. jclep ro. 2023. 138508 58. Marin Rivera R, Zante G, Hartley JM, Ryder KS, Abbott AP (2022) Catalytic dissolution of metals from printed circuit boards using a calcium chloride–based deep eutectic solvent, Green Chem 24: 3023–3034. https:// doi. org/ 10. 1039/ D1GC0 4694B. 59. Sabzkoohi HA, Kolliopoulos G (2021) Green zero-waste metal extraction and recycling from printed circuit boards. In: Inter- national Conference on Raw Materials and Circular Economy, MDPI, Basel Switzerland, p 39. https:// doi. org/ 10. 3390/ mater proc2 02100 5039. 60. Krishnamoorthy S, Ramakrishnan G, Dhandapani B (2021) Recovery of valuable metals from waste printed circuit boards https://doi.org/10.3390/su15031837 https://doi.org/10.3390/su15031837 https://doi.org/10.1016/j.matpr.2022.07.364 https://doi.org/10.1016/j.matpr.2022.07.364 https://doi.org/10.1016/j.resconrec.2018.08.007 https://doi.org/10.1016/j.envc.2021.100073 https://doi.org/10.1016/j.chemosphere.2021.132230 https://doi.org/10.1016/j.chemosphere.2021.132230 https://doi.org/10.1016/j.jenvman.2022.116133 https://doi.org/10.1002/anie.202214453 https://doi.org/10.15406/jbmoa.2018.06.00212 https://doi.org/10.52305/HFEO4188 https://doi.org/10.52305/HFEO4188 https://doi.org/10.1016/j.jwpe.2023.103724 https://doi.org/10.1016/j.jwpe.2023.103724 https://doi.org/10.1016/j.mineng.2020.106484 https://doi.org/10.1016/j.mineng.2020.106484 https://doi.org/10.1007/s42824-020-00010-2 https://doi.org/10.1007/s42824-020-00010-2 https://doi.org/10.1016/j.seppur.2022.122699 https://doi.org/10.1016/j.wasman.2013.10.027 https://doi.org/10.1016/j.wasman.2013.10.027 https://doi.org/10.1016/j.jclepro.2015.02.045 https://doi.org/10.1016/j.jclepro.2015.02.045 https://doi.org/10.1016/j.wasman.2018.05.036 https://doi.org/10.37190/epe170405 https://doi.org/10.1007/s10163-021-01275-8 https://doi.org/10.1007/s10163-021-01275-8 https://doi.org/10.1016/j.hydromet.2021.105730 https://doi.org/10.1016/j.hydromet.2021.105730 https://doi.org/10.1016/j.jclepro.2020.120368 https://doi.org/10.1016/j.hydromet.2017.11.004 https://doi.org/10.1016/j.hydromet.2017.11.004 https://doi.org/10.1016/j.seppur.2022.122533 https://doi.org/10.1016/j.seppur.2022.122533 https://doi.org/10.1016/j.seppur.2022.122427 https://doi.org/10.1016/B978-0-323-99905-2.00004-2 https://doi.org/10.1016/B978-0-323-99905-2.00004-2 https://doi.org/10.1016/j.molliq.2023.121372 https://doi.org/10.1016/j.molliq.2023.121372 https://doi.org/10.1021/acs.est.2c07689 https://doi.org/10.3390/molecules27154984 https://doi.org/10.1016/j.jclepro.2023.138508 https://doi.org/10.1039/D1GC04694B https://doi.org/10.3390/materproc2021005039 https://doi.org/10.3390/materproc2021005039 1367Journal of Material Cycles and Waste Management (2024) 26:1349–1368 using organic acids synthesised by Aspergillus niveus. IET Nano- biotechnol 15:212–220. https:// doi. org/ 10. 1049/ nbt2. 12001 61. Jadhav U, Su C, Hocheng H (2016) Leaching of metals from large pieces of printed circuit boards using citric acid and hydrogen peroxide. Environ Sci Pollut Res 23:24384–24392. https:// doi. org/ 10. 1007/ s11356- 016- 7695-9 62. Anwer S, Panghal A, Majid I, Mallick S (2022) Urban min- ing: recovery of metals from printed circuit boards. Int J Environ Sci Technol 19:9731–9740. https:// doi. org/ 10. 1007/ s13762- 021- 03662-y 63. Nagarajan N, Panchatcharam P (2023) Cost-effective and eco- friendly copper recovery from waste printed circuit boards using organic chemical leaching. Heliyon 9:e13806. https:// doi. org/ 10. 1016/j. heliy on. 2023. e13806 64. Ozairy R, Rastegar SO, Beigzadeh R, Gu T (2022) Optimiza- tion of metal bio-acid leaching from mobile phone printed circuit boards using natural organic acids and H2O2. J Mater Cycles Waste Manag 24:179–188. https:// doi. org/ 10. 1007/ s10163- 021- 01302-8 65. Lu J, Dreisinger D (2021) Lead electrowinning from methane sulfonic acid. Hydrometallurgy 203:105623. https:// doi. org/ 10. 1016/j. hydro met. 2021. 105623 66. Binnemans K, Jones PT (2023) Methanesulfonic acid (MSA) in hydrometallurgy. J Sustain Met 9:26–45. https:// doi. org/ 10. 1007/ s40831- 022- 00641-6 67. Gernon MD, Wu M, Buszta T, Janney P (1999) Environmental benefits of methanesulfonic acid. Green Chem 1:127–140. https:// doi. org/ 10. 1039/ a9001 57c 68. Palden T, Onghena B, Regadío M, Binnemans K (2019) Meth- anesulfonic acid: a sustainable acidic solvent for recovering metals from the jarosite residue of the zinc industry. Green Chem 21:5394–5404. https:// doi. org/ 10. 1039/ C9GC0 2238D 69. Guthrie JP, Gallant RT (2000) Thermodynamics of methanesul- fonic acid revisited. Can J Chem 78:1295–1298. https:// doi. org/ 10. 1139/ v00- 134 70. Jadhao PR, Mishra S, Singh A, Pant KK, Nigam KDP (2023) A sustainable route for the recovery of metals from waste printed circuit boards using methanesulfonic acid. J Environ Manage 335:117581. https:// doi. org/ 10. 1016/j. jenvm an. 2023. 117581 71. Dietzen DJ (2018) Amino acids, peptides, and proteins, In: Principles and applications of molecular diagnostics, Else- vier, p 345–380. https:// doi. org/ 10. 1016/ B978-0- 12- 816061- 9. 00013-8. 72. Scott T (1988) Chemistry and biochemistry of the amino acids. Biochem Educ 16:118. https:// doi. org/ 10. 1016/ 0307- 4412(88) 90106-9 73. Khezri M, Rezai B, Akbar Abdollahzadeh A, Molaeinasab M, Wilson BP, Lundström M (2020) Glycine leaching of Sarchesh- meh chalcopyrite concentrate at high pulp densities in a stirred tank reactor. Miner Eng 157. https:// doi. org/ 10. 1016/j. mineng. 2020. 106555. 74. Barani K, Kogani Y, Nazarian F (2022) Leaching of com- plex gold ore using a cyanide-glycine solution. Miner Eng 180:107475. https:// doi. org/ 10. 1016/j. mineng. 2022. 107475 75. Barani K, Dehghani M, Azadi MR, Karrech A (2021) Leach- ing of a polymetal gold ore and reducing cyanide consumption using cyanide-glycine solutions. Miner Eng 163:106802. https:// doi. org/ 10. 1016/j. mineng. 2021. 106802 76. Altinkaya P, Wang Z, Korolev I, Hamuyuni J, Haapalainen M, Kolehmainen E, Yliniemi K, Lundström M (2020) Leaching and recovery of gold from ore in cyanide-free glycine media. Miner Eng 158. https:// doi. org/ 10. 1016/j. mineng. 2020. 106610. 77. Wang J, Wang R, Pan Y, Liu F, Xu Z (2022) Thermodynamic analysis of gold leaching by copper-glycine-thiosulfate solu- tions using Eh-pH and species distribution diagrams. Miner Eng 179:107438. https:// doi. org/ 10. 1016/j. mineng. 2022. 107438 78. Oraby EA, Li H, Eksteen JJ (2020) An alkaline glycine-based leach process of base and precious metals from powdered waste printed circuit boards. Waste Biomass Valori 11:3897–3909. https:// doi. org/ 10. 1007/ s12649- 019- 00780-0 79. Li H, Oraby E, Eksteen J (2020) Extraction of copper and the co-leaching behaviour of other metals from waste printed circuit boards using alkaline glycine solutions. Resour Conserv Recycl 154:104624. https:// doi. org/ 10. 1016/j. resco nrec. 2019. 104624 80. Han Y, Yi X, Wang R, Huang J, Chen M, Sun Z, Sun S, Shu J (2020) Copper extraction from waste printed circuit boards by glycine. Sep Purif Technol 253:117463. https:// doi. org/ 10. 1016/j. seppur. 2020. 117463 81. Li H, Oraby E, Eksteen J (2021) Recovery of copper and the deportment of other base metals from alkaline glycine leachates derived from waste printed circuit boards (WPCBs). Hydromet- allurgy 199:105540. https:// doi. org/ 10. 1016/j. hydro met. 2020. 105540 82. Mokhlis H, Daoudi RD, Azzi M (2021) Selective leaching of copper from waste printed circuit boards (Pcbs) using glycine as a complexing agent. Global Nest Journal 23. https:// doi. org/ 10. 30955/ gnj. 003361. 83. Broeksma CP, Dorfling C (2023) Evaluating glycine as an alter- native lixiviant for copper recovery from waste printed circuit boards. Waste Manage 163:96–107. https:// doi. org/ 10. 1016/j. wasman. 2023. 03. 030 84. Rezaee M, Saneie R, Mohammadzadeh A, Abdollahi H, Kord- loo M, Rezaee A, Vahidi E (2023) Eco-friendly recovery of base and precious metals from waste printed circuit boards by step-wise glycine leaching: process optimization, kinetics modeling, and comparative life cycle assessment. J Clean Prod 389:136016. https:// doi. org/ 10. 1016/j. jclep ro. 2023. 136016 85. Khetwunchai N, Akeprathumchai S, Thiravetyan P (2023) Recovery of copper and gold from waste printed circuit boards using monosodium glutamate supplemented with hydrogen per- oxide. Minerals 13:321. https:// doi. org/ 10. 3390/ min13 030321 86. Li H, Oraby EA, Eksteen JJ (2022) Development of an inte- grated glycine-based process for base and precious metals recovery from waste printed circuit boards. Resour Conserv Recycl 187:106631. https:// doi. org/ 10. 1016/j. resco nrec. 2022. 106631 87. Li H, Deng Z, Oraby E, Eksteen J (2023) Amino acids as lixivi- ants for metals extraction from natural and secondary resources with emphasis on glycine: a literature review, Hydrometallurgy 216. https:// doi. org/ 10. 1016/j. hydro met. 2022. 106008. 88. Li H, Oraby E, Eksteen J (2022) Extraction of precious met- als from waste printed circuit boards using cyanide-free alka- line glycine solution in the presence of an oxidant. Miner Eng 181:107501. https:// doi. org/ 10. 1016/j. mineng. 2022. 107501 89. Hao J, Wang X, Wang Y, Guo F, Wu Y (2023) Study of gold leaching from pre-treated waste printed circuit boards by thio- sulfate-cobalt-glycine system and separation by solvent extrac- tion. Hydrometallurgy 221:106141. https:// doi. org/ 10. 1016/j. hydro met. 2023. 106141 90. Godigamuwa K, Okibe N (2023) Gold leaching from printed circuit boards using a novel synergistic effect of glycine and thiosulfate. Minerals 13:1270. https:// doi. org/ 10. 3390/ min13 101270 91. Gulliani S, Volpe M, Messineo A, Volpe R (2023) Recovery of metals and valuable chemicals from waste electric and elec- tronic materials: a critical review of existing technologies. RSC Sustainability 1:1085–1108. https:// doi. org/ 10. 1039/ D3SU0 0034F 92. Perea CG, Baena OJR, Ihle CF, Estay H (2021) Copper leach- ing from wastes electrical and electronic equipment (WEEE) using alkaline monosodium glutamate: thermodynamics and https://doi.org/10.1049/nbt2.12001 https://doi.org/10.1007/s11356-016-7695-9 https://doi.org/10.1007/s11356-016-7695-9 https://doi.org/10.1007/s13762-021-03662-y https://doi.org/10.1007/s13762-021-03662-y https://doi.org/10.1016/j.heliyon.2023.e13806 https://doi.org/10.1016/j.heliyon.2023.e13806 https://doi.org/10.1007/s10163-021-01302-8 https://doi.org/10.1007/s10163-021-01302-8 https://doi.org/10.1016/j.hydromet.2021.105623 https://doi.org/10.1016/j.hydromet.2021.105623 https://doi.org/10.1007/s40831-022-00641-6 https://doi.org/10.1007/s40831-022-00641-6 https://doi.org/10.1039/a900157c https://doi.org/10.1039/a900157c https://doi.org/10.1039/C9GC02238D https://doi.org/10.1139/v00-134 https://doi.org/10.1139/v00-134 https://doi.org/10.1016/j.jenvman.2023.117581 https://doi.org/10.1016/B978-0-12-816061-9.00013-8 https://doi.org/10.1016/B978-0-12-816061-9.00013-8 https://doi.org/10.1016/0307-4412(88)90106-9 https://doi.org/10.1016/0307-4412(88)90106-9 https://doi.org/10.1016/j.mineng.2020.106555 https://doi.org/10.1016/j.mineng.2020.106555 https://doi.org/10.1016/j.mineng.2022.107475 https://doi.org/10.1016/j.mineng.2021.106802 https://doi.org/10.1016/j.mineng.2021.106802 https://doi.org/10.1016/j.mineng.2020.106610 https://doi.org/10.1016/j.mineng.2022.107438 https://doi.org/10.1007/s12649-019-00780-0 https://doi.org/10.1016/j.resconrec.2019.104624 https://doi.org/10.1016/j.seppur.2020.117463 https://doi.org/10.1016/j.seppur.2020.117463 https://doi.org/10.1016/j.hydromet.2020.105540 https://doi.org/10.1016/j.hydromet.2020.105540 https://doi.org/10.30955/gnj.003361 https://doi.org/10.30955/gnj.003361 https://doi.org/10.1016/j.wasman.2023.03.030 https://doi.org/10.1016/j.wasman.2023.03.030 https://doi.org/10.1016/j.jclepro.2023.136016 https://doi.org/10.3390/min13030321 https://doi.org/10.1016/j.resconrec.2022.106631 https://doi.org/10.1016/j.resconrec.2022.106631 https://doi.org/10.1016/j.hydromet.2022.106008 https://doi.org/10.1016/j.mineng.2022.107501 https://doi.org/10.1016/j.hydromet.2023.106141 https://doi.org/10.1016/j.hydromet.2023.106141 https://doi.org/10.3390/min13101270 https://doi.org/10.3390/min13101270 https://doi.org/10.1039/D3SU00034F https://doi.org/10.1039/D3SU00034F 1368 Journal of Material Cycles and Waste Management (2024) 26:1349–1368 dissolution tests. Clean Eng Technol 5:100312. https:// doi. org/ 10. 1016/j. clet. 2021. 100312 93. Oraby EA, Eksteen JJ, O’Connor GM (2020) Gold leaching from oxide ores in alkaline glycine solutions in the presence of permanganate. Hydrometallurgy 198:105527. https:// doi. org/ 10. 1016/j. hydro met. 2020. 105527 94. Oraby EA, Eksteen JJ (2014) The selective leaching of copper from a gold–copper concentrate in glycine solutions. Hydromet- allurgy 150:14–19. https:// doi. org/ 10. 1016/j. hydro met. 2014. 09. 005 95. Oraby EA, Eksteen JJ (2015) Gold leaching in cyanide-starved copper solutions in the presence of glycine. Hydrometallurgy 156:81–88. https:// doi. org/ 10. 1016/j. hydro met. 2015. 05. 012 96. Eksteen JJ, Oraby EA (2015) The leaching and adsorption of gold using low concentration amino acids and hydrogen perox- ide: effect of catalytic ions, sulphide minerals and amino acid type. Miner Eng 70:36–42. https:// doi. org/ 10. 1016/j. mineng. 2014. 08. 020 97. Jadhao P, Chauhan G, Pant KK, Nigam KDP (2016) Greener approach for the extraction of copper metal from electronic waste. Waste Manage 57:102–112. https:// doi. org/ 10. 1016/j. wasman. 2015. 11. 023 98. Zhu N, Xiang Y, Zhang T, Wu P, Dang Z, Li P, Wu J (2011) Bioleaching of metal concentrates of waste printed circuit boards by mixed culture of acidophilic bacteria. J Hazard Mater 192:614–619. https:// doi. org/ 10. 1016/j. jhazm at. 2011. 05. 062 99. Huy Do M, Tien Nguyen G, Dong Thach U, Lee Y, Huu Bui T (2023) Advances in hydrometallurgical approaches for gold recovery from E-waste: a comprehensive review and perspec- tives, Miner Eng 191:107977. https:// doi. org/ 10. 1016/j. mineng. 2022. 107977. Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. https://doi.org/10.1016/j.clet.2021.100312 https://doi.org/10.1016/j.clet.2021.100312 https://doi.org/10.1016/j.hydromet.2020.105527 https://doi.org/10.1016/j.hydromet.2020.105527 https://doi.org/10.1016/j.hydromet.2014.09.005 https://doi.org/10.1016/j.hydromet.2014.09.005 https://doi.org/10.1016/j.hydromet.2015.05.012 https://doi.org/10.1016/j.mineng.2014.08.020 https://doi.org/10.1016/j.mineng.2014.08.020 https://doi.org/10.1016/j.wasman.2015.11.023 https://doi.org/10.1016/j.wasman.2015.11.023 https://doi.org/10.1016/j.jhazmat.2011.05.062 https://doi.org/10.1016/j.mineng.2022.107977 https://doi.org/10.1016/j.mineng.2022.107977 Recent chemical methods for metals recovery from printed circuit boards: A review Abstract Introduction Recovery methods for metals from DPCBs Recovery of metals mediated by ionic liquids Recovery of metals mediated by deep eutectic solvents Recovery of metals mediated by organic acids Recovery of metals mediated by amino acids Factors affecting the recovery of metals from DPCBs Effect of contact time and temperature Effect of concentration of leaching agents Effect of presenceconcentrationnature of oxidants Effect of pH Effect of solidliquid ratio Effect of particle size Conclusion and future directions Acknowledgement(s) References