An Old Technique with A Promising Future: Recent Advances in the Use of Electrodeposition for Metal Recovery

Although the first published works on electrodeposition dates from more than one century ago (1905), the uses of this technique in the recovery of metals are attracting an increasing interest from the scientific community in the recent years. Moreover, the intense use of metals in electronics and the necessity to assure a second life of these devices in a context of circular economy, have increased the interest of the scientific community on electrodeposition, with almost 3000 works published per year nowadays. In this review, we aim to revise the most relevant and recent publications in the application of electrodeposition for metal recovery. These contributions have been classified into four main groups of approaches: (1) treatment and reuse of wastewater; (2) use of ionic liquids; (3) use of bio-electrochemical processes (microbial fuel cells and microbial electrolysis cells) and (4) integration of electrodeposition with other processes (bioleaching, adsorption, membrane processes, etc.). This would increase the awareness about the importance of the technology and would serve as a starting point for anyone that aims to start working in the field.


Introduction
Electrodeposition (ED) has been studied as one of the key electrochemical technologies for more than a century. According to the Scopus database, the first article about electrodeposition was published in 1905 [1] and studied the electrodeposition of copper upon iron. Since then, the scientific community has added new materials, methods, conditions and many other elements in order to improve this technique and its results and to widen the range of potential applications. This growing interest has caused an exponential increase in the number of publications regarding electrodeposition. Thus, Figure 1 shows the number of works published, according to the Scopus database, related to the terms "electrodeposition" or "electrochemical deposition".
As it can be seen in Figure 1, there is an exponential increase in the number of articles published in this topic, reaching a total amount of almost 56,000 papers and 3000 articles in the recent 2020 (1771 in 2021 until 23 July). This evolution means that, although electrodeposition is a mature technology, the range of applications in which this technique can be applied is widening, increasing the interest of the scientific community on its development.
In order to focus on the topic of this review, the term "metal recovery" was included to refine this search. The results of this refined search in the Scopus database were analyzed by the software VOSviewer, a free software developed by Nees Jan van Eck and Ludo Waltman at Leiden University's Centre for Science and Technology Studies (CWTS). Two figures were created in order to show the countries that have devoted more interest to this topic ( Figure 2) and the most used keywords ( Figure 3). Moreover, the connections between papers are also shown in these figures. In the case of Figure 2, it was restricted to countries with a minimum of 13 publications, meanwhile for Figure 3, a minimum number In order to focus on the topic of this review, the term "metal recovery" was included to refine this search. The results of this refined search in the Scopus database were analyzed by the software VOSviewer, a free software developed by Nees Jan van Eck and Ludo Waltman at Leiden University's Centre for Science and Technology Studies (CWTS). Two figures were created in order to show the countries that have devoted more interest to this topic ( Figure 2) and the most used keywords ( Figure 3). Moreover, the connections between papers are also shown in these figures. In the case of Figure 2, it was restricted to countries with a minimum of 13 publications, meanwhile for Figure 3, a minimum number of occurrences of an author keyword of 13 was also used in order to limit the complexity of the figures.  In order to focus on the topic of this review, the term "metal recovery" was included to refine this search. The results of this refined search in the Scopus database were analyzed by the software VOSviewer, a free software developed by Nees Jan van Eck and Ludo Waltman at Leiden University's Centre for Science and Technology Studies (CWTS). Two figures were created in order to show the countries that have devoted more interest to this topic ( Figure 2) and the most used keywords ( Figure 3). Moreover, the connections between papers are also shown in these figures. In the case of Figure 2, it was restricted to countries with a minimum of 13 publications, meanwhile for Figure 3, a minimum number of occurrences of an author keyword of 13 was also used in order to limit the complexity of the figures. Almost 1300 papers were found for this topic. As it can be observed in Figure 2, China appears as the country which has devoted the highest interest in the development of electrodeposition for metal recovery. It is followed by the United States, United Kingdom, India and Japan. Regarding Figure 3, apart from electrodeposition and metal recovery (terms included in the search restrictions), recycling, leaching, different metals, ionic liquids, microbial electrolysis cell, microbial fuel cell and wastewater treatment are among the most common topics in this research field.
The use of electrodeposition for the recovery of metal ions is a topic closely related to environmental applications, as the presence of heavy metals represents an environmental issue of increasing concern, due to the massive use of metals in electronics and other applications and its inherent bioaccumulation and potential risks for the human health [2][3][4][5][6]. If this treatment is performed by electrodeposition, the removal of heavy metals is performed together with its recovery, working on the field of circular economy, an essential part of the main research programs worldwide, including Horizon Europe of the European Commission. Almost 1300 papers were found for this topic. As it can be observed in Figure 2, China appears as the country which has devoted the highest interest in the development of electrodeposition for metal recovery. It is followed by the United States, United Kingdom, India and Japan. Regarding Figure 3, apart from electrodeposition and metal recovery (terms included in the search restrictions), recycling, leaching, different metals, ionic liquids, microbial electrolysis cell, microbial fuel cell and wastewater treatment are among the most common topics in this research field.
The use of electrodeposition for the recovery of metal ions is a topic closely related to environmental applications, as the presence of heavy metals represents an environmental issue of increasing concern, due to the massive use of metals in electronics and other applications and its inherent bioaccumulation and potential risks for the human health [2][3][4][5][6]. If this treatment is performed by electrodeposition, the removal of heavy metals is performed together with its recovery, working on the field of circular economy, an essential part of the main research programs worldwide, including Horizon Europe of the European Commission.
Based on this, the main aim of this paper is to review the most recent and relevant applications of electrodeposition, focusing on the recovery of metals. According to the most common topics of recent research work, the articles have been classified into four different groups: (1) treatment and reuse of wastewater; (2) use of ionic liquids; (3) use of bio-electrochemical processes (microbial fuel cells and microbial electrolysis cells) and (4) integration of electrodeposition with other processes (bioleaching, adsorption, electrodialysis, etc.).

Treatment and Reuse of Wastewater by Electrodeposition
Wastewater from industries, hospitals and domestic applications are an environmental issue of increasing concern nowadays [7-10]. As it is stated in several of the United Nations Global Development Goals and it all the research programs worldwide, it is compulsory to look for water treatments in order to obtain clean water without metals, plastics and microplastics, pesticides and other pollutants [11][12][13].
In this field, electrodeposition serves as a plausible technology to recover a wide spectrum of metals and to remove them from the target effluents to be treated. Table 1 gathers some of the most recent works in this field, including the main objective of the work, the effluent treated and the main conclusions obtained. Based on this, the main aim of this paper is to review the most recent and relevant applications of electrodeposition, focusing on the recovery of metals. According to the most common topics of recent research work, the articles have been classified into four different groups: (1) treatment and reuse of wastewater; (2) use of ionic liquids; (3) use of bio-electrochemical processes (microbial fuel cells and microbial electrolysis cells) and (4) integration of electrodeposition with other processes (bioleaching, adsorption, electrodialysis, etc.).

Treatment and Reuse of Wastewater by Electrodeposition
Wastewater from industries, hospitals and domestic applications are an environmental issue of increasing concern nowadays [7-10]. As it is stated in several of the United Nations Global Development Goals and it all the research programs worldwide, it is compulsory to look for water treatments in order to obtain clean water without metals, plastics and microplastics, pesticides and other pollutants [11][12][13].
In this field, electrodeposition serves as a plausible technology to recover a wide spectrum of metals and to remove them from the target effluents to be treated. Table 1 gathers some of the most recent works in this field, including the main objective of the work, the effluent treated and the main conclusions obtained.
Generally, the articles published in the field are divided between those treating real wastewater [14][15][16][17] and those that elaborate a synthetic one [18][19][20][21][22] in order to simulate real effluents to set the material required for the installation, operational conditions, parameters to measure, elements to quantify and the equipment to use for it.
The common presence of copper in both electronic or deplating wastewaters joined to its high value of reduction potential (+0.34 V vs. SHE) and its consequent facile electrodeposition. This metal is the most commonly studied in papers regarding the recovery of metals by electrodeposition, although the recovery of other metals such as Co, Ni, Pd, Pb, Zn or Te is also evaluated, as can be observed in Table 1.
In the recent papers published about metal recovery by electrodeposition, it is possible to find several approaches to enhance the overall behavior of the process. The first interesting approach is coupling the cathodic recovery of target metals to the anodic oxidation of organic matter, as it is the case of the work of Gu et al. [14], who studied the reduction of chemical oxygen demand (COD) by oxidation of plastic deplating wastewater, obtaining a reduction of COD from 1360 mg L −1 to 378 mg L −1 after the electrochemical oxidation and a subsequent oxidation by H 2 O 2 . In the same line, Gu et al. [16] evaluated the simultaneous removal of phenol and recovery of several heavy metals from petrochemical wastewaters. Maximum electric power density of 10.5 W·m −2 using a spontaneous Fe/Cu 2+ galvanic cell. 98.6% recovery Cu. Electric energy production of 3.3 kW·h using a 1.0 m 2 area membrane produces 1 kg copper and 100 L of water in 122 h of operation.
[20]  [16] Electrochemical recovery of copper and nickel from acid pickling solutions at pilot scale.
Very low Ni removal (<20%). Optimum energy consumption: 2 kWh·kg −1 Cu. Promising results for Cu recovery from cost analysis. [23] Copper recovery from treated wood waste by sulfuric acid extraction and electrodeposition.
92% Cu deposition at 10 A for 90 min. 65 US$·tn −1 of profit for wood waste treatment using electrodeposition according to an economic analysis. [17] Tellurium recovery from spent Te electrolytes by cyclone electrowinning. 99.07% Pd recovery with a purity of 94.02% [25] To recover copper by electro-electrodialysis (EED) from ammonia solutions.
Almost 80% current efficiency in Cu recovery. Ammonia complexing agent can be reused. [26] To obtain high-purity copper deposits from complex mixtures by electrodeposition with a centrifuge electrode. 99.9% copper purity obtained. The rest of metals can be also separated. [27] To recover uranium from aqueous solution into magnetite (Fe 3 O 4 ) formed by iron anode dissolution and electrodeposition.
Maximum removal of U (88%) at pH: 2.6, 30 V, 8-10 cm electrode gap. [28] To recover gold from cyanide solutions using a static batch electrochemical reactor operating in an electrogenerative mode.
Cathode: three-dimensional cathodes: porous graphite and reticulated vitreous carbon (RVC) and two-dimensional cathode materials: copper and stainless steel plates). Anode: zinc. >90% of gold can be recovered in 3 h of experiment for the cathodes studied. More than 99% gold was recovered in 1 h of operation using activated RVC. [29] The second way to improve the performance of electrodeposition is by proposing novel reactor concepts with the aim of maximizing mass transfer or enhancing the reuse of the treated effluents. One interesting approach is that used by Campenedo de Morais Nepel and coworkers [15], who propose a pulsed electrodeposition with a rotating electrode in order to improve the removal of copper and the quality of the deposit. An additional system was designed by Ning and coworkers [21], who proposed a jet design (jet electrodeposition) that maximizes mass transfer by a direct injection of the solution on the cathode surface or Wang et al., who propose a centrifuge electrode to enhance the recovery of copper into high-purity solids from complex matrixes [27]. With this approach, it is possible to work at very high current densities (and thus at high recovery rates) maintaining also high recovery percentages and current efficiencies. A similar objective is obtained with the design of cyclone cells, that promote high flow rate of electrolyte on the cathode surface in order to enhance mass transfer of the process [24]. An additional interesting approach is the so-called electro-electrodialysis (EED), that combines the proper use of ion exchange membranes inside an electrochemical cell in order to allow the recovery of the metal and the simultaneous reuse of a solution of interest [26]. Finally, Lu and coworkers proposed a system that promotes the adsorption of uranium on magnetite (Fe 3 O 4 ), that is produced on a graphite cathode from the Fe(II) dissolved from an iron anode [28]. Specific details about the metal recovery percentages and efficiencies can be consulted in Table 1.
Additionally, a very recent approach to upgrade the potential uses of electrodeposition for metal recovery is coupling this process with the generation of energy. This is the case of the work of Wang et al. [19], who propose to apply the process called bimetallic thermally regenerative electro-deposition battery (B-TREB), which uses waste heat to regenerate an ammonia solution that is used as the anolyte of a spontaneous galvanic cell that produce energy by oxidation of zinc and electrodeposition of copper. The similar aim is reached by using an electrochemical-osmotic system (EOS), that take advantage of the high salinity of a copper containing wastewater to produce energy by promoting a spontaneous electrochemical reaction involving the electrodeposition of copper and the oxidation or iron. Moreover, by placing an osmosis membrane between electrodes, it is possible to produce reclaimed water in the anodic compartment [20].

Use of Ionic Liquids in Electrodeposition
Ionic liquids (ILs) can be defined as organic ionic salts that are liquid at ambient or near ambient temperature. Among its properties, it is worth noting their low or negligible volatility, high thermal and chemical stability, high ionic conductivity, high solubility, low flammability, moderate viscosity and high polarity [30]. This interesting combination of properties have attracted an exponential increasing interest of the scientific community, with a growing number or articles published and an increasing spectrum of potential applications [31]. Among these applications, the low volatility and high electrical conductivity of ionic liquids, their potential applications electrochemical processes is continuously increasing, including their use as electrolyte in batteries and fuel cells, electrode materials for batteries/supercapacitors and carbon precursors for electrode catalysts [31][32][33][34][35].
Electrodeposition is one of these potential applications of ionic liquids in electrochemistry. The use of ILs in electrodeposition mainly tends to enhance the recovery yield of heavy metals due to the high conductivity of ILs together with their wide electrochemical window, that prevent the concurrence of hydrogen evolution [36].
Thus, Table 2 resumes the most recent and relevant papers published in the field, including the main objective, the most relevant process conditions and conclusions reached from these works and focusing mainly on those works devoted to the recovery of metals.
As it can be observed, ionic liquids are mainly used to replace aqueous environments in order to enhance the recovery percentage of high value-added metals, as gold, palladium, copper or platinum and increasing current efficiency. In general terms, using a certain percentage of ILs enhances the recovery of the target metal and allows obtaining this metal as a power of a controlled size or morphology [37][38][39][40], including some works regarding the formation of metal nanotubes [41]. It is also common to observe that the metal recovery does not improve when the concentration of the IL increases above a given threshold.
The use of ILs not only improves the recovery and morphology of the target metal, but also increases the percentage of metal extracted from mineral ores or from electronic waste as waste printed circuits or used mobile phones [37,38,42]. In this line, an approach has been done when using mixtures of ILs to prepare a deep eutectic solvent mixture, which can be used to efficiently extract metals from mineral power and further recover them by electrodeposition [43].
Although the use of ILs in electrodeposition is gaining an increasing interest, some issues should still be solved, as is the case of the low thermal and chemical stability of ILs, which hinder its reusability and thus increase its cost and environmental impact, and the high viscosity of ILs, which may lead to low mass transfer coefficients and, consequently, to mass transfer limitations [44,45].
The cost of ionic liquids is also an additional key aspect to consider for its use in the recovering of metals by electrodeposition. Although there is not a comprehensive study about the cost/benefit of using ionic liquids combined with electrodeposition, some works have presented partial conclusions on this topic. Thus, Abbott et al. conclude that the high cost and viscosity of ionic liquids make them better suited for the concentration of metals from large volumes of aqueous metal solutions to reduced volumes of ionic liquids concentrated in the target metals to be recovered [36]. Additionally, Magina et al. pointed out in their recent review about the challenges and opportunities in the use of ionic liquids, that the high cost of ionic liquids is one of the main drawbacks for the use of these group of compounds [31]. In the same line, Parmentier et al. concluded that the use of tetraoctylphosphonium oleate for cobalt concentration was more expensive in both CAPEX and OPEX than a conventional process using ion exchange resins, although they pointed out that it could be a promising alternative for the recovery of precious metals or when the brine disposal is a matter of concern [46].
To sum up, although the use of ionic liquids is a promising alternative, a detailed study of the environmental and cost issues should be carefully performed for any application to be developed.  Extraction and recovery of neodymium from acidic medium using extraction by novel undiluted and non-fluorinated ILs and electrodeposition.
Gold extraction in the presence of thiourea is high yielding (86%). Current efficiency of 30% for the 20% b mim HS0 4 solution at a current density of 5 mA·dm −2 . [50] Use ionic liquids as deep eutectic solvents (DES), for the recovery of different metals by electrodeposition.
With a small concentration of DES added, the recovery of Cu, Fe, Pb, Ni, Zn, Al, Au, Co and Mn increased. Ag and Cr recovery increases with a high quantity of DES. [43] Removal of iron and boron by ionic liquid extraction and recovery of neodymium by ED from voice coil motors (VCMs) waste.  To study gold(I) recovery from alkaline cyanide solutions by using hydrophobic imidazolium-based ionic liquids as extractants.

Use of Bio-Electrochemical Processes
The bio-electrochemical process is defined as a system where the electrochemical reactions are taking place with the contribution of a living system. In most of the cases, the living systems are based on microorganisms, but also plants and higher organisms can be used. In the bio-electrochemical processes, the living organisms mainly contribute to the oxidation processes but can also contribute to the reduction processes.
Depending on reaction spontaneity of the bio-electrochemical systems, these systems can be defined as Microbial Fuel Cells (MFCs) or Microbial Electrolysis Cells (MECs). On the one hand, MFCs carry out spontaneous oxidative and reductive half reactions, therefore exerting a net energy flow. From the oxidative and the reductive processes, chemicals can be oxidized and reduced, leading to the removal of pollutants, the recovery of precious chemical, such as metals, etc. On the other hand, MECs carry out non-spontaneous oxidative and reductive half reactions, being necessary and an energy supply to carry out the reactions and causing, therefore, a net energy consumption [53,54].
When using bio-electrochemical systems for metal recovery, single, dual or multiple electrochemical deposition cells can be used. The single configuration is the simplest. In this configuration, the air acts as the final electron acceptor, its availability on the cathode being necessary [55]. In the dual configuration, the anode and cathode are electrically connected by an external conductor, whereas both compartments are separated by a membrane [55][56][57][58][59][60]. Finally, the multiple configuration requires the participation of several units electrically connected in series or parallel and hydraulically connected individual, cascade, etc. With the multiple-cells configuration, a higher energy production and chemical transformations can be obtained. Nevertheless, it has multiple electrical and hydraulic connections, so this can hinder the assembly [55,61,62].
In the literature, during the last years different new models or configurations have been developed with the aim to improve the performance of the bio-electrochemical systems. These modifications are very important because, depending on the shape (cylindrical, rectangular, etc.), the size, the superficial characteristics, and many other parameters, the mass transfer can be enhanced, leading to an increase in the performance yields.
In Table 3, the most relevant publications related to the metal recovery by using bio-electrochemical processes are presented. In these publications, the main operational parameters and performance of the bioelectrochemical systems have been studied. Table 3. Bio-electrochemical processes for metal recovery.

Objective
Process Conditions Conclusions Ref.
Treating smelting wastewater by a bioelectrochemical system (BES) coupled with a thermoelectric generation (TEG), which uses the simulated heat potentially available in the smelting factories. Voltage output: 0-212 mV.
>90% recover Fe with abiotic, glucose, acetate, ethanol and lactate. [65] Elaborate a bioelectrochemically assisted electrodeposition system for the removal and recovery of Pb and Zn.  To demonstrate the feasibility of chloramphenicol (CAP) removal and Ag recovery. EC: The Fe recovery of >94% was at 500 mA and 60 min. EC in its optimal conditions (Fe-electrodes, 400 mA and 60 min) was found to be more cost effective than MFC and more suitability for large-scale use. EC energy consumption is 9.6 and 3.6 kWhm −3 for Fe and Al, respectively. Operating cost for EC is 0.56 and 0.81 $·m −3 for Fe and Al, respectively. MFC: Power density between 2 and 20 W·m −3 . The Fe removal of >99% was achieved thanks to a 1000 mg·L −1 acetate at the beginning. Total cost of consumables for MFC 2731 $·m −3 . [68] Study applied cell potential, initial cobalt concentration, pH of the catholyte, and the mesh size of the cathode on the performance of a new design of flow-by fixed bed bio-electrochemical reactor.
The best removal efficiency of cobalt is >99% with 1.8 V; 50 ppm initial concentration; pH of 7; a tack of stainless no. 30 steel mesh as a packed bed cathode; energy consumption 1564 kWh·kg −1 cobalt. Use of polyaniline coated electrodes to recover cadmium in semiconductor processing WW.
92% turbidity removal, 89% of TSS; 90% Cd recovery and 72% of water reclaimed in 40 min. Polyaniline coated electrodes exhibit high capability for the electrochemical recovery of cadmium. [59] Study the effect on copper electrowinning of the cyanide/copper molar ratio, the potential applied to the electrolytic cell, the temperature and circulation rate of the solution, and the use of synthetic solutions using electrolytic reactors in discontinuous, discontinuous with recirculation, and continuous flow regimes. The maximum copper removal efficiency of 95.4 ± 0.9% was obtained thanks to maximum current density, initial concentration of copper of 5 mg·L −1 and 0.8 V applied.
[71] The recovery percentage depends on the starting concentration. Recovery of Cu 60-95%. The recovery was low when the anode was supplied with copper depleted distillery waste. [72] Recover platinum through charring biofilms in MFCs, studying its distribution and generating Pt/C catalyst. 5 rectors, one of them was the abiotic. The reactors differ from Pt initial concentration.
Around 40% Pt was recovered. Each batch process worked for 24 h. Maximum power density of 844.0 mW·m −2 , having 2.11 mg·L −1 Pt(IV). [73] Nowadays, the bio-electrochemical systems seem to be a very interesting option of metal recovery from effluents. This is because these systems allow us to reach simultaneously two objectives: on the one hand, the metal recovery and on the other hand, the energy generation. The bioelectrochemical processes can be coupled with other technologies or processes, like coupling with a thermoelectric generation [63] or electrocoagulation to increase the pH and remove iron [63,68]. With the last, more than 94 and 99% Fe was recovered by electrocoagulation and MFC, respectively. Both obtained great results, but the MFC configuration yielded better results [68].
In the scientific studies, different substrates were fed to the microbial culture of the anodic chamber, those presenting higher yield in terms of electricity generation and metal recovery being the most biodegradable [65,73]. In other cases, real wastewaters have been coupled with the metals recovery in both synthetic and real AMD effluents [59] obtaining high recovery yields of the metals presenting the highest reduction potentials, mainly Cd, Cu, Fe, Al, Zn and Pb [64,66]. The coupling between anode and cathode has also been implemented with the objective of the antibiotic removal from hospital wastewater saving 478.88 Wh·m −3 of energy [67].
When using electrically enhanced systems, microbial electrolysis cells, higher metal recovery yields were obtained. These systems have the advantage that they can be done in situ [71]. Novel flow-by fixed bed bio-electrochemical reactors are also under development, allowing us to reach higher metal recoveries and energy generation efficiencies [69,70].

Integration of Electrodeposition with Other Processes
The last approach that will be covered in this review is the combination of electrodeposition with other technologies. In general terms, the main aim of combining other technologies with electrodeposition is either transferring the metal from a solid to a liquid phase or improving the process of metal recovery by a previous stage of concentration/purification. Table 4 shows a selection of recent combination of electrodeposition with other technologies for metal recovery.
As it can be observed, the use of leaching or bioleaching to extract the metal from a solid to a liquid phase and thus allowing its recovery by electrodeposition is a common approach. Bioleaching consists in the extraction of metals from their ores using living organisms [74]. This technology has been extensively studied in order to enhance the metal recovery from many different solid matrixes polluted with metals [75][76][77][78][79][80][81]. Therefore, the coupling of bioleaching with electrodeposition has been extensively evaluated to recover copper from printed circuit boards, obtaining very high copper purities in the final deposits and metal recoveries ranging from 75.8% to 92.85% [82][83][84]. Additionally, this leaching can be also performed chemically, as it is the case of the work published by Wang et al., who propose the combination of chemical leaching (by a combination of acid and H 2 O 2 addition) to obtain ultra-pure Ag deposits from spent silver oxide button batteries [85]. To recover silver from spent silver oxide batteries by acid leaching and electrodeposition.
Real WW from nickel electroplating: 500 mg·L −1 Ni(II  To recover Cu(II) from wastewater through an ion exchange process coupled with electrodeposition.
Electrodeposition is a viable via for polymer regeneration in PEUF. 100% copper recovery. Polymer used in PEUF is not affected by electrodeposition. [89] Another process with results that are interesting for enhancing the efficiency of electrodeposition technique is ion exchange. The main aim of using an ion exchange resin is to increase the concentration of the metal to be recovered in order to increase the efficiency of the subsequent electrodeposition stage. Thus, when using an ion exchange, it works in cycles of operation (production of a treated solution)-regeneration. This latter regeneration stage is performed by adding an acid stream, and it produces a concentrated metal solution that is further treated by electrodeposition. With this approach, it is possible to recover nickel with very high current efficiency (95.6%) [86] and to recover Cu from wastewater with high purity (96.38%) and low energy consumption (0.6 kWh·kg −1 Cu) [88].
Further approaches have been done including the combination of membrane processes with electrodeposition. In this group, it is worth noting the combination of a membrane capacitive deionization with electrodeposition in order to simultaneously promote the desalination of a water stream with the recovery of copper, concluding that the recovery of copper is possible by the combination of technologies proposed [87]. Another example of combination of membrane technologies and electrodeposition was proposed by Camarillo et al., who combined the so-called polymer enhanced ultrafiltration technique (PEUF) with electrodeposition as an efficient process for efficient copper recovery [89]. In this latter work, PEUF uses a water-soluble polymer to recover and concentrate Cu(II) by an ultrafiltration membrane. This concentrated is further treated by electrodeposition, a technology that allows us to recover copper and to simultaneously regenerate the polymer, that can be used in a further PEUF stage.
To sum up, great recovering percentages, acceptable energy consumption and high metal purities can be obtained from both solid and liquid wastes by properly combining electrodeposition with leaching or concentration techniques.

Conclusions
This review covers the most relevant and recent uses of electrodeposition for metal recovery. From the information included in this this review, the following conclusions can be deduced:

•
The scientific interest in the use of electrodeposition is continuously growing since it was first reported in 1904.

•
The use of electrodeposition in water treatment and reuse is probably the most important topic regarding the use of the technology for metal recovery, with the most recent works mainly devoted to the development of novel reactor configurations of enhanced mass transfer characteristics. • When using electrodeposition combined with ionic liquids, it is possible to obtain an elevated yield of value-added metals recovery and a controlled morphology and size of the deposits. The cost, stability and reusability of ILs is a matter of improvement for the development of the technology.

•
The attention devoted to the use of bio-electrodeposition systems has increased within the last years, as the selection of the reactor configuration, operational conditions and source of the inoculum are critical in order to obtain the best performance of these systems.

•
Electrodeposition is commonly coupled to other technologies that allow either extracting the metals from a solid phase or concentrating them in the liquid phase, leading to an upgrade of the metal recovering while saving energy.
Funding: This research was funded by Junta de Comunidades de Castilla-La Mancha's project SBPLY/19/180501/000254 and from the Spanish Ministry of Science and Innovation through the project PID2019-107282RB-I00.

Conflicts of Interest:
The authors declare no conflict of interest.