Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems

Abstract

The rapid technological advancements and the shift towards clean energy have significantly increased the demand for metals, leading to an increasing metal pollution problem. This review explores recent advances in bioelectrochemical systems (BES) for metal recovery from waste, especially Acid Mine Drainage (AMD) and Electrical, Electronic Wastes (EEW) and waste from smelters, highlighting their potential as a sustainable and economically viable alternative to traditional methods. This study addresses the applications and limitations of current BES recovery techniques. BES, including microbial fuel cells (MFCs), microbial electrolytic cells (MECs), and Microbial Desalination Cells (MDCs), offer promising solutions by combining microbial processes with electrochemical reactions to recover valuable metals while reducing energy requirements. This review categorizes recent research into two main areas: pure BES applications and BES coupled with other technologies. Key findings include the efficiency of BES in recovering metals like copper, chromium, vanadium, iron, zinc, nickel, lead, silver, and gold and the potential for integrating BES with other systems to enhance performance. Despite significant progress in BES application for metal recovery, challenges such as high costs and slow kinetics remain, necessitating further research to optimize materials, configurations, and operational conditions. The work also includes an economic assessment and guidelines for BES development and upscale. This review underscores the critical role of BES in advancing sustainable metal recovery and mitigating the environmental impact of metal pollution.

1. Introduction

The rapid development and expansion of technology have made electronic devices indispensable in our daily lives [1]. This increase in the demand for electronic devices, coupled with the shift towards clean energy and decarbonization, has elevated the importance of metals in this technological era. The accelerated pace of technological advancements and the frequent replacement of existing technologies are causing electronic products to become obsolete prematurely, generating a significant amount of electrical and electronic waste (EEW) [1,2]. In fact, EEW is one of the fastest-growing waste streams across the world. Unfortunately, this problem will be worsened in the near future due to three main aspects: (1) The increase in the global population, (2) the extensive range of applications for electrical and electronic devices, and (3) the modern lifestyle (driven by the need for fast information and communication access). These three aspects will increase the demand for electrical and electronic devices in the near future and, consequently, the EEW generation [3].

In 2019, the global generation of EEW surpassed 53 million metric tons, with projections suggesting this figure could exceed 74 million metric tons by 2030 [4]. EEW encompasses a wide range of electrical and electronic equipment, including large and small household appliances, electronic tools, medical devices, monitoring and testing equipment, and telecommunication devices [5,6,7]. The EEW is classified as hazardous due to its high content of metals such as lead, cadmium, nickel, chromium, and copper, which pose substantial environmental risks. Additionally, EEW contains substances like brominated flame retardants, persistent organic pollutants, and polychlorinated biphenyls, which could cause environmental and health problems [8,9]. Although metals are naturally present in the environment, anthropogenic activities are the primary source of metal contamination due to the substantial volume of minerals processed as well as the metal wastes generated daily [10]. Metal pollution has become a significant environmental issue, polluting water and soil. Heavy metals—characterized by their high atomic weight, e.g., Pb, Hg, As, Cr, Ni, Tl, Co, Cu, and Zn—are not biodegradable and tend to bioaccumulate in plants and animals, affecting ecosystems and causing environmental problems. They also pose serious health risks in humans, including cancer, neurological damage, kidney damage, respiratory and cardiac issues, and may contribute to diseases like Alzheimer’s [1,10,11].

Beyond metal pollution, there is a critical need for a reliable supply of metals to manufacture new generations of electrical and electronic devices. Consequently, the metal extracting and processing industry faces new challenges in achieving the required metal production within the environmental and economic frames [2,7]. Despite increasing mineral extraction rates to meet today’s high consumption demands, the depletion of high-quality ore deposits is leading to a decline in the quality of extracted materials, making it more challenging and costly to obtain the necessary metals. This situation forces metal extracting and processing companies to rely on lower-quality ores as raw materials for production [12,13]. Therefore, it is essential to consider alternative metal resources such as low-grade ores, aqueous reserves, slag, but also EEW [7,14]. In particular, EEW represents a significant secondary metal reserve within the industry [1,13]. These wastes are rich sources of valuable metals, including rare earth elements, precious metals like gold, and base metals such as iron and copper [8]. As a result, the utilization of EEW as a metal source has increased in recent decades and is now recognized as a viable secondary source for obtaining these crucial materials for the technological industry [2,15].

Due to the scarcity of metal, as well as the environmental and health problems caused by metal pollution, it is imperative to implement comprehensive regulatory frameworks to manage EEW, as most of it is currently processed through unregulated and unsafe recycling practices. This prevalent practice results in a high percentage of EEW ending up in landfills, undergoing manual disposal, acid leaching, open burning, or illegal management and deposit [2,9,11]. These disposal methods can release harmful toxins and toxic gases into the environment, polluting the atmosphere, soil, and water resources. Workers in these recycling facilities are exposed to hazardous substances, particulate matter, high noise levels, and other health risks, with low-income countries being the most affected [2,16,17]. The transformation of EEW by natural processes is often hindered, leading to inappropriate disposal practices that pose significant environmental and health concerns. Therefore, developing sustainable management strategies for EEW is paramount [2,16,18]. Inadequate recycling practices result in the loss of valuable metals contained in EEW, contributing to environmental and human health impacts [9,11]. Building the necessary infrastructure for effective EEW management presents a significant techno-economic challenge, requiring incentives to transform the processes [19]. The continued use of traditional technologies for metal disposal and recovery necessitates the development of more cost-effective and environmentally friendly management strategies [17]. In the literature, the use of process technology for metal waste management is usually coupled to the metal recovery within the frame of environmental applications and circular economy. This trend is driven by environmental concerns and the massive use of metals in electrical and electronic devices. Consequently, metal recovery serves not only to mitigate environmental issues but also as a valuable secondary resource, offering economic benefits.

In the literature, a variety of techniques have been employed for the recovery of metals, including pyrometallurgy, hydrometallurgy, flotation, chemical precipitation, ion exchange, osmosis, and bioleaching, amongst others [10,20,21,22]. Nowadays, the most established technique is pyrometallurgy, which relies on high-temperature processes to extract and refine metals. This approach typically involves smelting, roasting, and refining steps, where metal-containing ores or wastes are heated to temperatures that facilitate the physical and chemical separation of valuable metals from impurities. One of its key advantages is that it minimizes the need for extensive pre-treatment of the waste, as temperatures break down complex matrices and volatilize or oxidize non-metallic components. This makes pyrometallurgy a relatively fast and adaptable processing methodology. However, pyrometallurgy presents several drawbacks, including high energy consumption, low selectivity, loss of strategic metals, and the emission of polluting gases into the atmosphere [23].

During the last few years, hydrometallurgy has gained prominence due to its lower economic costs and reduced environmental impact compared with pyrometallurgy. It enables superior control of impurities, leading to high recovery rates and the obtention of high-purity metals. Hydrometallurgy is predominantly based on leaching, where metals are dissolved into solutions using inorganic or organic acids, followed by chemical precipitation, which selectively recovers the metals as solid compounds. Nevertheless, the extensive use of chemical reagents in these processes often leads to significant secondary pollution issues, e.g., the considerable volume of wastewater generated and the emission of pollutants such as chlorine gas [24,25].

To address these challenges, alternative technologies have emerged, including bioelectrochemical systems (BESs). The combination of electrochemical technology with bioprocesses stands up as an economical and sustainable alternative for metal recovery from wastes while simultaneously addressing environmental remediation. BESs combine a variety of microorganisms, such as fungi, bacteria, and archaea, with electrochemical processes to facilitate the solubilization and recovery of metals while minimizing energy consumption or enabling energy generation [2,10,25,26,27]. Table 1 compares the different technologies used for metal recovery, highlighting their efficiencies, advantages, disadvantages, and typical applications. As illustrated, BESs stand out for their potential to combine metal recovery with energy generation; however, their full-scale application remains limited due to slow kinetics and a low level of technological readiness.

Over the last few decades, BESs have been increasingly studied as one of the key electrochemical technologies for metal recovery [28,29,30,31,32]. Extensive research efforts have introduced new materials, methods, operating conditions, and design improvements to enhance performance and expand their range of potential applications. Figure 1A presents the number of papers published related to “Metal”, “Recovery”, and “Bioelectrochemical” appearing in either abstract, title, or keywords categorized as Microbial Fuel cell (MFC) or Microbial Electrolysis Cell (MEC). As can be observed, there is a significant increase in the number of articles published on this topic, reflecting the growing interest of the scientific community and the versatility of this technology. Figure 1B displays the number of metal recovery studies reported in the literature during the last years when operating with MFC and MEC configurations. The data show a steady rise in publications in recent years, underscoring the increasing interest and relevance of metal recovery through BES systems [33].

Table 1. Comparative analysis of metal recovery techniques.

Technology	Process Description	Advantages	Disadvantages
Pyrometallurgy [23]	High-temperature processes to extract and refine metals from ores or waste (e.g., smelting, roasting).	- Minimal pre-treatment required - Rapid processing - Adaptable to different metal sources	- High energy consumption - Low selectivity - Loss of strategic metals - Emission of polluting gases
Hydrometallurgy [22,34]	Metal extraction via aqueous solutions primarily involving leaching, solvent extraction, and precipitation.	High purity, effective impurity control, lower energy costs	- Extensive use of chemicals - Secondary pollution and wastewater generation
Ion Exchange [35]	Metal ions are exchanged with ions on a solid resin, allowing for selective recovery.	- High selectivity - Reusable resins	- High operational cost - Requires regeneration of resins - Less effective for high metal concentrations
Electrochemical Recovery/electrowinning [36,37]	Metals are plated onto cathodes via redox reactions in electrochemical cells.	- High purity of recovered metals - Can be coupled with renewable energy	- Reduced efficiency for dilute solutions due to mass transport limitations.
Membrane Filtration [35,38]	Separates metals via physical barriers (e.g., reverse osmosis, nanofiltration, ultrafiltration).	- High removal efficiency - Suitable for a wide range of metals	- High CAPEX and OPEX - Prone to membrane fouling - Requires high pressure and energy input
Bioleaching [22,39]	Uses microorganisms to extract metals through biological oxidation/reduction processes.	- Environmentally friendly - Cost-effective for low-grade ores - Minimal energy input	- Slow process - Requires specific conditions (e.g., pH, temperature) - Limited to certain metals
Bioelectrochemical Systems (BES) [28,40]	Integrates bioprocesses with electrochemical techniques for metal solubilization and recovery.	- Energy-efficient or energy-generating - Simultaneous wastewater treatment and metal recovery	- Slow kinetics due to microbial activity - Limited scalability - Low technological readiness level

Table 1. Comparative analysis of metal recovery techniques.

Technology	Process Description	Advantages	Disadvantages
Pyrometallurgy [23]	High-temperature processes to extract and refine metals from ores or waste (e.g., smelting, roasting).	- Minimal pre-treatment required - Rapid processing - Adaptable to different metal sources	- High energy consumption - Low selectivity - Loss of strategic metals - Emission of polluting gases
Hydrometallurgy [22,34]	Metal extraction via aqueous solutions primarily involving leaching, solvent extraction, and precipitation.	High purity, effective impurity control, lower energy costs	- Extensive use of chemicals - Secondary pollution and wastewater generation
Ion Exchange [35]	Metal ions are exchanged with ions on a solid resin, allowing for selective recovery.	- High selectivity - Reusable resins	- High operational cost - Requires regeneration of resins - Less effective for high metal concentrations
Electrochemical Recovery/electrowinning [36,37]	Metals are plated onto cathodes via redox reactions in electrochemical cells.	- High purity of recovered metals - Can be coupled with renewable energy	- Reduced efficiency for dilute solutions due to mass transport limitations.
Membrane Filtration [35,38]	Separates metals via physical barriers (e.g., reverse osmosis, nanofiltration, ultrafiltration).	- High removal efficiency - Suitable for a wide range of metals	- High CAPEX and OPEX - Prone to membrane fouling - Requires high pressure and energy input
Bioleaching [22,39]	Uses microorganisms to extract metals through biological oxidation/reduction processes.	- Environmentally friendly - Cost-effective for low-grade ores - Minimal energy input	- Slow process - Requires specific conditions (e.g., pH, temperature) - Limited to certain metals
Bioelectrochemical Systems (BES) [28,40]	Integrates bioprocesses with electrochemical techniques for metal solubilization and recovery.	- Energy-efficient or energy-generating - Simultaneous wastewater treatment and metal recovery	- Slow kinetics due to microbial activity - Limited scalability - Low technological readiness level

The search results from the Scopus database were analyzed using the free software VOSviewer^®, version 1.6.20, and the findings are presented in Figure 2, which provides a co-occurrence analysis of keywords associated with “Metal”, “Recovery”, and “Bioelectrochemical” from published research articles. This visualization highlights the most frequently used keywords as nodes, with the size of each node representing the frequency of the keyword’s appearance in the dataset, while the connections between nodes indicate the strength of their co-occurrence. The analysis reveals distinct clusters that reflect major themes within the field, such as environmental applications like “wastewater treatment,” “heavy metals” and “sewage” (blue clusters); microbial and bioelectrochemical applications including “microbial fuel cells” and “bio-electrochemical” (green clusters); electrochemical processes and performance metrics such as “electrodeposition”, “energy utilisation” and “cathodes”. This network demonstrates the multidisciplinary nature of research in bioelectrochemical systems, showcasing the integration of biological, chemical, and engineering aspects while highlighting emerging trends in energy efficiency, sustainability, and hybrid technologies. About 200 papers were identified on this topic. In addition to the terms “Metal”, “Recovery”, and “Bioelectrochemical”, which were included as search criteria, other frequently occurring keywords in this research field include “recovery”, “metal recovery”, “recycling”, and “electrodeposition”, reflecting the central focus and themes within the field.

In this context, the main aim of this paper is to review the most recent developments in bioelectrochemical technology with applications for metal recovery from waste, with a focus on the last 5 years. According to the most common topics of recent research work, the articles have been classified into two main groups: (1) the use of pure bioelectrochemical systems for metal recovery and (2) the coupling of bioelectrochemical systems with other technologies for metal recovery.

2. Bioelectrochemical Systems for Metal Recovery

Since the 1990s, BESs have been the subject of extensive research with the objective of improving the efficiency of metal recovery, as well as achieving simultaneous energy generation. Over the years, research has explored various mechanisms and configurations to optimize performance, leading to significant advancements. These efforts are summarized in Table 2, which highlights key studies showcasing progress in BES for metal recovery. Figure 3 further classifies BESs into distinct categories based on their metal recovery principles, providing a comprehensive overview of the diverse strategies employed.

The electroactive or electrogenic biofilm developed either on the anode, the cathode, or both facilitates the occurring redox reactions due to bioelectrocatalysis. Usually, the typical BES systems rely on biotic anodic reactions and abiotic cathodic reactions. The coupling of the (bio)anodic and cathodic reactions enables the development of a potential difference that could lead to electricity generation due to the oxidation and reduction reactions taking place in the BES [10,41,42]. The efficacy of the BES depends on the specific microbial strain employed, the anodic and cathodic potentials, the reactor design (e.g., geometry, electrode distance, electrode material, separator, etc.), and a wide range of technical and operational variables (e.g., electrolyte pH, conductivity, temperature, the composition of the basal medium, etc.) [10,43,44].

BESs can be classified based on different criteria. Regarding energy demand, BESs are primarily divided into microbial fuel cells (MFCs) and microbial electrolytic cells (MECs). MFCs operate with a positive net cell potential (E_cathode − E_anode > 0), allowing electrons to flow spontaneously from the anode to the cathode, thereby generating an electrical current. MFCs have been widely studied for their ability to oxidize biodegradable pollutants at the (bio)anode while simultaneously facilitating reduction reactions with coupled electrical current generation [45]. In contrast, MECs require an external energy input to drive the reactions, overcoming thermodynamic and kinetic barriers. In other instances, MECs can also be seen as an extension of MFC performance, where the spontaneous redox reactions occurring between the anode and cathode are enhanced by an external energy input. This allows for the achievement of higher reaction rates and electrical current outputs, surpassing the limitations of short-circuit conditions in MFCs (illustrated graphically in Figure 4 and further discussed in Section 4). This additional energy input allows MECs to achieve processes such as hydrogen evolution or enhanced metal recovery that are not feasible under the spontaneous conditions of MFCs.

Both systems are commonly employed for the oxidation of biodegradable pollutants at the anode, a process that enhances the system’s energy balance by achieving a sufficiently low anode potential. At the cathode, target species undergo reduction, enabling applications such as metal recovery, e.g., through electrodeposition (Figure 3a) [46]. In both MFCs and MECs, the bioelectroactive anodic process drives the cathodic reduction of soluble metals in the electrolyte, leading to their deposition in their elemental oxidation state (e.g., via electrodeposition). MFCs accomplish this with net energy generation, while MECs achieve it with lower energy consumption due to the external energy input. Another cathodic process associated with bioelectroactive anodes is metal recovery through bioprecipitation or immobilization in a biocathode, either as oxides or in their zero-valent state. This occurs through biocatalysis at the cathode, with mechanisms varying based on the biocathode configuration. In electroactive biocathodes, the electrode itself serves as an electron donor, and the soluble metal acts as the electron acceptor, as illustrated in Figure 3b [32]. In non-electroactive biocathodes (Figure 3c), bacteria utilize H₂—electrogenerated at the cathode via water splitting—as the electron donor while the soluble metal acts as the electron acceptor. Examples of these processes include the reduction of Cr⁶⁺ to Cr³⁺ using a Shewanella oneidensis biocathode [47] and the reduction of Au³⁺ to Au⁰ using S. oneidensis or C. metallidurans [48], both of which rely on H₂ generated electrochemically at the cathode of a MEC. An alternative pathway for metal precipitation at the cathode involves increasing the catholyte pH and redox potential. This can occur, for example, through the oxygen reduction reaction (ORR) at a gas diffusion electrode, which produces H₂O₂ (a high redox potential species capable of oxidizing metals) and OH⁻, raising the pH. These changes result in the bulk precipitation of metals as oxides or hydroxides. This cathodic process has been previously referred to as Gas-Diffusion Electrocrystallisation [49].

BESs can also be categorized based on their application. For instance, microbial desalination cells (MDCs), which typically operate in MFC mode, are primarily used for desalinating saline waters through ion migration across selective membranes. This process simultaneously enables the concentration of salts and metals, which can subsequently be recovered [38,50]. Another notable application is microbial electrosynthesis (MES), which focuses on the production of valuable products such as organic acids and alcohols from CO₂ reduction and, in some cases, metal-complexed chemicals. However, MES is less commonly used for metal recovery [51,52,53,54]. MDCs have been applied for metal recovery in various configurations. In some cases, metals of interest, initially present in the middle compartment, are concentrated in the cathodic compartment through electromigration facilitated by selective membranes [55,56], as illustrated in Figure 3d. In other configurations, a standalone MDC system desalinates the middle compartment while simultaneously performing metal electrodeposition at the cathode (Figure 3e), such as in the case of copper recovery [38]. Additionally, MDCs can operate to desalinate normal brine without heavy metals, functioning in MFC mode with an aerated abiotic cathode while powering a connected MEC that recovers metals at its cathode via electrodeposition (e.g., lead [50]).

Designing an effective BES requires careful consideration of factors such as the number and combination of BES chambers, their shape and size, the distance between electrodes, the materials used for electrodes, the circuit configuration (e.g., resistor connections), the presence or absence of a separator (e.g., diaphragm or ion exchange membrane), biofilm formation conditions, temperature, pH, and whether direct or alternating current is applied. These design elements significantly influence system performance, scalability, and costs [57].

Regarding the reactor design, BES can be categorized into various configurations, including single-chamber MFC, double-chamber MFC, stacked systems, upflow reactors, tubular reactors, etc. In general, single-chamber MFCs are more economically affordable with lower Capital Expenditures (CAPEX) since their design eliminates the need for a membrane. This simplification reduces construction and maintenance costs, making them easier to scale up [58,59]. Single-chamber systems also typically achieve higher power densities due to reduced internal resistance. However, they are less suitable for research requiring precise separation of anodic and cathodic environments, particularly in cases where incompatibilities between the anolyte and catholyte exist or where inhibitory effects on biofilms occur due to cross-contamination. This limitation is especially relevant when dealing with systems like acid mine drainage and electronic waste, where maintaining distinct pH levels or preventing interactions between redox environments is critical for optimal performance. Challenges such as oxygen leakage into the anode compartment can reduce efficiency, and open design limits their effectiveness in certain applications, including energy recovery [60]. When air cathodes are used in MFCs, a membrane can be integrated into a zero-gap configuration with the cathode, streamlining the design by using only the anolyte as the electrolyte. However, this setup is not suitable for applications like water treatment, which require strict separation of anodic and cathodic environments to avoid cross-contamination and maintain process efficiency [61].

In contrast, dual-chamber MFCs remain the most widely studied configuration, as evidenced by the literature review in Table 2. These systems consist of separate anodic and cathodic chambers, divided by a proton exchange membrane that facilitates proton transfer while maintaining the separation of redox reactions. This design provides greater control over the reaction environment, making it versatile for diverse applications such as metal recovery and microbial electrosynthesis [58]. Despite these advantages, dual-chamber MFCs face scalability challenges due to their high CAPEX and operational costs (OPEX), which are driven by the expense of membranes and a higher internal resistance of the system. Consequently, they are predominantly utilized in laboratory settings, with limited progression to commercial-scale applications [61].

Stacked MFCs offer a practical method for enhancing power generation, as illustrated in Figure 3f,g. By connecting multiple MFCs in series or parallel, overall performance can be improved. In a series configuration, the total voltage of the stack is the sum of the individual cell voltages, while the current remains the same as that of a single cell. In contrast, in a parallel configuration, the total current is the sum of the currents from each cell, while the voltage remains equal to that of a single cell [62]. In practice, parallel configurations often outperform series setups in terms of power output. This is because parallel arrangements allow each cell to operate independently, minimizing the impact of performance mismatches or resistance differences between cells. In series configurations, the overall current is limited by the weakest-performing cell, which can significantly reduce efficiency [63].

Alternative BES configurations, such as upflow MFCs and tubular MFCs, provide additional options for specific applications. Upflow MFCs are cylindrical reactors designed for continuous operation, with the anode positioned at the base and the cathode at the top, separated by layers of glass or glass wool. While this system does not present issues regarding proton transfer, it is characterized by two significant disadvantages: the considerable distance between electrodes and the high cost associated with the pump. Finally, tubular MFCs are more robust and suitable for scaling up. However, they are challenging to operate in continuous mode and lose energy efficiency over extended periods of time [45,64,65,66].

The choice of electrodes is a crucial aspect of the development of MFCs. The evaluation of materials, surface area, porosity, durability, biocompatibility, and electrochemical properties is essential. Depending on the intended application, graphene, titanium, copper, nickel, silver, or carbon foil can be employed. The carbonaceous electrodes are currently the most prevalent, with modifications to enhance their conductivity and performance. The pH of the medium is another factor that exerts a determining influence on the performance of the MFC. Studies have demonstrated that the behavior of the MFC varies depending on the pH level. Typically, higher anodic pH levels result in lower yields due to the reduced number of protons migrating to the cathode. Neutral anodic pH levels tend to favor the growth of biofilms. However, for cathodic metal recovery, a lower pH is typically required to prevent the precipitation of metals in the form of hydroxides and enhance cell performance by increasing the availability of protons that can be transported [10,64,67,68,69].

Overall, the literature review presented in Table 2 underscores significant progress and persistent challenges in applying BES for metal recovery. Integrating BES with complementary technologies, such as microbial electrodialysis and photo-assisted processes, has improved recovery rates and operational efficiency, while advancements in carbon-based and functionalized electrode materials have enhanced conductivity, biofilm formation, leading to improved power output, process rates, and selectivity for metals like copper, nickel, and zinc. Multi-chamber and stacked designs now enable simultaneous metal recovery, desalination, and pollutant degradation, showcasing the versatility of BESs.

Table 2. Key studies highlighting advances in metal recovery using BES.

Objective	Reactor Setup	Operational Conditions	Milestones	Ref.
Use of MFC for recovery of metals from AMD	System: Dual-chamber MFC Anode: Carbon felt (CF) and doped variants with non-activated (CFnaH) or activated hydrochar (CFaH), electrode dimensions (2.5 × 2.5 × 0.8 cm) Cathode: Copper (2.5 × 2.5 cm) Membrane: Bipolar (Fumasep® FBM)	Anolyte: Synthetic medium with acetate (1000 mg/L, pH 7) Catholyte: Synthetic AMD (400 mg/L Cu2+, 400 mg/L Fe3+, 50 mg/L Ni2+, 50 mg/L Sn2+, pH 2.5) Rext: 120 Ω	- Cu recovery: 91% (CFaH), 85% (CFnaH), 48% (CF) - Faster Cu recovery (3 days vs. 4 days) with doped anodes - Fe3+ reduced from 400 to 10 mg/L - Current density: 1.2 A/m2	[10]
Copper recovery by saline MFC with polypyrrole-based cathodes	System: Dual-chamber MFC Anode: Carbon felt (6 mm thick, 3 × 3 cm2) Cathode: Stainless steel with Polypyrrole (SS/PPy), Polypyrrole with Phytic Acid (SS/PPy-PA), Polypyrrole with Carbonised Cellulose (SS/PPy-CC), and combinations (SS/PPy-PA-CC) Membrane: Nafion	Anolyte: Synthetic wastewater with acetate (pH 7) Catholyte: CuSO4 solution (256 mg/L Cu) in 2% NaCl pH: 3.0 Temperature: 30 °C Rext: Various (10–20,000 Ω)	- 97% Cu recovery in 3 days—SS/PPy-PA cathode: higher efficiency but corroded over time - SS/PPy suitable for large-scale MFCs	[70]
Metal removal using algae and algal biochar as cathode catalysts in Photosynthetic dual-chamber MFC (PMFCs)	System: Photosynthetic dual-chamber MFC Anode: Carbon felt (CF) (110 cm2) Cathode: CF (130 cm2), CF coated with Cu-accumulated ABC700 (activated biochar prepared from algae biomass thermally activated at 700 °C), and CF coated with Co-accumulated ABC700 Membrane: Clayware cylinder wall as proton exchange separator	Anolyte: 125 mL cultivated algae solution (pH 7) Catholyte: 400 mL with Cu2+ or Co2+ (25, 50, 75, and 100 mg/L, pH 7) Rext: 100 Ω Temperature: Room temperature	- Cu2+ recovery: 94% - Co2+ recovery: 88% - Highest power density and COD removal achieved with Co-accumulated ABC700 cathode - Algal biochar cathodes contributed to reducing pollutants and supporting energy generation	[68]
Heavy metal removal and energy recovery using pyrite-enhanced MnCo/CNF anode	System: Dual-chamber “H”-Type MFC Anode: Carbon felt (CF), Carbon nanofibers (CNF), MnCo/CNF, MnCo/CNF with pyrite (5 × 2 cm) Cathode: Carbon felt (5 × 2 cm) Membrane: Nanofiber membrane	Anolyte: Acetate and glucose-based medium Catholyte: 25 mM K3[Fe(CN)6] solution Rext: 1000 Ω Temperature: 20 ± 5 °C	- 99% Sb removal with MnCoPy-MFC - Maximum power density hierarchy: CF-MFC < CNF-MFC < MnCoCNF-MFC < MnCoPy-MFC - MnCoPy-MFC exhibited the highest energy recovery performance	[71]
Cr6+ recovery using upflow MFC	System: Upflow MFC Anode: Carbon felt with granular activated carbon (45 g, 377 cm2) Cathode: Carbon cloth (1130 cm2) Membrane: Proton exchange (Nafion® 117)	Anolyte: Mineral medium (pH 7), 0.98 L Catholyte: Cr(VI)-containing medium (pH 4), 4.2 L Temperature: 34 °C Rext: 1000 Ω	- 97.7% Cr(VI) removal with optimal 800 mg/L influent, 2-day retention time, and pH 7 (anode)/pH 4 (cathode). - Cr(III) precipitation reduced MFC efficiency over time. - Parallel configuration outperformed series	[72]
Cr6+ recovery using a combined wetland and MFC system	System: Vertical upward-flow MFC Anode/Cathode: 100-mesh stainless steel wrapped with activated carbon Membrane: None	Structure: Bottom (zeolite), anode (activated carbon), middle (ceramsite), cathode (activated carbon), wetland plants Simulated Wastewater: Cr(VI) (40–120 mg/L), acetate, NH4Cl, NaHCO3, nutrients Rext: 1000 Ω	- 97.7% Cr(VI) removal efficiency - Effective for chromium pollution reduction in wastewater - Integrated system design leveraging wetland plants and vertical MFC layers	[73]
Environmental impact investigation on Cr6+ recovery and electricity generation	System: Dual-chamber MFC Anode: Carbon felt Cathode: Carbon cloth, carbon brush, carbon felt Membrane: Proton exchange membrane	System: Dual-chamber MFC Anode: Carbon felt Cathode: Carbon cloth, carbon brush, carbon felt Membrane: Proton exchange	System: Dual-chamber MFC Anode: Carbon felt Cathode: Carbon cloth, carbon brush, carbon felt Membrane: Proton exchange	[74]
Chromium nano-mining using electrified bioreactors (EBS)	System: Two-compartment EBS (0.125 L per compartment) separated by CEM (Ultrex CMI-700, 64 cm2) Electrodes: Stainless Steel mesh (4.8 cm2)	Catholyte: 250 mL K2Cr2O7, pH 7, inoculated with S. oneidensis MR-1 Cathode polarization: –0.8 VSHE Flow rate: Batch recirculation at 2.5 L/h	- Cr(VI) removal: 76% in 24 h - Total Cr removal: 61% - Cr nanoparticles (19–36 nm) observed around bacterial cells - Energy consumption: 0.8 kJ - Maximum removal capacity: 1121 mg Cr/g cells	[47]
Silver recovery from photovoltaic panels	System: Dual-chamber MFC Anode: Carbon felt (5 × 5 cm) Cathode: Graphite paper (3 × 3 cm) Membrane: Nafion® 115	Anolyte: Phosphate buffer + glucose Catholyte: Synthetic extract solution (AgNO3, CuCl2, AlCl3) Rext: 100 Ω/Temperature: 30 °C	- Ag: 100% recovery (7 h) - Cu: 60% recovery (7 h) - Al: 15% recovery (7 h)	[75]
Investigating the effect of Ag concentrations on current production and recovery	System: Dual-chamber H-type MFC Anode: Carbon cloth (25 cm2) Cathode: Carbon cloth (25 cm2) with Pt (0.5 mg/cm2) Membrane: Cation exchange membrane (Nafion® 117)	Anolyte: Synthetic wastewater (acetate 1 g/L, trace elements, neutral pH) Catholyte: Ag solution (6–95 mg/L) Rext: 100 Ω Temperature: 25 °C	- Maximum Ag removal: 99.8% at 100 mg/L Ag - Ag recovery: 75% in 2 h at <30 mg/L, 90% in 6 h at >50 mg/L - Batch operation up to 8 h investigated for varying Ag concentrations	[76]
Feasibility of MFC technology for silver recovery from synthetic PV hydrometallurgical wastewater	System: Dual-chamber MFC Anode: Graphite paper (2.5 × 3.8 cm) Cathode: Plain graphite paper Membrane: Proton exchange membrane (Nafion® 117, 3.77 cm2)	Anolyte: Synthetic wastewater with 50 mg/L Ag Catholyte: Phosphate buffer (pH 7, 0.16 g/L KCl) Rext: 100 Ω/Temperature: 32 °C	- Ag recovery: >93% with NaClO4 as the supporting electrolyte - 100% Ag recovery at pH 2.00 in 3 h and pH 7.00 in 5 h - Lower efficiency when KCl was used as a supporting electrolyte	[77]
Gold Recovery and Nanoparticle Synthesis in Microbial Systems Using Fractional Factorial Design	Serum bottle reactors (120 mL), 50 mL solution, sealed with butyl rubber stoppers; Shewanella oneidensis MR-1 and Cupriavidus metallidurans CH34 as bacterial strains Electrochemical tests: Two-compartment BES Electrodes: Carbon electrodes Electron donor: In situ hydrogen from cathodic water reduction	Factors: cell concentration (OD 0.5 or 1), temperature (28 °C or 37 °C), anoxic/oxic, pH (1 or 5), Au3+ concentration (0.2 mM or 2 mM), electron donor (electrogenerated H2 or lactate), bacterial species; batch incubation for 72 h. Electrochemical tests for H2 generation: Carbon cathode poised at −0.3 V vs. Ag/AgCl	- Highest Au removal (88.2%) achieved with S. oneidensis MR-1 at 0.2 mM Au3+, pH 5, anoxic conditions, and (electrogenerated) H2 as the electron donor. - Targeted Au nanoparticle size (50 nm) obtained under optimal conditions - Significant role of pH and Au3+ concentration in removal efficiency	[48]
Alternative approach for Au recovery using microbial electrochemical snorkel (MES)	System: Dual-chamber MFC Anode: Bioanode Cathode: Gold foil and graphitized paper (8 cm2) Membrane: Proton exchange (Nafion® 117)	Anolyte: Aqueous phase above freshwater sediment Catholyte: Gold-containing solution (1 g/L Au3+) Rext: 510 Ω Temperature: 25 °C	- In MES-mode (short-circuited), >50% Au recovered in 2 h, 100% recovered in 24 h - Gold deposited as an elemental state - Similar performance for both cathodes, with graphite offering a cost advantage	[78]
Assessing Castellaniella inoculum for cathodic metal recovery in BES	System: Dual-chamber MFC Anode: Biofilm of mixed Castellaniella inoculum Cathode: Carbon cloth Membrane: Anion exchange membrane	Anolyte: 50 mM PBS with Wolfe’s vitamins and trace elements Catholyte: Pb–Zn smelting wastewater (COD: 2.26 mg/L, Cu2+: 183.19 mg/L, Hg2+: 78.44 mg/L, Pb2+: 206.94 mg/L, Zn2+: 126.92 mg/L, pH 2.38) Rext: 10 Ω/Temperature: 30 °C	Mode MFC: - Cu2+: 99.86% recovery in 60 h - Hg2+: 99.98% recovery in 17 h - Pb2+, Zn2+ unchanged Mode MEC: - 1 V: Pb2+: 93.49% in 36 h, Zn2+ unchanged - 2 V: Pb2+: 99.98% in 24 h, Zn2+: 99.17% in 24 h	[79]
Investigating heavy metal removal with varying metal concentrations and dissolved oxygen conditions	System: Dual-chamber MFC Anode: Carbon felt (diagonal placement) Cathode: Titanium sheet and graphite plate (vertical placement) Membrane: Proton exchange (Nafion® 117)	Anolyte: Anaerobic sludge with nutrient solution (acetate 1.0 g/L, trace elements) Catholyte: CuCl2 (100 mg/L) and K2Cr2O7 (10–150 mg/L) Rext: 200 Ω Volume: 125 cm3 (both chambers)	- Cu2+ recovery: 98.34% at 10 mg/L Cr6+ with graphite cathode - Cr6+ recovery: 99.92% at 10 mg/L Cr6+ with graphite cathode - Graphite cathode outperformed titanium: Cu2+: 98.09% vs. 88.79%, Cr6+: 86.13% vs. 51.13% - Higher Cr6+ conc. reduced recovery efficiency	[80]
Recovery of metals from AMD via electrodialysis followed by MFC and MEC modes	System: Dual-chamber MFC Anode: Carbon felt (KFA 10, 2.5 × 2.5 × 1.1 cm) Cathode: Titanium (2.5 × 2.5 cm) Membrane: Bipolar membrane (Fumasep® FBM)	Anolyte: 100 mL activated sludge (50%) mixed with nutrient medium (acetate 1 g/L, Na2HPO4 3 g/L, trace elements) Catholyte: 100 mL concentrated AMD Rext: 120 Ω (MFC)	MFC mode: - Fe3+ reduced to Fe2+ in 2 days - Cu2+ reduced to Cu0 in 8 days MEC mode: - Ec = –0.5 V: Negligible recovery in 48 h - Ec = –1.0 V: Mixed metal recovery (Cd2+, Ni2+, Fe2+, Zn2+) in 96 h - Ec = –1.5 V: Remaining Fe2+ and Zn2+ recovered in 96 h	[81]
Model comparison of factors affecting Cu2+ deposition in MFCs	System: Dual-chamber MFC Anode: Carbon fiber brush (2.5 cm diameter, 2.5 cm length) Cathode: Rough graphite plate (4 cm2) Membrane: Bipolar membrane (BPM-I)	Anolyte: Acetate with 50 mM phosphate buffer (27 mL) Catholyte: Simulated Cu2+ wastewater (17.5 mL) Rext: 50 Ω/Temperature: 30 °C	- Cu2+ recovery: 84.59% after 16 h. - Electromigration identified as a significant factor in Cu2+ mass transfer - Mathematical model was successfully validated	[82]
Simultaneous bioelectricity and Cu recovery	System: Dual-chamber MFC Anode: Carbon felt (16 cm2) Cathode: Carbon felt (16 cm2) Membrane: Nafion® 117	Anolyte: Culture medium with anaerobic sludge Catholyte: Artificial Cu2+ solution (1 g/L) pH: 7.0 (anode), 3.0 (cathode) Temperature: 35 °C/Rext: 1000 Ω	- 99.7% Cu recovery in 192 h - 50% recovery achieved within 48 h	[69]
Purification of fracturing flowback water (FFW) and Cu2+ removal using dual-anode MFCs	System: Dual-anode MFC (DA-MFC) with two glass chambers (180 mL total volume) Anode: Bioanode, circular carbon felt (Φ4 cm, 3 mm thick) Cathode: Wet-proofed carbon cloth (Φ5 cm) with Pt (0.5 mg/cm2) Membrane: Proton exchange membrane (Nafion® 117)	Anolyte: Synthetic FFW (1298.78 ± 1.22 mg/L COD, 247.5 ± 23.0 mg/L SO42−, 2 mg/L Cu2+, 12,000 mg salinity, pH 7) Catholyte: Phosphate buffer (50 mM, pH 7, 150 mL) Voltage: 0.05–0.2 V Rext: 100 Ω	- Cu2+ removal: 99.9 ± 0.5% in 5.5 days at 0.1 V - Removal at other voltages: 99.5 ± 0.3% at 0.05 V, 99.8 ± 0.7% at 0.2 V - Optimum performance at 0.1 V	[83]
Simultaneous mineralization of organic compounds and heavy metal recovery using photo-assisted BES	System: Dual-chamber MFC and MEC (MFC in situ-MEC and MFC1-MFC2-MEC) Anode: Porous graphite felt (1 × 1 × 1 cm) Cathode: Graphite felt with WO3/MoO3/g-C3N4 heterojunctions (2 × 2 × 0.25 cm) Membrane: None	Anolyte: 26 mL with 5 mM NaH2PO4, acetate, NH4NO3, KNO3, and trace elements Catholyte: 13–26 mL; soluble COD: 306 mg/L, Cu2+: 25.1 mg/L, Zn2+: 5.1 mg/L, Ni2+: 8.2 mg/L, pH 2.98 Rext: 10 Ω	- Optimal setup: 2 MFCs in series (13 mL cathodes) feeding a 26 mL cathode MEC, 6 h HRT Cu2+ reduced on MFC cathodes, Zn2+ precipitated in MEC–Metal removal: Metal removal: Cu2+, Ni2+ nearly 100%; Zn2+: 86% - Absence of light enhanced removal efficiency	[84]
Recovery of Zn from bioleachate using a MEC	System: Dual-chamber H-cell MEC Anode: Carbon felt (12.5 cm2) Cathode: Graphite foil (12.5 cm2) Membrane: Proton exchange membrane	Anolyte: 220 mL nutrient medium with COD: 675 mg/L (acetate, glucose, peptone, yeast extract, sewage sludge, pH 7.2) Catholyte: 220 mL, 36 mM phosphate buffer (pH 7.2) Potential: –100 mV	- Zn2+ reduced from 444 mg/L to 245 mg/L in 4 days - Al3+ reduced from 270 mg/L to 10 mg/L in 4 days - Simultaneous Fe reduction observed - Lower energy consumption compared to electrowinning	[85]
Recovery of Cr, Cu, and Cd from industrial water using Castellaniella species in MFC and MEC	System: Dual-chamber MFC and MEC Anode: Electroactive biofilms of Castellaniella species Cathode: Rectangular carbon cloth (2.5 × 0.9 cm) Membrane: Cation exchange membrane	Anolyte: 28 mL (2.5 g/L NaCl, 1.25 g/L yeast extract, 2.5 g/L peptone, pH 7) Catholyte: 15 mL simulated industrial wastewater (Cr6+: 134.88 mg/L, Cd2+: 130.18 mg/L, Cu2+: 130.78 mg/L, pH 1.8) Rext: 10 Ω/Temperature: 30 °C	MFC mode removal efficiency: - Cr6+: 99.6%, Cu2+: 99.9% MEC mode removal efficiency: - Cd2+: 99.9% - Mixed culture biofilm outperformed pure strains.	[86]
Wastewater treatment and elemental telluride recovery in a dual-chamber MFC	System: Dual-chamber MFC Anode: Graphite (38 cm2) Cathode: Graphite (38 cm2) Membrane: Proton exchange (Nafion® 117, 4 × 4 cm)	Anolyte: Acid-pretreated inoculum (10% v/v, VSS: 2.0 g/L) in nutrient medium (glucose: 3 g/L, pH 6) Catholyte: Sodium tellurite (0.011–0.044 g/L, pH 7) Rext: 30–0.05 kΩ/Temperature: 28 ± 2 °C	- Highest elemental tellurium (Te0) recovery: 45.3%. - Simultaneous Te4+ removal: 54.7% - Cathodic terminal electron acceptors enhanced microbial anodic oxidation and metal detoxification	[71]
Recovery and separation of Cu2+, Ni2+, and Zn2+ from Etching Terminal Wastewater (ETW) using photocomposition-assisted BES	System: Dual-chamber MFC (MFCCu) and MEC (MECNi, MECZn) Anode: Graphite felt Cathode: Graphite felt with WO3/MoO3/g-C3N4 heterojunctions Membrane: Cation exchange membrane (CM1-7000)	Anolyte: Nutrient solution (acetate 1 g/L, KH2PO4 4.4 g/L, trace elements, 26 mL) Catholyte: Actual ETW (SCOD 306 mg/L, Cu2+ 25.2 mg/L, Zn2+ 5.1 mg/L, Ni2+ 8.2 mg/L, Cr6+ 0.2 mg/L, pH 2.98) Power Source: 0.3 V and 0.6 V (MEC) Rext: 10 Ω (MFC) Temperature: 25 ± 3 °C	- Metal recovery: Cu2+ 85.8% (MFCCu), Ni2+ 71.6% (MECNi), Zn2+ 67.7% (MECZn) - Complete removal of Cr6+ and total chromium - Light irradiation and current application worked synergistically for mineralizing recalcitrant organic compounds and metals present in ETW - Current application improved metal recovery by 35.8–112.8% compared to light irradiation alone.	[87]
Investigating Cu, Ni, and Zn removal using BES under MFC and MEC modes	System: Dual-chamber MFC and MEC Anode: Carbon felt (25 cm2, Sigracell® KFD2.5) Cathode: Carbon fiber fabric (25 cm2, Sigratex® C U200) with PMF-011904 catalyst (2 mg/cm2) Membrane: Anion exchange membrane (AMI-7001)	Anolyte: Acetate-based mineral medium (ABMM, 0.5 L) Catholyte: 1 L solutions: Cu2+ (1.1 mM), Ni2+ (1.1 mM), Zn2+ (1.6 mM) Modes: Short-circuit MFC (Ecell = 0 V), MEC (Ec = –0.4 V vs. Ag/AgCl)	- Copper (24 h): MEC: 97.1% (64.1 g/d·m3), MFC: 88.1% (53.3 g/d·m3) - Nickel (48 h): MFC: 50.7% (17.4 g/d·m3), MEC: 41.0% (13.1 g/d·m3) - Zinc (24 h): MEC: 73.2% (40.25 g/d·m3), MFC: 74.5% (39.03 g/d·m3)	[88]
Recovery of cadmium and nickel from wastewater using a biocathode in MFC	System: Dual-chamber MFC Anode: Graphite (15 × 9 cm) Cathode: Graphite (15 × 9 cm) Membrane: Proton exchange membrane (Nafion® 117, 4 cm diameter)	Anolyte: Distillery wastewater (33,750 mg/L COD, pH 6, inoculum from Nazafgarh wetland) Catholyte: Ni2+, Cd2+, or Ni2+–Cd2+ solutions (10–25 mg/L, pH neutral) Rext: 200 Ω/Temperature: 25 ± 3 °C	- Abiotic cathode: Ni recovery: 71.5% (10 mg/L), 28.6% (25 mg/L); Cd recovery: 68.2% (10 mg/L), 20.6% (25 mg/L) - Biocathode: Ni recovery: 91.7% (10 mg/L), 48% (25 mg/L); Cd recovery: 86.9% (10 mg/L), 33% (25 mg/L)	[89]
Removal of zinc from industrial effluents using MFCs	System: Half-cell MFC Anode: Plain carbon felt (3 × 3 cm2) Cathode: Plain carbon felt (3 × 3 cm2) Membrane: Anion exchange membrane (FAB-PK-130, Fumatech)	Anolyte: 1.0 g/L CH3COONa, 3.0 g/L NaCl (7 × 7 × 2 cm3 working volume) Catholyte: 1.9 mM ZnCl2, 3.0 g/L NaCl (7 × 7 × 2 cm3 working volume) Rext: 10 Ω Temperature: 22 ± 3 °C	- Zn2+ removal: 96% for synthetic and industrial samples (<2.0 mM Zn2+) in 22 h - Cathode recovery: 83% (synthetic), 46% (industrial) - Electrodeposition predominated over chemical deposition	[90]
Recovery of metals (Fe, Cu, Sn, Ni) from synthetic AMD using BES in MFC and MEC modes	System: Dual-chamber MFC and MEC Anode: Carbon felt (KFA10, 2.5 × 2.5 × 0.8 cm, porosity: 0.95) Cathode: Copper (MFC), Titanium (MEC) Membrane: Bipolar membrane (Fumasep® FBM)	Anolyte: 0.1 L (acetate-based medium, pH 7.48) Catholyte: 0.1 L synthetic AMD (Fe3+: 500 mg/L, Cu2+: 500 mg/L, Sn2+: 50 mg/L, Ni2+: 50 mg/L, pH 2.5) Rext: 120 Ω Temperature: 25 °C	MFC mode: - Fe3+ reduced to Fe2+ within 1 day - Cu2+ to Cu0 recovery: 100% in 4 days MEC mode: - At –0.7 V: Sn2+ recovery: >80% in 0.3 days - After 72 h: Ni2+: 77%, Fe2+: 60% - BES required 0.5 V, significantly less than the 1.8 V needed in the abiotic blank system	[91]
Synchronized recovery of high-purity Fe and S from sulfide tailings using triple-chamber MFC	System: Triple-chamber MFC (two cathode chambers flanking the anode chamber) Anode: Pretreated carbon felt (7.1 cm2) Cathode: Pretreated carbon felt (7.1 cm2) Membranes: CEM (first cathode chamber) and CEM (second cathode chamber), area: 7.1 cm2	Anolyte: 28 mL (25 g/L sulfide tailing model, 1 g/L NaHCO3) Catholyte: 28 mL (Na2SO4: 0.67 g, simulating ferrous- and sulfate-laden streams) Rext: 500 Ω Temperature: 30 ± 1 °C	- Recovery in 50 h: Fe: 80% as Fe(OH)3, S: 22.1% as S0 - Purities: Fe(OH)3: 93.1%, S0: 90.2% - Facilitates simultaneous removal of iron and sulfate ions, reducing waste and enabling direct reuse of recovered materials in metallurgical processes	[92]
Modeling and validating metal removal efficiencies and energy consumption in BES vs. conventional techniques	Model: “Grey-box” combining Monod kinetics (MFC) and Butler-Volmer equations (MEC), predicting concentration profiles, current generation, and metal recovery. System: Dual-chamber MFC and MEC Anode: Carbon felt (2.5 × 2.5 × 0.8 cm) Cathode: Copper (MFC), Titanium (MEC) Membrane: Bipolar membrane	Anolyte: 0.1 L (acetate-based medium, pH 7.48) Catholyte: 0.1 L (Cu2+: 500 mg/L, Fe3+: 500 mg/L, Sn2+: 50 mg/L, Ni2+: 50 mg/L, pH 2.5) Rext: 120 Ω Temperature: 25 °C	- Model Validation: High accuracy for Cu2+ recovery, slight deviations for Fe3+. Captures concentration profiles and current density trends - MFC Mode: Complete Cu2+ and Fe3+ removal in 4 days, linking recovery with electricity generation - MEC Mode: >80% Sn2+ recovery in 1 day; Ni2+ (15 mg/L) and Fe2+ (30 mg/L) recovered over 4 days	[93]
Recovery of Se4+ oxyanion at the cathode of a dual-chamber BES	System: Dual-chamber MFC Anode: Graphite (38 cm2) Cathode: Graphite (38 cm2) Membrane: Nafion 117 (4 × 4 cm)	Anolyte: 10% inoculum, 3 g/L glucose, pH 6 ± 0.01 Catholyte: Na2SeO3∙5H2O (0.025–0.102 g/L), pH 7 ± 0.01 Rext: 30 to 0.05 kΩ/Temperature: 28 ± 2 °C	- Highest Se0 recovery: 26.4% in BES-SeC with 73.6% selenium removal from catholyte - Selenium formed at the cathode was amorphous. - Increased selenium concentration enhanced microbial dehydrogenase activity	[94]
Recovery of Cr6+, Cu2+, and V5+ contaminants using MFC technology	System: Dual-chamber MFC Anode: Carbon cloth (1.5 cm2) Cathode: Carbon cloth (12 cm2) Membrane: Bipolar (Fumasep®, 7 cm2)	Anolyte: Domestic wastewater with nutrient medium, 150 mL, pH 7.00 Catholyte: Metal solutions (K2Cr2O7, CuCl2, NaVO3), 150 mL, pH 7.00 Rext: 500 Ω/Temperature: 30 °C	- Metal concentrations reduced from 1 g/L to 0.02 g/L in 8 days - Highest recovery: Cr6+ (80%) > Cu2+ (74%) > V5+ (70%). - Current density decreased with metal concentration reduction	[95]
Use of Pseudomonas sp. E8 in MFC and MEC mode for Cu2+ and Cd2+ recovery in simulated AMD	MFC (single chamber): - Anode: Carbon brush - Cathode: Carbon cloth (7.07 cm2) - Membrane: None MEC (dual chamber): - Anode: Carbon felt - Cathode: Carbon cloth (2.5 × 0.9 cm) - Membrane: Anionic exchange membrane	MFC & MEC Anolyte: 28 mL, peptone (2.5 g/L), yeast extract (1.25 mg/L), NaCl (2.5 g/L), pH 1.80. Pseudomonas sp. E8 biofilm MEC Catholyte: 15 mL, cadmium sulfate with leachate of chalcopyrite Rext: MFC: 1000 Ω, MEC: 10 Ω Temperature: 30 °C	- MFC mode: Cu2+ rapidly reduced from 184.78 to 56.43 mg/L in 10 h, achieving 99.95 ± 0.09% recovery in 48 h - MEC mode: Cd2+ recovery of 99.86 ± 0.04% at 1.20 V applied voltage - Sequential recovery enabled by switching from MFC to MEC mode	[96]
Removal of Cu2+ and Zn2+ from industrial wastewater using MDC	System: Batch-operated dual-chamber MDC Anode: Carbon graphite (4 × 1 × 14 cm) Cathode: Carbon graphite Membrane: Polyester-based CEM (Fumasep FTCM-E) and AEM (Fumasep FTAM-E) Chamber Volumes: 2.6 L each	Anolyte: Activated sludge and municipal wastewater Middle Chamber: Synthetic Cu2+ and Zn2+ solution (100 mg/L) at pH 7 Catholyte: 0.1 M phosphate buffer DO in cathode chamber: 4.4 mg/L Retention Time: 120 min Temperature: 26 °C (mesophilic conditions)	- Removal efficiency: Cu2+: 79.7%, Zn2+: 79.6% (synthetic samples) - Voltage: 0.9 V (Cu2+), 0.8 V (Zn2+) - Removal efficiency (real samples): Cu2+: 68.37%, Zn2+: 70.64% - Maximum removal at mesophilic phase, 4.4 mg/L DO, and 120 min retention time	[56]
Copper removal and desalination using an MDC-ED integrated system	System: Dual-chamber MDC-ED (electrodialysis) integrated system Anode: Carbon felt Cathode: Carbon felt Membrane: AEM and CEM	Anolyte: Synthetic wastewater with 1 g/L CH3COONa (pH 7) Catholyte: Cu2+ solution (100 mg/L, pH 2) Desalination Chamber: NaCl solution (5 g/L) Rext: 10 Ω/Temperature: 30 °C	- Cu recovery efficiency: 88% - Salt removal efficiency: 47% - Energy-efficient desalination with simultaneous Cu2+ recovery - Current density: 2.1 A/m2	[55]
Simultaneous Cu and desalination in MDC	System: Four-chamber MDC (FMDC) Anode: Carbon felt Cathode: Carbon felt Membrane: AEM and CEM	Anolyte: Synthetic Cu wastewater (100–800 mg/L Cu2+, pH 7) Desalination chamber: 5 g/L NaCl solution Temperature: Room temperature	- 94% Cu recovery - 43% salt removal in desalination - Electricity generation with maximum current density: 2.0 A/m2	[97]
Copper removal using ZIF-8 nanocomposite MDC	System: Dual-chamber MDC Anode: Carbon felt (3 × 3 cm) Cathode: Carbon felt Membrane: ZIF-8 nanocomposite	Anolyte: Acetate solution (1 g/L) (pH 7) Catholyte: Cu(NO3)2 solutions (pH 2) Temperature: Room temperature	- Cu removal efficiency increased to 292 mg/g due to ZIF-8 membrane adsorption capacity - Desalination efficiency of 56% achieved - Significant biofouling prevention due to reduced microbial adhesion on the ZIF-8 surface	[38]
Coupling MDC-MEC for Pb recovery	System: MDC-MEC integrated system MDC: Carbon brush anode, carbon cloth cathode, Nafion, and AEM membranes MEC: Carbon brush anode, carbon cloth cathode for Pb reduction	MDC and MEC Anolyte: Synthetic wastewater (acetate, pH 7) MDC Catholyte and middle compartment: Brine solution (NaCl) MEC Catholyte: Pb(NO3)2 solution (pH 3)	- Lead recovery: 96% in 48 h - Energy generated by MDC stored in capacitors to power MEC	[50]

Table 2. Key studies highlighting advances in metal recovery using BES.

Objective	Reactor Setup	Operational Conditions	Milestones	Ref.
Use of MFC for recovery of metals from AMD	System: Dual-chamber MFC Anode: Carbon felt (CF) and doped variants with non-activated (CFnaH) or activated hydrochar (CFaH), electrode dimensions (2.5 × 2.5 × 0.8 cm) Cathode: Copper (2.5 × 2.5 cm) Membrane: Bipolar (Fumasep® FBM)	Anolyte: Synthetic medium with acetate (1000 mg/L, pH 7) Catholyte: Synthetic AMD (400 mg/L Cu2+, 400 mg/L Fe3+, 50 mg/L Ni2+, 50 mg/L Sn2+, pH 2.5) Rext: 120 Ω	- Cu recovery: 91% (CFaH), 85% (CFnaH), 48% (CF) - Faster Cu recovery (3 days vs. 4 days) with doped anodes - Fe3+ reduced from 400 to 10 mg/L - Current density: 1.2 A/m2	[10]
Copper recovery by saline MFC with polypyrrole-based cathodes	System: Dual-chamber MFC Anode: Carbon felt (6 mm thick, 3 × 3 cm2) Cathode: Stainless steel with Polypyrrole (SS/PPy), Polypyrrole with Phytic Acid (SS/PPy-PA), Polypyrrole with Carbonised Cellulose (SS/PPy-CC), and combinations (SS/PPy-PA-CC) Membrane: Nafion	Anolyte: Synthetic wastewater with acetate (pH 7) Catholyte: CuSO4 solution (256 mg/L Cu) in 2% NaCl pH: 3.0 Temperature: 30 °C Rext: Various (10–20,000 Ω)	- 97% Cu recovery in 3 days—SS/PPy-PA cathode: higher efficiency but corroded over time - SS/PPy suitable for large-scale MFCs	[70]
Metal removal using algae and algal biochar as cathode catalysts in Photosynthetic dual-chamber MFC (PMFCs)	System: Photosynthetic dual-chamber MFC Anode: Carbon felt (CF) (110 cm2) Cathode: CF (130 cm2), CF coated with Cu-accumulated ABC700 (activated biochar prepared from algae biomass thermally activated at 700 °C), and CF coated with Co-accumulated ABC700 Membrane: Clayware cylinder wall as proton exchange separator	Anolyte: 125 mL cultivated algae solution (pH 7) Catholyte: 400 mL with Cu2+ or Co2+ (25, 50, 75, and 100 mg/L, pH 7) Rext: 100 Ω Temperature: Room temperature	- Cu2+ recovery: 94% - Co2+ recovery: 88% - Highest power density and COD removal achieved with Co-accumulated ABC700 cathode - Algal biochar cathodes contributed to reducing pollutants and supporting energy generation	[68]
Heavy metal removal and energy recovery using pyrite-enhanced MnCo/CNF anode	System: Dual-chamber “H”-Type MFC Anode: Carbon felt (CF), Carbon nanofibers (CNF), MnCo/CNF, MnCo/CNF with pyrite (5 × 2 cm) Cathode: Carbon felt (5 × 2 cm) Membrane: Nanofiber membrane	Anolyte: Acetate and glucose-based medium Catholyte: 25 mM K3[Fe(CN)6] solution Rext: 1000 Ω Temperature: 20 ± 5 °C	- 99% Sb removal with MnCoPy-MFC - Maximum power density hierarchy: CF-MFC < CNF-MFC < MnCoCNF-MFC < MnCoPy-MFC - MnCoPy-MFC exhibited the highest energy recovery performance	[71]
Cr6+ recovery using upflow MFC	System: Upflow MFC Anode: Carbon felt with granular activated carbon (45 g, 377 cm2) Cathode: Carbon cloth (1130 cm2) Membrane: Proton exchange (Nafion® 117)	Anolyte: Mineral medium (pH 7), 0.98 L Catholyte: Cr(VI)-containing medium (pH 4), 4.2 L Temperature: 34 °C Rext: 1000 Ω	- 97.7% Cr(VI) removal with optimal 800 mg/L influent, 2-day retention time, and pH 7 (anode)/pH 4 (cathode). - Cr(III) precipitation reduced MFC efficiency over time. - Parallel configuration outperformed series	[72]
Cr6+ recovery using a combined wetland and MFC system	System: Vertical upward-flow MFC Anode/Cathode: 100-mesh stainless steel wrapped with activated carbon Membrane: None	Structure: Bottom (zeolite), anode (activated carbon), middle (ceramsite), cathode (activated carbon), wetland plants Simulated Wastewater: Cr(VI) (40–120 mg/L), acetate, NH4Cl, NaHCO3, nutrients Rext: 1000 Ω	- 97.7% Cr(VI) removal efficiency - Effective for chromium pollution reduction in wastewater - Integrated system design leveraging wetland plants and vertical MFC layers	[73]
Environmental impact investigation on Cr6+ recovery and electricity generation	System: Dual-chamber MFC Anode: Carbon felt Cathode: Carbon cloth, carbon brush, carbon felt Membrane: Proton exchange membrane	System: Dual-chamber MFC Anode: Carbon felt Cathode: Carbon cloth, carbon brush, carbon felt Membrane: Proton exchange	System: Dual-chamber MFC Anode: Carbon felt Cathode: Carbon cloth, carbon brush, carbon felt Membrane: Proton exchange	[74]
Chromium nano-mining using electrified bioreactors (EBS)	System: Two-compartment EBS (0.125 L per compartment) separated by CEM (Ultrex CMI-700, 64 cm2) Electrodes: Stainless Steel mesh (4.8 cm2)	Catholyte: 250 mL K2Cr2O7, pH 7, inoculated with S. oneidensis MR-1 Cathode polarization: –0.8 VSHE Flow rate: Batch recirculation at 2.5 L/h	- Cr(VI) removal: 76% in 24 h - Total Cr removal: 61% - Cr nanoparticles (19–36 nm) observed around bacterial cells - Energy consumption: 0.8 kJ - Maximum removal capacity: 1121 mg Cr/g cells	[47]
Silver recovery from photovoltaic panels	System: Dual-chamber MFC Anode: Carbon felt (5 × 5 cm) Cathode: Graphite paper (3 × 3 cm) Membrane: Nafion® 115	Anolyte: Phosphate buffer + glucose Catholyte: Synthetic extract solution (AgNO3, CuCl2, AlCl3) Rext: 100 Ω/Temperature: 30 °C	- Ag: 100% recovery (7 h) - Cu: 60% recovery (7 h) - Al: 15% recovery (7 h)	[75]
Investigating the effect of Ag concentrations on current production and recovery	System: Dual-chamber H-type MFC Anode: Carbon cloth (25 cm2) Cathode: Carbon cloth (25 cm2) with Pt (0.5 mg/cm2) Membrane: Cation exchange membrane (Nafion® 117)	Anolyte: Synthetic wastewater (acetate 1 g/L, trace elements, neutral pH) Catholyte: Ag solution (6–95 mg/L) Rext: 100 Ω Temperature: 25 °C	- Maximum Ag removal: 99.8% at 100 mg/L Ag - Ag recovery: 75% in 2 h at <30 mg/L, 90% in 6 h at >50 mg/L - Batch operation up to 8 h investigated for varying Ag concentrations	[76]
Feasibility of MFC technology for silver recovery from synthetic PV hydrometallurgical wastewater	System: Dual-chamber MFC Anode: Graphite paper (2.5 × 3.8 cm) Cathode: Plain graphite paper Membrane: Proton exchange membrane (Nafion® 117, 3.77 cm2)	Anolyte: Synthetic wastewater with 50 mg/L Ag Catholyte: Phosphate buffer (pH 7, 0.16 g/L KCl) Rext: 100 Ω/Temperature: 32 °C	- Ag recovery: >93% with NaClO4 as the supporting electrolyte - 100% Ag recovery at pH 2.00 in 3 h and pH 7.00 in 5 h - Lower efficiency when KCl was used as a supporting electrolyte	[77]
Gold Recovery and Nanoparticle Synthesis in Microbial Systems Using Fractional Factorial Design	Serum bottle reactors (120 mL), 50 mL solution, sealed with butyl rubber stoppers; Shewanella oneidensis MR-1 and Cupriavidus metallidurans CH34 as bacterial strains Electrochemical tests: Two-compartment BES Electrodes: Carbon electrodes Electron donor: In situ hydrogen from cathodic water reduction	Factors: cell concentration (OD 0.5 or 1), temperature (28 °C or 37 °C), anoxic/oxic, pH (1 or 5), Au3+ concentration (0.2 mM or 2 mM), electron donor (electrogenerated H2 or lactate), bacterial species; batch incubation for 72 h. Electrochemical tests for H2 generation: Carbon cathode poised at −0.3 V vs. Ag/AgCl	- Highest Au removal (88.2%) achieved with S. oneidensis MR-1 at 0.2 mM Au3+, pH 5, anoxic conditions, and (electrogenerated) H2 as the electron donor. - Targeted Au nanoparticle size (50 nm) obtained under optimal conditions - Significant role of pH and Au3+ concentration in removal efficiency	[48]
Alternative approach for Au recovery using microbial electrochemical snorkel (MES)	System: Dual-chamber MFC Anode: Bioanode Cathode: Gold foil and graphitized paper (8 cm2) Membrane: Proton exchange (Nafion® 117)	Anolyte: Aqueous phase above freshwater sediment Catholyte: Gold-containing solution (1 g/L Au3+) Rext: 510 Ω Temperature: 25 °C	- In MES-mode (short-circuited), >50% Au recovered in 2 h, 100% recovered in 24 h - Gold deposited as an elemental state - Similar performance for both cathodes, with graphite offering a cost advantage	[78]
Assessing Castellaniella inoculum for cathodic metal recovery in BES	System: Dual-chamber MFC Anode: Biofilm of mixed Castellaniella inoculum Cathode: Carbon cloth Membrane: Anion exchange membrane	Anolyte: 50 mM PBS with Wolfe’s vitamins and trace elements Catholyte: Pb–Zn smelting wastewater (COD: 2.26 mg/L, Cu2+: 183.19 mg/L, Hg2+: 78.44 mg/L, Pb2+: 206.94 mg/L, Zn2+: 126.92 mg/L, pH 2.38) Rext: 10 Ω/Temperature: 30 °C	Mode MFC: - Cu2+: 99.86% recovery in 60 h - Hg2+: 99.98% recovery in 17 h - Pb2+, Zn2+ unchanged Mode MEC: - 1 V: Pb2+: 93.49% in 36 h, Zn2+ unchanged - 2 V: Pb2+: 99.98% in 24 h, Zn2+: 99.17% in 24 h	[79]
Investigating heavy metal removal with varying metal concentrations and dissolved oxygen conditions	System: Dual-chamber MFC Anode: Carbon felt (diagonal placement) Cathode: Titanium sheet and graphite plate (vertical placement) Membrane: Proton exchange (Nafion® 117)	Anolyte: Anaerobic sludge with nutrient solution (acetate 1.0 g/L, trace elements) Catholyte: CuCl2 (100 mg/L) and K2Cr2O7 (10–150 mg/L) Rext: 200 Ω Volume: 125 cm3 (both chambers)	- Cu2+ recovery: 98.34% at 10 mg/L Cr6+ with graphite cathode - Cr6+ recovery: 99.92% at 10 mg/L Cr6+ with graphite cathode - Graphite cathode outperformed titanium: Cu2+: 98.09% vs. 88.79%, Cr6+: 86.13% vs. 51.13% - Higher Cr6+ conc. reduced recovery efficiency	[80]
Recovery of metals from AMD via electrodialysis followed by MFC and MEC modes	System: Dual-chamber MFC Anode: Carbon felt (KFA 10, 2.5 × 2.5 × 1.1 cm) Cathode: Titanium (2.5 × 2.5 cm) Membrane: Bipolar membrane (Fumasep® FBM)	Anolyte: 100 mL activated sludge (50%) mixed with nutrient medium (acetate 1 g/L, Na2HPO4 3 g/L, trace elements) Catholyte: 100 mL concentrated AMD Rext: 120 Ω (MFC)	MFC mode: - Fe3+ reduced to Fe2+ in 2 days - Cu2+ reduced to Cu0 in 8 days MEC mode: - Ec = –0.5 V: Negligible recovery in 48 h - Ec = –1.0 V: Mixed metal recovery (Cd2+, Ni2+, Fe2+, Zn2+) in 96 h - Ec = –1.5 V: Remaining Fe2+ and Zn2+ recovered in 96 h	[81]
Model comparison of factors affecting Cu2+ deposition in MFCs	System: Dual-chamber MFC Anode: Carbon fiber brush (2.5 cm diameter, 2.5 cm length) Cathode: Rough graphite plate (4 cm2) Membrane: Bipolar membrane (BPM-I)	Anolyte: Acetate with 50 mM phosphate buffer (27 mL) Catholyte: Simulated Cu2+ wastewater (17.5 mL) Rext: 50 Ω/Temperature: 30 °C	- Cu2+ recovery: 84.59% after 16 h. - Electromigration identified as a significant factor in Cu2+ mass transfer - Mathematical model was successfully validated	[82]
Simultaneous bioelectricity and Cu recovery	System: Dual-chamber MFC Anode: Carbon felt (16 cm2) Cathode: Carbon felt (16 cm2) Membrane: Nafion® 117	Anolyte: Culture medium with anaerobic sludge Catholyte: Artificial Cu2+ solution (1 g/L) pH: 7.0 (anode), 3.0 (cathode) Temperature: 35 °C/Rext: 1000 Ω	- 99.7% Cu recovery in 192 h - 50% recovery achieved within 48 h	[69]
Purification of fracturing flowback water (FFW) and Cu2+ removal using dual-anode MFCs	System: Dual-anode MFC (DA-MFC) with two glass chambers (180 mL total volume) Anode: Bioanode, circular carbon felt (Φ4 cm, 3 mm thick) Cathode: Wet-proofed carbon cloth (Φ5 cm) with Pt (0.5 mg/cm2) Membrane: Proton exchange membrane (Nafion® 117)	Anolyte: Synthetic FFW (1298.78 ± 1.22 mg/L COD, 247.5 ± 23.0 mg/L SO42−, 2 mg/L Cu2+, 12,000 mg salinity, pH 7) Catholyte: Phosphate buffer (50 mM, pH 7, 150 mL) Voltage: 0.05–0.2 V Rext: 100 Ω	- Cu2+ removal: 99.9 ± 0.5% in 5.5 days at 0.1 V - Removal at other voltages: 99.5 ± 0.3% at 0.05 V, 99.8 ± 0.7% at 0.2 V - Optimum performance at 0.1 V	[83]
Simultaneous mineralization of organic compounds and heavy metal recovery using photo-assisted BES	System: Dual-chamber MFC and MEC (MFC in situ-MEC and MFC1-MFC2-MEC) Anode: Porous graphite felt (1 × 1 × 1 cm) Cathode: Graphite felt with WO3/MoO3/g-C3N4 heterojunctions (2 × 2 × 0.25 cm) Membrane: None	Anolyte: 26 mL with 5 mM NaH2PO4, acetate, NH4NO3, KNO3, and trace elements Catholyte: 13–26 mL; soluble COD: 306 mg/L, Cu2+: 25.1 mg/L, Zn2+: 5.1 mg/L, Ni2+: 8.2 mg/L, pH 2.98 Rext: 10 Ω	- Optimal setup: 2 MFCs in series (13 mL cathodes) feeding a 26 mL cathode MEC, 6 h HRT Cu2+ reduced on MFC cathodes, Zn2+ precipitated in MEC–Metal removal: Metal removal: Cu2+, Ni2+ nearly 100%; Zn2+: 86% - Absence of light enhanced removal efficiency	[84]
Recovery of Zn from bioleachate using a MEC	System: Dual-chamber H-cell MEC Anode: Carbon felt (12.5 cm2) Cathode: Graphite foil (12.5 cm2) Membrane: Proton exchange membrane	Anolyte: 220 mL nutrient medium with COD: 675 mg/L (acetate, glucose, peptone, yeast extract, sewage sludge, pH 7.2) Catholyte: 220 mL, 36 mM phosphate buffer (pH 7.2) Potential: –100 mV	- Zn2+ reduced from 444 mg/L to 245 mg/L in 4 days - Al3+ reduced from 270 mg/L to 10 mg/L in 4 days - Simultaneous Fe reduction observed - Lower energy consumption compared to electrowinning	[85]
Recovery of Cr, Cu, and Cd from industrial water using Castellaniella species in MFC and MEC	System: Dual-chamber MFC and MEC Anode: Electroactive biofilms of Castellaniella species Cathode: Rectangular carbon cloth (2.5 × 0.9 cm) Membrane: Cation exchange membrane	Anolyte: 28 mL (2.5 g/L NaCl, 1.25 g/L yeast extract, 2.5 g/L peptone, pH 7) Catholyte: 15 mL simulated industrial wastewater (Cr6+: 134.88 mg/L, Cd2+: 130.18 mg/L, Cu2+: 130.78 mg/L, pH 1.8) Rext: 10 Ω/Temperature: 30 °C	MFC mode removal efficiency: - Cr6+: 99.6%, Cu2+: 99.9% MEC mode removal efficiency: - Cd2+: 99.9% - Mixed culture biofilm outperformed pure strains.	[86]
Wastewater treatment and elemental telluride recovery in a dual-chamber MFC	System: Dual-chamber MFC Anode: Graphite (38 cm2) Cathode: Graphite (38 cm2) Membrane: Proton exchange (Nafion® 117, 4 × 4 cm)	Anolyte: Acid-pretreated inoculum (10% v/v, VSS: 2.0 g/L) in nutrient medium (glucose: 3 g/L, pH 6) Catholyte: Sodium tellurite (0.011–0.044 g/L, pH 7) Rext: 30–0.05 kΩ/Temperature: 28 ± 2 °C	- Highest elemental tellurium (Te0) recovery: 45.3%. - Simultaneous Te4+ removal: 54.7% - Cathodic terminal electron acceptors enhanced microbial anodic oxidation and metal detoxification	[71]
Recovery and separation of Cu2+, Ni2+, and Zn2+ from Etching Terminal Wastewater (ETW) using photocomposition-assisted BES	System: Dual-chamber MFC (MFCCu) and MEC (MECNi, MECZn) Anode: Graphite felt Cathode: Graphite felt with WO3/MoO3/g-C3N4 heterojunctions Membrane: Cation exchange membrane (CM1-7000)	Anolyte: Nutrient solution (acetate 1 g/L, KH2PO4 4.4 g/L, trace elements, 26 mL) Catholyte: Actual ETW (SCOD 306 mg/L, Cu2+ 25.2 mg/L, Zn2+ 5.1 mg/L, Ni2+ 8.2 mg/L, Cr6+ 0.2 mg/L, pH 2.98) Power Source: 0.3 V and 0.6 V (MEC) Rext: 10 Ω (MFC) Temperature: 25 ± 3 °C	- Metal recovery: Cu2+ 85.8% (MFCCu), Ni2+ 71.6% (MECNi), Zn2+ 67.7% (MECZn) - Complete removal of Cr6+ and total chromium - Light irradiation and current application worked synergistically for mineralizing recalcitrant organic compounds and metals present in ETW - Current application improved metal recovery by 35.8–112.8% compared to light irradiation alone.	[87]
Investigating Cu, Ni, and Zn removal using BES under MFC and MEC modes	System: Dual-chamber MFC and MEC Anode: Carbon felt (25 cm2, Sigracell® KFD2.5) Cathode: Carbon fiber fabric (25 cm2, Sigratex® C U200) with PMF-011904 catalyst (2 mg/cm2) Membrane: Anion exchange membrane (AMI-7001)	Anolyte: Acetate-based mineral medium (ABMM, 0.5 L) Catholyte: 1 L solutions: Cu2+ (1.1 mM), Ni2+ (1.1 mM), Zn2+ (1.6 mM) Modes: Short-circuit MFC (Ecell = 0 V), MEC (Ec = –0.4 V vs. Ag/AgCl)	- Copper (24 h): MEC: 97.1% (64.1 g/d·m3), MFC: 88.1% (53.3 g/d·m3) - Nickel (48 h): MFC: 50.7% (17.4 g/d·m3), MEC: 41.0% (13.1 g/d·m3) - Zinc (24 h): MEC: 73.2% (40.25 g/d·m3), MFC: 74.5% (39.03 g/d·m3)	[88]
Recovery of cadmium and nickel from wastewater using a biocathode in MFC	System: Dual-chamber MFC Anode: Graphite (15 × 9 cm) Cathode: Graphite (15 × 9 cm) Membrane: Proton exchange membrane (Nafion® 117, 4 cm diameter)	Anolyte: Distillery wastewater (33,750 mg/L COD, pH 6, inoculum from Nazafgarh wetland) Catholyte: Ni2+, Cd2+, or Ni2+–Cd2+ solutions (10–25 mg/L, pH neutral) Rext: 200 Ω/Temperature: 25 ± 3 °C	- Abiotic cathode: Ni recovery: 71.5% (10 mg/L), 28.6% (25 mg/L); Cd recovery: 68.2% (10 mg/L), 20.6% (25 mg/L) - Biocathode: Ni recovery: 91.7% (10 mg/L), 48% (25 mg/L); Cd recovery: 86.9% (10 mg/L), 33% (25 mg/L)	[89]
Removal of zinc from industrial effluents using MFCs	System: Half-cell MFC Anode: Plain carbon felt (3 × 3 cm2) Cathode: Plain carbon felt (3 × 3 cm2) Membrane: Anion exchange membrane (FAB-PK-130, Fumatech)	Anolyte: 1.0 g/L CH3COONa, 3.0 g/L NaCl (7 × 7 × 2 cm3 working volume) Catholyte: 1.9 mM ZnCl2, 3.0 g/L NaCl (7 × 7 × 2 cm3 working volume) Rext: 10 Ω Temperature: 22 ± 3 °C	- Zn2+ removal: 96% for synthetic and industrial samples (<2.0 mM Zn2+) in 22 h - Cathode recovery: 83% (synthetic), 46% (industrial) - Electrodeposition predominated over chemical deposition	[90]
Recovery of metals (Fe, Cu, Sn, Ni) from synthetic AMD using BES in MFC and MEC modes	System: Dual-chamber MFC and MEC Anode: Carbon felt (KFA10, 2.5 × 2.5 × 0.8 cm, porosity: 0.95) Cathode: Copper (MFC), Titanium (MEC) Membrane: Bipolar membrane (Fumasep® FBM)	Anolyte: 0.1 L (acetate-based medium, pH 7.48) Catholyte: 0.1 L synthetic AMD (Fe3+: 500 mg/L, Cu2+: 500 mg/L, Sn2+: 50 mg/L, Ni2+: 50 mg/L, pH 2.5) Rext: 120 Ω Temperature: 25 °C	MFC mode: - Fe3+ reduced to Fe2+ within 1 day - Cu2+ to Cu0 recovery: 100% in 4 days MEC mode: - At –0.7 V: Sn2+ recovery: >80% in 0.3 days - After 72 h: Ni2+: 77%, Fe2+: 60% - BES required 0.5 V, significantly less than the 1.8 V needed in the abiotic blank system	[91]
Synchronized recovery of high-purity Fe and S from sulfide tailings using triple-chamber MFC	System: Triple-chamber MFC (two cathode chambers flanking the anode chamber) Anode: Pretreated carbon felt (7.1 cm2) Cathode: Pretreated carbon felt (7.1 cm2) Membranes: CEM (first cathode chamber) and CEM (second cathode chamber), area: 7.1 cm2	Anolyte: 28 mL (25 g/L sulfide tailing model, 1 g/L NaHCO3) Catholyte: 28 mL (Na2SO4: 0.67 g, simulating ferrous- and sulfate-laden streams) Rext: 500 Ω Temperature: 30 ± 1 °C	- Recovery in 50 h: Fe: 80% as Fe(OH)3, S: 22.1% as S0 - Purities: Fe(OH)3: 93.1%, S0: 90.2% - Facilitates simultaneous removal of iron and sulfate ions, reducing waste and enabling direct reuse of recovered materials in metallurgical processes	[92]
Modeling and validating metal removal efficiencies and energy consumption in BES vs. conventional techniques	Model: “Grey-box” combining Monod kinetics (MFC) and Butler-Volmer equations (MEC), predicting concentration profiles, current generation, and metal recovery. System: Dual-chamber MFC and MEC Anode: Carbon felt (2.5 × 2.5 × 0.8 cm) Cathode: Copper (MFC), Titanium (MEC) Membrane: Bipolar membrane	Anolyte: 0.1 L (acetate-based medium, pH 7.48) Catholyte: 0.1 L (Cu2+: 500 mg/L, Fe3+: 500 mg/L, Sn2+: 50 mg/L, Ni2+: 50 mg/L, pH 2.5) Rext: 120 Ω Temperature: 25 °C	- Model Validation: High accuracy for Cu2+ recovery, slight deviations for Fe3+. Captures concentration profiles and current density trends - MFC Mode: Complete Cu2+ and Fe3+ removal in 4 days, linking recovery with electricity generation - MEC Mode: >80% Sn2+ recovery in 1 day; Ni2+ (15 mg/L) and Fe2+ (30 mg/L) recovered over 4 days	[93]
Recovery of Se4+ oxyanion at the cathode of a dual-chamber BES	System: Dual-chamber MFC Anode: Graphite (38 cm2) Cathode: Graphite (38 cm2) Membrane: Nafion 117 (4 × 4 cm)	Anolyte: 10% inoculum, 3 g/L glucose, pH 6 ± 0.01 Catholyte: Na2SeO3∙5H2O (0.025–0.102 g/L), pH 7 ± 0.01 Rext: 30 to 0.05 kΩ/Temperature: 28 ± 2 °C	- Highest Se0 recovery: 26.4% in BES-SeC with 73.6% selenium removal from catholyte - Selenium formed at the cathode was amorphous. - Increased selenium concentration enhanced microbial dehydrogenase activity	[94]
Recovery of Cr6+, Cu2+, and V5+ contaminants using MFC technology	System: Dual-chamber MFC Anode: Carbon cloth (1.5 cm2) Cathode: Carbon cloth (12 cm2) Membrane: Bipolar (Fumasep®, 7 cm2)	Anolyte: Domestic wastewater with nutrient medium, 150 mL, pH 7.00 Catholyte: Metal solutions (K2Cr2O7, CuCl2, NaVO3), 150 mL, pH 7.00 Rext: 500 Ω/Temperature: 30 °C	- Metal concentrations reduced from 1 g/L to 0.02 g/L in 8 days - Highest recovery: Cr6+ (80%) > Cu2+ (74%) > V5+ (70%). - Current density decreased with metal concentration reduction	[95]
Use of Pseudomonas sp. E8 in MFC and MEC mode for Cu2+ and Cd2+ recovery in simulated AMD	MFC (single chamber): - Anode: Carbon brush - Cathode: Carbon cloth (7.07 cm2) - Membrane: None MEC (dual chamber): - Anode: Carbon felt - Cathode: Carbon cloth (2.5 × 0.9 cm) - Membrane: Anionic exchange membrane	MFC & MEC Anolyte: 28 mL, peptone (2.5 g/L), yeast extract (1.25 mg/L), NaCl (2.5 g/L), pH 1.80. Pseudomonas sp. E8 biofilm MEC Catholyte: 15 mL, cadmium sulfate with leachate of chalcopyrite Rext: MFC: 1000 Ω, MEC: 10 Ω Temperature: 30 °C	- MFC mode: Cu2+ rapidly reduced from 184.78 to 56.43 mg/L in 10 h, achieving 99.95 ± 0.09% recovery in 48 h - MEC mode: Cd2+ recovery of 99.86 ± 0.04% at 1.20 V applied voltage - Sequential recovery enabled by switching from MFC to MEC mode	[96]
Removal of Cu2+ and Zn2+ from industrial wastewater using MDC	System: Batch-operated dual-chamber MDC Anode: Carbon graphite (4 × 1 × 14 cm) Cathode: Carbon graphite Membrane: Polyester-based CEM (Fumasep FTCM-E) and AEM (Fumasep FTAM-E) Chamber Volumes: 2.6 L each	Anolyte: Activated sludge and municipal wastewater Middle Chamber: Synthetic Cu2+ and Zn2+ solution (100 mg/L) at pH 7 Catholyte: 0.1 M phosphate buffer DO in cathode chamber: 4.4 mg/L Retention Time: 120 min Temperature: 26 °C (mesophilic conditions)	- Removal efficiency: Cu2+: 79.7%, Zn2+: 79.6% (synthetic samples) - Voltage: 0.9 V (Cu2+), 0.8 V (Zn2+) - Removal efficiency (real samples): Cu2+: 68.37%, Zn2+: 70.64% - Maximum removal at mesophilic phase, 4.4 mg/L DO, and 120 min retention time	[56]
Copper removal and desalination using an MDC-ED integrated system	System: Dual-chamber MDC-ED (electrodialysis) integrated system Anode: Carbon felt Cathode: Carbon felt Membrane: AEM and CEM	Anolyte: Synthetic wastewater with 1 g/L CH3COONa (pH 7) Catholyte: Cu2+ solution (100 mg/L, pH 2) Desalination Chamber: NaCl solution (5 g/L) Rext: 10 Ω/Temperature: 30 °C	- Cu recovery efficiency: 88% - Salt removal efficiency: 47% - Energy-efficient desalination with simultaneous Cu2+ recovery - Current density: 2.1 A/m2	[55]
Simultaneous Cu and desalination in MDC	System: Four-chamber MDC (FMDC) Anode: Carbon felt Cathode: Carbon felt Membrane: AEM and CEM	Anolyte: Synthetic Cu wastewater (100–800 mg/L Cu2+, pH 7) Desalination chamber: 5 g/L NaCl solution Temperature: Room temperature	- 94% Cu recovery - 43% salt removal in desalination - Electricity generation with maximum current density: 2.0 A/m2	[97]
Copper removal using ZIF-8 nanocomposite MDC	System: Dual-chamber MDC Anode: Carbon felt (3 × 3 cm) Cathode: Carbon felt Membrane: ZIF-8 nanocomposite	Anolyte: Acetate solution (1 g/L) (pH 7) Catholyte: Cu(NO3)2 solutions (pH 2) Temperature: Room temperature	- Cu removal efficiency increased to 292 mg/g due to ZIF-8 membrane adsorption capacity - Desalination efficiency of 56% achieved - Significant biofouling prevention due to reduced microbial adhesion on the ZIF-8 surface	[38]
Coupling MDC-MEC for Pb recovery	System: MDC-MEC integrated system MDC: Carbon brush anode, carbon cloth cathode, Nafion, and AEM membranes MEC: Carbon brush anode, carbon cloth cathode for Pb reduction	MDC and MEC Anolyte: Synthetic wastewater (acetate, pH 7) MDC Catholyte and middle compartment: Brine solution (NaCl) MEC Catholyte: Pb(NO3)2 solution (pH 3)	- Lead recovery: 96% in 48 h - Energy generated by MDC stored in capacitors to power MEC	[50]

3. BES Coupling with Other Technologies for Metal Recovery

BESs can be integrated with complementary technologies to achieve upgraded metal recovery/utilization and electricity generation. These hybrid approaches offer improved efficiencies by leveraging bioelectrocatalysis and additional mechanisms such as photocatalysis, redox reactions, and advanced oxidation processes. Additionally, innovative pre-treatment methods for waste substrates have enabled more efficient BES integration into renewable material and energy systems. Such approaches facilitate the transformation of waste into valuable resources while mitigating environmental impact. In the literature, the research has resulted in the development of numerous hybrid BES systems and technologies associated with these systems [98,99].

3.1. MFC–MEC Coupling System

Metal recovery from wastewaters containing ions with varying redox potentials benefits significantly from the coupling of MFCs with MECs. Depending on its origin, some metal-polluted wastewaters contain metal ions with high redox potentials, such as Cr⁶⁺, V⁵⁺, Cu²⁺, or precious metals (i.e., Ag⁺, Au³⁺, and platinum group metals), which can be readily reduced at the cathode of an MFC, producing energy. Conversely, metals like Zn²⁺, Pb²⁺, or Ni²⁺, which have low redox potentials, require external energy input for reduction, which is achieved by operating under MEC mode.

The coupling of MFC and MEC systems offers a synergistic approach where the electricity generated by the MFC can partially or fully power the MEC, enabling the reduction of low-redox potential metals. This dual configuration facilitates the simultaneous recovery of metals with both high and low redox potentials, optimizing energy efficiency while addressing the complexities of treating wastewater containing diverse metal species [100,101]. Moreover, stacking multiple MFCs—via series and parallel configurations, as depicted in Figure 4a—can significantly enhance the power output, providing sufficient energy to drive MEC reactions at rates that are both efficient and practical for metal recovery applications.

3.2. Coupling MFC with Photocatalytic Fuel Cells

Despite the significant advancements in MFCs, considerable constraints in terms of power generation and bacterial start-up time still remain. Given the slow metal recovery rates at the cathode due to slow microbial kinetics at the anode, research trends are currently focused on the incorporation of photocatalysts into the BES configuration. The (bio)anode is either supplemented with a photocatalyst or complemented with an additional photoanode, a semiconductor material such as TiO₂, WO₃, or CdS, which generates electron–hole pairs upon exposure to light [102]. The positive holes facilitate the decomposition of organic waste or water oxidation at the anode (in parallel with the bioanodic reactions), while the photogenerated electrons are transferred to the cathode, as displayed in Figure 4b.

The synergy of bio- and photo-generated electrons effectively overcomes the kinetic limitations of the cathodic process, significantly enhancing the system’s overall performance [103,104]. Additionally, the use of micro- and macro-algae for bioremediation of toxic heavy metals is being explored to further improve these systems [105]. Algae offer advantages such as low operational and maintenance costs, natural abundance, and high efficiency in metal removal via biosorption and bioaccumulation. The integration of these biological and photocatalytic components not only optimizes wastewater management but also generates substantial environmental benefits, promoting a more sustainable and cost-effective approach to resource recovery and pollution mitigation [103,106,107,108].

3.3. Coupled Redox Fuel Cells

In coupled redox fuel cells, substances with low redox potential are oxidized at the anode, releasing protons and electrons. Conversely, substances with high redox potential are reduced at the cathode, transforming the chemicals into different species and generating electrical energy. Organic pollutants in wastewater, such as urea, phenol, and ethanol, typically exhibit low standard potentials and are prime candidates for anodic oxidation deposits [109]. Methanol is frequently used in fuel cell systems due to its high energy density and ease of handling; however, its oxidation (as in direct methanol fuel cells) typically requires noble metal catalysts, such as platinum, to overcome the high activation energy and facilitate efficient reaction kinetics [110]. However, within BES frameworks, microbial activity at the bioanode (electroactive biofilm) facilitates the degradation of such organic compounds without the need for expensive catalysts.

The microbial metabolism at the bioanode drives these oxidation reactions, transferring electrons to the electrode and generating a flow of current to the cathode. In some cases, the efficiency of this process can be enhanced by adding soluble redox mediators, such as methylene blue, neutral red, or anthraquinone-2,6-disulfonate (AQDS) [111,112,113,114,115]. These mediators shuttle electrons between the microorganisms and the electrode surface, effectively bridging the gap for electron transfer. Importantly, the redox potential of these mediators must be slightly higher than that of microbial cytochromes (typically −0.2 to −0.5 V vs. SHE) to ensure electron flow toward the electrode. Nature operates remarkably close to thermodynamic limits, allowing mediators with minimal overpotentials to efficiently drive the electron transfer process [111]. In this mechanism, bacteria utilize the mediators for cellular respiration, consuming organic substrates such as urea, phenol, and ethanol. The oxidized mediators are subsequently reduced on the electrode, completing the cycle [112].

At the abiotic cathode, metals like Cr⁶⁺, Au³⁺, Ag⁺, and Cu²⁺ are reduced to their elemental states or less toxic forms. These reduction reactions are driven by the electrons generated at the bioanode, traveling through an external circuit. For instance, Cr⁶⁺ can be reduced to Cr³⁺, a less toxic species, while Ag⁺ and Cu²⁺ can be recovered as metallic deposits [109,116].

3.4. Catalyst-Free Biologically Driven ORR

The oxygen reduction reaction is traditionally catalyzed by platinum-based materials on gas diffusion electrodes. However, biologically mediated ORR, driven by acidophilic iron-oxidizing bacteria (Acidithiobacillus ferrooxidans), presents a sustainable, catalyst-free alternative. These bacteria facilitate ORR by oxidizing ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) under acidic conditions, utilizing dissolved O₂ as the electron acceptor. Fe³⁺ is subsequently reduced at the cathode to provide the necessary redox potential for cathodic reactions. Unlike electroactive biofilms, the bacteria remain in suspension, acting as a biocatalyst, while the Fe²⁺/Fe³⁺ redox couple serves as a mediator, and hence, the overall reaction is the oxygen reduction to water. The use of gas diffusion electrodes as a cathode in this configuration enhances oxygen mass transport from air, enabling efficient passive oxygen dissolution into the catholyte without requiring external aeration [117].

While this approach slightly compromises cell voltage due to the higher redox potential of the O₂/H₂O couple (E⁰ = +1.23 V vs. SHE) compared to Fe³⁺/Fe²⁺ (E⁰ = +0.77 V vs. SHE), the latter is kinetically more favored. Fe³⁺ reduction involves a single-electron transfer, which is faster and more efficient than the sluggish, four-electron ORR. This kinetic advantage compensates for the voltage trade-off, improving reaction rates and overall system performance. Additionally, this approach removes the dependency on precious metals, substantially lowering CAPEX and enhancing the system’s economic viability at modest scales [118].

One of the notable advantages of this system is the potential to utilize Fe-containing wastewater, such as acid mine drainage, as the catholyte. AMD, naturally enriched with Acidithiobacillus ferrooxidans, can serve as both the catholyte and the inoculum, simplifying system operation and further lowering costs. Moreover, coupling a bioanode with a biocathode in an MFC system introduces additional opportunities for sustainable applications. The bioanode can process organic waste or other pollutants, while the biocathode catalyzes ORR using Fe²⁺ from wastewater, providing a second life to waste streams and decreasing the overall environmental impact.

3.5. MFC-Fenton Hybrid Systems

Bio-electro-Fenton systems represent a promising avenue for the treatment of recalcitrant organic pollutants due to their ability to couple bioelectrochemical energy generation with advanced oxidation processes. The classical (homogeneous) electro-Fenton process takes place in the cathodic chamber, where oxygen is partially reduced at the cathode (e.g., air diffusion cathode), yielding H₂O₂ (two-electron transfer reaction); subsequently, ferrous ions present in solution or added react with the peroxide to produce hydroxyl radicals and ferric ions, as illustrated in Figure 4c,d. Hydroxyl radicals (·OH) possess a high oxidizing power and may degrade a multitude of organic molecules, including pharmaceuticals, dyes, and phenolic compounds. However, the generation of ferric ions during the process may lead to the formation of iron³⁺ hydroxide if no proper pH control is kept (i.e., catholyte acidification), posing challenges such as catalyst/electrode deactivation and fouling [119,120].

The location of the electro-Fenton reactions dictates whether the process is in situ or ex-situ. In ex situ systems, microbial catalysis and the electro-Fenton process occur in different reactors, the latter being powered by a single or stacked MFC, as illustrated in Figure 4d. In this case, electrons generated at the bioanode are transferred to an external electro-Fenton cathode chamber, enhancing the separation of reactions but introducing additional challenges, such as increased energy demands and the need for effective electron transfer mechanisms. In contrast, in situ systems (Figure 4e) integrate bio-oxidation at the anode and electro-Fenton reactions at the cathode within the same reactor. This design simplifies the system but may compromise reaction efficiency due to competing processes and the inherent limitations of coupled microbial kinetics [84,92,93].

Integrating microbial bioanodes reduces the dependency on external energy inputs, making bio-electro-Fenton systems more sustainable and energy-efficient. One promising avenue for bio-electro-Fenton systems is the utilization of metal-containing wastes as the catholyte, such as AMD or iron-rich industrial effluents, which naturally contain ferrous ions that can act as Fenton catalysts for the degradation of organic pollutants. This strategy not only enhances pollutant removal but also valorizes waste streams as a resource for environmental remediation [121,122]. Heterogeneous electro-Fenton systems, where solid catalysts replace soluble iron, offer an additional layer of versatility by reducing issues related to the dissolution and reprecipitation of iron ions. Materials such as goethite, hematite, and magnetite are frequently employed in heterogeneous Fenton reactions due to their catalytic efficiency and stability [123]. Recent studies have demonstrated that these materials can be synthesized using electrochemically assisted oxidative alkaline precipitation (previously referred to as Gas-Diffusion Electrocrystallisation, a concept described in Figure 3d) from Fe²⁺-containing solutions, a process that can be powered by anodic bioelectrocatalysis. This would enable the remediation of AMD via BESs, with the resulting solid catalysts being repurposed for heterogeneous (bio)-electro-Fenton applications [124].

4. Techno-Economic Feasibility of BES for Metal Recovery

A conceptual framework for evaluating the energy efficiency of bioelectrochemical systems compared to conventional (abiotic) electrochemical technologies for cathodic processes, such as metal recovery (examples illustrated in Figure 3), is presented in Figure 5. This framework illustrates the operational regions where BES, driven by anodic bioelectrocatalysis, can outperform classical electrolysis technologies. The diagram highlights critical intersections in cell voltage vs. current density and power density vs. current density curves, comparing BES systems with their abiotic counterparts where the anodic reaction is the oxygen evolution reaction (OER) using either carbon felt (without any electroactive biofilm) or precious metal catalysts based (e.g., Pt, IrO₂, RuO₂) anode materials. These intersections delineate two key regions that define the economic feasibility of each technology.

In region (a), BES demonstrates a clear operational advantage, characterized by lower power consumption or even power production within the MFC operation region, making it particularly suitable for applications prioritizing energy efficiency and reduced operational costs. The MFC operation spans from open circuit voltage (OCV) to short-circuit conditions, leveraging the spontaneity of the redox processes. Beyond this region, BES transitions into MEC mode, where an external voltage is applied to drive non-spontaneous reactions, enabling the system to exceed the natural operational limits of MFCs and achieve higher reaction rates.

A key advantage of BESs lies in their significantly lower CAPEX, as carbon-based anode materials (e.g., carbon felt) used for the development of the electroactive biofilm are far less expensive than the platinum group metals used in conventional systems for OER catalysis. Additionally, carbon-based materials avoid long-term stability challenges, such as catalyst deactivation or dissolution, often faced by noble metal-based anodes. These factors further enhance the economic feasibility of BES configurations. In contrast, region (b) represents operational conditions where classical electrolytic technologies, particularly those based on catalyzed OER, become more advantageous due to their higher efficiency at elevated current densities.

The improved energy balance of BES systems over conventional electrochemical technology can be further expanded by optimizing system design and operation. For instance, increasing the anode-to-cathode surface area ratio or improving reactor geometry enhances the performance of BES, enabling it to reach the extended regions (a′) and (b′). These improvements allow BES to operate more efficiently under a broader range of conditions (expanded region (a’)), maintaining competitiveness and reducing energy demands.

However, despite these advantages, BESs face inherent limitations when compared to conventional systems, particularly at larger scales. The microbial catalysis that powers BESs while reducing energy demand or driving spontaneity under certain conditions is constrained by slower reaction kinetics. When BESs are pushed to higher current densities or productivity levels, their performance plateaus due to these kinetic limitations. Consequently, at high current densities (or productivity per electrode surface area), the efficiency of BESs is surpassed by their abiotic electrochemical counterparts, transitioning from region (a) to region (b), as depicted in Figure 5, where pure electrochemical technologies become more efficient. To maintain operation within region (a) under milder conditions, BESs would require a larger number of cells in a stack to achieve the same productivity as other technologies. This increases the complexity of practical implementation and significantly impacts CAPEX, posing challenges to the scalability of BESs for industrial applications. Addressing these limitations requires innovative strategies to enhance microbial kinetics and reactor configurations, ensuring BESs remain competitive for large-scale metal recovery and related processes.

This diagram provides a versatile tool for evaluating bioelectrochemical systems and identifying strategies to enhance their applicability, facilitating decision-making to maximize BES performance for real potential applications. BESs excel in low-current-density scenarios, where their energy efficiency and lower CAPEX, enabled by inexpensive and stable carbon-based anodes, make them ideal for decentralized and small-scale operations. However, at higher current densities, BES performance is constrained by slower microbial kinetics, leading to plateaus that necessitate larger stacks to achieve comparable productivity to abiotic systems, thereby increasing CAPEX and operational complexity. Although abiotic technologies demonstrate greater efficiency at higher current densities, BESs remain competitive in niche applications that prioritize energy efficiency, sustainability, and cost-effectiveness over large-scale productivity.

BES systems demonstrate significant advantages in low-current-density scenarios and specific applications where energy efficiency, low maintenance, and inexpensive materials are critical; however, they face inherent limitations when reaction rate is the performance indicator. Although BESs have a lower CAPEX at modest current densities, their costs increase more steeply compared to pure electrochemical reactors as current densities rise. However, BESs remain competitive for modest-scale, domestic, or localized applications where slow reaction rates are acceptable, simplicity is valued, and the use of catalyst-free, inexpensive materials is an advantage.

The primary limitation of BES systems is the reaction rates of electroactive biofilms, which are constrained by the limiting current density imposed by the inherent kinetics of microbial processes. Further polarization of the cell—such as increasing the anode potential beyond +0.4 V vs. Ag/AgCl—can disrupt biofilms due to oxygen generation and a drop in the local pH within the biofilm [126]. To mitigate this, larger anode surface areas relative to the cathode may be required (e.g., tubular reactors with the outer part as the anode). In some cases, decoupling electrochemistry from bioprocesses offers a more effective alternative.

In some cases, decoupling electrochemistry from bioprocesses offers a more effective alternative. While BES systems rely on direct extracellular electron transfer facilitated by bioelectrocatalysis, in some cases, Electrified Biological Systems (EBS) offer an alternative approach by decoupling electrochemistry from biological processes, making them suitable for certain applications. For instance, the bioreduction of Cr⁶⁺ to Cr³⁺ or Au³⁺ to Au⁰ can be achieved using electrogenerated H₂ as an electron donor in an EBS configuration (as discussed in Section 2 and displayed in Figure 3c, thereby bypassing the need for an electroactive biofilm developed on the cathode and addressing some of its associated challenges. Another example of an EBS configuration is the precious-metal-free biocatalyzed oxygen reduction reaction mediated by Acidithiobacillus ferrooxidans and iron, as described in Section 3 and depicted in Figure 4c [117]. In this approach, Fe³⁺ is electrochemically reduced at the cathode, while the bacteria oxidize Fe²⁺ back to Fe³⁺ using O₂ as the electron acceptor.

EBSs use electrogenerated intermediary products and redox mediators, enabling simplicity in scalability and decoupling the electrochemical and biological processes. Conversely, BESs rely on direct electron transfer via electroactive biofilms, excelling in energy-efficient biocatalysis with streamlined configurations that rely solely on electrodes as electron donors or acceptors. Hence, the selection of BESs or EBSs is context-dependent and determined by the specific requirements of the application.

Rather than aiming to replace established industrial processes, efforts should concentrate on applications where bioelectrochemical systems can offer measurable advantages. When applied to address specific challenges, BES can complement traditional technologies, creating synergies that drive innovation and efficiency in targeted applications [127].

5. Conclusions

The recovery of metals from wastes has become increasingly critical in addressing the dual challenges of growing metal demand and environmental pollution. Wastes offer a valuable secondary metal resource, particularly as conventional mineral extraction struggles to meet current demands. Proper waste treatment not only mitigates environmental pollution but also maximizes the recovery of metals and, when feasible, energy.

While traditional methods such as pyrometallurgy and hydrometallurgy are effective, they often generate secondary pollution and come with significant operational costs. In contrast, BESs present an environmentally and economically sustainable alternative. These systems combine microbial activity with electrochemical processes to recover metals while reducing or even generating energy. BESs have demonstrated the ability to recover metals such as copper, cobalt, chromium, and iron, among others. Double-chamber MFC bioelectrochemical systems remain the most commonly used because of their superior flexibility and precise control, which enable enhanced performance and efficient metal recovery.

Despite significant advancements, large-scale implementation of BESs remains a challenge. The microbial catalysis that drives these systems often exhibits slow reaction kinetics, limiting productivity, especially at higher current densities. This kinetic constraint limits BES performance, requiring operation under milder conditions to maintain efficiency. While these conditions align with the energy-saving advantages of BESs, meeting higher demand or treatment rates comparable to other technologies requires significantly larger system configurations, such as stacked cells. This approach complicates practical implementation and increases CAPEX, posing additional barriers to scalability.

Future advancements in BESs for metal recovery must address critical challenges to enhance their scalability, efficiency, and economic viability. Key priorities include optimizing reactor designs to maximize metal recovery rates, developing cost-effective and sustainable materials for electrodes and membranes, and improving the robustness of microbial communities to withstand industrial conditions. Economic evaluations underscore the importance of reducing operational costs, particularly by minimizing energy demands and material expenses, to ensure competitiveness with established technologies.

The unique advantage of BESs lies in their dual ability to recover valuable resources while simultaneously remediating polluted environments. This combination positions BES as a promising solution for decentralized applications and niche markets where traditional technologies may be impractical or inefficient. Hybrid and modular systems, which integrate BESs with other technologies, represent a significant opportunity to expand their applicability and address scalability limitations.