# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 2 Bioelectrochemical Systems for Metal Recovery
However, they are less suitable for research requiring precise separation of anodic and cathodic environments, particularly in caseswhere 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.