# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 2 Bioelectrochemical Systems for Metal Recovery
(**b**) Bioelectroactive anode coupled to bioelectroactive cathode, where electrochemically active microorganisms use the cathode as electron donor to reduce a metal ion in solution to a more reduced form; (**c**) bioelectroactive anode coupled to a non-bioelectroactive biological cathode, where the cathode provides H2 from water reduction, and the microorganisms in suspension use it as an electron donor to, biologically, reduce the metals in solution; then the reduced metal is precipitated in zero-valent state or as an oxide; (**d**) bioelectroactive anode coupled to the oxygen-reduction-assisted metal precipitation: at the gas diffusion cathode (GDC), oxygen reduction generates hydrogen peroxide (H2O2) and hydroxide ions (OH\({}^{-}\)), the former oxidizing the metal ions in solution, and the later increasing the pH in solution, leading to alkaline metal precipitation; (**e**) microbial desalination cell (also, microbial electrodialysis), in a three-chamber reactor design: the central compartment houses a metal-containing solution, where anions (A\({}^{-}\)) migrate to the anode through an anion exchange membrane (AEM) and metal cations (M\({}^{n+}\)) migrate to the cathode through a cation exchange membrane (CEM), hence the catholyte is concentrated in M\({}^{n+}\); (**f**) stack of BES reactors connected in series; (**g**) stack of BES reactors connected in parallel. 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  
Figure 4: Schematic overview of BES coupling with advanced technologies for metal recovery. (**a**) MFC–MEC Coupling System, where metals with high redox potential, \(\mathrm{M_{1}}\) (e.g., Cu\({}^{2+}\) or precious metals), are reduced at the MFC cathode (in this example, electrodeposition), generating electricity. The voltage produced powers an MEC for the reduction of metals with low redox potential (e.g., Zn\({}^{2+}\), Ni\({}^{2+}\)). MFC stacks can be used to enhance the voltage output for MEC operation. (**b**) Coupling MFC with photocatalytic fuel cells, in which the bioanode pairs with a photoanode, where photocatalysts generate electrons and holes. Bioanode oxidation and photogenerated electrons enhance metal reduction at the cathode, while holes split water or degrade organics at the photoanode. (**c**) Catalyst-free biologically driven ORR, where acidophilic iron-oxidizing bacteria oxidize Fe\({}^{2+}\) to Fe\({}^{3+}\) using dissolved O\({}_{2}\) as the electron acceptor, while Fe\({}^{3+}\) is electrochemically reduced at the cathode. (**d**) Electro-Fenton reactor powered by an MFC (ex situ bio-electro-Fenton), where the electro-Fenton reactions are driven by the electricity generated in the MFC. Stacking multiple MFC units, as shown in (**a**), can enhance the reaction rates by increasing the applied potential. (**e**) In situ bio-electro-Fenton reactor, in which the electro-Fenton reactions are powered directly by the bioanodic process of the BES. Abbreviations: CF, carbon felt; CEM, cation exchange membrane; GDC, gas-diffusion cathode. 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 MFCs achieve it with lower energy consumption due to the external energy input.