# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 3 BES Coupling with Other Technologies for Metal Recovery
### 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 H\({}_{2}\)O\({}_{2}\) (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 iron\({}^{3+}\) 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 Electro crystallisation, a concept described in Figure 3d) from Fe\({}^{2+}\)-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].