# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 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]. Figure 2: Co-occurrence analysis of keywords of papers published per year, including the terms “Metal recovery” and “Bioelectrochemical systems”. Author keywords, Minimum number of occurrences: 6, Number of keywords selected: 100. Figure made with VOSviewer software. 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 (\(\text{E}_{\text{cathode}}-\text{E}_{\text{anode}}>0\)), 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, which are also enhanced by the external energy input. Figure 3: Classification of BESs into categories depending on the metal recovery mechanism. (**a**) Bioelectroactive anode coupled to the abiotic cathode, where a metal in solution undergoes electrochemical reduction or electrodeposition onto the cathode.