# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 2 Bioelectrochemical Systems for Metal Recovery
Another cathodic process associated with bioelectroactive anodes is metal recovery through bioprecipitation or immobilization in a bioethode, either as oxides or in their zero-valent state. This occurs through biocatalysis at the cathode, with mechanisms varying based on the bioethode 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 H\({}_{2}\)--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 C\({}^{6+}\) to C\({}^{3+}\) using a _Shevanella oneidensis_ bioethode [47] and the reduction of Au\({}^{3+}\) to Au\({}^{0}\) using _S. oneidensis_ or _C. metallidurans_[48], both of which rely on H\({}_{2}\) 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 H\({}_{2}\)O\({}_{2}\) (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 hydroxies. 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 CO\({}_{2}\) 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.