Microbial Fuel Cells for Heavy Metal Wastewater Treatment: Mechanisms, Applications, and Challenges

Microbial fuel cells (MFC), as a sustainable and eco-friendly technology, exhibit a dual advantage of "waste-to-resource conversion" in the remediation of heavy metal-laden wastewater. This approach leverages electroactive microorganisms to metabolize organic substrates for bioelectricity generation while concurrently driving the reduction and immobilization of toxic heavy metals. This review systematically elucidates the mechanisms, reactor configurations, and microbial synergies involved in MFC-mediated heavy metal removal. Research findings demonstrate that MFC cathodes enable the bioelectrochemical reduction of highly toxic metal ions (e.g., Cr⁶⁺, Cu²⁺) into less toxic forms (e.g., Cr³⁺, Cu⁰) or elemental states, coupled with immobilization via hydroxide or sulfide precipitation. Dual-chamber MFCs achieve up to 92% reduction efficiency for high-concentration wastewater, while single-chamber and biocathode configurations facilitate cost-effective or sulfide-mediated multi-metal removal (e.g., PbS, CuS). Microbial consortia play a critical role: direct electron-transferring microorganisms (e.g., Shewanella, Geobacter) utilize cytochromes for metal reduction; sulfate-reducing bacteria generate S²⁻ to promote sulfide precipitation; and adsorbent microbes (e.g., Pseudomonas) immobilize metals via extracellular polymeric substances. However, practical applications remain constrained by challenges such as inefficient electron transfer, multi-metal competition, and long-term operational instability. Future research should prioritize innovations in electrode materials (e.g., MXene/biochar composites), strategies for electron allocation in multi-metal systems, and modular system integration to enhance treatment efficiency and resource recovery. This review offers theoretical and technical guidance for optimizing MFC in heavy metal wastewater treatment.