Heavy metals, particularly toxic elements like chromium, uranium, and selenium, pose persistent risks to ecosystems and human health due to their long-term stability and mobility in natural environments. Under oxidative conditions, these pollutants readily migrate and bioaccumulate, leading to severe adverse effects including carcinogenicity, neurotoxicity, and genotoxicity (Adnan et al., 2024; Waqas et al., 2024). Developing green and efficient strategies for heavy metal removal and immobilization therefore remains a critical challenge in environmental remediation. Microbial reduction has emerged as a promising alternative, as microorganisms can transform soluble heavy metals into insoluble and less toxic forms via Extracellular Electron Transfer (EET) pathways. This process enables bio-reductive immobilization through direct or indirect reduction of metal species, limiting their mobility and environmental risk (Wang et al., 2024a). Compared to conventional chemical reagents and engineered materials, microbial bio-reduction operates under milder conditions with lower energy demand, offering a more sustainable remediation approach (Agrawal et al., 2024).

However, EET-dependent heavy metal bio-reduction, primarily mediated by Electroactive Microorganisms (EAMs) such as Shewanella oneidensis and Geobacter sulfurreducens, is often constrained by intrinsically low electron transfer efficiency, hindering remediation performance. To address this bottleneck, recent studies have proposed diverse EET enhancement strategies, including conductive materials (e.g., iron minerals, carbon-based materials) (Yu et al., 2023; Zhang et al., 2024a), exogenous electron transfer mediators (Zhang et al., 2022a), and external stimuli such as electric fields or light (Chen et al., 2022a; Chen et al., 2023). While these approaches can markedly improve EET efficiency and heavy metal bio-reductive immobilization, they may introduce secondary pollution risks and often oversimplify microbe–soil interactions, restricting their applicability under field conditions. Therefore, further research is needed to integrate eco-friendly materials and advanced genetic tools (Li et al., 2024b) with microbial community interactions (Kost et al., 2023; Zhang et al., 2024b) and mechanistic insights into EET pathways (Norman Michael et al., 2023; Tian et al., 2024), thereby supporting the development of environmentally sustainable and practically viable EET enhancement strategies.

This review systematically summarizes recent progress in microbial EET-driven heavy metal bio-reduction and immobilization. It emphasizes mechanistic insights, enhancement strategies and implications for practical application. Specifically, this review: (i) outlines the fundamental mechanisms of microbial EET; (ii) analyzes representative case studies of heavy metal bio-reduction with a focus on metal-specific reduction pathways; (iii) summarizes current EET enhancement strategies and their effects on bio-reductive performance; and (iv) discusses remaining challenges and future perspectives, highlighting green EET promoters, genetic approaches to improve microbial performance, and the roles of interspecies interactions and electron transfer pathways in achieving sustainable remediation.