Immobilization of Heavy Metals by Microbial Extracellular Polymeric Substances: Key Mechanisms, Driving Factors, and...

Microbial extracellular polymeric substances (EPS) are increasingly recognized as promising biogenic matrices for heavy metal immobilization, but their role is still frequently interpreted through an adsorption-centered perspective. This interpretation limits a mechanistic understanding of why EPS-rich systems often show greater stability and adaptability than conventional biosorbents under complex wastewater conditions. This study proposes to view EPS-mediated heavy metal immobilization as a coupled and adaptive process that integrates rapid adsorption, selective coordination, precipitation, biomineralization, biofilm protection, and microbial regulation. Evidence from recent studies indicates that EPS immobilize heavy metals through ion exchange, electrostatic attraction, coordination complexation, direct precipitation, and metabolically induced biomineralization, while these pathways are jointly governed by EPS composition, functional-group distribution, spatial stratification, pH, salinity, redox state, nutrient balance, metal stress, and quorum sensing. A key point of this study is that it attempts to go beyond describing what EPS do and to explain how EPS convert transient metal capture into more stable immobilization by linking extracellular chemistry, microbial metabolism, and biofilm-scale organization. This framework suggests a different interpretation of prior studies: high adsorption capacity alone should not be treated as a universal performance indicator; instead, EPS performance should be evaluated by capacity, selectivity, post-binding stability, regeneration potential, and tolerance to mixed-metal and saline wastewater conditions. By critically comparing mechanisms, performance ranges, and engineering trade-offs, this review provides a more analytical basis for controllable EPS production, functional optimization, composite design, and resource-oriented wastewater treatment. EPS should be regarded not merely as passive biosorbents, but as adaptive biological interfaces with strong potential for sustainable and scalable heavy metal remediation.