Interactions of Microbial Extracellular Polymeric Substances with Iron Oxides: Adsorption, Redox, and Mineral Transformation

Iron (Fe) is the fourth most abundant element in the earth’s crust and plays an important role in both biological and chemical processes. Iron oxides naturally interact with microorganisms, especially through extracellular polymeric substances (EPS), which are a major part of microbial biomass. These processes significantly contribute to nutrient cycling and long-term retention of carbon. The pathways, mechanisms, and environmental consequences of EPS and Fe oxide interactions have gained increasing attention in recent decades. Here, we propose and apply a unified mechanistic framework that maps EPS–Fe interactions onto an adsorption–redox-transformation cascade spanning molecular, interfacial, and mineral scales. Especially, this review synthesizes advances in characterizing EPS adsorption onto Fe oxides and EPS-induced Fe mineral transformation. These interactions are not merely passive surface processes but involve dynamic, multistep mechanisms governed by the molecular heterogeneity of EPS. Such processes influence Fe mineral stability, carbon retention, and contaminant fates in aqueous environments. EPS-induced Fe transformations depend on the physicochemical properties of both EPS and Fe oxides and proceed through adsorption and binding, along with stabilization, reductive dissolution, and secondary mineral formation of Fe. We also discuss a range of practical and advanced techniques used in this field. Finally, we identify key knowledge gaps and propose future research directions: EPS functional-group specificity, EPS-induced long-term Fe mineral transformation processes, and their roles in contaminant fates and Fe cycling.