Reconciling the reactivity-biocompatibility trade-off in nanoscale zero-valent iron with an amorphous core and...

Nanoscale zero-valent iron (nZVI) serves as an effective electron donor to enhance anaerobic methanogenesis, yet its high reducibility often induces microbial nanotoxicity, creating a fundamental conflict between reactivity and biocompatibility. Herein, we synthesized an extracellular polymeric substance (EPS)-modified nZVI bio-composite (nZVI@EPS) via one-step liquid-phase reduction, with a focus on the structural characteristics and functional interplay of its core-shell architecture in bio-nano systems. We found that EPS decelerated precursor reduction kinetics, suppressing supersaturation-induced crystallization and favoring the formation of an amorphous iron core with elongated Fe-Fe bonds that enhanced its thermodynamic driving force for electron donation. Simultaneously, the resulting EPS layer served as a biocompatible and pseudocapacitive biointerface, physically shielding microorganisms from direct contact and electrochemically buffering electron surge from the highly reductive iron core through a storage and controlled-release mechanism. Hydrogen evolution experiments confirmed that the amorphous core ensured sufficient electron supply, while the EPS biointerface merely regulated the electron release kinetics without sacrificing ultimate utilization efficiency. In the anaerobic digestion of waste activated sludge, the optimized core-interface synergistically enhanced methane yield and biogas purity by 31.11 % and 37.42 %, respectively. Such improvements were underpinned by enhanced enzymatic activities, reinforced energy conservation, and a redirected methanogenic metabolic flux toward the hydrogenotrophic pathway. This study leverages insights from iron core-interface functional decoupling to propose a synchronized optimization strategy, establishing a universal design framework for engineering nZVI materials that integrate high reactivity with biocompatibility for efficient waste-to-energy conversion.