Humic acid promotes Fe(II) dynamics and hydroxyl radical generation via interfacial regulation on nanoscale zerovalent iron for imidacloprid degradation.

Interfacial activation of dissolved oxygen by nanoscale zerovalent iron (nFe0) through corrosion-derived Fe(II) offers an effective pathway for oxidative remediation of contaminated water. However, its performance is limited by aggregation, surface passivation, and inefficient Fe(II) utilization, which hinder oxygen activation and oxidative transformation. Herein, humic acid-modified nFe0 (HA-nFe0) was constructed via a Fe(II) pre-coordination strategy for efficient imidacloprid (IMI) removal. Synchrotron-based analyses revealed that HA introduced low-coordination Fe-O structures and redistributed interfacial charges, significantly promoting electron transfer. The optimized HA-nFe0(1/40) achieved 91.5% IMI removal, with a ∼3.5-fold higher reaction rate constant than pristine nFe0. Crucially, the enhanced reactivity was governed not simply by increased Fe(II) production, but by the HA-regulated speciation and dynamic evolution of interfacial Fe(II). Through Fe coordination and electron shuttling, HA regulated the dynamic redistribution of strongly bound, weakly adsorbed, and dissolved Fe(II), thereby sustaining reactive interfacial Fe(II) and promoting the sequential conversion of oxygen intermediates to sustain ·OH generation. DFT calculations further showed that HA lowered *OH adsorption energy for rapid radical release, facilitating ·OH release and sustained oxidative reactivity. Continuous-flow experiments confirmed excellent stability over 28 days with reduced ecotoxicity of transformation products. This work establishes a mechanistic link between HA-regulated interfacial Fe(II) dynamics and oxygen activation, providing an interfacial perspective for regulating the oxidative reactivity of Fe-based environmental materials via natural organic matter.