Interface Reconstruction of nZVI with a Mesoporous Mg(OH)2 Shell: Coupled Mass Transport and Interfacial Electron Transfer During Perchlorate Transformation

Perchlorate (ClO4−) is kinetically persistent in aqueous environments because of its highly symmetric tetrahedral structure and the high activation barrier for Cl−O bond cleavage. Although nanoscale zero−valent iron (nZVI) is a strong reductant, particle aggregation, surface passivation, and inefficient interfacial electron transfer limit its reactivity toward ClO4−. Here, a mesoporous nZVI@Mg(OH)2 core–shell−like composite was fabricated by a one−step liquid−phase in situ deposition strategy to couple substrate accessibility with interfacial electron transfer. At a nominal Mg(OH)2/nZVI mass ratio of 10%, the Mg(OH)2−modified composite showed substantially reduced particle aggregation and increased the BET specific surface area from 4.57 to 67.29 m2·g−1 and the pore volume from 0.0242 to 0.2542 cm3·g−1. The hydroxylated Mg(OH)2 interface may additionally favor local ClO4− accumulation under acidic−to−near−neutral conditions. Under benchmark conditions, the system removed 98.37% of 10 mg·L−1 ClO4− from the aqueous phase within 48 h. The reaction also showed an apparent activation energy of 50.96 ± 6.04 kJ·mol−1. XRD, Raman, and XPS analyses showed progressive Fe0 consumption and the development of a Fe3O4−dominated crystalline reaction layer with a more oxidized and hydroxylated Fe(III)−rich outer region, providing independent evidence for a strongly reducing and dynamically reconstructed Fe−containing interface. Electrochemical measurements revealed a substantially more negative open−circuit potential for nZVI@Mg(OH)2, while equivalent−circuit fitting showed that the total polarization resistance decreased from 15.21 to 10.78 kΩ relative to bare nZVI, consistent with a more electrochemically accessible interface. In situ OCP measurements further showed an immediate electrochemical response upon ClO4− addition. Together with TBA scavenging and Fe2+ control experiments, these results support a predominantly heterogeneous interfacial transformation process and are consistent with a major contribution from direct interfacial electron transfer. The enhanced reactivity is attributed to the coupling of improved substrate accessibility with an electronically accessible, low−resistance Fe−containing interface. This work provides a mechanistic basis for the interfacial design of Fe−based materials with coordinated mass−transport and electron−transfer characteristics toward kinetically persistent oxyanions.