Bidentate Hydrogen Bonds Accelerate Interfacial Proton-Coupled Electron Transfer Kinetics by Reshaping Dynamic Water Networks on Zero-Valent Iron Interfaces

Abstract

Proton-coupled electron transfer (PCET) at the solid–liquid interface is crucial for addressing the efficiency reduction of zerovalent iron (ZVI) materials in environmental pollution control, which stems from hindered iron cycling. In this study, we report a tannic acid-modified ZVI (TA-ZVI) that accelerates PCET by engineering an interfacial hydrogen-bonding network at the outer Helmholtz plane (OHP). Grafting ortho-phenolic hydroxyl groups onto ZVI reconstructs the interfacial water network, increases the fraction of weakly bound/free water, and enhances interfacial solvation/polarization relaxation, thereby lowering the kinetic barrier for PCET and promoting the generation and utilization of H*. The H* is subsequently delivered through the interfacial hydrogen-bonding network via a thermodynamically favorable Grotthuss-like pathway to the Fe(III) sites, thereby accelerating the iron cycle and enhancing the activity of the surface-bound Fe(II)-mediated 2e– ORR-Fenton reaction. Mechanistic investigations using phenolic analogues identify that ortho-phenolic hydroxyl groups are uniquely effective relative to para-/meta-configurations, owing to the formation of bidentate hydrogen bonds that confine and stabilize H*. Using sulfamethazine (SMT) as a model contaminant, TA-ZVI achieves 85% removal, substantially outperforming pristine ZVI, which removes only 19.8%. TA-ZVI also maintains effective and continuous SMT removal in real wastewater matrices and sustains stable operation for 1000 min in a continuous-flow membrane reactor. This work establishes dynamic hydrogen-bond-network engineering as a molecular strategy for regulating interfacial PCET and enhancing ZVI-based oxidative remediation.