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.