Interfacial hydrophobicity-induced desolvation effects on reductive dehalogenation by Zero-Valent Iron

Modifications of zero-valent iron (ZVI) that increase surface hydrobobicity are known to increase the rate constants and electron efficiency (ε_e) for non-polar contaminant reduction, yet the effect of hydrophobicity-driven desolvation on transformation of polar contaminants remains unclear. This study provides the first comprehensive characterization of interfacial desolvation's role in enhancing ZVI-based dehalogenation performance, using the amphiphilic pharmaceutical compound florfenicol (FF) as a target contaminant. ZVI with controlled hydrophobicity was prepared by introducing boron mechanochemically, resulting two distinct materials: sB-ZVI with a hydrophobic BO_x surface layer (water contact angle is approximately 77°) and B-ZVI without it (water contact angle <25°). Attenuated total reflection Fourier transform infrared (ATR-FTIR) analysis showed the hydrophobic BO_x layer disrupts interfacial hydration, decreasing coordination by water around FF's hydrophilic amide and sulfonyl groups. This desolvation activates FF's CCl bonds partly by lowering their lowest unoccupied molecular orbital (LUMO) energies from about -2.1 eV to -2.3 eV, which increases the rates of electron transfer-mediated dehalogenation kinetics (2-5-fold compared to B-ZVI). The BO_x layer also weakens interactions between sB-ZVI and water, which suppresses the rate of hydrogen evolution by about 80%. The net result of these two effects are increased ε_e by 38%. The relevance of these desolvation-induced benefits on FF dehalogenation was verified by batch experiments done with a representative set of groundwater samples and 150-day aging tests (retaining >80% Fe° content and ≤20% kinetic reduction).