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 C-Cl 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).