Engineering a gold(iii)-modified cathode for electro-Fenton degradation of ciprofloxacin: linking structure,...

A structurally well-defined gold(iii) complex, [AuCl(κ³-dien)]Cl₂, was successfully synthesized and structurally characterized by single-crystal X-ray diffraction, revealing a distorted square-planar coordination geometry stabilized by a tridentate amine ligand and an extended hydrogen-bonding network. Complementary spectroscopic and thermal analyses further confirmed the structural integrity and stability of the complex. The synthesized compound was subsequently employed as a functional modifier of a carbon felt cathode in a boron-doped diamond (BDD)-based Electro-Fenton system for the degradation of ciprofloxacin. Under optimized operating conditions (pH = 3, current density = 30 mA cm^-2, Fe²⁺ concentration = 0.10 mM, and CIP concentration = 20 mg L^-1), the resulting BDD/Au@CF system achieved approximately 99% CIP removal within 20 min and 92% TOC removal after 240 min, significantly outperforming the unmodified cathode. The influence of key operational parameters was systematically investigated, demonstrating their critical role in regulating reactive oxygen species generation and overall process performance. Kinetic studies revealed rapid pseudo-first-order degradation behavior, while mineralization and ecotoxicological assessments confirmed extensive organic matter removal accompanied by substantial detoxification of the treated effluent. Mechanistic analysis based on the identified transformation products indicated that ciprofloxacin degradation proceeds through successive hydroxyl-radical attacks, leading to molecular fragmentation, aromatic ring opening, and the formation of low-molecular-weight carboxylic acids prior to complete mineralization into CO₂, H₂O, and inorganic ions. The superior performance of the system is attributed to the synergistic interaction between the BDD anode and the Au-modified cathode, which facilitates interfacial electron-transfer processes and enhances the utilization of Electro-generated oxidizing species. Overall, this study demonstrates that structurally engineered gold(iii) coordination complexes constitute promising cathodic modifiers for next-generation Electro-Fenton systems and provide an efficient and sustainable strategy for the remediation of pharmaceutical contaminants in aquatic environments.