Electrochemical Degradation of Xenobiotics on BDD Electrodes

The persistence of xenobiotic pollutants such as antibiotics and per- and polyfluoroalkyl substances (PFAS) in aquatic environments poses serious ecological and public health risks, largely due to their resistance to conventional wastewater treatment processes. This thesis investigates the electrochemical degradation of two representative pollutant classes; oxolinic acid (OA), a first-generation quinolone antibiotic, and two model PFAS compounds, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonamide (PFOSA) using boron-doped diamond (BDD) electrodes. The overall aim was to evaluate the degradation efficiency, kinetics, and energy consumption of these systems under various operational conditions, and to elucidate the role of electrolyte composition and electrode configuration in enhancing treatment performance. In the first part of the work, the electrochemical degradation of OA (10 μM) was studied under galvanostatic conditions in both simulated and real seawater using BDD electrodes (9 cm2). A range of supporting electrolytes typical of seawater including NaBr, NaCl, Na2SO4, Na2CO3, NaHCO3, and Na2B4O7·10H2O were evaluated to assess their influence on degradation efficiency. After two hours of electrolysis, degradation ranged from 50% to 99%, with NaBr delivering the highest performance, achieving over 90% removal even at a low current density of 1.1 mA cm−2. The addition of other salts to NaBr maintained high degradation levels while improving energy efficiency. Degradation rates were largely independent of current density, though slightly enhanced at higher currents with increased energy consumption. In real seawater, nearly complete OA removal (>99%) was achieved at 1.1 mA cm−2, with specific energy consumption as low as 0.71 kWh m−3, demonstrating the potential of BDD electrochemical oxidation as an effective and energy-efficient strategy for saline wastewater treatment. The second part of the thesis focused on the degradation of PFAS, a class of anthropogenic chemicals of extreme persistence that have exceeded planetary boundaries due to environmental accumulation. The exceptional strength of the C–F bond renders direct oxidation inefficient for many PFAS, necessitating strategies that combine oxidative degradation with fluoride capture. PFOA and PFOSA were selected as representative carboxylate and sulfonamide PFAS, respectively. Their degradation was studied using BDD anodes and either conventional BDD or hydroxyapatite-coated glassy carbon cathodes under various electrolytes, with and without calcium ions, across different current densities. PFOSA exhibited rapid and nearly complete degradation under all tested conditions, while PFOA showed slower kinetics and a stronger dependence on operational parameters. The addition of calcium ions and the use of hydroxyapatite cathodes enhanced PFOA degradation at low current densities by promoting fluoride sequestration as insoluble CaF2, shifting reaction equilibria and improving degradation. Overall, this work provides a systematic investigation of the electrochemical degradation of structurally different xenobiotic pollutants, highlighting the influence of electrolyte chemistry and electrode materials on degradation pathways, kinetics, and energy consumption. The findings contribute to the development of more effective and energy efficient electrochemical degradation processes for the treatment of saline and contaminated waters, addressing critical challenges in environmental remediation and pollutant abatement