Degradation of salicylic acid using electrochemically assisted UV/chlorine process: Effect of operating conditions, reaction kinetics, and mechanisms

Salicylic acid (SA), which is an emerging pollutant, usually undergoes minimal decomposition during conventional wastewater treatment. To tackle the formidable pollutant, an electrochemically assisted UV/chlorine reaction system in which oxidants can be continuously produced in real time without extra dosing agents was proposed. Free chlorine in the form of hypochlorite ions and hypochlorous acid, which are electrogenerated from chloride ions exhibited enhanced ability to oxidize SA. The process is technically and chemically simple, allowing a facile and efficient treatment of SA-containing wastewater. The novelty of this work resides in the integration of electrogenerated free chlorine with UV irradiation for improving the degradation of SA and the elucidation of SA degradation in terms of reaction kinetics and mechanisms. Optimal conditions for the electrochemical degradation of SA were determined at a current density of 5 mA cm-2, chloride ion concentration of 0.05 M and pH 4, leading to a degradation efficiency (DE) of 96%. The DE of SA can be raised to > 99% using electrochemical treatment with UVC irradiation owing to free radical production. The free radicals reduced the activation energy for SA degradation, thus resulting in excellent DE. The degradation of SA by the electrochemically assisted UV/chlorine system also conformed to the proposed pseudo-first-order reaction. The system also exhibited high stability against SA degradation during repetitive treatments. Based on results obtained using liquid chromatography-tandem mass spectrometry, possible SA degradation pathways were proposed, revealing that SA degradation was dominated by reactions associated with hydroxyl radicals rather than reactive chlorine species.