Electrochemical oxidation of Naproxen in aqueous matrices: Elucidating the intermediates’ eco-toxicity, by assessing its degradation pathways via experimental and density functional theory (DFT) approaches

The removal of the non-steroidal anti-inflammatory drug (NSAID) Naproxen (NAX) in water by hydroxyl radicals (•OH) was performed by electrochemical advanced oxidation processes either with Pt or BDD anodes and a 3D carbon felt cathode. The degradation of NAX by (•OH vs electrolysis time) was well fitted to a pseudo-first-order reaction rate kinetic. The detected reaction intermediates (aromatic compounds and carboxylic acids) were experimentally monitored during the process via LC, while density functional theory (DFT) was applied to uncover undetected intermediates, some for the first time in literature. The formation of toxic intermediates with higher toxicity than NAX were identified, such as IMS4b (6-Methoxy-1-[1-(6-methoxynaphthalen-2-yl) ethyl] naphthalen-2-ol), catechol, and glycolic acid. Based on these data, a detailed oxidation pathway of NAX by •OH was proposed. The evolution of solution toxicity indicated that the formed toxic intermediates were subsequently removed during the TOC removal process. Finally, almost complete mineralization of NAX was achieved in simulated urine or wastewater by the electro-Fenton treatment, with an optimized dose of iron as catalyst, showing the EAOPs' potential to efficiently remove NAX even from challenging matrices. In extension, the strategies developed can be applied to the treatment of other NSAIDs.