A review on the application of single and combined Fenton, photo-Fenton, and electrochemical advanced oxidation processes to remove diclofenac from aqueous media
Enric Brillas
Abstract
Diclofenac (DCF) is a nonsteroidal anti-inflammatory drug (NSAID) with antipyretic, anti-inflammatory, and analgesic properties for treating a variety of acute and chronic pains and inflammatory diseases. It is frequently detected in surface waters, groundwaters, and influents and effluents of WWTPs, and its prolonged exposure to its environmental concentrations is a risk by its adverse effects on aquatic life, then being needed its effective removal from waters. This article presents a comprehensive critical review on the degradation/mineralization of DCF in aqueous matrices by advanced oxidation processes (AOPs) like Fenton and photo-Fenton and electrochemical AOPs (EAOPs) up to October 2024. Aromatic intermediates giving final carboxylic acids were always formed. The formation of recalcitrant Fe(III)-carboxylate species inhibit the mineralization in Fenton. In contrast, solar photo-Fenton and solar photoelectro-Fenton allowed achieving total mineralization due to the photolysis of the final Fe(III)-carboxylate complexes produced. The use of the more powerful boron-doped diamond anode and other potent modified SnO 2 ones in anodic oxidation is described. The treatment by homogeneous electro-Fenton with Fe 2 + catalyst is reported, along with the action of solid iron catalysts or functionalized iron cathodes in heterogeneous electro-Fenton. Photoelectrocatalysis treatments with TiO 2 , modified TiO 2 , and other photoanodes are detailed. Combined processes with photo-Fenton, anodic oxidation, electro-Fenton, and photoelectrocatalysis, as well as microbial fuel cells and chemical wet peroxide oxidation are described. The role of the oxidants generated ( · OH, O 2 ·− , and/or 1 O 2 ), the reusability of the system, and the by-products formed are discussed. The change of toxicity in the treatments is finally analyzed. • Degradation/mineralization of diclofenac by Fenton, photo-Fenton, and EAOPs is summarized. • Solar photo-Fenton and solar photoelectron-Fenton gave the best performance. • · OH as main oxidant in most procedures, O 2 ·− and 1 O 2 formed with catalysts. • Removal in real waters < in pure waters: inhibition with common inorganic ions and NOM. • Large detoxification when aromatic by-products are converted into final carboxylic acids.