Advances in the decontamination of wastewaters with synthetic organic dyes by electrochemical Fenton-based processes

Large amounts of diverse colored synthetic organic dyes are discharged from industrial effluents in the environment causing non-esthetic pollution and serious health-risk problems. Dyes are very stable upon solar irradiation, temperature, and biological treatment, then being non-removed in conventional wastewater treatment plants. The decontamination of dyeing effluents by potent methods has received increasing attention over the last years, showing a marked relevance in the electrochemical Fenton-based processes consisting of H₂O₂ electro-generation and its catalytic decomposition with Fe²⁺ to form ^∙OH. This paper presents a comprehensive review of these processes in the period 2018–2022 dealing with the discoloration and mineralization of synthetic solutions and real wastewaters with single or mixed synthetic organic dyes. Homogeneous electro-Fenton (EF), heterogeneous EF with solid catalysts or functionalized cathodes, photoelectro-Fenton (PEF), microbial fuel cells, other hybrid processes with adsorption, nanofiltration, electro-peroxone, or photoelectrocatalysis, and sequential processes with electrocoagulation or sonication are summarized. The principles of methods are explained to understand the role of the oxidizing agents generated. The H₂O₂ accumulation at pH 3.0 is described with cathodes of raw and modified carbonaceous materials as well as gas-diffusion electrodes, and the effect of the anode, the electrolyte composition, the applied current density or cathodic potential, and the Fe²⁺ and dye contents on the homogeneous EF performance are discussed. Rapid discoloration and much slower mineralization were found by homogeneous EF. Similar behavior was obtained for heterogeneous EF, although allowed operating at a more neutral pH without sludge precipitation. The dye removal was more improved in PEF and solar PEF (SPEF) due to the positive photolysis of Fe(III) species by UVA light and sunlight, respectively. SPEF can be envisaged as the best and more cost-effective electrochemical Fenton-based treatment for dyes destruction.