Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review

Effect of anodic materials on solar photoelectro-Fenton process using a diazo dye as a model contaminant

Recalcitrant and toxic organic pollutants from wastewaters are scarcely removed in conventional wastewater treatment plants. To preserve the water quality, organics need to be removed by developing powerful oxidation technologies. Our laboratory proposed in 2007 a potent electrochemical advanced oxidation process (EAOP) for wastewater remediation, so-called solar photoelectro-Fenton (SPEF). This review summarizes the advances of this emerging technology up to 2022, making evident its effectiveness and cost-efficiency for the destruction of usual organic pollutants. The simultaneous action of generated hydroxyl radicals and the photolysis by sunlight explains the high oxidation power of SPEF respect to other EAOPs. The review is initiated by describing the fundamentals of the process to remark the role of the produced oxidants and the benefits of using solar irradiation in its performance. The photoelectrochemical systems used (bench tank reactor and solar pre-pilot flow plant) and the assessment of the operating variables are discussed. The characteristics of the most common homogeneous SPEF for the degradation and mineralization of several synthetic solutions of industrial chemicals, herbicides, pharmaceuticals, and synthetic organic dyes, as well as of some real wastewaters, are further described. The influence of the photoelectrochemical cell, electrodes, solution pH, electrolyte composition, Fe2+ and pollutant concentration, and current density is analyzed. The performance of a homogeneous SPEF-like process with active chlorine and heterogeneous SPEF processes with solid catalysts such as Fe3O4 and sodium vermiculite is also discussed. Finally, the advances of homogeneous SPEF combined with other techniques like solar photocatalysis, solar photoelectrocatalysis, anaerobic digestion, and nanofiltration are reported.

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.

Application of electrochemical advanced oxidation to bisphenol A degradation in water. Effect of sulfate and chloride ions

The present study was stimulated by an authoritative review on decontamination of wastewaters containing synthetic organic dyes by electrochemical methods published in Martínez-Huitle and Brillas (2009). As reviewed by the authors, there have been significant efforts on investigating the decontamination of wastewaters containing synthetic dyes by electrochemical methods, and currently, more studies are being published. A high number of electrodes have been tested in this method, including boron doped diamond (BDD) anodes. In this context, many papers have demonstrated that the use of a BDD thin film in electrochemical oxidation provides total mineralization with high current efficiency of different organics in real wastewaters. And this synthetic material deposited on several supports has been recently applied to dyestuff treatment. Although, in the last two years, more reports have been published treating electrochemically synthetic dyes wastewaters using BDD, there are few reports on the use of electrooxidation processes to degrade real textile effluents. The aim of this paper is to summarize and discuss the most important and recent results available in the literature about the application of BDD electrodes for removing azo dyes in synthetic and real wastewaters.

Application of electrochemical advanced oxidation processes to the mineralization of the herbicide diuron

The fate of pharmaceuticals in the aquatic environment is significantly affected by the presence of humic acids (HA). In this work, the synergistic effect of electrochemical oxidation (EO) and ultraviolet irradiation (UVI) was evaluated for HA removal and for the simultaneous degradation of three pharmaceuticals (carbamazepine, propranolol and sulfamethoxazole) in presence of HA. The effectiveness of EO, UVI and their combination for HA removal was assessed using different operating parameters, such as type of anode (Nb/BDD and Ti/IrO2), supporting electrolyte (NaCl, NaBr and Na2SO4), current density (8.1, 16.1, 28.2, 40.3, and 48.4 mA/cm2), pH (3, 7 and 9) and NaCl electrolyte concentration (7, 14 and 21 mM). The use of non‐active anode Nb/BDD, NaCl electrolyte and combination EO‐UVI was the most efficacious option for HA removal, due to the production of hydroxyl radicals as well as active chlorine species (HClO, Cl● and ClO−) generated by anodic oxidation and by UVI. The effectiveness of the EO process was enhanced coupling EO with UVI, however the energetic consumption increased. The composition of the electrolyte was the pivotal parameter since a complete degradation of the pharmaceuticals was achieved by both processes EO and EO‐UVI using NaCl as electrolyte; this is attributed to the indirect oxidation by electrogenerated active chlorine which dominates the pharmaceuticals degradation.

Comparison of electrochemical advanced oxidation processes for the treatment of complex synthetic dye mixtures

Advanced oxidation of real sulfamethoxazole + trimethoprim formulations using different anodes and electrolytes

Electro-Fenton, UVA photoelectro-Fenton and solar photoelectro-Fenton degradation of the drug ibuprofen in acid aqueous medium using platinum and boron-doped diamond anodes

Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: A general review

Incorporation of electrochemical advanced oxidation processes in a multistage treatment system for sanitary landfill leachate

Electrochemical oxidation of reverse osmosis concentrate on boron-doped diamond anodes at circumneutral and acidic pH

This study shows the good performance of a sequential electrochemical methodology, consisting in electrocoagulation (EC) followed by an electrochemical advanced oxidation process (EAOP), to treat raw cheese whey wastewater at laboratory and pre-pilot scales. In EC, different electrode materials like Fe, Al and stainless steel (AISI 304 and ASI 316L) were tested. Among EAOPs, photoelectro-Fenton (PEF) and electrochemical oxidation (EO) with active anodes like Pt or DSA® and non-active ones like boron-doped diamond (BDD) were studied. At both scales, the optimum anode/cathode combination in EC was Fe/AISI 304, which yielded the highest total organic carbon (TOC) removal of 22.0–27.0%. This is due to various effects on organic compounds: (i) coagulation promoted by Fe(OH)3 flocs, (ii) cathodic reduction, and (iii) oxidation with generated active chlorine. At small scale, the resulting wastewater was further treated by PEF at pH 3.0. The highest TOC removal was achieved using the BDD, owing to the great oxidation power of hydroxyl radicals. In contrast, total nitrogen was abated much more rapidly with active anodes because of the attack of active chlorine on N-compounds. At pre-pilot scale, the post-treatment of conditioned wastewater made by EO with a BDD/Pt flow cell combined with UVA irradiation yielded the highest TOC removal, i.e., 49.1%. The high energy consumed by the UVA lamp would be a drawback at industrial scale, which could be overcome by using sunlight.

Chlorophenols are typical priority pollutants listed by USEPA (U.S. Environmental Protection Agency). The removal of chlorophenol could be carried out by a combination of electrochemical reduction and oxidation method. Results showed that it was feasible to degrade contaminants containing chlorine atoms by electrochemical reduction to form phenol, which was further degraded on the anode by electrochemical oxidation. Chlorophenol removal rate was more than 90% by the combined electrochemical reduction and oxidation at current of 6 mA and pH 6. The hydrogen atom is a powerful reducing agent that reductively dechlorinates chlorophenols. The instantaneous current efficiency was calculated and the results indicated that cathodic reduction was the main contributor to the degradation of chlorophenol.