Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

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With the aim of bringing the heterogeneous electro-Fenton (EF) treatment one step closer to a more realistic operation, the scaling-up of that technology was evaluated. Assays were performed firstly at lab scale in a stirred-tank reactor and then at bench scale in a flow setup including a jet aerator and a microfluidic flow-through electrochemical cell. A fluidized-bed reactor was added to the bench-scale installation in order to retain the solid catalyst, iron-containing alginate beads. To the best of the authors' knowledge, there are no precedent studies reporting a heterogeneous EF treatment in a similar bench scale-configuration. Hydrogen peroxide generation and clofibric acid removal were assessed at both scales at current intensities of 0.12 and 0.25 A. Results showed that the scaled-up treatment was more efficient and cost-effective: at bench scale 18 times more volume was treated, the mass production of hydrogen peroxide was 28 higher and the specific cost for the removal of clofibric acid was cut by more than half. The most efficient treatment turned out to be the EF performed at 0.12 A at bench scale. Those results highlighted the importance of the reactor design in the scaling-up process. Additionally, aromatic intermediates were detected by liquid chromatography-mass spectrometry (LC-MS) and a degradation route was suggested. Carboxylic acids were also measured by HPLC confirming that the pollutant is mineralizing.

Concern about the current pollution of water environments and the inefficacy of conventional water treatments for the elimination of refractory contaminants has placed electrochemistry in the spotlight. With the objective of demonstrating the diverse applications that electrochemical techniques can have in the area of water remediation, this study is focused on the use of three different methods: (i) electro-Fenton process with heterogeneous catalyst as the treatment for the degradation of the target compounds; (ii) cyclic voltammetry for the characterization of the electrochemical system, and (iii) differential pulse voltammetry for the monitoring of the evolution of the degradation process. Four organic compounds were selected as target pollutants: the ionic liquid 1,3-Bis(2,4,6-trimethylphenyl)imidazolinium chloride, Mesitol, Mesidine and 2,5-Xylidine. Results were corroborated and complemented with chromatographic and total organic carbon (TOC) measurements. After 420 min of heterogeneous electro-Fenton treatment, almost 80% of TOC abatement was achieved for the ionic liquid and more than 90% for Mesitol, Mesidine and 2,5-Xylidine. Cyclic voltammetry studies for Mesitol and Mesidine suggested the formation of a polymeric film which remains adsorbed on the electrode surface. Finally, it was possible to conclude that the coupling of differential pulse voltammetry with the heterogeneous electro-Fenton process provides useful This contribution has been peer-reviewed.