Degradation of Organic Contaminants by Advanced Electrochemical Oxidation Methods

Abstract

Advanced electrochemical oxidation processes (AEOPs) constitute promising technologies for the treatment of organic pollutants in waters. They are based on the production of oxidant hydroxyl radical (·OH) from water oxidation on the surface of a high O2-overvoltage anode and/or from Fenton’s reaction between added Fe2+ and hydrogen peroxide electrogenerated at the cathode by two-electron O2 reduction. In this paper, fundamentals of AEOPs such as anodic oxidation, electro-Fenton, photoelectro-Fenton and peroxi-coagulation are described, and comparative degradation of aqueous solutions with aromatic pollutants, such as aniline, 4-chlorophenol and several chlorophenoxyacetic and chlorobenzoic acids, in 0.05 M Na2SO4 + H2SO4 of pH 3.0 by these techniques using an undivided electrolytic cell with an O2-diffusion cathode under galvanostatic conditions is discussed. The decay kinetics of chlorophenoxyacetic acids and the evolution of their aromatic intermediates and generated carboxylic acids are also reported to clarify their mineralization processes by the different AEOPs. Anodic oxidation with a Pt anode yields poor decontamination of pollutants, while alternative anodic oxidation with a boron-doped diamond (BDD) anode leads to total mineralization of all solutions due to the greater production of ·OH on the BDD surface. Electro-Fenton with a Pt anode has high oxidation ability at short electrolysis times, but the formation of stable Fe3+-oxalate complexes limits the degradation of aromatic contaminants. These products are completely oxidized in electro-Fenton with a BDD anode or photodecomposed by the action of UVA light in photoelectro-Fenton with a Pt anode. Peroxi-coagulation with an Fe anode also gives fast degradation with generation of small amounts of stable Fe3+ complexes, since organics are mainly retained in the Fe(OH)3 precipitate formed.