Direct And Mediated Anodic Oxidation of Organic Pollutants

Literature results summarized in this review show that there are two main strategies for anodic oxidation of organic compounds. The first is to perform electrolysis at a high anodic potential in the region of water discharge, with the participation of intermediates of electrogenerated hydroxyl radicals. The nature of electrode material strongly affects both process selectivity and efficiency. In particular, anodes with low oxygen evolution overpotential, such as graphite, IrO2, RuO2 and Pt permit only partial oxidation of organics, while anodes with high oxygen evolution overpotential, such as SnO2, PbO2 and BDD, favour complete oxidation of organics to CO2, therefore they are ideal electrodes for wastewater treatment. Among them, BDD anodes show the highest removal rate and stability, and are thus promising anodes for industrial-scale wastewater treatment. The second strategy is to oxidize pollutants by indirect electrolysis, generating a redox reagent in situ as chemical reactant. The inorganic mediator can be a metallic redox couple, used for waste material disposal with over 15-20% organic content, or a chemical reagent (e.g. chlorine, ozone, peroxides). With indirect electrolysis there are no mass transfer limitation problems, but it either requires separation of metallic redox couples or produces secondary pollution. Although laboratory and pilot tests have been successful, industrial applications of these methods are still limited, due to relatively high energy consumption of the electrochemical methods. However, thanks to the development of new electrode materials, electrochemical oxidation could be increasingly applied in the future, due to specific advantages for certain applications over other technologies. Moreover, energy consumption could be reduced using so-called “advanced electrochemical oxidation processes”, based on the combination of anodic and cathodic electrogeneration of highly oxidizing hydroxyl radicals.

A study was conducted to demonstrate direct and mediated anodic processes for the oxidation of organic pollutants. The study proposed different expressions of current efficiency, such as instantaneous current efficiency (ICE), electrochemical oxidation index (EOI), general current efficiency (GCE), and mineralization current efficiency (MCE). ICE of electro-oxidation was determined by the oxygen flow rate (OFR) method or chemical oxygen demand (COD). GCE represented an average value of current efficiency between the initial time t = 0 and t. Efforts were made to perform electrolysis at a high anodic potential in the region of water discharge, using intermediates of electrogenerated hydroxyl radicals. Another strategy involved oxidizing pollutants by indirect electrolysis, generating a redox reagent in situ as a chemical reactant.