Past and present of electrochemical treatment of organic pollutants

Oxidative electrochemical technologies offer an alternative solution to many environmental problems in the process industry, because electrons provide a versatile, efficient, cost-effective, easily automatable, and clean reagent. In electro-oxidation, organic pollutants can be removed by different methods: (i) Direct electrolysis: the pollutants (R) are oxidized after adsorption on the anode surface without the involvement of any substance other than electrons: Rads → Pads + ze- (ii) Oxidation via intermediates of oxygen evolution: organic compounds are oxidised near the anode surface (M) at high potentials in the region of water discharge due to the participation of intermediates of oxygen evolution: M + H2O → M(•OH) + H+ + e- R + M(•OH) → M + CO2 + H2O Anodes with high oxygen evolution overpotential, such as SnO2, PbO2 or boron-doped diamond (BDD) are ideal electrodes for the complete oxidation of organics to CO2 in wastewater treatment. (iii) Indirect electrolysis mediated by oxidizing agents generated anodically: organic pollutants are removed through the mediation of some electroactive species generated at the anode surface, which act as intermediaries for electrons shuttling between the electrode and the organic compounds. The main oxidizing chemicals electrogenerated anodically are active chlorine and persulfates, that are produced by the oxidation of chloride and sulphates ions commonly present in wastewaters: 2Cl- → Cl2 + 2e- 2SO42- → S2O82- + 2H+ + 2e- (iv) Electro-Fenton processes: the pollutants are removed by the •OH produced in the bulk of the solution using the electrogenerated Fenton's reagent where H2O2 is supplied in situ from the two-electron reduction of O2 on cathodes such as gas diffusion electrodes (GDE), reticulated vitreous carbon (RVC) or graphite-felt, and Fe2+ is continually regenerated from Fe3+ reduction: Fe2+ + H2O2 → Fe3+ + OH- + •OH O2 + 2H+ + 2e- → H2O2 Fe3+ + e- → Fe2+ (v) Coupled anodic and cathodic Processes: using an undivided cell, the contaminants are treated by H2O2 generated on the cathode and oxidizing agents or •OH generated at the anode. Process selection depends on the nature of the electrode material, experimental conditions, and electrolyte composition. This lecture focuses on recent progress in the most promising electrochemical tools for the treatment of wastewater contaminated by organic pollutants.