Irradiation-assisted electrochemical processes for the removal of persistent organic pollutants from wastewater

Abstract

This work focused on the removal of persistent organic pollutants (POPs) from wastewater using irradiation-assisted electrochemical technologies, i.e. sonoelectrolysis, photoelectrolysis and sono-photoelectrolysis. Single-irradiation processes (sonolysis and photolysis) and electrochemical oxidation using conductive diamond anodes (current density of 30 mA cm−2) were also evaluated for comparison. Three POPs with different molecular structures (sulfamethoxazole, metoprolol and caffeine, initial concentration of 100 mg dm−3) were studied to evaluate the robustness of the selected technologies and the oxidation mechanisms involved in each case. Results show that the single application of the irradiation technologies led to the removal of only a small amount of POPs and no mineralisation, with the nature of the pollutant showing a marked effect; the opposite was observed for the single application of conductive diamond electrochemical oxidation (CDEO), which is a highly robust and efficient technology for the degradation of all types of POPs. Sonoelectrolysis, photoelectrolysis and sono-photoelectrolysis processes (ultrasound conditions: 200 W, ultraviolet conditions: 254 nm, 4 W) may show synergistic, antagonistic or nil effects with respect to a single electrochemical oxidation event, depending on the nature of the treated molecule. The differences observed may be related to the different chemical nature of the organic species studied, indicating an important role of mediated oxidation processes, which may be enhanced with ultrasounds and ultraviolet radiation techniques.