Conductive diamond electrochemical oxidation of caffeine-intensified biologically treated urban wastewater

• CDEO shows a high efficiency in the removal of caffeine in urban wastewater.
• The presence of electrogenerated oxidants increases the efficiency in real medium.
• The N atoms present in the molecule of caffeine are oxidized to nitrate.
• Ammonium and hypochlorite can react to form chloramines.
• Operation mode must be modified to minimize the formation of chlorate and perchlorate.

In this work, the usefulness of Conductive Diamond Electrochemical Oxidation (CDEO) to degrade caffeine in real urban wastewater matrixes was assessed. The oxidation of actual wastewater intensified with caffeine (from 1 to 100 mg L−1) was studied, paying particular attention to the influence of the initial load of caffeine and the differences observed during the treatment of caffeine in synthetic wastewater. The results showed that CDEO is a technology that is capable of efficiently degrading this compound even at very low concentrations and that it can even be completely depleted. Profiles of the ionic species of S (SO42−), N (NH4+, NO3−) and Cl (ClO−, ClO3− and ClO4−) were monitored and explained for plausible oxidation mechanisms. It was observed that the efficiency achieved is higher in the treatment of real wastewater than in the oxidation of synthetic wastewater because of the contribution of electrogenerated oxidant species such as hypochlorite. The formation of chlorate and perchlorate during electrochemical processes was observed, and a combined strategy to prevent this important drawback was successfully tested based on the application of low current densities with the simultaneous dosing of hydrogen peroxide.