Electrochemical degradation of naproxen from water by anodic oxidation with multiwall carbon nanotubes glassy carbon electrode

Naproxen (NPX) degradation was investigated by anodic oxidation both at constant potential and by cyclic voltammetry, using this last technique for optimizing reaction conditions and catalyst properties. Three multiwall carbon nanotubes (MWCNTs)-promoted electrodes were used (MWCNT, MWCNT-COOH and MWCNT-NH2) and a two steps oxidation process was observed in all the cases. At the optimized conditions (volume of MWCNT = 15 μL), the influence of the scan rate indicates the diffusion–adsorption control of the process. Likewise, the kinetic study of NPX degradation at fix potential, considering two different stirring speeds (250 and 500 rpm), indicates that degradation rate increases with the stirring speed. After 20 h, NPX is degraded even an 82.5%, whereas the mineralization reaches almost 70%, as it was obtained from total organic carbon analysis. The pH effect was also analysed, in the range 5–11, observing a positive effect at low pH. Concerning the surface chemistry of the electrode, MWCNT-NH2, with the highest isoelectric point (4.70), is the most promising material due to the improved interactions with the reactant. From these observations, a pathway is proposed, which includes two steps of electrochemical oxidation followed by subsequent oxidation steps, until mineralization of the NPX, attributed mainly to active chlorine species and ·OH.