Electrochemical Advanced Oxidation Process for the Degradation of Acetaminophen, Naproxen and their Transformation Products in Surface Water

Abstract

The use of electrochemical advanced oxidation processes has been studied in the last decades to develop an effective and affordable method for the degradation of pharmaceuticals and their transformation products. In this research, acetaminophen, naproxen and their transformation products were totally degraded in surface water by electrochemical oxidation using stainless-steel electrodes. Evaluation of pH and current density effects were assessed, anodic oxidation and oxidation with active chlorine species mechanisms described, and degradation kinetics explained. Analysis of generated iron hydroxides due the use of stainless-steel electrodes was don; mineralization percentages based on total organic carbon and chemical oxygen demand removals were also obtained. Total degradation of acetaminophen and naproxen was achieved at 2.5 minutes reaction time at pH 3 and 20.3 mA/cm2. Higher current densities and lower pH values promoted faster degradations. Acetaminophen transformation products were totally degraded at 7.5 minutes reaction time at pH 5 and 16.3 mA/cm2; naproxen transformation products required 10 minutes reaction time at pH 3 and 20.3 mA/cm2 for total degradation. Oxidation with active chlorine species achieved shorter reaction times compared to anodic oxidation. This might be because the degradation mechanism with “active” anodes of SST was enhanced by Cl- and HClO alone or in combination with hydroxyl radicals instead of only oxidizing the pharmaceuticals by AO or with “active oxygen” species. Degradation of pharmaceuticals followed first order kinetics with rate constants (k) were highly correlated to pH and the applied current density. Higher rate constants were achieved at lower pH values and higher current densities. Electrochemical oxidation with active chlorine species created a higher amount of transformation products for both pharmaceuticals than anodic oxidation. In case of acetaminophen seven transformation products were detected when oxidation with active chlorine species took place compared to three when anodic oxidation governed the degradation process. For naproxen, eighteen transformation products were detected compared to five respectively. Nevertheless, shorter reaction times for total degradation of the transformation products with active chlorine species were needed. Identification of these compounds was out of the scope of this research. It was found that iron hydroxides generated during the electrochemical oxidation process, trapped some transformation products of acetaminophen and naproxen and consequently required longer reaction times for their complete degradation. However, at the end such transformation products were totally oxidized. Thus, iron hydroxides formation delays the oxidation of transformation products. Energy consumption per milligram of APAP, NAP and their transformation products for total degradation was obtained. Optimal operational conditions were used; energy consumptions of 0.68 kWh/mg and 1.22 kWh/mg were required for APAP and NAP respectively.