Electrochemical conversion of contaminant p-nitrophenol to p-aminophenol and high-value p-quinones: Mechanism, kinetics and modeling

Abstract

Understanding the chemistry of contaminant p-nitrophenol (p-NP) in electrochemical systems is of importance to remove it from wastewater and to produce high-value products such as quinones. In this study, redox experiments in a system initially containing an aqueous solution of sodium chloride and p-NP at pH 5 and 25 °C were performed in an electrochemical reactor with low-cost steel electrodes. The reactor was operated at 0.1 and 0.2 A, and at initial p-NP concentrations of 4.04×10−5, 6.97×10−5 and 10.3×10−5 M. Air and argon were bubbled at the bottom of the cathode to have solutions saturated with O2 (7.7 mg L−1) and free of dissolved O2. At these conditions p-NP was completely converted to less toxic and more valuable p-hydroquinone and p-aminophenol in an hour. The reaction rates were individually controlled by the steps of electron transfer and adsorption of p-NP at the cathode surface. In the presence of dissolved O2, p-hydroquinone and p-aminophenol were intermediate species oxidized to p-benzoquinone, and p-quinoneimine in the greatest amount, respectively. The rate constants for the reactions were determined on the basis of kinetic results obtained by liquid chromatography and spectrophotometry. The suggested mechanism, and the innovative kinetic model involving Langmuir-Hinshelwood rate expressions for the surface reactions, were consistent with results from mass spectrometry and total organic carbon. For the first time for this kind of kinetic model, it was also able to reproduce a large set of kinetic data of electroreduction of p-NP with iron electrodes obtained by independent researchers at several different conditions.