Oxidative degradation of acetaminophen using superoxide ion generated in ionic liquid/aprotic solvent binary system

Abstract

The environmental contamination by pharmaceuticals is on a continual rise, ensuing levels which are imminent globally. While the alleged harmful effects of drug waste are being rapidly substantiated at present, the development of effective and ‘greener’ techniques to degrade pharmaceuticals is a new challenge. This study explores a method using superoxide ion (O2−) as an oxidant for degradation of acetaminophen (ACTM) as a prototype of active pharmaceutical ingredients (APIs). The binary system consisting of bis(trifluoromethylsulfonyl)imide [TFSI−] anion-based hydrophobic ionic liquids (ILs) and acetonitrile (AcN) as an aprotic solvent were used in varied combinations to generate O2− for subsequent in situ degradation of ACTM. The O2− was chemically generated by dissolution of potassium superoxide (KO2) in [BTEAmm+][TFSI−]/AcN and [EMIm+][TFSI−]/AcN systems to achieve a complete degradation of the drug. A characteristic peak at the wavelength of 258 nm in UV–Visible spectrophotometry was indicative of the stable generation of O2− species, which confirms its presence in certain reaction media used. Cyclic voltammetry (CV) was used in order to further validate O2− as a major reactive oxygen species generated in selected aprotic media, as evidently indicated by the oxygen reduction peak in the cyclic voltammograms. In [BTEAmm+][TFSI−]/AcN, more than 98% of ACTM was eliminated within 2 h. The ILs were recycled and further employed for up to 5 replica cycles of degradation, depicting high efficacy of the environmentally benign regenerated media. Moreover, evaluation of TOC decay determined that complete mineralization of ACTM was achieved under optimum conditions. A degradation pathway was proposed based on the identification of intermediate transformation products resulting from ACTM oxidation by means of LCMS. This work will serve to instigate further progression in the direct use of O2− as a suitable alternative approach for environmental remediation appertaining to pharmaceutical contaminants.