Elucidating the catalytic degradation of enrofloxacin by copper oxide nanoparticles through the identification of the reactive oxygen species Ishai Dror a, * , Lea Fink a, c , Lev Weiner b , Brian Berkowitz a a Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, 7610001, Israel b Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, 7610001, Israel c Currently at the Institute of Chemistry and Center for Nanoscience and Nanotechnology, Edmond J. Safra Campus, Givat Ram, The Hebrew University of Jerusalem, Jerusalem, 9190401, Israel highlights graphical abstract  CuO nanoparticles degrade persistent organic contaminants in aqueous solutions.  The reaction mechanism and cata- lytic cycle of CuO nanoparticles is elucidated.  ESR study demonstrates production of reactive oxygen species by CuO nanoparticles.  The distribution of radical species shows high production of superoxide radicals.  The CuO nanoparticle catalytic cycle mimics superoxide dismutation enzyme activity. article info Article history: Received 24 February 2020 Received in revised form 22 April 2020 Accepted 29 May 2020 Available online 5 June 2020 Handling Editor: Jun Huang Keywords: Reactive oxygen species Enrofloxacin Oxidation Superoxide dismutase Superoxide radicals abstract Copper oxide nanoparticles (CuO-NPs) have been suggested as effective catalysts to degrade many persistent organic contaminants. In parallel, CuO-NPs are considered toxic to soil microorganisms, plants and human cells, possibly because they induce oxidative stress and generation of reactive oxygen species (ROS). However, the mechanism of the catalytic process and the generated ROS are poorly understood. Here we discuss the reaction mechanism of CuO-NPs during the catalytic degradation of enrofloxacin e an antibiotic pharmaceutical used in this study as a representative persistent organic compound. The degradation of an aqueous solution of the enrofloxacin exposed to CuO-NPs and hydrogen peroxide was studied showing fast removal of the enrofloxacin at ambient conditionsns. ROS production was identified by electron spin resonance and a spin trapping technique. The distribution of the free radical species indicated production of a high percentage of superoxide (O 2 . ) radicals as well as hydroxyl radicals; this production is similar to the “radical production” activity of the superoxide dismutase (SOD) enzyme in the presence of hydrogen peroxide. This activity was also tested in the opposite direction, to examine if CuO-NPs show reactivity that potentially mimics the classical SOD enzymatic activity. The CuO-NPs were found to catalyze the dismutation of superoxide to hydrogen peroxide and oxygen in a set of laboratory experiments. © 2020 Elsevier Ltd. All rights reserved. * Corresponding author. E-mail address: ishai.dror@weizmann.ac.il (I. Dror). Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere https://doi.org/10.1016/j.chemosphere.2020.127266 0045-6535/© 2020 Elsevier Ltd. All rights reserved. Chemosphere 258 (2020) 127266