Research Article Received: 13 December 2011 Revised: 18 January 2012 Accepted: 18 January 2012 Published online in Wiley Online Library: (wileyonlinelibrary.com) DOI 10.1002/jctb.3750 Metronidazole photodegradation in aqueous solution by using photosensitizers and hydrogen peroxide Manuel S ´ anchez-Polo, ∗ Jos ´ e Rivera-Utrilla, Gonzalo Prados-Joya and Ra ´ ul Ocampo-P ´ erez Abstract BACKGROUND: The effectiveness of UV radiation on the indirect photodegradation of metronidazole (MNZ) conducting a kinetic study of the process in the presence of radical promoter (H 2 O 2 ) and photosensitizer (Sens) promoters (duroquinone, tetrahydroxyquinone, tetrachloro-1,4-benzoquinone, 4-carboxy-benzophenone, 2,4-dihydroxybenzophenone, 4-hydroxybenzophenone, and 4,4 ′ -dihydroxybenzophenone) was investigated. RESULTS: The electron-donating groups of photosensitizers may favor MNZ photodegradation through electron transfer. However, the photosensitizers studied showed low effectiveness of MNZ photodegradation in aqueous phase. The presence of H 2 O 2 substantially increased MNZ photodegradation rate, which was enhanced by increasing the concentration of hydrogen peroxide in the system. The contribution of direct photolysis of MNZ in the UV/H 2 O 2 system was drastically decreased with increasing H 2 O 2 concentrations. CONCLUSIONS: The low effectiveness obtained with most of the Sens used demonstrates that metronidazole cannot be photoxidized by indirect photooxidation in aqueous solution. c  2012 Society of Chemical Industry Keywords: photooxidation; sensitizers; hydrogen peroxide; metronidazole INTRODUCTION Antibiotics are among the most important pharmaceuticals used in medicine. Antibiotics used for humans and animals can enter different environmental compartments as parent compounds or metabolites, mainly through their excretion, their improper disposal into sewage systems when expired or surplus, and their use in agriculture and aquaculture. Accordingly, various antibiotics and their metabolites have been found in municipal wastewater treatment plants, surface waters and groundwaters. 1,2 The numerous types of antibiotics available include nitroim- idazoles, which are widely used to treat infections caused by anaerobic and protozoan bacteria, e.g. Trichomonas vaginalis and Giardia lamblia. 3,4 Besides their use in humans, nitroimidazoles are also added to poultry and fish feed, leading to their accumulation in the animals, in the water of fish-farms, and, especially, in efflu- ents from meat industries. 5 Nitroimidazoles are among the most widely produced and prescribed antibiotics in many countries and have been detected at concentrations of several mg L −1 in sewage treatment plants. In general, nitroimidazoles show high solubility in water, low biodegradability, 6,7 and high toxicity. 8 They have mutagenic and carcinogenic characteristics 9–12 and are therefore potentially dangerous compounds whose persistence in aqueous medium permits their bioaccumulation. Our research group has used different methods to satisfactorily remove nitroimidazoles in aqueous solution, including adsorp- tion on activated carbon, ozonation, ozonation catalyzed with activated carbon, gamma radiation, and bioadsorption. 13–15 We have also analyzed the efficacy of ultraviolet (UV) radiation in the direct photodegradation of nitroimidazoles (metronidazole [MNZ], dimetridazole, tinidazole, ronidazole) in ultrapure, surface, ground and waste waters, examining the effects of natural organic matter components on this process. 16 The UV irradiation applied was not effective to remove nitroimidazoles from aqueous medium and showed very low quantum yields for the four nitroimidazoles; according to the R 254 values obtained, the dose habitually used in water disinfection is not sufficient for this type of pharmaceutical; therefore, higher doses of UV irradiation or longer exposure times are required for their removal. Furthermore, the time course of TOC and toxicity showed that oxidation by-products are not oxidized to CO 2 to the desired extent, generating oxidation by-products that are more toxic than the initial product. Results obtained indicated that the gallic acid (GAL), tannic acid (TAN), and humic acid (HUM) in natural organic matter may act as promoters and/or inhibitors of HO • radicals via photoproduction of H 2 O 2 . ∗ Correspondence to: Manuel S´ anchez-Polo, Department of Inorganic Chemistry. Faculty of Science. University of Granada. 18071, Granada, Spain. E-mail: mansanch@ugr.es Department of Inorganic Chemistry. Faculty of Science. University of Granada. 18071, Granada, Spain J Chem Technol Biotechnol (2012) www.soci.org c  2012 Society of Chemical Industry