Free Radical Destruction of -Blockers in Aqueous Solution WEIHUA SONG,* ,† WILLIAM J. COOPER, † STEPHEN P. MEZYK, ‡ JOHN GREAVES, § AND BARRIE M. PEAKE 4 Urban Water Research Center, Department of Civil and Environmental Engineering, University of California, Irvine, Irvine, California 92697-2175, Department of Chemistry and Biochemistry, California State University at Long Beach, 1250 Bellflower Blvd. Long Beach, California 90840, Department of Chemistry, University of California, Irvine, Irvine, California 92697-2025, Chemistry Department, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand Received September 6, 2007. Revised manuscript received November 13, 2007. Accepted November 23, 2007. Many pharmaceutical compounds and metabolites are currently found in surface and ground waters which indicates their ineffective removal by conventional water treatment technologies. Advanced oxidation/reduction processes (AO/ RPs) are alternatives to traditional water treatment, which utilize free radical reactions to directly degrade chemical contaminants. This study reports the absolute rate constants for reaction of three -blockers (atenolol, metoprolol, and propranolol) with the two major AO/RP radicals; the hydroxyl radical (•OH) and hydrated electron (e - aq ). The bimolecular reaction rate constants for •OH are (7.05 ( 0.27) × 10 9 , (8.39 ( 0.06) × 10 9 , and (1.07 ( 0.02) × 10 10 , and for e - aq they are (5.91 ( 0.21) × 10 8 , (1.73 ( 0.03) × 10 8 , and (1.26 ( 0.02) × 10 10 , respectively. Transient spectra were observed for the intermediate radicals produced by hydroxyl radical reactions. In addition, preliminary degradation mechanisms and major products were elucidated using 60 Co γ-irradiation and LC-MS. These data are required for both evaluating the potential use of AO/RPs for the destruction of these compounds and for studies of their fate and transport in surface waters where radical chemistry may be important in assessing their lifetime. Introduction The presence of active pharmaceutical ingredients (APIs) in surface waters is an emerging environmental issue and provides a new challenge to drinking water, wastewater, and water reuse treatment systems (1). Most of the APIs admin- istered to patients are excreted either as metabolites or as the unchanged parent compounds (2). Another common practice is to dispose of outdated medicines “down the drain”, but either way they end up in the wastewater treatment plants. The presence of pharmaceuticals in the aquatic environ- ment was reported as early as the beginning of the 1980s (3). Studies of various treatment technologies for the removal of APIs have been reported, including membrane filtration (4), activated carbon adsorption (5), and reverse osmosis (6). The effectiveness of these processes is also influenced by the amount and type of natural organic matter (NOM) present in the wastewater which results in higher treatment costs (7). Although partial removal of APIs can be achieved through these processes, recent studies have demonstrated that conventional water treatment processes are relatively inef- ficient in treating APIs (8). In addition, these technologies require the disposal of wastes such as membrane retentate and spent activated carbon generated during the treatment. Advanced oxidation/reduction processes (AO/RPs) are alternatives to traditional treatment and have recently received considerable attention for API removal (1). AO/RPs typically involve the formation of hydroxyl radicals (•OH) as oxidizing species and either hydrated electrons (e - aq ) or hydrogen atoms (H•) as reducing species, all of which can be utilized in the destruction of organic pollutants present in drinking or wastewater. AO/RPs are effective in the treatment of a variety of anthropogenic pollutants including APIs (9–11). However, to provide a fundamental under- standing of the applicability of these processes to degrade APIs, it is necessary to determine the bimolecular reaction rate constants between the reactive species and the chemicals of interest. Additionally, the environmental fate of APIs in natural waters is attracting increasing attention. In surface waters, while biodegradation may be important (12), it is likely that abiotic processes, such as phototransformation (13–15) and partitioning to sediments (16, 17) may actually have a greater impact on reducing aqueous concentrations of APIs. Hydroxyl radicals, especially in photosensitized oxidation, are also expected to play a key role during environmental degradation (18). Beta-blockers belong to a group of cardiovascular APIs and are generally used in the treatment of disorders such as hypertension, angina, and arrhythmias. The activity of these compounds is to block the action of epinephrine and norepinephrine on the -adrenergic receptors in the body, primarily in the heart (19). Among the -blockers, atenolol, metoprolol, and propranolol have been in long-term use in Europe and North America, and they have also been detected in the aquatic environment (16, 20–22). Less than 10% of atenolol and metoprolol is removed by conventional waste- water treatment using activated sludge (23). Although there are some reports on the degradation of -blockers by AO/ RPs (24–26), there appears to be no reported aqueous kinetic data and only limited reporting of free-radical-based deg- radation byproducts (27). The objective of this study was to determine the absolute rate constants for the reaction of the hydroxyl radical and hydrated electron with the three -blocker pharmaceutical compounds: atenolol, propranolol, and metoprolol. In this work, transient free radical spectra produced by hydroxyl radical reaction with these three species were also recorded over the time period of 1–200 µs after irradiation to provide a better understanding of the nature of the intermediate radical species produced. Finally, detailed product studies of the free-radical-induced degradation pathways of these -blockers using γ-irradiation in aerated solution were conducted to provide preliminary insight into the mecha- nisms that might occur under typical water treatment conditions. Materials and Methods Materials. The -blocker pharmaceuticals: atenolol, pro- pranolol hydrochloride, and (()-metoprolol (+)-tartrate salt * Corresponding author e-mail: wsong@uci.edu. † Department of Civil and Environmental Engineering, University of California, Irvine. ‡ California State University at Long Beach. § Department of Chemistry, University of California, Irvine. 4 University of Otago. Environ. Sci. Technol. 2008, 42, 1256–1261 1256 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 4, 2008 10.1021/es702245n CCC: $40.75  2008 American Chemical Society Published on Web 01/16/2008