Degradation Kinetics of Atrazine and Its Degradation Products with Ozone and OH Radicals: A Predictive Tool for Drinking Water Treatment JUAN L. ACERO, † KONRAD STEMMLER, AND URS VON GUNTEN* Swiss Federal Institute for Environm ental Science and Technology (EAWAG), Ueberlandstrasse 133, CH-8600 Du ¨bendorf, Switzerland The present study investigates the degradation of atrazine (2-chloro-4-(ethylamino)-6-isopropylamino- s-triazine) by ozone and OH radicals during ozonation and advanced oxidation processes, with the identification of the main degradation products. Besides the dealkylated and amide degradation products (6-amino-2-chloro-4-isopropylamino- s-triazine, 6-amino-2-chloro-4-(ethylamino)- s-triazine, 4-acetamido-2-chloro-6-isopropylamino- s-triazine, 4-acet- amido-6-amino-2-chloro- s-triazine, and chlorodiamino- s- triazine), two new degradation products with an imine group were identified (2-chloro-4-ethylimino-6-isopropylamino- s- triazine and 6-amino-2-chloro-4-ethylimino- s-triazine). The contribution of the different pathways (direct ozone and OH radical reaction) to the overall degradation process has been quantified, and the rate constants of the reactions of atrazine and its main degradation products with both oxidants have been measured. The ethyl group is more reactive than the isopropyl group (i.e. 19 times during ozonation and four times during OH radical attack). The ethyl group reacts in higher proportion through oxidation to acetamide or imine derivates than to dealkylation. In contrast, the isopropyl group reacts mainly through dealkylation to the free amino group. Acetamido and imino groups are found to be resistant to chemical oxidation. These reactivities were corroborated by the measured values of the rate constants with both oxidants. A combination of product distribution and the kinetic parameters together with ozone and OH radical concentrations allowed us to calculate the evolution of the concentration of the degradation products for a given ozonation process. Introduction The European Union has set pesticide standards for drinking waters, at a maximum permissible concentration for a particular pesticide at 0.1 ppb and the sum of all pesticides at 0.5 ppb (1). The new regulation establishes not only a maximum concentration ofpesticides in drinking water but also includestheirdegradation productsafterdrinkingwater treatment (2). There are no global maximum concentration levels (MCLs) for pesticides in the U.S.A. The regulation is based on toxicological evidence of single compounds. For atrazine the MCL in the U.S.A. is 3 μg/ L (3). Due to their potential toxicity, the application of pesticides has been restricted in Switzerland over the last 10 years (4). There is a change in mentality of the convenience of agricultural control versus pollution of water resources with pesticides. Therefore, the development of ecological farming in Swit- zerland is encouraged. If source control does not lead to satisfactory pesticides levels, water treatment has to be applied to comply with the drinkingwater standards.There are severalprocesses which are currently used: activated carbon filtration, ozonation, microbial action, hydrolysis, photodecomposition, and ad- vanced oxidation processes (AOPs) (5-11). In the present study we will focus on ozone-based oxidation processes for atrazine degradation.Atrazine isone ofthe most widelyused agricultural herbicides (12) (i.e. about 36 000 metric t/year are applied onlyin the U.S.A.(13)).Because ofits persistence and the large use, traces of triazine herbicides have been found in many ground and surface waters. Very often a 1:1 mixture of atrazine and deethylatrazine is found (14, 15). Due to the low reactivity of atrazine with molecular ozone (16), AOPs have a higher potential for the elimination of not onlyatrazine but also itsfirst degradation byproducts.Ozone- based AOPs involve the generation ofradicalintermediates, in particular the hydroxyl radicals ( • OH), which are highly reactive with most organic compounds including atrazine (17). The combination O3/H2O2 is the most widely applied AOP in drinking water treatment, because of the simple adaptation ofconventionalozonation processes to this AOP by addition of hydrogen peroxide in the ozonation reactor (18). Attempts to identify and quantify the formation of degradation products during this AOP have been made (6- 8). The most abundant degradation products found during degradation of atrazine with O3 and • OH are listed in Table 1. In general, dealkylation and alkyl chain oxidation to acetamide are the predominant pathwayswhen ozone,AOPs, photolytic degradation, and Fenton’s reagent are applied (6-11).The formation ofthese degradation products can be explained by attacks of the side chains with ozone (19)and/ or • OH (20).An attackofozone to the N or the R-Catom leads to the formation of N-dealkyl and acetamide derivates (19, 21) over an unknown reaction pathway. OH radicals attack the R-C through hydrogen abstraction, forming a carbon centered radical.Thisradical,afteraddition ofoxygen forming the peroxyl radical, decomposes through bimolecular reac- tions to N-dealkyl and acetamide derivates (20, 21). In contrast, when photodecomposition is utilized, hydroxy triazinesarethepredominantsubproducts(10).Even though there is quite a lot of qualitative information about possible reaction pathways, there is still lack of information about the quantitative contribution ofthe different transformation pathways when ozone and • OH are present as oxidants. The knowledge of the quantitative evolution of degradation products during ozonation or an AOP will allow assessing the treatment with respect to drinking water regulations. To define and calibrate an ozonation process with respect to its oxidation capacity,it is necessaryto estimate the oxidant concentrations. Whereas ozone can be readily measured, there are no fast and easy methods to determine the • OH concentration duringozonation processes.An experimental approach has been developed in order to determine the *Corresponding author phone: +41-1-8235270; fax: +41-1- 8235028; e-mail: vongunten@eawag.ch. † Present address: Departamento de Ingenieria Quimica, Uni- versidad de Extremadura, 06071 Badajoz, Spain. Phone: +34-924- 289385; fax: +34-924-271304; e-mail: jlacero@unex.es. Environ. Sci. Technol. 2000, 34, 591-597 10.1021/es990724e CCC: $19.00  2000 American Chemical Society VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 591 Published on Web 01/07/2000