Carbon, Hydrogen, and Nitrogen Isotope Fractionation During Light

Jul 9, 2008 - Photooxidation of atrazine is accompanied by moderately large, normal 2H fractionation compared to inverse 13C and 15N isotope effects ...
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Environ. Sci. Technol. 2008, 42, 7751–7756

Carbon, Hydrogen, and Nitrogen Isotope Fractionation During Light-Induced Transformations of Atrazine ´ E. HARTENBACH,† AKANE T H O M A S B . H O F S T E T T E R , * ,† PETER R. TENTSCHER,† SILVIO CANONICA,# MICHAEL BERG,# ´ P. SCHWARZENBACH† AND RENE Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zurich, Universitatstrasse 16, CH 8092 Zurich, Switzerland, and Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH 8600 Dubendorf, Switzerland

Received February 5, 2008. Revised manuscript received April 20, 2008. Accepted May 8, 2008.

The 13C, 2H, and 15N fractionation associated with lightinduced transformations of N-containing pesticides in surface waters was investigated using atrazine as a model compound. In laboratory model systems, bulk isotope enrichment factors C, H, and N were determined during the photooxidation of atrazine by excited triplet states of 4-carboxybenzophenone (34-CBBP*), by OH radicals, and during direct photolysis at 254 nm. Moderately large 2H fractionations, quantified by H values of -51.2 ( 2.5‰ and -25.3 ( 1.7‰, were found for the transformation of atrazine by 34-CBBP* and OH radicals, respectively. 13C and 15N enrichment factors were rather small (-0.3‰ > C, N > -1.7‰). The combined δ13C, δ2H, and δ15N analysis suggests that isotope effects are most likely due to H abstraction at the N-H and C-H bonds of the N-alkyl side chains. Direct photolysis of atrazine yielding hydroxyatrazine as main product was characterized by inverse 13C and 15N fractionation (C ) 4.6 ( 0.3‰, N ) 4.9 ( 0.2‰) and no detectable 2H fractionation. We hypothesize that isotope effects from photophysical processes involving the excited states of atrazine as well as magnetic isotope effect originating from the magnetic interactions of spin-carrying C and N nuclei have contributed to the observed inverse fractionation. Our study illustrates how compound-specific isotope analysis can be used to differentiate between important direct and indirect phototransformation pathways of agrochemicals in the environment.

Introduction Light-induced transformations of organic pollutants, which encompass direct and indirect photolysis, are important degradation pathways for many agrochemicals such as pesticides and herbicides in surface waters as well as at soil and plant surfaces (1, 2). During direct photolysis, light absorption generates reactive compounds with excited * Corresponding author phone: +41 44 632 83 28; fax: +41 44 633 11 22; e-mail: [email protected]. † Institute of Biogeochemistry and Pollutant Dynamics. # Swiss Federal Institute of Aquatic Science and Technology. 10.1021/es800356h CCC: $40.75

Published on Web 07/09/2008

 2008 American Chemical Society

electronic states that are susceptible to chemical transformation. Indirect or sensitized photolysis, in contrast, leads to contaminant transformations through energy transfer or chemical reactions with transient species formed in the presence of light, in particular with photooxidants such as hydroxyl radicals (•OH), singlet oxygen (1O2), and excited triplet states of dissolved organic material (3DOM*) with oxidative character (2–6). The identification of these photochemical reactions in the environment might be compromised by the fact that light-induced reactions and other (bio)degradation processes of agrochemicals can take place simultaneously and sometimes lead to the same products. Moreover, quantifying the contribution of direct and indirect photolysis to the overall transformation of the contaminant with reactant and product concentration measurements is very difficult because nondegradative processes such as dilution or adsorption also contribute to the attenuation of agrochemicals in aquatic environments. Compound-specific isotope analysis (CSIA) of C, H, and N has been shown to offer new avenues for the identification and quantification of transformation pathways of different organic water contaminants including fuel constituents, chlorinated solvents, and nitroaromatic compounds (7–15). Changes of contaminants’ stable isotope signatures measured by CSIA are due to kinetic isotope effects (KIEs) at the reacting bonds, which are pertinent to a given reaction mechanism. Therefore, one can expect an isotopic enrichment in the remaining substrate that is characteristic for each reaction pathway. The corresponding measured bulk isotopic enrichment factors, , quantify the observed enrichment and can relate changes in isotope signatures to the extent of transformation. Furthermore, the more elements are included in the isotopic analysis of a given transformation process the greater the chance to assess unequivocally the underlying reaction mechanism(s). This is particularly important for cases in which competing reactions occur such as in phototransformation processes of agrochemicals, where different functional groups of a molecule might be involved. To date, isotope fractionation during light-induced transformations of organic contaminants has barely been investigated. The goal of this work was therefore to assess 13C, 2H, and 15N fractionation associated with important photochemical transformation pathways. Atrazine was chosen as model compound because this herbicide can be transformed by various light-induced reactions yielding different products (16), that is primarily N-dealkylated compounds and sidechain oxidation products for oxidation by transient photooxidants, and hydroxyatrazine for direct photolysis. Therefore, the two types of reactions can be expected to exhibit different isotope fractionation patterns. In this study, bulk C, H, and N values were determined in laboratory model systems for the photooxidation of atrazine by OH radicals and by triplet states of 4-carboxybenzophenone (34-CBBP*), a surrogate molecule for 3DOM*, as well as for the direct photolysis of atrazine in the low UV range at 254 nm. Hydroxyl radicals and 34-CBBP* react both via initial electron/hydrogen abstraction mechanism with aromatic and aliphatic amines (17–20). Therefore, photooxidation of atrazine by these two reactants should be accompanied by a significant 2H fractionation (21). Estimates of the isotope fractionation during the direct photolysis of atrazine are, in contrast, less straightforward because isotope fractionation associated with photoexcitation processes has hardly been investigated. This study, therefore, presents the first data set of this type of reactions. VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Pseudo-First Order Rate Constants of Light-Induced Transformation of Atrazine, kobs, Bulk 13C, 2H, and 15N Enrichment Factors, EC, EH, and EN, for Photooxidation of Atrazine by Triplet States of 4-Carboxybenzophenone (34-CBBP*), by OH Radicals, and During Direct Photolysis of Atrazine at 254 nm (DP254); ∆δ2H/∆δ13C, ∆δ2H/∆δ15N, and ∆δ15N/∆δ13C are the Slopes Obtained from the Two-Dimensional Isotope Analysis kobs (s-1)

EC (‰)a

Photooxidation 34-CBBP* 3.20 × 10-4 •OH 1.06 × 10-4

-1.73 ( 0.07 -0.52 ( 0.22

Direct Photolysis DP254 8.60 × 10-4

4.60 ( 0.31

a

EH (‰)a -51.17 ( 2.47 -25.30 ( 1.69 no fractionation

∆δ2H/∆δ13Ca

∆δ2H/∆δ15Na

∆δ15N/∆δ13Ca

-0.69 ( 0.05 -0.26 ( 0.03

31 ( 2 27 ( 13

72 ( 9 93 ( 21

0.40 ( 0.04 0.24 ( 0.15

n.d.b

1.05 ( 0.14

4.85 ( 0.15

n.d.b

Uncertainties represent ( one standard deviation ((1σ) of the linear regression analysis.

Experimental Section Photochemical Experiments. Photooxidation experiments were carried out in 3 mM KH2PO4 solutions (pH 7) with initial atrazine concentrations between 90 and 130 µM. We used a DEMA 125 merry-go-round photoreactor (Hans Mangels, Bornheim, Germany), which was equipped with a mediumpressure mercury lamp (MP Hg) Hanau TQ718. The lamp was contained in a cooling jacket made of quartz, borosilicate glass and UVW-55 glass, which resulted in a 308-410 nm band-pass filter (main irradiation wavelength of 366 nm (3)). The photoreactor was filled with deionized water and kept at the constant temperature of 25.0 ( 0.2 °C. For the sensitized photolysis by 34-CBBP* (4-carboxybenzophenone), eight quartz tubes (i.d. 14 mm) were filled with 20 mL of a buffered atrazine solution (107 µM) containing 100 µM 4-CBBP. After introduction into the photoreactor, the first three tubes were withdrawn every 10 min, the remaining four were subsequently removed every 30 min, and one tube was kept in the dark as control. Additional control experiment performed without 4-CBBP showed that atrazine degradation by direct photolysis was negligible ( 300 nm (Supporting Information, Figure S1). For the atrazine oxidation by OH radicals, six quartz tubes were filled with 20 mL of a buffered atrazine solution (117 µM) containing 11.6 mM H2O2. Sample tubes were removed every 83 min during 8.3 h. Dark control experiment were performed to verify that atrazine loss due to direct oxidation by H2O2 was negligible ( C, N > -1.7‰, Table 1). These observations are consistent with the expected H abstraction from N-H and C-H bonds at the N-ethyl and N-isopropyl side chains of atrazine. Even though we cannot infer the reactive bond(s) from our data, a tentative calculation of apparent kinetic hydrogen isotope effect (AKIEH (7)) in these systems for selected boundary cases is informative. Assuming that either only the tertiary CR-H bond is reactive and isotopic for H abstractions vs the situation in which all N-H and

CR-H bonds are responsible for the observed 2H fractionation, one can hypothesize that AKIEH values lie between 1.7 to 2.7 and 1.3 to 1.5 for reactions of atrazine with 34-CBBP* and OH radicals, respectively. This range for AKIEH is in good agreement with the range of 1.2 to 1.9 reported for the photooxidation of tert-butylamine-N-d2, 2-aminobutane-Nd2 and 2-aminobutane-R-C-d by triplet state benzophenone (21). The somewhat smaller AKIEH values found for the OH radicals may be explained by the near diffusion-controlled reaction rates in this system (bimolecular rate constant of 2.6 × 109 M-1 s-1 (28)), which could partially mask the 2H isotope effect (29, 30). To check whether the two photooxidants react with atrazine via different mechanisms in addition to the abovementioned comparison of isotope enrichment factors, one can derive the relative δ13C, δ2H, and δ15N shifts during the reactions. Slopes of the two-dimensional isotope analysis, in this case the ∆δ2H/∆δ13C, ∆δ2H/∆δ15N, and ∆δ15N/∆δ13C correlation analysis, are proportional to the ratio of the corresponding bulk enrichment factors and thus indicative of the underlying reaction mechanism (15, 31, 32). As is shown in Table 1 and Figure S4, the slopes of the two-dimensional isotope analysis corresponded well with the ratio of -values. In addition, the slopes of the ∆δ2H/∆δ13C and ∆δ2H/∆δ15N correlations are identical within error ((1σ) for the two photooxidants, supporting the hypothesis that the two reactions largely take place via similar mechanisms of H abstraction from N-H and/or C-H bonds. The ∆δ15N/∆δ13C correlation, in contrast, is slightly larger in the 34-CBBP* system (0.40 ( 0.04 vs 0.24 ( 0.14), which may reflect the higher selectivity of this oxidant for the N-H vs C-H hydrogen abstraction as compared with reactions of OH radicals. The observed differences are, however, rather small. Therefore, an identification of the predominant reactive VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Isotope fractionation during atrazine photooxidation by OH radicals. Panels a, b, and c show the evolution of the δ13C, δ2H, and δ15N values vs the remaining fraction of atrazine (circles). Solid lines were calculated using the enrichment factors in Table 1. Panels d, e and f show the linearized isotope enrichment used to derive enrichment factors with eq 1. Cyanazine (internal standard, IS, diamonds). Uncertainties represent ( one standard deviation from triplicate measurements. positions for the reactions of the two photooxidants cannot mation. The significant inverse 13C and 15N fractionation, be accomplished with the presented data. however, are inconsistent with the hydrolysis step leading to Direct Photolysis of Atrazine. Figure 3f shows that hydroxyatrazine because the C-Cl bond cleavage is expected atrazine transformation followed first-order kinetics. High to cause normal isotope fractionation for C and only a initial atrazine concentration (94 µM) required for isotope secondary effect on N (35). Therefore, other elementary analysis resulted in solutions with nonvanishing absorbance process(es) preceding the hydrolysis step are likely responat λ ) 254 nm (i.e., A254 . 0.07), and therefore first-order rate sible for the observed inverse 13C and 15N isotope effects constants (kobs of 8.6 × 10-4 s-1, Table 1) were somewhat during the direct photolysis of atrazine. Scheme 1 illustrates smaller compared to experiments performed at low atrazine that prior to the hydrolysis step the excited singlet and triplet concentration (23). Hydroxyatrazine was identified as the states of atrazine (S1 and T1) can return to the electronic ground-state via a variety of photophysical processes (36). major degradation product (16), amounting to up to 80% of initial atrazine concentration. Two minor unidentified reacSCHEME 1 tion products were detected during HPLC analysis, whose absorption spectra were similar to that of hydroxyatrazine (Figure S5), which is in agreement with observations reported in the literature (33, 34). As shown in Figure 3a and 3b, direct atrazine photolysis resulted in an inverse isotope fractionation of 13C and 15N, that is the substrate was enriched with the light isotopologues during the reaction. The lack of δ13C and δ15N shifts observed for cyanazine (open diamonds) confirms that the observed fractionation was associated with the direct photolysis of atrazine. Note that the slope of 1.05 ( 0.14 from twodimensional isotope analysis of 15N vs 13C, ∆δ15N/∆δ13C, was significantly different from those observed for photooxidation Isotope effects associated with photophysical excitation of atrazine with 34-CBBP* and OH radicals (Table 1). In and relaxation processes (Scheme S1) could have caused the contrast to 13C and 15N, we observed a negligible enrichment observed isotope fractionation, but little is known that would of 2H in atrazine after 80% reactant conversion, which was allow one to confirm such hypotheses. As an alternative, virtually identical to δ2H trends of the cyanazine standard. magnetic isotope effects (MIE) could be invoked if magnetic Therefore, the changes of atrazine δ2H were presumably not interactions between spin carrying nuclei (i.e., 13C (spin 1/2) 2H enrichment associated with direct photolysis, and the bulk vs 12C isotopologues (no spin) or 15N (spin 1) vs 14N (spin factor is likely close to zero. 1/2)) and unpaired electrons of excited atrazine radicals The absence of 2H fractionation is in agreement with the contribute to the lifetime of the intermediate species (37, 38). observation of hydroxyatrazine as the predominant photolysis Assuming, for example, that the photodissociation of atrazine product, indicative for a reaction mechanism in which mainly generates radical pairs in the singlet state, light isotopologue the triazine ring is involved in the photochemical transforradical pairs might predominantly recombine while heavy 7754

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FIGURE 3. Isotope fractionation during atrazine direct photolysis at λ ) 254 nm. Panels a, b, and c, show the evolution of the δ13C, δ2H, and δ15N values vs the remaining fraction of atrazine (circles). Solid lines were calculated using the enrichment factors in Table 1. Panels d and e show the linearized isotope enrichment used to derive enrichment factors with eq 1. Cyanazine (internal standard, IS, diamonds). Panel f shows atrazine transformation kinetics during direct photolysis. Uncertainties represent ( one standard deviation from triplicate measurements. isotopologue pairs will undergo S-T conversion via hyperfine coupling in atrazine and react to hydroxyatrazine. Such magnetic field effects have already been suggested for photochemical reactions involving species with unpaired electrons, such as those resulting from homolytic bond cleavages or molecule in their excited triplet state (37). Postulating magnetic interactions offers interesting avenues to explain our observations. The significance of such effects for the direct photolysis of atrazine, however, cannot be assessed from the available literature on MIEs and requires further study. Implications for the Assessment of Light-Induced Transformations of Triazine Herbicides. Our work shows that the distinction between direct and indirect photolysis of atrazine by CSIA is facilitated by the very different bulk isotope enrichment factors. Similar effects can be expected for other triazine herbicides assuming that photooxidations affect the N-alkyl side chains vs the direct photolysis affecting primarily the triazine ring. For a more comprehensive application of CSIA to this important class of water contaminants (39–42), however, knowledge of the isotope fractionation associated with the various biodegradation pathways is necessary. To our knowledge, biodegradation of atrazine and other triazine herbicides has not been assessed yet by means of CSIA. Enzyme-catalyzed N-dealkylations yielding desethyl- and desisopropylatrazine via electron transfer or H abstraction are likely to result in 13C, 2H, and 15N fractionation patterns similar to that of photooxidations. Enzymatic hydrolysis of atrazine, in contrast, will presumably show normal isotope fractionation for the cleavage of the C-Cl bond (43) and might, therefore, be differentiated from the hydroxyatrazine formation via direct photolysis. With additional studies on the isotope fractionation during biodegradation of triazine herbicides including the analysis of transformation products, CSIA is likely to provide key information for the distinction

of microbial vs photochemical degradation pathways in the environment.

Acknowledgments We thank Jakov Bolotin for analytical support and Anatoly Buchachenko for valuable comments. This work was supported by the Swiss National Science Foundation (200020116447/1).

Supporting Information Available Analytical methods and identified reaction products during indirect and direct photolysis experiments, two-dimensional isotope analysis for 13C, 2H, and 15N fractionation of indirect photolysis, and figures illustrating photophysical processes of atrazine.This material is available free of charge via the Internet at http://pubs.acs.org.

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