Pesticide Biotransformation in Plants and Microorganisms - American

Chapter 21

Chloropropionic Acid Photoreduction in Solutions of Na S and Quinones 2

Kenley K. Ngim and Donald G. Crosby

Downloaded by COLUMBIA UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: December 1, 2000 | doi: 10.1021/bk-2001-0777.ch021

Department of Environmental Toxicology, University of California, Davis, CA 95616

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Photoreductive dechlorinations mediated by H S/HS are possible environmental mechanisms or may be applicable to the remediation of contaminated waters. Carfentrazone-chloropropionic acid {I; α,2-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]-4-fluorobenzenepropanoic acid} is dechlor inatedto its propionic acid {II; 2-chloro-5-[4-(difluoromethyl)-4,5dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]-4-fluorobenzenepropanoic acid} in California flooded rice fields, and I and II are found in water and soil. While reductive dechlorinations are known to occur microbially, our purpose was to determine the reducing conditions promoting this reaction abiotically. Solutions of Na S alone and with quinones or natural rice field e carriers that were irradiated by U V were shown to dechlorinate I, yielding 1.6-28.4% of II; II was produced immediately, and maximum yields were obtained within 650 h. Dark reactivity was minor (0-2.5% of II). Only Na S with 1,4-benzoquinone produced a higher yield (28.4%) than Na S alone (19.8%), where the yield of II was directly proportional to Na S starting concentration and temperature. Minor dechlorination was demonstrated also in 4-chlorophenoxyacetic acid. Overall, Na S solution constituents were the primary photoreductants, while reduced quinones were more important in the photosensitized decomposition of I. 2

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Photoreductions are used in organic synthesis and may be applicable as an environmental decomposition mechanism. Pesticide photoreductions in irradiated natural water solutions have been demonstrated extensively by our laboratory (1,2,3), © 2001 American Chemical Society

In Pesticide Biotransformation in Plants and Microorganisms; Hall, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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implying that reaction with reduced dissolved organic matter microenvironments occurs. Recent findings of mirex dechlorination in humic acid solutions irradiated by U V support this assumption (4). Also, findings of D D E dechlorination in irradiated sediment suspensions (5) extend the microenvironment theory to particulates. The difficulty with the above studies is that the photoreductants have not been characterized. Furthermore, it is unfortunate that virtually all studies using known reductants (6,7,8,9) were intended as synthetic methods. Consequently, organic solvent systems were used, making extrapolation to natural waters difficult. Conversely, studies of "dark" reductions have been conducted in aqueous solution for certain types of chemicals (i.e., nitroaromatics, polychlorinated aliphatics), and enrichment with known reductants prevents any uncertainty over e" donating species. In fact, the pathway of nitro reduction has been determined (10), and all such abiotic reductions are believed to involve similar sequential e" transfer to an 8 atom, followed by protonation. Such reactions are believed to occur via redox coupling, analogous to biological e" transfer in photosynthesis or in oxidative metabolism. In the environment, the bulk e" donors for abiotic reductions include sulfides (i.e., H S) and Fe(II) (11), and these are often used in in vitro studies. Certain functionalities may serve as e" carriers, including quinones and organometal complexes, due to inherent interchangeability of e" acquisition and release; these may be constituents of aquatic organic matter (i.e., dissolved, colloidal, particulate) or soil surfaces. While redox coupling with other oxidized mediators is possible, e" transfer to a reducible group on a pollutant molecule normally terminates this chain. Reduction under such schemes has been demonstrated for polyhaloaliphatics (12,13) and substituted nitrobenzenes (14,15). The notion that carfentrazone-chloropropionic acid {I; ot,2-dichloro-5-[4(difluoromethyl)-4,5-dihycho-3-methyl-5-oxo- IH-1,2,4-triazol-1 -yl]-4-fluorobenzenepropanoic acid} may be reduced in a similar manner was prompted by observations of rapid hydrolysis of carfentrazone-ethyl herbicide {ethyl a,2-dichloro5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo- IH-1,2,4-triazol-1 -yl]-4-fluorobenzenepropanoate} to I and identification of the corresponding propionic acid {II; 2-chloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo- IH-1,2,4-triazol-1 -yl]-4fluorobenzenepropanoic acid} as the major degradation product in California rice field water and soil (16) (Figure 1). However, the polarity imparted by a single CI is considerably less than that of typically used chemicals, so we anticipated that the dark reaction would occur far more slowly, or maybe not at all. Our objective was to characterize reducing conditions that would facilitate such an unfavorable reaction, so we used U V radiation as a driving force. This study would allow a better understanding of the environmental fate of carfentrazone-ethyl, while utilizing known reductants and solutions relevant to natural waters. +

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Materials & Methods Standard Photoreduction Parameters Triplicate U V exposures were conducted in glass-stoppered Pyrex glass reactors (17), each outfitted with a single F40/350BL lamp (Sylvania-GTE). Solution temperature (35°C) was maintained by a plastic tubing jacket connected to a contant temperature circulator. Dark trials were conducted in Teflon-capped, amber glass bottles (1 L ) maintained in a water bath. Solutions were flushed with N for 30 min (30 mL/min) to purge 0 ; dark control headspace was purged after each opening. U V trials were run for 600-800 h or >3 dissipation half-lives of I, while dark controls lasted as long as corresponding U V trials. Model solutions consisted of 0.1 M phosphate buffer (pH 7) containing Na S (26 mM), e" carrier, and I (1.4 juM). Quinone e" carriers (0.13 mM) included benzoquinone {1,4-benzoquinone}, juglone {5-hydroxy-1,4-naphthoquinone}, or lawsone {2-hydroxy-1,4-naphthoquinone}. A 20% rice straw extract in 0.1 M phosphate buffer and unbuffered rice field water were used for comparison. The straw extract was made by incubating 150 g of dried rice straw (Colusa County, C A ) in 3.75 L distilled deionized water for 6 months, at 24°C in Teflon-capped, amber glass jugs. Field water and straw extracts were filtered (0.22 pm) prior to use. Controls consisted of dark trials for the above and UV/dark trials of Na S alone, reduced quinones alone, e" carrier alone, and phosphate buffered solutions of I. Reduced quinones included hydroquinone and hydrojuglone; the latter is not commercially available and was synthesized according to a published method (18). Samples (25 mL) were acidified (pH 2-2.5) with 10% H S 0 (1.0 mL) and passed through 3 cc, 500 mg, C-18 solid-phase extraction (SPE) cartridges (Varian, Harbor City, C A ) . Ether eluates (2 mL) were methylated with diazomethane in ether, evaporated to near dryness under N , reconstituted in hexane (2 mL), and adjusted to 1 m L by further evaporation and volumetric glassware. Analysis of I, II, and other products (i.e., benzoic, cinnamic acids) was by G C with a thermionic specific detector (Varian, Walnut Creek, C A ) fitted with a 15 m x 0.32 mm i.d. column containing a 0.25 jum film of DB-1701 (J&W, Folsom, C A ) . Reduction product identity was verified by G C with a mass-selective detector (Hewlett Packard, Wilmington, D E ) operated in scan mode and fitted with a 30 m x 0.25 mm i.d. column containing a 0.25 fim film of DB-5 (J&W). Mass spectra from authentic standards were used for confirmation. 2


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Supplemental Studies Changes from the above standard parameter trials were necessary to confirm the photoreducing activity of Na S with benzoquinone and better understand the 2


400 mechanism of photoreductive dechlorination. To validate photoredox coupling between I and reduced quinoids, the Na S starting concentration was lowered from 26 m M to 1.3 m M , while benzoquinone remained constant (0.13 mM). Next, since Na S appeared to be the primary reductant, trials without e" carriers were run as follows: concentration (1.3 m M and 91 mM), p H (5 and 9), temperature (24°C and 50°C) in the dark; findings contrasted with those from Na S standard condition controls (26 m M , p H 7, 35°C). 4-Chlorophenoxyacetic acid (0.13 mM) was investigated as a model monochloroaromatic compound to determine how universal the photoreduction mechanism might be. This experiment was conducted with Na S (no e' carrier) under standard conditions. Samples (50 mL) were analyzed by acidification with 20% H S 0 and C-18 SPE. Cartridges were thoroughly dried prior to elution with methanol (2 mL). Analytes (phenoxyacetic acid, 4-chlorophenol, and phenol) were detected using reverse-phase H P L C with U V / V i s detection (Isco, Lincoln, NE) at 237 nm, with acetonitrile:H 0:acetic acid (50:49:1) as the mobile phase. Separation was achieved with a 250 mm x 4.6 mm i.d. Alltima column packed with 5 pm C-18 particles (Alltech, Deerfield, IL). 2

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Chloropropionic Acid Dissipation Dissipation of I under U V irradiation was dictated by degree of photosensitized degradation, photoreduction, and a dark reaction. Half-life (t ) (Table I) was calculated from pseudo-first-order kinetics. The least stability was observed with juglone and lawsone (ti 3.58 h and 8.42 h, respectively), indicating that combinations with Na S act as potent photosensitizing agents in the decomposition of I and possibly of the reduction product. Indeed, the photosensitizing activity of quinoids is known (19), and that of hydroquinone on methoxychlor solutions was demonstrated (7). Rice straw extract and benzoquinone (t, 88.2 h and 260 h, respectively) were less effective. The rice straw extract was intended to simulate anaerobic straw decomposition in a rice field. Natural organic matter contains polyphenolic groups, so quinoid functions may be present in the rice straw extract. Also, as the extract was incubated under nonsterile conditions, various microbial metabolites may be present, as well. A s the composition of the rice straw extract was not characterized, however, the dissipation of I cannot be attributed conclusively to photosensitized degradation with reduced quinones. Rice field water and Na S alone (t 528 h and 467 h, respectively) produced no effects. The field water may contain insufficient organic matter to enhance sensitized photolysis. The dissipation of I is illustrated for Na S alone and with juglone (Figure 2). ]/2

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401 CI I R-CH CH-COOEt 2


Carfentrazone-ethyl

H0 • • 2

C! I R-CH CH-COOH 2

Carfentrazonechloropropionic acid (I)

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R-CH CH -COOH 2

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Carfentrazonepropionic acid (II)

Figure 1. Primary carfentrazone-ethyl decomposition route in California rice field water and soil. Base hydrolysis to carfentrazone-chloropropionic acid (I) precedes reduction to corresponding propionic acid (II).

Time [h]

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Figure 2. Dissipation of carfentrazone-chloropropionic acid (squares) to corresponding propionic acid (circles) in solutions ofNa2S (A) and Na2S with juglone (B) irradiated by UV light. In Pesticide Biotransformation in Plants and Microorganisms; Hall, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

402 Table I. Chloropropionic Acid Stability and Yield of Its Propionic Acid Metabolite i n Various Na S Solutions Irradiated by U V Light 2

Trial®

tj/2 (h)

N ^ S only Benzoquinone Juglone Lawsone Rice Straw Solution (20%) Rice Field Water (unbuffered) n = 3 for each

467 +/-•88 260 +/-• 16 3.58 +/- 0.53 8.42 +/- 3.37 88.2 +/- 10.9 528 +/- 108

Yield of Compound II (% of Initial I) 19.8 +/- 6.9 28.4 +/- 10.2 8.48 +/- 1.63 1.64+/- 0.51 18.4+/-•5.9 14.8 +/-•2.9


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I was generally more stable in the corresponding dark controls for this experiment (t 360 h or greater). Juglone and lawsone provided stability, as both were run only as long as their U V trials (50-60 h) and I did not dissipate in this short period. In the presence of rice straw, benzoquinone, and rice field water I was less stable (t 360 h, 435 h, and 568 h, respectively). The exception was Na S (t 465 h), which produced comparable U V dissipation kinetics, suggesting that a dark reaction occurs. Since e" donation from H S/HS" is sequential, free-radical sulfur species may be involved. The dissipation in e" carrier controls (no Na S) ranged from giving t values comparable to corresponding Na S, trials to being stable; dark trials also were stable. Stability of I was greater in hydrojuglone and hydroquinone UV/dark controls (20% degraded after 53 h and t 337 h, respectively) than corresponding Na S trials with the quinone. The differences might indicate that hydroquinones had been oxidized more rapidly in the absence of Na S to less photoactive quinones or that Na S and quinones work in tandem as reactants. Also, it is known that mercaptoquinones are formed in solutions of Na S and quinones via Michael addition (12). Thus, these may be acting as even more potent photosensitizing agents than are the corresponding hydroquinones. Dissipation of I in benzoquinone with 1.3 m M Na S was far greater than corresponding 26 m M trials (t 10.4 h vs. 260 h, respectively), indicating that H S/HS" disrupts photosensitized decomposition by reduced quinones. This agrees with the more rapid decomposition in direct photolysis trials (t 358 h). Stability of I in Na S trials at 1.3 m M and 91 m M (

Pesticide Biotransformation in Plants and Microorganisms - American

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