Article pubs.acs.org/JAFC
Comparison of Direct and Indirect Photolysis in Imazosulfuron Photodegradation Caitlin Rering,*,† Katryn Williams,‡ Matt Hengel,‡ and Ronald Tjeerdema‡ †
Center for Medicinal and Veterinary Entomology, U.S. Department of Agriculture, 1600 S.W. 23rd Drive, Gainesville, Florida 32608, United States ‡ Environmental Toxicology Department, University of CaliforniaDavis, 4138 Meyer Hall, One Shields Avenue, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Imazosulfuron, a sulfonylurea herbicide used in rice cultivation, has been shown to undergo photodegradation in water, but neither the photochemical mechanism nor the role of indirect photolysis is known. The purpose of this study was to investigate the underlying processes that operate on imazosulfuron during aqueous photodegradation. Our data indicate that in the presence of oxygen, most photochemical degradation proceeds through a direct singlet-excited state pathway, whereas tripletexcited state imazosulfuron enhanced decay rates under low dissolved oxygen conditions. Oxidation by hydroxyl radical and singlet oxygen were not significant. At dissolved organic matter (DOM) concentrations representative of rice field conditions, fulvic acid solutions exhibited faster degradation than rice field water containing both humic and fulvic acid fractions. Both enhancement, via reaction with triplet-state DOM, and inhibition, via competition for photons, of degradation was observed in DOM solutions. KEYWORDS: indirect photolysis, dissolved organic matter, reactive oxygen species, herbicide, imazosulfuron
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INTRODUCTION Herbicide application both reduces labor costs and increases crop yields, but their use also has ecological repercussions, including contamination of surface water and air, drift damage to other crops and vegetation, and toxicity to nontarget organisms.1−3 Sunlight-induced photodegradation, either by absorption of a photon (direct photolysis) or by reaction with transient reactive species (indirect photolysis), is an important process driving the attenuation of many contaminants in water. Environmental fate studies have successfully determined the role of photolysis in overall fate for many commonly applied herbicides in pure or buffered waters.4 This foundation invites a more mechanistic approach, particularly for those contaminants for which photodegradation is expected to be a significant environmental dissipation contributor and which have high mobility and are therefore able to disperse into different environments with dissimilar water quality. Understanding the relative contribution of direct and indirect photoreactions facilitates more accurate rate estimations, which may change with fluctuations of ubiquitous water constituents such as dissolved organic matter (DOM) and molecular oxygen.5,6 DOM photolysis produces triplet excited-state DOM (3DOM) and other species, such as singlet oxygen (1O2), peroxyl radical, hydrated electrons, and hydroxyl radical (•OH).7−13 DOM may also inhibit degradation by acting as a sink for reactive species, competing for photons, or quenching excited states via energy transfer. Here we examine the effects of these species on imazosulfuron photodegradation under simulated sunlight (Figure 1). Sulfonylurea herbicides such as imazosulfuron are known for their high phytotoxicity at low application rates.14 After recent registration in California, imazosulfuron has not yet been © XXXX American Chemical Society
Figure 1. Structures of imazosulfuron (pKa = 4.0) and its photodegradation products.
widely applied by cultivators (only 1600+ lb of imazosulfuron active ingredient applied in 2014).15 However, in other areas of the world including the Midwestern United States, Canada, and Japan, imazosulfuron and other sulfonylureas have been detected in agricultural drains, rivers, and groundwater, indicative of off-site transport with more extensive use.16−18 Field and laboratory studies performed by our group and others have confirmed that imazosulfuron readily undergoes direct photolysis upon exposure to simulated solar radiation.19,20 However, the degree to which photolysis may be mediated by other photochemical processes, including reaction with reactive oxygen species (ROS) and 3DOM, is not yet known. The purpose of this study was to evaluate the importance of these Received: Revised: Accepted: Published: A
January 10, 2017 March 31, 2017 April 3, 2017 April 3, 2017 DOI: 10.1021/acs.jafc.7b00134 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
Susceptibility of imazosulfuron to reaction with singlet oxygen (1O2) was determined by thermal generation of this species to avoid interference from direct photolysis. Solutions (n = 5, pH 10) were prepared containing molybdate ions (1 mM MoO42−) and H2O2 (10 mM) and wrapped in foil to protect them from light. At each time point a 100 μL aliquot was removed and added to a saturated βcarotene in acetone solution (10.0 mL, 37.3 mM) to quench the reaction; analysis followed immediately. A furfuryl alcohol probe confirmed the production of 1O2 in solution (steady state 1O2 concentration = 0.9 pM). Photochemical Mechanism. The relative importance of triplet excited state imazosulfuron to degradation was evaluated using a selective scavenger to increase the abundance of triplet imazosulfuron in solution. The addition of Cs+ ion (1 M CsCl) increases the intersystem crossing rate by quenching singlet excited states, resulting in enhanced triplet state concentrations.26,27 Analysis. Analysis was performed using an Agilent 1200 series high-pressure liquid chromatograph coupled to a 6420 triplequadrupole mass spectrometer run in positive ion ESI multiple reaction monitoring mode as described elsewhere.21 Briefly, samples were injected onto the analytical column, a 150 mm × 4.6 mm i.d., 5 μm, Eclipse XDB-C18 (Agilent, Santa Clara, CA, USA). A gradient mobile phase consisting of water (0.25% formic acid, solvent A) and acetonitrile (0.25% formic acid, solvent B) was used with the following conditions: 70:30 A/B at 0 min, 62:38 A/B at 3 min, 10:90 A/B at 4 min, flow rate = 0.25 mL/min, total run time = 27.5 min. The column was rapidly equilibrated to starting conditions at 1 mL/min. For each analyte, one quantification and two qualification ions were monitored. Data Analysis. Statistical analysis was performed using JMP 11.0 (SAS Institute, Cary, NC, USA), and differences between treatments were determined using an ANCOVA test for equality of slopes at 95% confidence.
processes and to determine the photochemical mechanism of direct photolysis.
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MATERIALS AND METHODS
Chemicals and Organic Matter Isolates. Imazosulfuron (1-(2chloroimidazo[1,2-a]pyridin-3-ylsulfonyl)-3-(4,6-dimethoxypyrimidin2-yl) urea) was purchased from Santa Cruz Biotechnologies, Inc. (Santa Cruz, CA, USA). 2-Amino-4,6-dimethoxypyrimidine (98%) 4nitroacetophenone; (98%), pyridine (99.8%), and high-purity water for TOC analysis were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). The degradation products 2-chloroimidazo[1,2-α]pyridine-3-sulfonamide and 2-ureido-2,6-dimethoxypyrimidine were synthesized, purified, and characterized by our collaborators at California State UniversityChico and are described elsewhere.21 Suwannee River and Pony Lake fulvic acid standards (SRFA and PLFA, respectively) were purchased from the International Humic Substances Society (St. Paul, MN, USA). Formic acid, acetonitrile, and TOC-free and Optima grade water were purchased from Fisher Scientific (Hampton, NH, USA). All standard solutions were prepared in acetonitrile and stored at −20 °C, and all solvents were of LC-MS/ MS grade. Water Collection and Characterization. Water (pH 8.37) was collected on May 16, 2015, from a rice field in Glenn county, California (39.454467° N, 122.078344° W) and prescreened for imazosulfuron residues. Specific absorbance (SUVA254), a measure of solution aromaticity, was calculated by normalizing the UV absorbance of a solution at 254 nm with the concentration of DOM (mg C/L).22 Absorbance units were converted to absorption coefficients as a=
2.303A l
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where a is the absorption coefficient (m−1), l is the path length (m), and A is the absorbance. Photolysis of Imazosulfuron. Exposure solutions were prepared from buffered high-purity water for TOC analysis (5 μM KH2PO4 adjusted to pH 9 with 0.1 N NaOH) with SRFA or PLFA at 5 mg C/ L. Rice field water, filtered with a 0.45 μm membrane (EMD Millipore, Darmstadt, Germany), was diluted from 9.2 to 5 mg C/L with highpurity water to facilitate direct comparison to the fulvic acid isolate solutions. A survey of California rice fields reported the mean DOC value for rice field water throughout the growing season (4.32 mg L−1), which was the basis for the selected DOC concentration.23 All prepared solutions were fortified with ca. 250 μg L−1 imazosulfuron (0.48 μM; approximately U.S. field application rate assuming 10 cm flood depth). Imazosulfuron solutions (75 mL) were placed in borosilicate type-200 vials for photoexposed samples (capable of filtering UV < 290 nm); dark controls were foil-wrapped. Samples and controls were placed in two exposure chambers outfitted with 10 broad-spectrum UV lamps (8 W; 300 ± 50 nm) (Southern New England Ultraviolet Co., Branford, CT, USA) mounted 25 cm above the samples, and exposures were conducted at 25.0 (±1.5) °C. Irradiation was monitored using a portable radiometer (Ultra Violet Products, Upland, CA, USA) and 4-nitroacetophenone-pyridine chemical actinometry. Samples were collected (n = 5) at each time interval (0, 3, 6, 12, 18, 24, 36, and 60 (±0.5) h), briefly vortexed, and stored at −20 °C prior to analysis. Dark controls (n = 3) were wrapped in foil to protect from light and run concurrently. Dark samples were collected at 0, 36, and 60 (±0.5) h). All glassware was autoclaved (121 °C, 15 psi, 1 h) prior to use. All reaction rate coefficients were corrected for solution absorbance.24 Reaction with Selected ROS. A preliminary investigation into the role of ROS in imazosulfuron photolysis was carried out under lowoxygen conditions. Oxygen was removed from both rice field and ultrapure laboratory water by purging with argon gas (5 mL/min, 2 h); water was then tightly sealed in a glovebox. The role of hydroxyl radical (•OH) was assessed by preparing solutions with 25 mM isopropanol, a selective •OH scavenger; samples were collected as described above.
RESULTS AND DISCUSSION Rice Field Water Characterization. DOM differs in composition and reactivity on the basis of the substrate from which it was derived. Allochthonous DOM, originated from terrestrial plants, typically has higher aromaticity and greater optical density due to the incorporation of lignin from cell walls. Autochthonous DOM, produced from macrophytes and algae, has more nitrogen and sulfur moieties and lower molecular weight. Both the character and concentration of DOM in rice fields vary depending on inflow and outflow rates, water reuse, and disposal of residual rice straw by either burning or tilling. Krupa et al.23 conducted a survey of DOM composition in California rice field outflows and watersheds and found that dissolved organic carbon (DOC) levels spike upon initial flooding, as bulk stores of organic matter are rapidly mobilized. DOC levels then decreased over the course of the growing season, as constant water flow stripped away organic matter available for mobilization within the soil. This pattern is consistently observed within terrestrial environments that undergo regular wet and dry cycles. The mean DOC value for the growing season was 4.32 mg/L and was the basis for the selected DOC concentration (5 mg/L) tested in this study. DOC content in the collected rice field water used in this study was 9.2 mg C/L, corresponding well with the typical values reported23 between May and June (Table 1). This is the time period both when imazosulfuron would be applied to rice and when the water used in this study was collected (midMay). DOM SUVA254 for the collected water was 4.3 L/(mg C m). DOM SUVA254 also peaks upon flooding and then decreases over the course of the growing season. The loss in aromaticity is likely caused by both microbial metabolism and photodegradation. B
DOI: 10.1021/acs.jafc.7b00134 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
DOM in these samples may be explained by a compensatory effect between quenching and reaction with DOM. To further investigate the influence of DOM concentration, SRFA was added in excess of typical field water values (50 mg C/L). The imazosulfuron degradation rate coefficient corrected for light absorbance by the solution was enhanced significantly (P < 0.0001, Figure 3), confirming that contributions to overall
Table 1. Characterization Data for Fulvic Acid Solutions and Rice Field Water [DOM] (mg C/L)
SUVA254a
S290−350b
PLFA
5
4.4
0.92
SRFA
5 50
9.9 8.0
0.84 0.33
4.8 4.3
0.96 0.87
sample type
rice field
5 9.2
a
SUVA254 is specific UV absorbance at 254 nm corrected by DOM concentration (L/(mg C m). bS290−350 is the screening factor calculated over 290−350 nm wavelength range.
DOM Influence on Imazosulfuron Photodegradation. DOM can either enhance (via production of secondary reactive species such as 3DOM) or slow (by competing for photons or reactive species within solution or quenching reactive species) the degradation of contaminants depending on concentration and type.25,28−35 3DOM has been shown to oxidize diverse chemical classes, including sulfonamide antibiotics, phenylurea herbicides, and amino acids.30,36,37 Reactions with organic pollutants fall into three general categories: by energy transfer, hydrogen abstraction, electron transfer, or some combination of these.5,38 Reaction with imazosulfuron by energy transfer is possible, because the average triplet energy of DOM is ∼250 kJ/mol, whereas sulfonylurea triplet energies are ∼215 kJ/ mol.7,39,40 At DOM concentrations representative of California rice fields (5 mg C/L), photodegradation rate coefficients in the two fulvic acid isolates were not significantly different from one another (P > 0.5), but were higher than those in the collected field water (P < 0.05, Figure 2). Differences in reactivity
Figure 3. Observed rate coefficients for imazosulfuron photolysis with various SRFA enrichments, corrected for solution absorbance. Histogram bars represent mean values (n = 5), whereas error bars represent 95% confidence interval. Values identified by different letters are statistically different (P < 0.05).
photodegradation via indirect photolysis are observed. Without correction for the absorbance of the 50 mg C/L SRFA solution, degradation was slower than ultrapure buffered water controls (kobs = 0.039 ± 0.006 h−1, P = 0.0004). These findings indicate that at high DOM concentrations, contributions via indirect photolysis, although significant, do not overcome the DOM “inner filter” effect. In the 5 mg C/L solutions, uncorrected rate coefficients were not significantly different from buffered ultrapure water (P > 0.06), indicating that inhibition of direct photolysis (via competition for photons and quenching) was approximately equal to the enhancement via indirect processes. For each particular DOM, it is likely a threshold concentration exists at which point light screening and quenching within solution are sufficient to slow imazosulfuron photolysis, and below this range enhancement is observed. Rate coefficient enhancement was observed with the tested fulvic acids, providing the first preliminary evidence of reaction between imazosulfuron and 3DOM. The light source used in this study provides high-energy photons (λ 290−350 nm), a low-intensity region of the actinic spectrum where both imazosulfuron and DOM absorb strongly. We assumed that higher energy photons produce more energetic and generally reactive 3DOM species that may go on to react more efficiently with imazosulfuron or other substrates. However, because DOM absorbs a much broader spectrum of light, contributions from 3DOM may be enhanced in sunlight relative to these findings. Additional studies are needed to address the input of DOM under low-energy irradiation. Overall, the data suggest that within a rice field, imazosulfuron photolysis will be efficient regardless of different DOM inputs arising from various management practices, so long as concentrations of DOM are low. Formation of Degradation Products. The photodegradation products of imazosulfuron have been identified previously and include 2-amino-4,6-dimethoxypyrimidine (3) and 2-chloroimidazo[1,2-α]pyridine-3-sulfonamide (2), formed via cleavage of the sulfonylurea bridge at the carbonyl, and 2ureido-2,6-dimethoxypyrimidine (1), formed by homolytic cleavage of the S−N bond (Figure 1).19,21 Concentration
Figure 2. Observed pseudo-first-order rate coefficients for imazosulfuron photodegradation determined from experiments conducted in buffered pure laboratory water alone, with fulvic acid isolates, and in collected rice field water adjusted to 5 mg C/L. Histogram bars represent mean values (n = 5), whereas error bars represent 95% confidence interval. Values identified by different letters are statistically different (P < 0.05).
between fulvic acid isolates and whole water samples have been observed previously.30 The exact mechanism for this dissimilar behavior is yet unknown, but may arise due to compositional differences resulting from the fulvic acid isolation process. Photodegradation in SRFA solution was faster than that in the pure buffered water control; however, rate coefficients in both rice field water and the PLFA solution were indistinguishable from the control. The apparent insensitivity of degradation to C
DOI: 10.1021/acs.jafc.7b00134 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry profiles of imazosulfuron degradation products were very similar in the tested solutions with the exception of the rice field water, which had lower abundances of all products, attributable to reduced sensitivity within the more complex matrix. Compound 2 was rarely observed, but compounds 3 and 1 were produced in approximately the same molar ratio in all tested solutions, ranging from 1.3 to 2.4 1/3 at their peak, suggesting product formation is not linked to specific photolytic pathways. Registrant studies performed in accordance with the California Department of Regulation show these products retain no phytotoxicity and are not harmful to animals and that they are themselves photolabile. Imazosulfuron Reaction with ROS. A simple method for investigation into the role of ROS in chemical photodegradation is to remove O2. In anoxic solution, imazosulfuron photolytic rate coefficients were enhanced by a factor of 10 in ultrapure laboratory water (Figure 4). Molecular oxygen levels
Figure 5. Observed pseudo-first-order rate constants of imazosulfuron determined in experiments using the hydroxyl radical scavenger isopropanol in pure laboratory water or rice field water. Histogram bars represent mean values (n = 5), whereas error bars represent 95% confidence interval. Values identified by different letters are statistically different. NS signifies no significant difference.
0.001) in close agreement with the hydrolytic half-life (4 ± 2 s−1; Figure 6).47 The data show that despite the ionic structure of imazosulfuron, it is not particularly susceptible to oxidation by 1O2.
Figure 4. Observed rate coefficients for imazosulfuron photolysis in ultrapure laboratory water in control and deaerated water samples. Histogram bars represent mean values (n = 5), whereas error bars represent 95% confidence interval. Values identified by different letters are statistically different (P < 0.05).
at the water/soil interface of a flooded rice field hover close to 125−140 μM (∼half-saturation).41,42 Under these conditions, degradation enhancement may occur. Aziz et al.43 also reported enhanced photochemical degradation rates of sulfonylureas in deaerated solution. The hydroxyl radical (•OH) is a nonspecific, powerful oxidant produced in natural waters by photolysis of nitrate and DOM at ranges between 10−14 and 10−15 M.13 To investigate the relevance of this species, experiments were conducted with 25 mM isopropanol, a selective •OH scavenger. In pure laboratory water, a significant decrease in the photolytic rate coefficient was observed in solutions added with the quencher (11%; P = 0.002), although this effect was small compared to results from similar scavenger studies where rate coefficients were reduced by over half (Figure 5).33 However, no difference in rate coefficients was observed between field water solutions spiked and not spiked with the scavenger. This finding suggests that in the presence of DOM, a known sink and source for ROS, •OH-mediated oxidation has little to no effect on overall photolysis (P > 0.6). 1 O2 is a selective electrophilic oxidant, capable of reaction with a wide range of compounds including phenols, amines, dienes, sulfides, and pyrimidine bases.9,44−46 To avoid loss of imazosulfuron via direct photolysis, 1O2 was formed in the dark via reaction with molybdate ions and hydrogen peroxide. Imazosulfuron concentrations decreased by 6% throughout the exposure time period, a significant decrease (3.2 ± 0.4 s−1, P
