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Jun 1, 2016 - Department of Environmental Toxicology, College of Agricultural and Environmental Sciences, University of California, One Shields. Avenu...
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Hydrolytic Activation Kinetics of the Herbicide Benzobicyclon in Simulated Aquatic Systems Katryn L. Williams*,† and Ronald S. Tjeerdema† †

Department of Environmental Toxicology, College of Agricultural and Environmental Sciences, University of California, One Shields Avenue, Davis, California 95616, United States S Supporting Information *

ABSTRACT: Herbicide resistance is a growing concern for weeds in California rice fields. Benzobicyclon (BZB; 3-(2-chloro-4(methylsulfonyl)benzoyl)-2-phenylthiobicyclo[3.2.1]oct-2-en-4-one) has proven successful against resistant rice field weeds in Asia. A pro-herbicide, BZB forms the active agent, benzobicyclon hydrolysate (BH), in water; however, the transformation kinetics are not understood for aquatic systems, particularly flooded California rice fields. A quantitative experiment was performed to assess the primary mechanism and kinetics of BZB hydrolysis to BH. Complete conversion to BH was observed for all treatments. Basic conditions (pH 9) enhanced the reaction, with half-lives ranging from 5 to 28 h. Dissolved organic carbon (DOC) hindered transformation, which is consistent with other base-catalyzed hydrolysis reactions. BH was relatively hydrolytically stable, with 18% maximum loss after 5 days. Results indicate BZB is an efficient pro-herbicide under aqueous conditions such as those of a California rice field, although application may be best suited for fields with recirculating tailwater systems. KEYWORDS: hydrolysis, herbicide, benzobicyclon, rice, pesticide, Show-Ace, Butte

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INTRODUCTION Benzobicyclon (Show-Ace and Butte in the US; Figure 1) acts on weeds with a different mode of action from current rice

BH and various amino-substituted compounds, with a half-life (t1/2) of approximately 13 days and minor soil mineralization.1 However, a lack of reported test conditions, soil characteristics, and experimental design inhibits comparison to other BZB fate investigations. Sekino et al. studied the mobility of the proherbicide in five Japanese soil types ranging from sandy loam to heavy clay, with movement confined to within 2 cm of application depth regardless of soil type.8 Mobility was measured as visual herbicidal activity of 1 cm sections of spiked soil, which did not account for BZB levels under the bioactivity threshold beneath the 2 cm soil layer. Additionally, visual interpretation is prone to error, precluding application of results beyond the study. Rice plants exposed to BZB in a South Korean rice field were extracted and analyzed for the parent and an amino-substituted product by Im et al.7 No significant translocation of either compound was observed in rice plants. To our knowledge, there are currently no published reports describing the aqueous transformation kinetics of BZB to BH and its subsequent fate, particularly under simulated flooded California rice field conditions. California rice culture currently uses flow-through water management to maintain loss from evapotranspiration and soil percolation.9 After an initial flood when seeds are sown in late spring, fields are frequently drained after 40 days and reflooded with constant inflow and discharge. Inflow is provided from either the Sacramento or Feather Rivers, and rice field aqueous temperatures can range from 13.3 °C to as much as 40 °C during the growing season.10,11 Drained field water (tailwater)

Figure 1. Benzobicyclon (BZB) and its hydrolysate (BH).

herbicides,1 and is currently undergoing registration by the California Environmental Protection Agency (CalEPA). Formulated by SDS Biotech K.K., BZB acts as an inhibitor of 4hydroxyphenylpyruvate dioxygenase (4-HPPD) in plants, leading to bleaching and death.1 BZB is a pro-herbicide, which upon reaction with water, forms the active herbicide benzobicyclon hydrolysate (BH; Figure 1).1 Toxicological studies indicate the parent exhibits low toxicity to rats, birds, fish, and algae.1 If approved for use by CalEPA, BZB would be the first triketone and pro-herbicide intended for use on California rice fields. Several California rice herbicides have indications of resistance,2−6 highlighting the need for agents with new modes of action such as BZB. The dissipation of BZB in soil and rice plants has been briefly studied under Korean7 and simulated Japanese rice field conditions.1,8 A proprietary study by Komatsubara et al. analyzed the degradation of the parent in both Japanese rice and soil and proposed rapid transformation in both media to © 2016 American Chemical Society

Received: Revised: Accepted: Published: 4838

February 3, 2016 May 9, 2016 June 1, 2016 June 1, 2016 DOI: 10.1021/acs.jafc.6b00603 J. Agric. Food Chem. 2016, 64, 4838−4844

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Journal of Agricultural and Food Chemistry

mM boric acid and 0.5 mM KCl. All were titrated to within ±0.03 of their intended value using 5 N HCl and 6 N NaOH and stored at room temperature less than 24 h prior to the experiment; field water remained at ambient pH. Preparation of Standard and Stock Solutions. A 20 mg L−1 (44.7 μM) stock solution of BZB in acetonitrile was prepared by mixing 2 mg of BZB with 100 mL of acetonitrile. Stock solution was freshly prepared for each experiment and stored at −20 °C. The BH stock was prepared in a similar manner. The calibration stock solution was prepared by dissolving 2 mg each of BZB and BH in 100 mL of acetonitrile. The stock was diluted to 1 mg L−1 (2.23 μM BZB, 2.82 μM BH) with acetonitrile, which was used as the working stock for all other standards. The calibration stock solution was stored at −20 °C and used for all analyses of buffer samples. A matrix-matched acetonitrile solution was prepared by extracting blank rice field water with acetonitrile via SPE (procedure detailed below) and used to make a separate calibration stock solution for the analysis of field water samples. Hydrolysis of Benzobicyclon. Experiments were designed as per OECD 111 and repeated for 15, 25, and 35 °C.26 Vials (Supelco Amber screw-top vials, 15 mL, Sigma-Aldrich), caps (Supelco 18 mm PTFE-lined screw caps, Sigma-Aldrich), and other necessary glassware were sterilized by autoclaving (30 min, 15 psi), having been previously washed and solvent-rinsed with acetonitrile. Aqueous samples were sterilized via filtration to 0.2 μm prior to the start of the experiment and were divided into treatment and blank solutions. Treatment solutions were spiked with 1 mL of freshly prepared 20 mg L−1 (44.7 μM) BZB stock solution to reach a final concentration of 0.025 mg L−1 (5.59 × 10−2 μM), approximately half of the aqueous solubility of BZB (0.052 mg L−1, 11.6 × 10−2 μM).28 Aqueous blanks were spiked with 0.25 mL of acetonitrile, equivalent to the aqueous treatment aliquot. Solutions were stirred for approximately 15 min to equally distribute BZB, after which time 15 mL aliquots were pipetted into individual vials and capped. Vials were immediately transferred into a Precision Scientific Precision reciprocal shaking bath set at the desired temperature (within 0.1 °C) and agitated gently to promote mixing. Five replicates and one blank were utilized for all treatments and time points. Samples were periodically removed from the water bath at eight time points and immediately extracted. BH Hydrolysis. A second experiment assessed the potential of BH to hydrolyze.26 The experimental design and execution generally followed those described above for benzobicyclon. Sterilized solutions were spiked with 0.375 mL of freshly prepared 20 mg L−1 (56.3 μM) BH in acetonitrile to reach a final concentration of 0.025 mg L−1 (7.05 × 10−2 μM). Samples were pipetted in 15 mL increments into sterilized vials and capped, which were then transferred to water bath and heated to 50 ± 0.1 °C and shaken gently to promote mixing. Samples (five replicates and one blank) were removed from the bath at 0 and 120 h and immediately extracted. SPE Extraction. SPE was performed in order to exchange BZB and BH from aqueous solution to acetonitrile using Supelco Visiprep manifolds (Sigma-Aldrich) for enhanced analysis. Briefly, SPE cartridges (HF Mega Bond Elut C18, 1 g in 6 mL, Agilent) were conditioned with methanol and washed with HPLC water. Aqueous samples (15 mL) were acidified to pH < 2 using 2.5 N HCl and then extracted at a rate of approximately 3 drops s−1. Cartridges were rinsed with water and allowed to dry for 30 min. Acetonitrile (3 mL) was added to each cartridge and allowed to elute slowly, and extracts were collected and stored at −20 °C prior to analysis. Recoveries of BZB and BH were within 100−129% and 83−159%, respectively (Supporting Information, Table S1). The pH of all buffered samples at the final time point were verified within ± 0.1 of the original solution value after removal from the shaking bath, except for pH 9 buffer samples at 25 °C, which averaged a final pH of 8.85 ± 0.1. Field water samples produced a larger pH fluctuation, with a change of as much as ± 0.3 from the original value observed. Analytical Parameters. Extracted samples were analyzed via an Agilent 1200 series HPLC (Santa Clara, CA) coupled to an Agilent 6420 triple quad mass spectrometer (MS) with an electrospray ionization (ESI) source in positive mode. The mobile phase was

eventually rejoins the Sacramento River, which is the major source of drinking water for Sacramento, CA. Dissolved species such as DOC, clay particles, and metals can have an effect on the dissipation of pesticides in aquatic systems via sorption and consequential enhancement or retardation of degradation and toxicity.12−15 DOC varies seasonally in California rice fields, with average concentrations of 6.8 and 14.9 mg L−1 during the growing and winter seasons, respectively, although concentrations as high as 77.7 mg L−1 have been reported in tailwater.16 Rice field tailwater significantly influences Sacramento River DOC content,9 which ranges between 1.48 and 1.92 mg L−1.17 Montmorillonite, kaolinite, and vermiculite, clay species typically found in California rice field soils,18 have been shown to have an effect on hydrolysis of various organic compounds.19,20 Rice field water contains several dissolved metal species capable of complexation with organic ligands, including copper, zinc, and iron.21,22 It is currently unknown whether rice field constituents contribute to BZB hydrolysis and to what extent. Additionally, field water loss through percolation and seepage is significant enough (approximately 14.5% of water input) to warrant assessment of groundwater leaching potential.23 According to the manufacturer’s proposed label, BZB is intended for postflood use on rice fields when flow-through water management is practiced. It is therefore crucial to understand the kinetics of BZB transformation to BH under California rice field conditions to understand the influx of the active ingredient into the field over time and prevent the release of contaminated tailwater into the Sacramento River. It is also unknown if the reaction is either acid- or base-catalyzed within the pH range of rice field water (7−10).24,25 In this investigation, we determined the rate of hydrolysis of BZB to BH under realistic conditions for time of application in a California rice field. Specifically, we determined hydrolysis rates as a function of temperature (15, 25, and 35 °C), pH (4, 7, and 9), and water type (HPLC and field). Relative hydrolytic stability of BH under the same aqueous treatments was also determined.26 Results from this investigation were used to evaluate the fate of both compounds in a California rice field and surrounding watersheds in addition to groundwater leaching potential.

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MATERIALS AND METHODS

Reagents. HPLC-grade water, acetonitrile, and methanol were purchased from Sigma-Aldrich (St. Louis, MO), while formic acid (LC-MS grade), HCl (ACS grade), potassium biphthalate, NaOH, KCl, and boric acid were purchased from Fisher Scientific (Hampton, NH). Benzobicyclon (98% purity) was obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and BH (97% purity) was custom-synthesized by GTM China Co., Ltd. (Changzhou, China). Field Water Collection and Characterization. Water was collected from a rice field in the Yolo Causeway Bypass Wildlife Area in December 2014, stored in 4 L amber glass containers at 4 °C with minimal headspace and used within 30 days of collection. Aliquots of 1 L were filtered to 0.2 μm (Nylon membrane filters, Whatman GE) and stored at 2 °C for less than 24 h before use. The pH (8.34, 6.72) and DOC content (33.4 mg L−1, 1.0 mg L−1) of field and HPLC water, respectively, used in this study were characterized by the UC Davis Analytical Laboratory.27 Preparation of Buffer Solutions. Three buffers were prepared in HPLC water, one for each desired pH (4, 7, and 9) as per the Organisation for Economic Co-operation and Development (OECD) Test Number 111:26 pH 4, 0.04 mM NaOH solution in 0.5 mM potassium biphthalate, pH 7, 2.96 mM NaOH solution in 0.5 mM potassium phosphate, and pH 9, 2.13 mM NaOH in solution with 0.5 4839

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Journal of Agricultural and Food Chemistry comprised of 0.25% formic acid (A) and 0.25% formic acid in acetonitrile (B), which were eluted through a Titan phenyl column (50 mm × 2.1 mm, 5 μm) with attached ThermoScientific Hypersil guard column (10 mm × 3 mm, 5 μm) at a flow rate of 0.4 mL min−1 with a 15 μL injection volume. Initially, the mobile phase was 80% A for one min. Over 7 min, %A was decreased to 5%, held for two min, and ramped back to 80% A for the rest of the 16 min run. BZB and BH were quantified using multiple reaction monitoring (MRM) mode with the optimized transitions 447 m/z → 256.9 m/z and 355 m/z → 165 m/z, respectively. Additional transitions were used for identification of BZB (447 m/z → 229 m/z, 447 m/z → 349 m/z) and BH (355 m/z → 81 m/z). Second-order calibration curves (R2 ≥ 0.9959) were constructed using mixed standards of BZB and BH in acetonitrile ranging in concentration from 0.005 mg L−1 to 1 mg L−1. Field water samples were analyzed separately using a calibration curve constructed in matrix-matched acetonitrile. Instrumental limits of detection (LOD) and quantitation (LOQ) were determined by multiplying the standard deviation of eight replicate injections of the lowest concentration standard that met the criteria of ≤10% residual error by 3 (LOD) and 10 (LOQ). The instrumental LOD was found to be 0.001 mg L−1 and 0.003 mg L−1 for BZB and BH, respectively, while the instrumental LOQ was found to be 0.004 mg L−1 and 0.009 mg L−1 for BZB and BH, respectively. Kinetic Analysis. Hydrolysis rate constants (kH) for each treatment were determined via the following equation:

Ct = C0 e−kHt

loss of BZB were examined using a weighted-least-squares three-way analysis of variance (ANOVA). Post hoc pairwise analyses were constructed using Tukey honest significant difference (HSD). All statistical analyses were done using JMP Pro 11 statistical software (Cary, NC), and the significance level was set to P ≤ 0.005 for the purpose of this experiment. Further details can be found in the Supporting Information.

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RESULTS AND DISCUSSION Effect of Aqueous Treatment on Hydrolysis. As BH was not present at the initial time point and BZB was not measurable at the final time point for each treatment, conversion efficiency was measured as the ratio of moles of BH in solution at the final time point to the initial moles of BZB in solution (eq 5). CE ranged between 0.76−1.29 for all treatments. An example time course for the transformation can be found in Figure 2. The same relationship was observed for

(1) −1

where C0 is the initial concentration of BZB (mg L ), Ct is the concentration of BZB (mg L−1) after time t (h), and kH is the pseudo first-order hydrolysis rate constant. These rate constants can then be used to find the resulting half-life:

t1/2 =

ln(2) 0.693 ≅ kH kH

(2)

where t1/2 is the half-life in hours. The hydrolysis rate constant, kH, can also be expressed in terms of a contribution of acid-catalyzed, neutral, and base-catalyzed hydrolysis rate constants (ka, kn, and kb, respectively) as seen below:29

Figure 2. Decay of BZB to form BH over time (pH 9 buffer, 35 °C). Error bars represent the standard error of the mean (n = 5).

⎡ K ⎤ kH = ka[H +] + k n + k b[OH −] = ka[H +] + k n + k b⎢ w+ ⎥ ⎣ [H ] ⎦

all treatments, suggesting BH is the main product of the reaction beside the phenylthiolate leaving group. A nucleophilic conjugate addition mechanism is proposed for BZB hydrolysis (Figure 3) and is supported by the literature.32,33 Under basic conditions, a hydroxyl ion is hypothesized to attack BZB at C4, inducing transient movement of the displaced electrons to either adjacent ketone (C1 or C3), loss of phenylthiolate, and consequent enol formation (BH). Hydrolysis under neutral and acidic conditions is expected to proceed similarly with water as the nucleophile instead of OH−. Basic conditions enhanced degradation of BZB compared to acidic and neutral media as evidenced by hydrolytic rates and half-lives (Table 1). A weighted least-squares three-way ANOVA comparison of aqueous treatments indicated statistical similarity between pH 7 and field water treatments, while both pH 4 and pH 9 buffer treatments were significantly different from all others (P < 0.0001). Transformation under basic conditions was significantly faster than in other media (P < 0.0001), indicating the reaction is primarily base-catalyzed. Furthermore, BZB hydrolytic rates under acidic conditions were significantly slower than all other treatments (P < 0.0001), indicating contribution of acid catalysis is negligible. Acidcatalyzed rate constants (ka) were therefore treated as zero at each temperature and eq 3 was simplified for the reaction as

(3) where Kw is the equilibrium constant for the dissociation of pure water.30 These additional rate constants were estimated using nonlinear regression in JMP statistical software (JMP Pro 11, Cary, NC). The activation energies for each treatment were calculated as described by Sharma et al.31 The Arrhenius equation was applied to the results in the form of the equation:

⎡⎛ − E ⎞⎛ 1 ⎞⎤ ln kH = ⎢⎜ A ⎟⎜ ⎟⎥ + ln A ⎣⎝ R ⎠⎝ T ⎠⎦

(4) −1

where EA is the activation energy (kJ mol ), R is the universal gas constant (0.008314 kJ mol−1 K−1), and A is the pre-exponential factor with the same units as kH (h−1). Plotting ln kH vs 1/T gave a straight line with a slope equal to −EA/R with an intercept of ln A. Conversion Efficiency. To determine whether BH was the sole product of BZB hydrolysis, conversion efficiency (CE) was measured as

CE =

mol BZBf + mol BH f mol BZBi + mol BHi

=

mol BH f mol BZBi

(5)

where molBZBf, molBZBi, molBHf, and molBHi, are mols of BZB at the final and initial time point, and mols of BH at the final and initial time point, respectively. Statistical Analysis. The effects of the three variables (aqueous treatment, temperature, and time) and their interactions on hydrolytic

⎡ K ⎤ kH ≅ k n + k b[OH−] = k n + k b⎢ w+ ⎥ ⎣ [H ] ⎦ 4840

(6)

DOI: 10.1021/acs.jafc.6b00603 J. Agric. Food Chem. 2016, 64, 4838−4844

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Journal of Agricultural and Food Chemistry

Figure 3. Proposed transformation of benzobicyclon to BH via a base-catalyzed 1,4 nucleophilic addition mechanism.

Additional confirmation of ka insignificance to BZB transformation can be found in Table 1. Hydrolytic rates would be expected to decrease from pH 4 to 7 if acid catalysis contributed significantly to the reaction, which was not observed. Instead a minor significant increase in kH occurred from pH 4−7, suggesting a lack of ka influence. Minimal contribution of acid-catalyzed hydrolysis has been observed previously for 2,4-dinitriophenylacetate and phenyl dichloroacetate.34 Buffers (pH 4 and pH 7) were shown to not have an effect on hydrolysis rate in preliminary studies. BZB hydrolysis in pH 9 buffer differed from HPLC water at pH 9 (data not shown), however, this is predicted to be due to improved buffer capacity rather than buffer catalysis. Perdue and Wolfe suggest borate buffer concentrations less than 0.005 M do not have a significant effect on hydrolysis rate.35 All constituents in the pH 9 buffer used in this study were 0.5 mM or below, therefore contribution of buffer catalysis to BZB hydrolysis is expected to be insignificant. DOC is known to affect both acid-catalyzed and basecatalyzed pesticide hydrolysis, with a negative influence reported for several base-catalyzed processes.13,36−39 Hydrophobic compounds such as BZB (log KOW 3.140) have been shown to bind with DOC in aqueous solutions.13,36,38,39 Above pH 4, DOC carries a negative charge due to deprotonated carboxylic acid functional groups, which repel hydroxyl ions and subsequently preclude hydrolysis.13,36,41 Field water pH (8.34) was closest to the pH 9 treatment, thus field water hydrolytic rates were expected to mimic those for pH 9. Instead, field water and pH 7 hydrolysis rates were statistically similar while both deviated significantly from pH 9 rates. Therefore, rice field DOC is proposed to have an inhibitory effect on formation of BH. As California rice field DOC dominates its content in surrounding watersheds,9 BZB hydrolysis may also be hindered to a similar extent if released through tailwater, seepage or percolation. Dissolved clay species are not expected to significantly influence hydrolysis of BZB. Research on base-catalyzed hydrolysis of carbamate pesticides in montmorillonite and vermiculite suspensions indicate neither had an effect.20 Additionally, saturation of montmorillonite and kaolinite with water significantly reduces their ability to influence hydrolysis.19 Aqueous sediment solutions have been shown to reduce organophosphate and endosulfan base-catalyzed hydrolysis rates due to preferential partitioning of the analyte into sediment DOC over clay and metal surfaces.38,42 El-Amamy and Mill also hypothesize that DOC species can bind to clay active sites, which reduces clay-influenced hydrolysis via competitive sorption of the reactant to DOC.19 Inhibition of hydrolysis due to dissolved metals is possible, but unlikely. Metals mainly influence hydrolysis via chelation to

the compound of interest and/or nucleophile, allowing the reactants to be in close proximity for increased reaction or stability of the leaving group or both.43 Both the carbonyl oxygen and thiophenyl sulfur are relatively poor ligand donors, and the surrounding functional groups are bulky, indicating complexation is not favorable. Sorption to DOC is far more likely. In addition, catalysis of hydrolysis of organic compounds in basic conditions has been observed for some compounds rather than inhibition.15,44 For these reasons combined, metal species are not expected to significantly affect the hydrolytic rate of BZB in field water. Further studies are needed to confirm the lack of effect of clays and dissolved metal species on BZB hydrolysis. Effect of Temperature on Hydrolysis. The hydrolytic rate constants for each temperature and pH (Table 1) indicate that temperature had a significant effect on the rate of BZB transformation. Indeed, half-lives at 35 °C ranged from approximately 5−8 h while at 15 °C half-lives were significantly slower, ranging 28−38 h. Three-way ANOVA confirms this, with pairwise analysis via Tukey HSD, resulting in significant differences between all temperatures (α = 0.05, P < 0.0001). The lowest hydrolytic rate (15 °C, pH 4) corresponded to complete (>98%) hydrolytic degradation of BZB after only 10 days. Under conditions observed during the California ricegrowing season, neutral to basic pH (7−9) and higher temperatures (25−35 °C), BZB is expected to completely hydrolyze to the active herbicide in even less time, indicating that the agent is an effective pro-herbicide under aquatic conditions such as a flooded California rice field. Neutral and base-catalyzed hydrolytic rate constants (Table 2) correlated positively with temperature, as a 4-fold and 2-fold increase in kn and kb, respectively, was observed from 15 to 35 °C. Values of kb were approximately 5 orders of magnitude larger than kn regardless of temperature. Predicted kH (Table 2 and eq 6) matched Table 1 values within 4%. Differences in experimental vs predicted kH could be due to fit of the nonlinear regression curve to the experimental kH. Activation Energies. Activation energies and pre-exponential factors for each treatment can be found in Table 3. The highest EA corresponded to the pH 9 buffer treatment, which produced the highest hydrolytic rates (Table 1). EA are not necessarily indicative of rates of reaction, only sensitivity to changes in temperature. In fact, several studies have observed seeming disparities between energies of activation and hydrolysis reaction rates.31,45−47 One explanation for this phenomenon lies with the pre-exponential factor, which is a function of collision frequency and orientation probability.48 The largest A value belonged to the pH 9 treatment, suggesting a higher collision rate and/or probability of correct orientation between reactants at alkaline conditions compared to neutral and acidic media. The hydroxyl ion is a superior nucleophile 4841

DOI: 10.1021/acs.jafc.6b00603 J. Agric. Food Chem. 2016, 64, 4838−4844

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BH percent loss (± SE)

5.46 (± 1.78) 17.5 (± 3.1) 16.9 (± 4.1) 5.59 (± 3.84) 0.990 0.993 0.991 0.995 8.25 (± 0.14) 7.93 (± 0.12) 5.14 (± 0.08) 7.05 (± 0.08) 0.0840 (± 0.0015) 0.0874 (± 0.0013) 0.135 (± 0.0023) 0.0983 (± 0.0012)

kn × 10−2 (h−1) (± SE × 10−2)

kb × 103 (M−1 h−1) (± SE × 103)

15 25 35

1.89 (± 0.04) 4.31 (± 0.17) 8.55 (± 0.15)

1.30 (± 0.17) 1.78 (± 0.29) 2.42 (± 0.13)

aqueous treatment pH 4 pH 7 pH 9 field

EA (kJ mol−1) (± SE) 57.2 55.6 62.7 58.2

(± (± (± (±

0.8) 2.6) 0.9) 2.3)

ln A (± SE) 19.9 19.3 22.5 20.4

(± (± (± (±

0.3) 1.06) 0.4) 0.9)

R2 0.9997 0.9978 0.9998 0.9984

compared to water as the former is smaller and carries a negative charge, meaning it has a higher likelihood of successful attack. As shown in eq 4, both EA and A influence reaction rate. Another explanation for the observed results concerns temperature. Above the isokinetic temperature (TI), the temperature at which reactions with different EA exhibit the same rate, large EA reactions proceed faster than those with small EA.49 All studied temperatures were above the highest calculated isokinetic temperature (−9 °C); therefore, the hydrolysis reaction at pH 9 had enough energy to surmount the large EA barrier and proceeded faster than the other studied pH. Additionally, the base-catalyzed reaction appears to be more sensitive to temperature changes than all others, as reactions with high EA are influenced by temperature to a greater degree than those with low EA. From Table 1, hydrolytic rates increased by a factor of 4.5 from 15 to 35 °C for pH 4 and pH 7, while pH 9 rates increased by a factor of 5.5 for the same temperature range (0.0247−0.135 h−1, respectively). Hydrolytic Potential of BH. The hydrolytic potential of BH was also measured at pH 4, 7, and 9 and in field water. Results are presented as percent concentration loss after incubation at 50 °C for 120 h in Table 1. A loss of less than 10%, or t1/2 (25 °C) > 1 year, indicates hydrolytic stability.26 With losses of less than 6%, BH was stable under both acidic and field water conditions but relatively less stable in neutral and basic media, with losses of 17.5% and 16.9%, respectively. These results are compatible with those for related triketone herbicides such as sulcotrione and mesotrione, which are weak acids (pKa ∼ 3) and show resistance to hydrolysis in their anionic state.50 The dissipative behavior of BH in flooded fields is still unknown, although this investigation indicates hydrolysis is not expected to be a significant contributor to the fate of BH in aquatic systems. Groundwater Leaching Potential. Hydrolysis t1/2 is a predictive factor used by the California Department of Food and Agriculture (CDFA) for groundwater leaching assessment.51 CDFA guidelines indicate a pesticide could leach if its aqueous solubility is greater than 3 mg L−1 and its hydrolysis t1/2 is greater than 14 days. The aqueous solubility of BH is not currently available, so it was estimated by the Estimation Program Interface (EPI) Suite by the US EPA to be 7.26 mg L−1.52 The predicted aqueous solubility combined with observed hydrolytic recalcitrance indicate BH has leaching

16.7 (± 0.5) 15.4 (± 0.2) 11.4 (± 0.1) 16.0 (± 0.3) 0.0414 (± 0.0012) 0.0449 (± 0.0007) 0.0606 (± 0.0006) 0.0434 (± 0.0008) 0.994 0.995 0.983 0.998 0.0185 (± 0.0002) 0.0194 (± 0.0002) 0.0247 (± 0.0006) 0.0203 (± 0.0002) pH 4 pH 7 pH 9 Field

temperature (°C)

Table 3. Activation Energies and Pre-Exponential Factors (A) for BZB by Aqueous Treatment; R2 Values Correspond to the Equation of the Line Denoted by eq 4

37.6 (± 0.5) 35.7 (± 0.4) 28.1 (± 0.6) 34.2 (± 0.3)

R t1/2 (h) (± SE) kH (h ) (± SE) R t1/2 (h) (± SE) kH (h ) (± SE) treatment

15 °C

Table 2. Neutral (kn) and Base-Catalyzed (kb) Hydrolytic Rate Constants of BZB as a Function of Temperature; SE Denotes Standard Error of the Mean (n = 5)

0.973 0.991 0.997 0.988

R2 t1/2 (h) (± SE) kH (h ) (± SE)

35 °C

−1 2

25 °C

−1 2

−1

Table 1. Hydrolysis Rate Constants (kH) and Half-Lives of BZB under Various Treatments and Loss of BH in Aqueous Solution after 120 h (Expressed as Percent Loss); SE Denotes Standard Error of the Mean (n = 5)

Journal of Agricultural and Food Chemistry

4842

DOI: 10.1021/acs.jafc.6b00603 J. Agric. Food Chem. 2016, 64, 4838−4844

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Journal of Agricultural and Food Chemistry potential, while the low solubility (0.052 mg L−1) and hydrolytic t1/2 of the parent suggest BZB does not.28 BZB is an efficient pro-herbicide under California rice field conditions. We predict complete degradation of BZB due to hydrolysis within 10 days of application and thus complete conversion to BH within the same time frame. BH was found to be comparatively recalcitrant to hydrolysis, indicating hydrolysis is not a major dissipation pathway and leaching into groundwater is possible. Application of BZB to fields maintained without tailwater recirculation is cautioned until a better understanding of the fate and toxicity of BH is known, although based on evidence from similar compounds, BH is hypothesized to be relatively nontoxic.1,53 Future studies will transfer focus to the dissipation of the active herbicide, BH, under aqueous conditions typical of California rice fields.

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Echinochloa phyllopogon (Stapf) Koss. from California. J. Plant Dis. Prot. 2006, 20, 95−100. (6) Yasuor, H.; TenBrook, P. L.; Tjeerdema, R. S.; Fischer, A. J. Responses to clomazone and 5-ketoclomazone by Echinochloa phyllopogon resistant to multiple herbicides in Californian rice fields. Pest Manage. Sci. 2008, 64, 1031−1039. (7) Im, S. J.; Abd El-Aty, A. M.; Lee, Y.-J.; Rahman, M. M.; Kim, S.W.; Choi, J.-H.; Shim, J.-H. Analysis of benzobicyclon and its metabolite in brown rice and rice straw after field application using liquid chromatography-tandem mass spectrometry. Food Chem. 2015, 168, 404−409. (8) Sekino, K.; Koyanagi, H.; Ikuta, E.; Yamada, Y. Herbicidal activity of a new paddy bleaching herbicide, benzobicyclon. J. Pestic. Sci. 2008, 33, 364−370. (9) Krupa, M.; Spencer, R. M.; Tate, K.; Six, J.; van Kessel, C.; Linquist, B. Controls on dissolved organic carbon composition and export from rice-dominated systems. Biogeochemistry 2012, 108, 447− 466. (10) Strand, L. L., Espino, L. A., Fischer, A. J., Godfrey, L. D., Greer, C. A., Hill, J. E., Marsh, R. E., Mutters, R. G., Eds.; Integrated Pest Management for Rice, 3rd ed.; University of California Agriculture and Natural Resources Publication 3280; University of California Statewide Integrated Pest Management Program: Oakland, CA, 2013. (11) Climate Data for Colusa County, CA; National Oceanic and Atmospheric Administration National Centers for Environmental Information: Asheville, NC, http://www.ncdc.noaa.gov/cdo-web/ datasets/GHCNDMS/stations/GHCND:USC00041948/detail (accessed July 28, 2015). (12) Bejarano, A. C.; Chandler, G. T.; Decho, A. W. Influence of natural dissolved organic matter (DOM) on acute and chronic toxicity of the pesticides chlorothalonil, chlorpyrifos and fipronil on the meiobenthic estuarine copepod Amphiascus tenuiremis. J. Exp. Mar. Biol. Ecol. 2005, 321, 43−57. (13) Macalady, D. L.; Tratnyek, P. G.; Wolfe, N. L. Influences of natural organic matter on the abiotic hydrolysis of organic contaminants in aqueousAquatic Humic Substances; Advances in Chemistry; American Chemical Society: Washington, DC, 1989; Vol. 219, pp 323−33210.1021/ba-1988-0219.ch021. (14) Hoffmann, M. R. Trace metal catalysis in aquatic environments. Environ. Sci. Technol. 1980, 14, 1061−1066. (15) Smolen, J. M.; Stone, A. T. Divalent Metal Ion-Catalyzed Hydrolysis of Phosphorothionate Ester Pesticides and Their Corresponding Oxonates. Environ. Sci. Technol. 1997, 31, 1664−1673. (16) Ruark, M. D. L.; Lindquist, B. A.; Six, J.; van Kessel, C.; Greer, C. A.; Mutters, R. G.; Hill, J. E. Seasonal losses of dissolved organic carbon and total dissolved solids from rice production systems in northern California. J. Environ. Qual. 2009, 39, 304−313. (17) Chow, A. T.; Dahlgren, R. A.; Harrison, J. A. Watershed Sources of Disinfection Byproduct Precursors in the Sacramento and San Joaquin Rivers, California. Environ. Sci. Technol. 2007, 41, 7645−7652. (18) Brandon, D. M.; Mikkelsen, D. S. Phosphorus Transformations in Alternately Flooded California Soils: I. Cause of Plant Phosphorus Deficiency in Rice Rotation Crops and Correctional Methods. Soil Sci. Soc. Am. J. 1979, 43, 989−994. (19) El-Amamy, M. M.; Mill, T. Hydrolysis kinetics of organic chemicals on montmorillonite and kaolinite surfaces as related to moisture content. Clays Clay Miner. 1984, 32, 67−73. (20) Wei, J.; Furrer, G.; Kaufmann, S.; Schulin, R. Influence of Clay Minerals on the Hydrolysis of Carbamate Pesticides. Environ. Sci. Technol. 2001, 35, 2226−2232. (21) Witter, A. E.; Mabury, S. A.; Jones, A. D. Copper (II) complexation in northern California rice field waters: an investigation using differential pulse anodic and cathodic stripping voltammetry. Sci. Total Environ. 1998, 212, 21−37. (22) Williams, J. F., Mutters, R. G., Greer, C. A., Horwath, W. R., Eds.; Rice Nutrient Management in California; University of California Agriculture and Natural Resources Publication 3516; The Regents of the University of California Agriculture and Natural Resources: Richmond, CA, 2010.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.6b00603. Extraction recoveries and in-depth description of the method of statistical analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 530-752-2534. E-mail: [email protected]. Funding

This research was supported by the California Rice Research Board (award number RP-5), the William G. Golden Jr. and Kathleen H. Golden International Agriculture Fellowship, and the Donald G. Crosby Endowment in Environmental Chemistry. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Hengel, C. Rering, D. Carr, and J. Seiber for their valuable assistance, and N. Willits for statistical guidance. ABBREVIATIONS USED BZB, benzobicyclon; BH, benzobicyclon hydrolysate; DOC, dissolved organic carbon; 4-HPPD, 4-hydroxyphenylpyruvate dioxygenase; kH, hydrolysis rate constant; ka, acid-catalyzed hydrolysis rate constant; kn, neutral hydrolysis rate constant; kb, base-catalyzed hydrolysis rate constant; t1/2, half-life; A, preexponential factor; EA, activation energy

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REFERENCES

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