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Agricultural and Environmental Chemistry

Aqueous photolysis of benzobicyclon hydrolysate Katryn L. Williams, Richie Kaur, Alexander S McFall, Jacob Kalbfleisch, Joshua Gladfelder, David B. Ball, Cort Anastasio, and Ronald S. Tjeerdema J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b01012 • Publication Date (Web): 12 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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Journal of Agricultural and Food Chemistry

Aqueous Photolysis of Benzobicyclon Hydrolysate

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Katryn L. Williams†#*, Richie Kaur‡, Alexander S. McFall‡, Jacob Kalbfleisch§, Joshua J.

4

Gladfelder§, David B. Ball§, Cort Anastasio‡, and Ronald S. Tjeerdema#

5 6



7

182 Steele Ave Extension, Gloversville, NY 12078

Now at: Hale Creek Field Station, New York State Department of Environmental Conservation,

8 9 10



Department of Land, Air, and Water Resources, One Shields Avenue, University of California,

Davis, CA 95616

11 12

§

13

State University, Chico, CA 95929

Department of Chemistry and Biochemistry, Physical Science Building, Room 216, California

14 15

#

16

Davis, CA 95616

Department of Environmental Toxicology, One Shields Avenue, University of California,

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*

Corresponding author (Tel: 518-773-7138 ext 3002; Email: [email protected])

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Abstract

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Benzobicyclon (3-(2-chloro-4-(methylsulfonyl)benzoyl)-2-phenylthiobicyclo[3.2.1]oct-2-en-4-

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one) is a pro-herbicide used against resistant weeds in California rice fields. Persistence of its

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active product, benzobicyclon hydrolysate, is of concern. An acidic herbicide, the neutral species

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photolyzed faster than the more predominant anionic species (t1/2 = 1 h and 320 h, respectively;

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natural sunlight), from a >10-fold difference in quantum yield. Dissolved organic matter in

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natural waters reduced direct photolysis and increased indirect photolysis compared to high-

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purity water. Light attenuation appears significant in rice field water and can slow photolysis.

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These results, used in the Pesticides in Flooded Applications Model (PFAM) with other

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experimental properties, indicate a floodwater hold time of 20 days could be sufficient for

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dissipation of the majority of initial aqueous benzobicyclon hydrolysate prior to release.

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However, soil recalcitrance of both compounds will keep aqueous benzobicyclon hydrolysate

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levels constant months after benzobicyclon application.

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Key Words

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Photolysis, herbicide; benzobicyclon; benzobicyclon hydrolysate; rice; PFAM

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Journal of Agricultural and Food Chemistry

Introduction

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Recently approved for use on California rice fields by the Environmental Protection

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Agency,1 the herbicide benzobicyclon (Butte) kills previously resistant weeds via the active

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herbicide, benzobicyclon hydrolysate, compound 1 (Figure 1). Benzobicyclon hydrolysate works

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through a new mode of action, inhibition of 4-hydroxyphenylpyruvate dioxygenase.2 The

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Environmental Protection Agency has advised that Butte, a granular formulation containing 0.27

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lbs/acre benzobicyclon, can be applied on up to 10% of California rice fields in the upcoming

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growing season, though the fate of compound 1 in a flooded rice field is still mostly informed

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from estimated parameters.1 Formation of compound 1 from benzobicyclon is rapid in water,

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half-life of 16 h at 25 oC,3 which impedes investigation of aqueous photolysis of the parent.1

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Previous studies have experimentally characterized hydrolysis of compound 1 and its

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behavior in soil, and suggest a significant portion will remain in the aquatic fraction of a field as

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an anion.3, 4 The Environmental Protection Agency registration documentation for benzobicyclon

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reports measured compound 1 photolysis half-lives of 12.9 and 7.8 d at 40 degrees latitude in pH

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5 and in field water, respectively.1 However, the dates of these experiments were not specified,

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thus the corresponding photon fluxes are unknown; these are important as they influence the rate

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of photolysis. As benzobicyclon will be applied during the early part of the growing season,

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approximately early May, photolysis of the active 1 should be assessed for both the spring

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equinox and summer solstice to better understand the range of compound 1 photolysis rates. In

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addition, compound 1 possesses an acidic hydrogen (pKa = 2.89).4 California rice field water pH

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ranges from 7 to 9.5,5, 6 favoring the anion. However rice field soil can reach as low as pH 4.5,4

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potentially allowing enough of the neutral compound to form at the soil-water interface to

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warrant investigation of both species.

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Based on the registration information and estimated physicochemical properties, the

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Environmental Protection Agency has recommended floodwater treated with Butte to be held in

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the field for 20 d prior to release into the Colusa Basin Drain and eventually the Sacramento

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River.1 Water holding periods are commonly used with pesticides applied to flooded rice fields,

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based on persistence data from the Environmental Protection Agency, and are designed to

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minimize toxicity to aquatic organisms downstream.7, 8 Neither benzobicyclon hydrolysate or its

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parent appear to show acute toxicity to aquatic fish or invertebrates, but chronic exposure was

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found to lead to reduced growth and reproduction rates.1 Effects of both compounds on non-

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target aquatic plants and algae remain unclear.1 Currently, the Butte label suggests a floodwater

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hold time between 0 – 5 d, which could preemptively release compound 1 into the Sacramento

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River before it is able to dissipate in the field. Thus, photolysis should be understood in both a

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flooded rice field and the Sacramento River.

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In this investigation, the photolysis of both neutral and anionic benzobicyclon

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hydrolysate was assessed in high purity water as well as in filtered rice field water and

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Sacramento River water, using both simulated and natural sunlight. Additionally, the molar

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absorptivities, quantum yields, and photolysis products of both species were characterized, which

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have not been previously described. Finally, we used our photolysis results, along with other

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experimental data, in the Pesticide in Flooded Applications Model (PFAM) to estimate a field

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water holding time for benzobicyclon hydrolysate to better inform safe use practices.

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Materials and Methods

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Chemicals. Water for TOC-free analysis (high-purity water), 4-nitroanisole, 4'-

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nitroacetophenone, 2-nitrobenzaldehyde, and sulcotrione, Pestanal grade (internal standard, IS)

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were purchased from Sigma Aldrich (St. Louis, MO). Optima LC-MS acetonitrile, Optima LC-

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MS water, Optima isopropanol, formic acid (LC-MS grade), 2-chloro-4-(methylsulfonyl)benzoic

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acid, compound 3 in Figure 1 (95% purity), hydrochloric acid (ACS grade), sodium hydroxide,

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and boric acid were obtained through Fisher Scientific (Hampton, NH). Compound 1 (3-[2-

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chloro-4-(methylsulfonyl)benzoyl]bicyclo[3.2.1]octane-2,4-dione) (97% purity) and

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bicyclo[3.2.1]octane-2,4-dione, compound 5 in Figure 1 (96% purity) were synthesized by GTM

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China Co., LTD. (Changzhou, China). Cyclopentane-1,3-dicarboxylic acid, compound 4 in

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Figure 1, (95% purity) was purchased through AK Scientific, Inc. (Union City, CA). 3-

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(methanesulfonyl)-5a,6,7,8,9,10a-hexahydro-6,9-methanobenzo[b]cyclohepta[e]pyran-10,11-

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dione, compound 2 in Figure 1, and 3-{3-[2-chloro-4-(methanesulfonyl)phenyl]-3-

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oxopropanoyl)cyclopentane-1-carboxylic acid were custom synthesized.

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Natural water collection and analysis. Sacramento River water was collected from the Elkhorn

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Boat Launch Facility while rice field water was obtained from a flooded rice field in the Yolo

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Bypass Wildlife Area in August 2017 using 4 L amber glass bottles. The rice field water was

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yellow in appearance, but relatively clear. Natural waters were stored at 4 oC, filtered to 0.2 µm

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prior to use, and used within 30 d of collection. The UC Davis Analytical Laboratory

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characterized the pH, dissolved organic carbon, bicarbonate concentration, and electrical

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conductivity (EC) of high-purity water (6.52, 0.9 mg/L, < 0.01 meq/L, and < 0.01 dS/m,

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respectively), rice field water (8.07, 10.4 mg/L, 5.9 meq/L, and 0.95 dS/m, respectively), and

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Sacramento River water (7.63, 4.2 mg/L, 1.1 meq/L, and 0.12 dS/m, respectively) used in this

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investigation.9

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Solutions. To assess differences in direct photolysis between neutral and anionic compound 1,

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two high purity water solutions, 0.2 M HCl (pH 0.70 ± 0.02) and 1 mM H3BO3 in 1 mM NaOH

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(pH 8.00 ± 0.02), were prepared such that compound 1 was present in either its neutral form or

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anionic form at greater than 99%, respectively. Both solutions were filtered to 0.2 µm as a

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sterilization measure before use.

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Molar absorptivities and screening factors. Molar absorptivities (ε) of neutral and anionic

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compound 1 species, screening factors, and absorbance of unfiltered rice field water were

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determined using a UV-2501PC UV-Vis spectrophotometer (Shimadzu, Kyoto, Japan). To

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determine εneutral,λ and ε anionic,λ, absorbances of seven solutions of compound 1, in either pH 0.7

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high purity water or pH 8.0 high purity water, ranging in concentration from 0.1 µM to 60 µM

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were measured for wavelengths from 200 to 800 nm in 1 cm quartz cuvettes. Molar

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absorptivities (Figure 2) were calculated from the Beer-Lambert Law by plotting concentration

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versus absorbance at each wavelength:

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ελ =

Aλ [X]l

(1)

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where Aλ is the absorbance of the solution at wavelength λ, [X] is the concentration of compound

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€ (cm). 1 in solution, and l is the path length

127 128

Filtered Sacramento River water and filtered rice field water absorbances were also collected between 200 – 800 nm to calculate screening factors via the following equation:10, 11

1 −10 −α λ l 1 −10 −A λ = Sλ = 2.303 • α λ l 2.303 • Aλ

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where Sλ is the screening factor at wavelength λ, αλ is the light attenuation coefficient at

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wavelength λ, and l is the path length of the experimental container: 2 cm for simulated sunlight

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experiments and 1.2 cm for outdoor experiments. Screening factors were averaged between 290

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and 340 nm, the region where anionic compound 1 was photolytically active (Table 1, Figure 2).

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Quantum yields. Neutral and anionic compound 1 quantum yields were determined at 302, 313,

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and 334 nm using a monochromatic illumination system (Spectral, Amsterdam, The

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Netherlands) equipped with a 1000-W Hg/Xe lamp and downstream monochromator. Capped

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quartz cuvettes (1 cm) were used to contain solutions, which were spiked to 10 µM compound 1,

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during illumination and were uncapped only when aliquots were removed for analysis. The

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temperature was controlled at 25 ± 0.5 oC using a custom Peltier-cooled copper chamber (Paige

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Instruments). Hydrolysis (dark) controls were employed during each illumination and contained

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identical solution in a foil-wrapped quartz cuvette, which was not exposed to light. At each

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timepoint, 200 µL aliquots of illuminated sample and hydrolysis control were transferred into

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HPLC vials containing 400 µL glass inserts and 50 µL 25 µM sulcotrione in acetonitrile was

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added as an internal standard. Samples were stored at -20 oC until analysis. Photon fluxes were

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determined using 2-nitrobenzaldehyde (10 µM) as an actinometer. Actinometer photodegradation

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was measured before and after each sample was illuminated to calculate experimental jB values.

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The molar absorptivities and quantum yield (0.41) for 2-nitrobenzaldehyde used to calculate

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compound 1 quantum yields were previously determined.12

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Quantum yields of both compound 1 species were calculated at 302, 313, and 334 nm using the equation:10, 11

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ΦA,λ =

j A,λε A ,λ Φ j B,λε B ,λ B

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where ΦA,λ is the quantum yield of compound 1 at wavelength λ, jA,λ and jB,λ are the photolysis

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rates of compound 1 and 2-nitrobenzaldehyde (/h), respectively, at wavelength λ, εA,λ and εB,λ are

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the molar absorptivities of compound 1 and 2-nitrobenzaldehyde (/M /cm), and ΦB is the

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quantum yield of 2-nitrobenzaldehyde (0.41) which is independent of wavelength from 290 –

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400 nm.12 Then ΦA,λ were plotted as a function of wavelength (Figure 2) to obtain quantum

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yields for compound 1 for all photolytically relevant wavelengths (290 – 400 nm).

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Simulated sunlight illumination. Photolysis of compound 1 was determined using a custom-

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built apparatus containing a 1000 W Xe arc lamp with downstream optical filters.12 Triplicate

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sterilized aqueous samples were spiked to 10 µM compound 1 by diluting 50 µL of a 10 mM

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compound 1 solution in acetonitrile to 50 mL with the desired solution matrix and illuminated in

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2 cm quartz cells (Spectrocell Inc., Orland, PA) which were constantly stirred and held at 25 ±

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0.5 oC using a refrigerated recirculating water bath. Quartz was used to block wavelengths below

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290 nm, which are not environmentally relevant. Hydrolysis control quartz cuvettes were

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wrapped in foil and placed in the illumination chamber for each experiment. Isopropanol (25

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µM) was added to neutral compound 1 in high purity water (pH 0.7) to reduce secondary

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photolysis processes as seen in preliminary experiments. At each timepoint, 200 µL aliquots

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were transferred into HPLC vials containing inserts and an internal standard, 50 µL 25 µM

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sulcotrione in acetonitrile, was added. Samples were stored at -20 oC until analysis. 2-

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nitrobenzaldehyde, 10 µM in respective solution, was used as an actinometer for all simulated

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sunlight illuminations.

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Compound 1 photolysis followed a pseudo first-order decay and rates were determined from a linear regression of:

⎛C ⎞ ln⎜ t ⎟ = − j A t ⎝ C0 ⎠ 8 € ACS Paragon Plus Environment

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where C0 and Ct are the concentration of compound 1 in solution initially and at time t,

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respectively, and jA is the pseudo first-order photolysis rate constant of compound 1 (/h).

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The photolysis rate constant of 2-nitrobenzaldehyde was used to normalize the photolysis rate constant for simulated sunlight samples:

⎛j ⎞ B j p = j A ⎜⎜ env ⎟⎟ j ⎝ B exp ⎠

(5)

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where jp is the normalized compound 1 photolysis rate constant (/h), jB_exp is the photolysis rate

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€ measured on the day of the experiment (/s), and jB_env is the constant of 2-nitrobenzaldehyde

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measured photolysis rate constant of 2-nitrobenzaldehyde in Davis, CA at the summer solstice

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(June 21, 2000; 0.013 /s) or fall equinox (September 22, 2000; 0.011 /s) at 12:00PM PST.13

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The half-life, t1/2 (h), can then be determined as:

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t1/ 2 =

ln(2) kp

(6)

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Natural sunlight photolysis. Experiments were conducted on the roof of Meyer Hall at UC

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€ 2017. No experiments were run during the eclipse, August Davis during August and September

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21, 2017. Quartz tubes (12 mm ID x 15 mm OD x 100 mm) (Technical Glass Products,

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Painesville, OH), and Teflon-lined screw caps were used to contain aqueous samples (12 mL).

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Materials and solutions were sterilized via autoclave, 30 min at 18 psi and 121 oC, and filtration

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to 0.2 µm, respectively. Compound 1 (10 µM) was prepared in either pH 0.7 high purity water,

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pH 8.0 high purity water, filtered rice field water, or filtered Sacramento River water by spiking

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50 µL of 10 mM compound 1 in acetonitrile into 50 mL respective solution in triplicate. Quartz

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tubes containing three hydrolysis control solutions per treatment were wrapped with aluminum

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foil to prevent sunlight exposure. Isopropanol (25 µM) was added to pH 0.7 high purity water

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samples to reduce secondary photolysis. 9

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Samples were irradiated on black wooden platforms inclined at 30o from the horizon

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facing due south, as recommended by Environmental Protection Agency guidelines.14 Ambient

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temperatures were monitored every 10 min using an OM-24 data logger, Omega Engineering

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(Norwalk, CT), and ranged from 8.5 – 66.3 oC during the experiment, with an overall average

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temperature of 31.8 oC and average temperature of 42.5 oC during the day. Eight aliquots (200

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µL) were removed over 4 h, for pH 0.7 high purity water, or 14 d, for all other treatments, to

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assess compound 1 photolysis. The volume removed was limited to 15% or less of the initial

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total volume to minimize effects of changing volume on photolysis. Aliquots were spiked with

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50 µL 25 µM sulcotrione in acetonitrile as an internal standard and stored in HPLC vials with

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glass inserts at -20 oC prior to analysis via liquid chromatography mass spectrometry.

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Photolysis rates were determined using equation 4, then corrected for increased light scattering in tubes compared to a natural water body surface using the following equation:10, 11

j p = 0.46 j A

(7)

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Half-lives were then determined as per equation 6.

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€ using a 1200 Series LC coupled to a 6420 triple quadrupole Analysis. Samples were analyzed

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mass spectrometer using electrospray ionization (Agilent Technologies, Santa Clara, CA). The

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column used was a 150 mm x 3.5 mm i.d., 3.5 µm, Kromasil 100A C18, with a 10 mm x 3 mm

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i.d. guard column of the same material (Sigma Aldrich, St. Louis, MO). The analytical method

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has been published previously,4 and was updated to include compound 2 and 3-{3-[2-chloro-4-

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(methanesulfonyl)phenyl]-3-oxopropanoyl)cyclopentane-1-carboxylic acid. All compounds were

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quantitatively analyzed via multiple reaction monitoring. Compounds 1, 2 and 5, as well as

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sulcotrione and 3-{3-[2-chloro-4-(methanesulfonyl)phenyl]-3-oxopropanoyl)cyclopentane-1-

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carboxylic acid were analyzed in positive mode while compounds 3 and 4 were analyzed in

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negative mode. The following mass transitions, fragmentor values, and collision energies were

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used for compound 2 and 3-{3-[2-chloro-4-(methanesulfonyl)phenyl]-3-

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oxopropanoyl)cyclopentane-1-carboxylic acid: m/z 319 ! m/z 240, 150, 37 (compound 2) and

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m/z 373.1 ! m/z 217, 45, 16 (3-{3-[2-chloro-4-(methanesulfonyl)phenyl]-3-

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oxopropanoyl)cyclopentane-1-carboxylic acid). Internal calibration curves were constructed

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using seven standards between 0.05 – 15 µM with residuals ≤ 15% and R2 ≥ 0.9946.

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Method detection limits (MLOD) and quantitation limits (MLOQ) were determined for

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compound 1 and observed degradation compounds in each treatment solution by spiking seven

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blank solutions per treatment to 50 nM and multiplying the standard deviation of the resulting

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concentrations by 3 for MLOD or 10 for MLOQ. MLOD (nM) were 3, 8, 15, 29, 4, and 20 for

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compounds, 1, 2, 3, 4, 5, and 3-{3-[2-chloro-4-(methanesulfonyl)phenyl]-3-

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oxopropanoyl)cyclopentane-1-carboxylic acid, respectively. MLOQ (nM) were 9, 28, 49, 96, 14,

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and 68 for compounds 1, 2, 3, 4, 5, and 3-{3-[2-chloro-4-(methanesulfonyl)phenyl]-3-

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oxopropanoyl)cyclopentane-1-carboxylic acid, respectively. The starting concentration of

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compound 1, 10 µM, was sufficiently large to allow for quantitation of resulting degradation

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compounds formed at ≥ 1% based on the concentration of compound 1.

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Analysis of 2-nitrobenzaldehyde was conducted using a 10AT LC-DAD (Shimadzu,

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Kyoto, Japan) with a detection wavelength of 277 nm. The column used was a 250 mm x 3 mm

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i.d., 5 µm, BetaBasic-18 C18 (ThermoScientific, Waltham, MA). Analysis was carried out using

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an isocratic mobile phase of 20% acetonitrile and 80% water at 0.7 mL/min with an injection

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volume of 50 µL. Peak heights were used to calculate jB values.

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Statistics. A two-way ANOVA with post hoc pair-wise Tukey HSD comparison method (α =

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0.05) was used to assess differences in photolysis rates and half-lives at a significance level of P

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≤ 0.05 in JMP Pro 13 statistical software (SAS, Cary, NC).

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Pesticides in Flooded Applications Model (PFAM). PFAM is a comprehensive fate model

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designed to assess the dissipation of pesticides in aquatic systems, particularly rice fields.15

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PFAM was used to estimate the formation and subsequent disappearance of compound 1 in a

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California rice field using experimentally derived values.3, 4 PFAM was also used to determine

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an approximate tailwater holding time for compound 1 prior to release downstream. Weather

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conditions were approximated using meteorological data for Sacramento, CA between 1961 –

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1990, which was provided by the Environmental Protection Agency.16

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Results and Discussion

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Molar absorptivities and quantum yields of neutral and anionic compound 1. Molar

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absorptivities and quantum yields of neutral and anionic compound 1 are presented in Figure 2A

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and B, respectively. Both forms of compound 1 absorb sunlight wavelengths to nearly 400 nm

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(Figure 2A). However, the quantum yields for neutral compound 1 were 17, 18, and 120 times

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greater compared to anionic compound 1 at 302, 313, and 334 nm, respectively (Figure 2B),

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indicating that photolysis of the neutral form was much more efficient than for the anion. This is

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to be expected, as the resonance-distributed electron density within the anion aids dissipation of

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absorbed energy through physical processes in lieu of bond breakage (Figure 1).

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Additionally, while anionic compound 1 quantum yields decreased quickly with increasing

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wavelength, those for neutral compound 1 only slightly decreased over the same wavelengths.

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These fluctuations may lend uncertainty to calculated photolysis rates.

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Molar absorptivities, photon flux, and quantum yields were used to calculate the

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photolysis rate constant at each wavelength, jA,λ for each compound 1 species. This action

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spectrum plot (Figure 2C) shows that wavelengths between 300 and 340 nm were primarily

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responsible for the photolytic loss of both forms of compound 1. Furthermore, the area under the

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jneutral,λ curve is approximately 16 times greater than that for janionic,λ, indicating the sunlight

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photolysis rate of neutral compound 1 should be much higher than that of the anion. This is

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mostly driven by the significant difference in quantum yields between the two species.

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Photolysis rates and half-lives. Compound 1 in hydrolysis controls was found to be stable for

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all treatments, as more than 90% of the original concentration was retained over the course of

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each experiment. Simulated sunlight photolysis rates and half-lives were converted to midday,

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summer solstice values since this is the time nearest to compound 1 formation in the field (May –

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June).1 Rates were also estimated at the fall equinox to provide a direct comparison to measured

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natural sunlight photolysis rate constants, jp, and to approximate light intensities during the

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spring equinox, as light intensities are the same during both equinoxes. Summer solstice rates

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derived from simulated sunlight experiments were faster than those for the fall equinox as light

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intensities are greater during the summer solstice. Since our normalized simulated sunlight rate

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constants are for midday sunlight, we calculated a conversion factor (CF) of 0.283, which relates

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the midday rate constant to a 24-h average rate constant. Multiplying the simulated sunlight

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midday fall equinox photolysis rate constant by CF gives an approximate 24-h exposure rate

284

constant. Generally, these estimated values are within a factor of 2 of the measured natural

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sunlight photolysis rates, e.g. 170 h vs. 320 h for anionic compound 1 photolysis (Table 1).

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Photolysis half-lives in natural sunlight were larger compared to estimated values from simulated

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sunlight, except for pH 0.7 high purity water (neutral compound 1). This may be an artifact of

288

the uncertainty of the calculation, potentially due to the variability in the modeled quantum

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yields, as they were measured at only three wavelengths. Another possibility is temperature

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dependence of the photolysis rate: −E a

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j A = Ae RT

(8)

292

where A is the frequency factor, Ea is the activation energy, R is the universal gas constant, and T

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€ is the temperature. Using this equation along with jA values from Table 1 and illumination

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temperatures, 25 oC for simulated sunlight samples and 43 oC, the average temperature during

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the day for natural sunlight samples, suggests that the activation energies are approximately 50

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kJ/mol for neutral compound 1 and 80 kJ/mol for the anionic species. The lower activation

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energy for neutral compound 1 suggests its photodegradation rate constant is less susceptible to

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temperature changes than the anionic form of compound 1. Outdoor samples were exposed to

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high temperatures, up to 66 oC, which could have increased the photolysis rates for both species

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compared to simulated sunlight illuminations. California rice fields typically experience

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temperatures ranging from 13 – 40 oC.17, 18 Therefore, compound 1 photolysis in a rice field may

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be slower than reported here for ambient experiments, although the average temperature during

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the day, 43 oC, was only slightly higher than the typical temperature range so this difference is

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expected to be minimal.

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Photolysis rates differed significantly between compound 1 species (P < 0.001). Neutral

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compound 1 photolysis resulted in the shortest half-lives of all treatments (Table 1). The

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photolysis rate constant of the neutral species increased over preliminary illumination time

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experiments; we minimized this effect by using the earlier time data to determine the rate. This

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increase in apparent photolysis rate constant is likely due to secondary chemistry such as the

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formation of reactive intermediates from direct photolysis of neutral compound 1, which then

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react with another molecule of neutral compound 1. To avoid this, isopropanol was added to pH

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0.7 high purity water solutions to scavenge hydroxyl radical, a possible intermediate. Though

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isopropanol appeared to stabilize photolysis of neutral compound 1 in simulated sunlight

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samples, it is unclear why it did not affect natural sunlight photolysis samples. It is possible that

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the amount added was incorrectly low or it formed acetone in natural sunlight samples, which

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acted as a photosensitizer.19 Neutral compound 1 photolysis was faster in natural sunlight, ~ 1 h

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half-life, compared to simulated sunlight ,~ 3 h half-life (Figure 4), which seems to corroborate

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this explanation. The photolysis rate increased with illumination time in both natural and

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simulated sunlight, though the effect was more pronounced for neutral compound 1 in natural

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sunlight. Regardless of isopropanol presence, neutral compound 1 photolysis is quite rapid.

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However, since rice field water, pH 7 – 9.5,5, 6 and Sacramento River water, pH 6.9 – 8.5,20 are

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above pH 5, neutral benzobicyclon hydrolysate photolysis is not expected to contribute

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significantly to the overall fate of compound 1 in these environments. At pH 7, the percent of

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anionic compound 1 is 99.992% while the amount of neutral compound 1 would make up

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0.008%. The mole fraction-weighted jA at pH 7 using neutral and anionic compound 1 jp values

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(Table 1) is therefore estimated to be 0.012 and 0.0049 /h in simulated and natural sunlight,

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respectively. The contribution of neutral compound 1 to these estimated rates is 0.1 and 2%,

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respectively. Though neutral compound 1 photodegrades much faster than the anion, its overall

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contribution to photolysis above pH 7 is minimal compared to the contribution of anionic

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compound 1. The neutral species may play a larger role at the soil-water interface, where the pH

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can be as low as 4.5,4 raising the estimated jA at pH 4.5 to 0.017 and 0.038 /h in simulated and

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natural sunlight, respectively. As this area comprises only a small fraction of a flooded field, the

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anion is still expected to dominate the overall fate of compound 1 in the field.

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Anionic compound 1 photolysis proceeded much more slowly (40 – 320 h) (Table 1),

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which can be attributed to the lower quantum yields of anionic compound 1 compared to the

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neutral species (Figure 2B). Photolysis of sulcotrione and other triketones follow a similar

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pattern.21, 22 The pH of rice field water used was 8.07 and Sacramento River water was pH 7.63,

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therefore the anion comprised at least 99.998% of the total amount of compound 1 in both

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waters. It appears that indirect photolysis strongly influenced photolysis of the anion, as the

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natural sunlight photolysis half-life dropped significantly from pH 8.0 high purity water (320 h)

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compared to filtered rice field water (91 h). Indirect photolysis was also observed in Sacramento

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River water samples (208 h in natural sunlight). These anionic compound 1 photolysis rates were

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significantly different from one another (P < 0.001) (Figure 3). Dissolved organic matter is

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known to drive indirect photolysis in natural waters, through both sensitizing and quenching

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processes involving production of reactive oxygen species and excited state dissolved organic

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matter.23-25 Correlation coefficients between dissolved organic carbon concentrations, 0.9, 4.2,

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and 10.4 mg/L for high purity, rice field, and Sacramento River waters, respectively, and

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compound 1 photolysis rates (Table 1) are 0.96 for simulated sunlight and 0.97 for natural

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sunlight. This indicates dissolved organic matter may enhance compound 1 photolysis through

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indirect means, especially in rice field water. Other contributors to indirect photolysis in the field

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may be photo-oxidized proteins and amino acids as well as carbonate radical from dissolved

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bicarbonate.26, 27

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Screening factors and their influence on compound 1 photolysis in natural waters.

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Screening factors (Equation 2 and Table 1) measure solution light absorption by dissolved humic

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substances,10 which decreases the volume-averaged photon flux in the water sample. A screening

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factor of 1 indicates no light absorption in solution, whereas a screening factor significantly less

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than 1 suggests light absorption by dissolved organic matter, which leads to lower rates of direct

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photolysis. Filtered rice field water and Sacramento River water screening factors are shown in

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Table 1. Screening factors were also derived for pH 0.7 high purity water and pH 8.0 high purity

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water, which were 0.998 in both solutions. In comparison, filtered rice field water and

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Sacramento River water had screening factors of 0.820 and 0.963, respectively, between the

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wavelengths of 290 – 340 nm (Figure 2C). Wavelengths below 290 nm are blocked from the

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Earth’s surface by the ozone layer and wavelengths above 400 nm did not have enough energy to

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significantly induce photolysis of compound 1. These screening factors directly correlate with

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measured dissolved organic carbon concentrations.

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Compound 1 photolysis rates and half-lives in both natural waters can be corrected using

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the screening factor to account for light absorption by dissolved organic matter (Table 1).

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Sacramento River water photolysis rates did not significantly shift, 208 h to 203 h in natural

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sunlight, which can be explained by the relatively high screening factor, 0.98 in natural sunlight,

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and low dissolved organic carbon content (4.2 mg/L). Compound 1 photolysis in rice field water,

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however, decreased significantly, 91 h to 80 h in natural sunlight (Table 1). This could be

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attributed to increased dissolved organic carbon content (10.4 mg/L) and lower screening factor,

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0.89 in natural sunlight, compared to other samples. In other words, light absorption played a

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bigger role in rice field water, likely due to the increase in dissolved organic matter compared to

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other treatments; thus direct photolysis of compound 1 in this solution was hindered the most.

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However, dissolved organic matter enhanced indirect photolysis in rice field water compared to

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the other treatments. The light absorbed by dissolved organic matter, reducing direct photolysis

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of compound 1, may have generated sensitizing agents including reactive oxygen species and

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excited state dissolved organic matter, which then reacted with compound 1 through secondary

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processes. This also held true for Sacramento River water, though to a lesser extent, as its

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dissolved organic carbon concentration was lower than in rice field water. Though treatments

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were sterilized in this investigation, aquatic microbial degradation may also play a role in

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dissipation of compound 1 in the field.

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Degradation products. Target photolysis products, compounds 2, 3, 5, and 3-{3-[2-chloro-4-

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(methanesulfonyl)phenyl]-3-oxopropanoyl)cyclopentane-1-carboxylic acid, were selected to

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reflect known sulcotrione photolysis products.21, 28, 29 Of these, compounds 2, 3, and 5, were

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observed in photolysis samples (Figure 1, Table 2), however 3-{3-[2-chloro-4-

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(methanesulfonyl)phenyl]-3-oxopropanoyl)cyclopentane-1-carboxylic acid was not detected.

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Product yields are reported in Table 2. Mass balance, > 90% total moles retained, was only

390

achieved during photolysis of neutral compound 1 in simulated sunlight, whereas the final total

391

mole percentage was 72% for the anion. Under the same condition, the final total mole

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percentages in rice field water and Sacramento River water were 55% and 67%, respectively.

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Neutral compound 1 photolysis produced the greatest amount of compound 2, 32 and

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41% by mole from natural and simulated sunlight, respectively (Table 2). Compound 5 was not

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detected in neutral compound 1 photolysis samples, which is similar to sulcotrione photolysis.30

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Formation of compound 3 from neutral compound 1 photolysis varied wildly between simulated

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and natural sunlight, 0.9 and 38 % by mole, respectively. Secondary photolysis may provide an

398

explanation. As discussed previously, isopropanol was used to reduce neutral compound 1 loss

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through secondary reactions, however its effects were less apparent in natural sunlight samples.

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Compound 2 formed first as compound 1 photolysis progressed, for all treatments, followed by

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compound 3, and compound 5 in anionic compound 1 photolysis samples (Figure 4). While

402

compound 3 remained photolytically stable, compound 2 disappeared over time, most likely

403

forming compound 3 in the process.

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Degradation product formation was less prolific from anionic compound 1 photolysis

405

though compounds 2, 3, and 5 were all observed, especially in natural waters (Table 2). As was

406

the case for neutral compound 1 photolysis, formation of compound 2 was initially rapid

407

followed by decay over time, while compound 3 was photolytically stable once formed (Figure

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4). As with compound 2, compound 5 degraded over time after it was formed, however the

409

process was slow. Though anionic compound 1 photolysis was fastest in rice field water (P