Aqueous Photolysis of Benzobicyclon Hydrolysate - ACS Publications

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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

†

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182 Steele Ave Extension, Gloversville, NY 12078

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

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‡

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

Davis, CA 95616

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§

13

State University, Chico, CA 95929

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

14 15

#

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Davis, CA 95616

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

17 18

*

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

1

ACS Paragon Plus Environment


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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-

27

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

28

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

29

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

30

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

32

levels constant months after benzobicyclon application.

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Key Words

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

36 37 38 39 40 41

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Introduction

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

45

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

46

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

47

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

48

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

49

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

51

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

52

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

54

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

57

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

58

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

59

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

61

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

62

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

65

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

75

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

76

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

79

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

80

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

81

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

82

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

83

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.

85 86

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)

90

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

91

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

92

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

93

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

94

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

95

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

96

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

97

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

98

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

99

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

100

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

101

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

102

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

103

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

104

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

105

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

107

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

108

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

110

investigation.9

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

112

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

113

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

114

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

115

sterilization measure before use.

116

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

117

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

118

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

119

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

120

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

121

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

122

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

123

versus absorbance at each wavelength:

124

ελ =

Aλ [X]l

(1)

125

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

126

€ (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

129

€ 6


(2)

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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

132

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

133

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

134

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

135

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

136

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

137

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

138

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

139

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

140

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

141

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

142

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

144

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

145

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

146

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

147

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

148

compound 1 quantum yields were previously determined.12

149 150

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

151

ΦA,λ =

j A,λε A ,λ Φ j B,λε B ,λ B

€ 7


(3)


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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

153

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

154

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

155

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

156

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

157

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

158

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

159

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

160

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

161

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

162

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

163

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

164

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

165

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

166

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

167

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

168

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

169

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

170

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

171

sunlight illuminations.

172 173 174

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

(4)

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

176

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

177 178 179

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)

180

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

181

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

182

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

183

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

184

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

185

t1/ 2 =

ln(2) kp

(6)

186

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,

189

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

190

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

191

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

192

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

193

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

194

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

195

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

196

samples to reduce secondary photolysis. 9



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

198

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

199

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

200

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

201

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

202

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

203

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

204

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

205

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

206

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

207 208 209

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.

211

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

212

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

214

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

215

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

217

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

218

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

219

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

221

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

222

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

223

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

224

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

225

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

226

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

227

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

229

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-

231

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

232

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

233

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

234

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

235

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,

237

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

238

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

239

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

240

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

11



241

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

242

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

243

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

244

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

245

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

248

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

249

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

250

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

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Results and Discussion

253 254

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

255

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

256

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

257

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

258

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

259

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

260

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

261

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

263

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

264

These fluctuations may lend uncertainty to calculated photolysis rates.

265

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

266

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

267

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

268

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

269

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

270

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

271

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

272

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

273

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

274

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

275

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

276

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

277

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

278

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

279

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

280

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

281

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

282

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

283

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).

286

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

287

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

289

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

290

dependence of the photolysis rate: −E a

291

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

compared to simulated sunlight illuminations. California rice fields typically experience

301

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

302

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

303

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

304

expected to be minimal.

305

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

306

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

307

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

309

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

310

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

311

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

312

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

313

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

314

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

315

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

316

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

317

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

318

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

319

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

320

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

321

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

322

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

323

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

324

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

325

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

326

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

327

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

328

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

329

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

330

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

15



331

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

332

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

333

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

334

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

335

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

336

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

337

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

338

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

339

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

340

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

341

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

342

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

343

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

344

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

345

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

346

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

347

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

348

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

349

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

350

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

351

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

352

bicarbonate.26, 27

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353

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

355

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

356

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

357

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

358

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

359

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

360

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

361

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

362

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

363

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

364

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

365

measured dissolved organic carbon concentrations.

366

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

367

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

368

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

369

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

370

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

371

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

372

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

373

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

374

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

375

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

17



376

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

377

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

378

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

379

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

380

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

381

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

382

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

383

dissipation of compound 1 in the field.

384

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

385

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

386

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

387

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

388

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

389

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

392

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

393

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

394

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

395

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

396

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

397

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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399

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

400

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

401

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.

404

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

408

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

Aqueous Photolysis of Benzobicyclon Hydrolysate - ACS Publications

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