Sunlight Photolysis of 2,4-D Herbicides in Systems Simulating Leaf Surfaces

Environ mental Scie nce Processes & I mpacts

Vi e w Arti cl e O nli n e PAPE R Vi e w J o ur n al

E mergi ng i nvestigat or series: s u nlig ht p h ot olysis of 2, 4- D herbicides in syste ms si mulating leaf Cit e t his: D OI: 10.1039/c8e m00186c s urfaces †

L ei S u, a J ohn D. Sivey b and Ning Dai * a

Pesticides are c o m m only applied on f oliage, f or ming dry deposits on the leaf cuticular wax. H o wever, their photoche mical transfor mation in this lipophilic environ ment is much less understood co mpared with that in surface water. In t his w ork, s unlig ht ph ot olysis of six chl ori nated p hen oxyacetic acid herbici des ( i. e. , 2, 4 - D and structural analogues) was evaluated in four organic solvents, on quartz, and on para ffi n w a x. I n s olvents of l o w polarity ( i. e. , n -heptane and 2-propanol), direct photolysis of 2,4- D herbicides was enhanced due t o the relatively high quantu m yields in these s olvents. Ph ot olysis on para ffi n wax was sl o wer t ha n ph ot olysis on quartz by a fact or of 3 – 9, but was c o mparable wit h t hat in s olve nts of l o w p olarity. Wit h envir on mentally relevant irradiation and surface l oading, the half-lives of 2, 4- D herbicides on para ffi n wax were 27 – 159 h, which are within the sa me range reported for biodegradation, the do minant dissipation path way in the current 2,4- D fate model. Product analyses sho wed that photoreductive dechlorination is the do minant path way in organic s olvents, acc ounting f or 68 – 100 % of parent co mpound decay. On quartz a nd para ffi n wax surfaces, ho wever, photoreductive dechlorination products accounted for <60 % of parent Received 23rd April 2 018 co mpound decay. Co mbining kinetic modeling and product analyses, it was sho wn that neither could the Accepted 28th June 2018 t wo additional putative path ways (photosubstitution of chlorine by hydroxyl group and cleavage of the D OI: 10.1039/c8e m00186c ether bond) fully account f or the t otal phototransf or mation on surfaces. These results suggest that rapid rs c.li/ es pi ph ot olysis on surfaces can be attributed t o unique path ways that are absent in the organic s olvent phase.

E nviro n me ntal sig ni  ca nce Pestici des are o  e n applied o n foliage, but t heir p hotolysis i n t his e nviro n me nt is muc h less u nderstood co mpared wit h t hat i n water. T his study syste matically i nvestigate d t he p hotolysis of 2,4- D herbici des i n syste ms si mulati ng t he reactio n e nviro n me nt of leaf surface, i ncludi ng no n-polar orga nic solve nts a n d q uartz a nd wax surfaces. We deter mi ned p hotolysis rate co nsta nts a nd qua ntu m yields, a nd a nalyzed p hotoproducts. Our results s ho wed t hat direct p hotolysis of 2,4- D Published on 28 June 2018. Do wnloaded by T O WS O N ST ATE U NI VE RSIT Y on 7/5/2018 4:29:29 P M. her bici des o n para ﬃ n wax ca n be as fast as t heir bio degra datio n i n water, t he do mi na nt degra datio n pat h way co nsi dere d i n t he c urre nt fate mo del. A d ditio nally, t he major reactio n pat h way i n orga nic solve nts a nd o n surfaces ( i. e. , p hotore d uctive dec hlori natio n) is disti nct fro m t hat previo usly reporte d i n water.

1 Introduction Pesticides that are applied directly on foliage for m dry deposits o n t he cuticular wax of leaves. 6 Nevert heless, t heir P hotoc he mical tra nsfor matio n is a major pat h way for pesticide photoche mical transfor mation on leaves is much less under- degra datio n. 1 In a recent revie w, more than half of the 160 stood co mpared with that in surface water. Lipophilic envi- pesticides evaluated u ndergo direct p hotolysis u nder su nlig ht, ro n me nts, suc h as cuticular wax, 7 have been sho wn to pro mote wit h half-lives as s h ort as 2 h. 2 For pestici des t hat do not a bsor b direct p hotolysis by exte ndi ng t he life-ti mes of excited state su nlig ht, i ndirect p hotolysis ( i. e. , photolysis sensitized by m ol e c ul e s. 8 Spruce needle wax was s ho w n to serve as a hydroge n natural organic matter, nitrate, or iron species) can be an do nor i n t he p hotoreductive dec hlori natio n of so me persiste nt i mportant dissipation path way. 3 – 5 orga nic poll uta nts ( e. g. , benzophenone and D DT). 9 A d diti o nally, w he n prese nt as dry deposits, pesticide molecules may ex hibit lig ht absorptio n properties di ﬀ ere nt fro m t hose i n t he solve nt p h a s e, 1 and may favor reaction path ways that mini mize

a Depart ment of Civil, Structural and Environ mental Engineering, University at Bu ff al o, confor mation change bet ween reaction inter mediates and The State University of Ne w York, 231 Jarvis Hall, Bu ff alo, Ne w York 14260, USA. parent co mpounds. 1 0 E- m ail: ni n g d ai @ b u ff alo.edu; Fax: +1-716-645-3667; Tel: +1-716-645-4015 Previo us st u dies 1 1 – 1 5 of t he p hotoc he mical tra nsfor matio n of b Depart ment of Che mistry, To wson University, To wson, Maryland 21252, USA pestici des o n leaves use d t wo ty pes of mo del syste ms. I n t he  r st † Electronic supple mentary infor mation (ESI) available. See DOI: type, organic solvents were used to represent speci  c 10.1039/c8e m00186c

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co mpo ne nts of t he cuticular wax. For exa mple, cyclo hexa ne a nd mec ha nistic i nterpretatio ns have bee n scarce d ue to i ns u ffi cie nt cyclohexene were used as surrogates of the saturated and experi mental controls in many of the studies e mploying u nsat urate d hy drocarbo ns, respectively, of c utic ular wax. 1 1, 1 2 T h e surfaces. For exa mple, surfactants are a co m mon adjuvant in direct p hotolysis of f u ngicide folpet a nd i nsectici de parat hio n co m mercial pesticide for mulations; ho wever, so me previous was 10 – 100 ti mes faster in cyclohexene than in cyclohexane, studies e mployed pure pesticides, 1 5, 2 9 w hic h could for m aggre- suggesting that their photolysis proceeds more e ffi cie ntly i n gates capable of i nterferi ng wit h lig ht absorptio n. Surfacta nts ol e  nic me dia. 1 1, 1 2 When D DT, methoxychlor, and anilazine have o nly rece ntly bee n co nsidered as a n i mporta nt adjuva nt i n were irra diated i n met hyl oleate, a s urrogate of t he octa dece noic p hotolysis experi me nts. 3 0 Additionally, most of the reported acids in cuticle wax, photoinduced addition of pesticides to irradiation intensity was based on the output of the light methyl oleate was observed, suggesti ng t hat t he solve nt mole- s o urces, 1 9, 2 0, 2 3 – 2 5 w hic h ca n not acco u nt for t he variatio n i n lig ht c ules directly participate d i n pestici de p hototra nsfor matio n. 1 3, 1 4 atte nuatio n attributable to di ff erences in reactor geo metry In methanol and 2-propanol representing the pri mary and bet wee n solve nt a nd surface experi me nts. secondary alcohol groups, respectively, co m mon in cuticular T he goal of t his work is to i nvestigate t he direct p hotolysis of wax, cyclohexanedione oxi me herbicides clethodi m and sethox- 2,4-dic hlorop he noxyacetic acid (2,4- D) a nd  ve relate d her bi- ydi m photodegraded 3 – 5 ti mes faster than in water. 1 5 T h e ci des ( Ta ble 1) u n der co n ditio ns releva nt to leaf s urfaces. 2,4- D p ote ntial i n  ue nce of solve nt polarity o n pesticide p hotolysis, is a n her bici de (fre q ue ntly for m ulate d as a n ester) t hat is wi dely ho wever, was not co nsidered i n t hese studies. 1 1 – 1 5 applied post-e mergence to control broadleaf weeds. Developed The second type of model syste m e mploys surfaces with in the 1940s, 2,4- D re mains the 7 t h most used agricultural di ff ere nt p hysicoc he mical c haracteristics, s uc h as glass, 1 6 – 1 9 sili c a pesticide and the most used pesticide in non-agricultural g el, 2 0, 2 1 or a t hi n layer of wax extracted fro m leaves. 2 0, 2 2 – 2 6 Di ff er- sect ors. 3 1 In surface water, sunlight photolysis of 2,4- D and e nces bet ween direct p hotolysis rates on surfaces co mpared to relate d her bici des is slo w. For exa m ple, t he p hotolysis half-life those in solutions vary a mong pesticides. Fungicide hexa- of 2,4- D i n water is 13 d, 3 2 w hile its bio degra datio n half-life is c hlorobe nze ne persisted o n glass surface a  er 5 mo nt hs of irra- 30 to 40 h. 3 3 On leaf surfaces, ho wever, photolysis may play di ati o n by arti  cial su nlig ht, b ut ac hieved 70 % decay i n met ha nol a more sig ni  ca nt role d ue to t he lo wer microbial activity a n d a  er 15 days u n der su nlig ht. 2 7 I n co ntrast, f u ngicide guazati ne- t he pote ntial e n ha nce me nt of p hotolysis by leaf surfaces. triacetate degraded 67 % a  er 84 h when prese nt as a t hi n  l m I n t his work, direct p hotolysis of six c hlori nated p he noxy- o n glass surface, but no degradation was observed in water or acetic acid herbicides was exa mi ned i n orga nic solve nts a nd o n m et h a n ol. 1 8 More recently, Richard a nd co- workers 2 6 develo pe d surfaces under si mulated sunlight. First, photolysis rate a procedure to create si mulated leaf surfaces by extracti ng wax co nsta nts, molar exti nctio n coe ffi cients, and quantu m yields fro m plants and reco nstituting it o n dishes. O n t he extracted were deter mi ne d i n fo ur orga nic solve nts wit h di ff ere nt p olarity maize and carnauba gray wax, photolysis rates of triketone and hydrogen-donating ability. Subsequently, photolysis herbici des were 10 – 900 ti mes faster t ha n t hose i n water. 2 6, 2 8 experi ments were conducted on quartz and para ffi n wax Although di ff erences in the photoche mical behavior of surfaces, and the reaction rate constants were co mpared with pesticides in water and on surfaces have been docu mented, those obtained fro m solvent experi ments. For selected Published on 28 June 2018. Do wnloaded by T O WS O N ST ATE U NI VE RSIT Y on 7/5/2018 4:29:29 P M. Ta bl e 1 Che mical structure of the six chl orinated phen oxyacetic acid herbicides investigated in this study

2,4- Dic hlorop he noxyacetic acid (2,4- D) 2,4,5- Tric hloro p he noxyacetic aci d (2,4,5- T)

2,4- D met hyl ester (2,4- D M E) 2,4,5- T met hyl ester (2,4,5- T M E)

2,4- D b utoxyet hyl ester (2,4- D B E E) 2,4,5- T b utoxyet hyl ester (2,4,5- T B E E)

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herbicides, p hotodegradatio n products i n orga nic solve nts a nd re move irradiatio n belo w 290 n m to si mulate direct su nlig ht at on surfaces were analyzed. Lastly, the photolysis mechanis ms t he Eart h's surface. T he la mp was set to a lig ht i nte nsity of releva nt to leaf s urfaces were disc usse d. 0.68 W m 2 at 340 n m, a nd t he total irradiatio n i nte nsity was deter mi ned usi ng c he mical acti no metry (see Sectio n 2.4 belo w). A circulating water bath was used to maintain sa mple te mper- 2 Materials and methods at ure at 2 6 3 C. For li q ui d sa m ples, eac h reactio n sol utio n (10 2.1 Materials mL) was placed in a quartz test tube capped with a silicone Detail i nfor matio n o n t he c he micals use d i n t his st u dy is s ho w n stopper. T he test tubes were placed i n t he water ta nk at a 45 i n Text S1. † a ngle. At selected ti me poi nts, 0.5 mL of reactio n solutio n was re moved fro m eac h test tube for a nalysis. For surface sa mples, dis hes loaded wit h pesticides were covered by quartz disks a nd 2.2 Photolysis experi ments placed in the sunlight test cha mber (half sub merged in the A total of six str uct urally relate d p he noxyacetic aci d her bici des circulati ng water bat h). Dis hes were re moved at selected ti me were used (Table 1). A mong the m, 2,4- D and 2,4- DBEE are poi nts for a nalysis. All experi me nts were co nducted i n dupli- widely used in co m mercial for mulations, while 2,4- D ME and cates a nd i ncluded dark co ntrol sa mples. 2,4,5-T a nd its esters are ba n ned i n t he U nited States due to t heir hig h volatility a n d/or hig h toxicity, 3 4 b ut are still use d i n ot her parts of t he worl d. 3 5 For p hotolysis experi me nts i n t he 2.3 Sa mple analysis solvent phase, pesticide stock solutions were prepared fro m For solvent experi ments, the 2,4- D and 2,4,5-T sa mples were pure co mpou nds or used as purc hased. T he solve nt of t he stock analyzed using high perfor mance liquid chro matography with solutions was selected based on pesticide solubility li mits. a diode array detector ( HPL C- DA D, Agilent 1260 In  nity), a n d Accordingly, so me reaction solutions contained more than one t he met hyl a nd butoxyet hyl ester sa mples fro m solve nt experi- solve nt ( Ta ble S1 † ), b ut t he co-solve nt was less t ha n 1.5 vol % i n ments were analyzed by gas chro matography- mass spectro m- all reactio n sol utio ns except for 2,4- D a n d 2,4,5- T. T he i nitial etry (Agilent Model 7890 B G C-240 MS) with che mical ionization co nce ntratio ns of 2,4- D a nd 2,4,5-T i n t he reactio n solutio ns using methanol. For surface experi ments, the residual pesti- were 20 m M and those of 2,4- D ME, 2,4,5-T ME, 2,4- DBEE, and cides were dissolved by methanol and analyzed by HPL C- DA D. 2,4,5-T BEE were 5 m M. Aqueous reaction solutions were bu ff - T he details of t he a nalytical met hods are s ho w n i n Text S2. † ered with phosphate (p H 7.0, 5 m M). Photoproducts for the trichlorinated phenoxyacetic acid For surface experi ments, quartz dishes (dia meter: 55 m m, herbicides were evaluated. HPL C- DA D was  rst use d to q ua ntify depth: 15 m m, capacity: 20 mL) were used to provide quartz t he dec hlori natio n products of 2,4,5-T, 2,4,5-T ME, a nd 2,4,5- surfaces. To prepare para ffi n wax s urfaces, glass Petri dis hes T B E E ( i. e. , 2,4- D, 2,4- D M E, 2,4- D B E E, a n d t heir iso mers), as well were dippe d i n melte d para ffi n wax (110 C for 20 mi n utes), a n d as 2,4,5-trichlorophenol, the potential product fro m the then placed horizontally and cooled at roo m te mperature to cleavage of the ether bond. Because the iso mers of 2,4- D and for m a  at s urface. T he wax s urfaces were i ns pecte d vis ually. No 2,4- DBEE were not co m mercially available, the total concen- solvent pooling was observed during sa mple loading (see tratio ns of iso mers were esti mated usi ng t he HPL C calibratio n

Published on 28 June 2018. Dobelo wnloaded by T O w). WS O T N ST he ATE Uwax NI VE RSIT coated Y on 7/5/2018 4:29:29 P M. dis hes have t he sa me di me nsio ns as t he curve for 2,4- D a nd 2,4- D B E E, respectively. Furt her discussio n q uartz dis hes. To apply pestici des o n t he s urfaces, a mixt ure of on this methodology is provided in Section 3.3. The methanol t he surfacta nt T wee n ® 20 a nd t he target pesticide, dissolved i n and 2-propanol solvent sa mples and the methanol extracted methanol or acetonitrile (3 mL), was added to the dishes. surface sa mples were directly analyzed. For the n - hepta ne Beca use para ﬃ n wax surface is more hydrop hobic t ha n quartz, sa mples of 2,4,5-TBEE, 1 mL of sa mple was blo wn do wn by a higher concentration of T ween ® 20 was used on wax (10 : 1 nitroge n gas to dry ness i n a n a mber vial, a nd reco nstituted i n molar ratio to t he pestici de) t ha n o n q uartz (1 : 1 molar ratio). 1 mL methanol and i m mediately analyzed by HPL C- DA D. The T he dis hes were store d i n t he dark for 3 – 12 h t o all o w s olve nt t o recovery of 2,4- DBEE using this method was 102.8 % 1. 3 %. evaporate. Experi me ntal veri  cati o n ( e. g. , vi a microsco py) of Met hyl esters were too volatile to be recovered usi ng solve nt surface dry ness follo wi ng t he solve nt evaporatio n ti me was not blo wdo wn; therefore, the n -heptane sa mples of 2,4,5-T ME perfor med. T he recoveries of pesticides fro m t he t wo surfaces photolysis were not analyzed for photoproducts. The HPL C were i n t he ra nge of 87.8 – 99.8 % (Table S2 † ), s uggesti ng t here met ho d for pro d uct a nalysis is describe d i n Text S2. † was no substantial loss during sa mple loading or analysis G C- MS (full scan) was used to explore other products for (described belo w i n Sectio n 2.3). T he pesticide surface loadi ng 2,4,5-T ME p hotolysis i n t he solve nts. Aut he ntic 2,4- D ME sta n- used i n t he experi me nts (3 1 0 9 m ol c m 2 ) was si milar i n dar d was use d to verify o ne of t he pro d uct peaks fro m 2,4,5- T M E mag nit u de as t he 2,4- D applicatio n rate i n t he  el d (265 g per p hotolysis i n solve nts. T he G C- MS met hod for product a nalysis hectare, e q uivale nt to 1.2 1 0 8 m ol c m 2 ). 3 6 D uri ng prepa- is descri be d i n Text S2. † ratio n of s urface sa mples, a so di u m la mp ( l ¼ 589 n m) was used The 2,4,5-T and 2,4,5-TBEE surface sa mples were also for laboratory illu mi natio n to mi ni mize p hotolysis loss. analyzed usi ng ultra-perfor mance liquid c hro matograp hy i nter- P hotolysis experi me nts were co nducted i n a Q-S U N Xe-1 test faced with high-resolution, quadrupole/ti me-of-  ight mass cha mber equipped with a Xenon arc la mp to produce the full spectro meter using an electrospray ionizatio n source (operated sunlight spectru m. A daylight- Q  lter (X-7460) was used to i n negative ionizatio n mode) ( UPL C-ESI( )-qT OF). Met ha nol was

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used to extract t he photoproducts fro m t he surfaces a nd blo wn light trans mission through wax. Additionally, because light do wn to dryness under nitrogen gas. The sa mples were recon- scatteri ng by pestici des a n d re  ectio n by t he q uartz s urface is stit uted i n aceto nitrile for U PL C- ESI( )-q T OF a nalysis. Product not accou nted for, t his measure me nt yields t he upper bou nd of i de nti  cation was based on accurate mass measure ment. The molar exti nctio n coe ffi cie nts for pestici des o n q uartz. T he molar details of t he U PL C- ESI( )-qT OF method are sho wn in Text S2. † exti nctio n of a pestici de i n soli d for m is calc ulate d by e q n (5). A 3 ¼ 1 0 3 L c m 3 ( 5) l q 2.4 Deter mination of quantu m yield E q n (1) descri bes t he ki netics of a p hotolysis reactio n. 3 7 w here A is the absorbance (di mensionless) measured by UV-Vis " # spectrophoto metry, and q is the surface concentration d C X E 0 ðl Þ 3 C ( mol c m 2 ). p ða l þ 3l C Þd l ¼ F l 1 1 0 ( 1) d t d a l þ 3 l C Che mical actino metry was used to deter mine the quantu m yiel ds of pestici des i n di ff ere nt reactio n e nviro n me nts. 2- Nitro- 1 w here C is t he pestici de co nce ntratio n ( mol L ) at ti me t ( s); F l be nzalde hyde (2- N B) was used as t he acti no meter, a nd its decay is the quantu m yield of pesticide decay (di mensionless) at was mo nitored. T he p hotolysis of 2- N B occurs i ntra molecularly 0 2 wavele ngt h l ; E p (l ) is t he i nci de nt lig ht i nte nsity ( Ei nstei n c m wit h a reported qua ntu m yield of 0.41. 3 8, 3 9 Studies have sho wn 1 s ) at wavele ngt h l ; d is t he sa mple pat hle ngt h (c m); a l i s t h e t hat t he quantu m yield of 2- NB is i ndepe nde nt of te mperature, 1 absorptio n coe ffi cie nt of t he reactio n matrix (c m ); 3 l i s t h e solvent, and the state of the molecule ( e. g. , dissolved or 1 1 molar exti nctio n coe ffi cie nt of t he pestici de (L mol c m ). s oli d). 3 8 – 4 2 2- N B ca n be used eit her at a hig h co nce ntratio n ( e. g. , Eq n (1) ca n be si mpli  ed to pseudo  rst-or der ki netics (e q n 0.1 M), w here it absorbs all i ncide nt lig ht a nd ex hibits pseudo (2)) if t he follo wi ng t wo co nstrai nts are met: (1) lig ht a bsorptio n zero-or der p hotolysis ki netics or at a lo w co nce ntratio n ( e. g. , 1 0 by t he reactio n matrix is not sig ni  c a nt ( i. e. , 3l C [ a l ), a n d ( 2) m M), w here it ex hi bits pse u do  rst-or der p hotolysis ki netics. 3 8, 4 2 total lig ht a bsorptio n by pestici de a n d reactio n matrix toget her is For t he solve nt p hase experi me nts, a lo w co nce ntratio n (10 m M) s m all, F sl < 0.1, where F sl is t he fractio n of lig ht a bsor be d by t he of 2- N B aqueous solutio n was use d, for w hic h eq n (2) applies for 3 7 syste m as deter mined fro m t he absorption spectru m (eq n (3)). its pse u do  rst-or der ki netic decay. T hus, t he q ua nt u m yiel d for d C X eac h pesticide ca n be calculated usi ng eq n (6). For t he solve nt ¼ 2 :3 0 3 F E 0 ðl Þ3 C ¼ k C ( 2) d t l p l experi me nts, t he si mulated sunlig ht i nte nsity was 320 W m 2 a s deter mi ned by t he acti no meter 2- NB.

(a l + 3 l C )d X F sl ¼ 1 1 0 ( 3) 0 k p esti ci d e E p ðl Þ 2- N B 3 l ;2- N B F p esti ci d e ¼ F 2- N B X ( 6) k E 0 ðl Þ 3 In our experi ments (sunlight spectru m), both criteria are 2- N B p p esti ci d e l ;p esti ci d e s atis  ed: t he solve nts have negligible absorba nce a nd t he reaction solutions ex hibited lo w optical density. For exa mple, t he To q ua ntify lig ht i nte nsity o n surfaces, 2- N B was applied o n maxi mu m F s l for 2,4- D i n p hosp hate-bu ff ere d water is 0.09 at t he dis h s urface i n t he sa me fas hio n a nd at t he sa me loadi ng as t he absorption maxi mu m 290 n m. Quantu m yield F l was assu med pestici des ( 3 1 0 9 m ol c m 2 ). T he molar ratios of T wee n ® 20 to to be wavelength independent within the sunlight spectru m. 3 7 Published on 28 June 2018. Do wnloaded by T O WS O N ST ATE U NI VE RSIT Y on 7/5/2018 4:29:29 P M. 2- N B was 1 : 1 a n d 10 : 1 for experi me nts o n q uartz a nd para ffi n Accor di ngly, q ua ntu m yiel d ca n be calc ulated as eq n (4). wax, respectively. P hotolysis of 2- NB was perfor med u nder t he k sa me irradiatio n condition as pesticide surface sa mples. A  er F ¼ X ( 4) 0 irradiatio n, 3 mL a n d 5 mL of deio nized water, res pectively, were 2 :3 0 3 E p ðl Þ3 l added to the dishes with quartz or para ffi n wax surfaces to w here k is the pseudo  rst-order rate co nsta nt of pesticide dissolve residual 2- N B, and sa mples were taken a  er 5 mi n for 1 p hotolysis reactio n (s ). analysis. The recovery of 2- NB on surfaces was bet ween 92.8 % T he molar exti nctio n coe ffi cie nts 3 l for pestici des i n solve nts a nd 95.7 % (Table S2 † ). 2- NB was analyzed with HPL C- DA D. 2- NB were calculated using the Beer – La mbert la w based on the eluted at 1.52 min wit h 60 % Milli- Q water and 40 % acetonitrile absorptio n spectra (200 – 800 n m) measured by an Agilent Cary ( o w rate 1 mL min 1 ) and was detected at 230 n m. The molar 60 UV-Vis spectrophoto meter. The background absorption of exti nctio n coe ffi cie nt of t he acti no meter 2- N B o n quartz surface the respective solvent was subtracted. The molar extinction was measure d i n t he sa me way as t hat for t he pesticides. c o e ffi cie nts of pestici des o n q uartz were also esti mate d: a q uartz dis h loaded wit h a pesticide co mpou nd (6.17 1 0 8 m ol c m 2 ; T ween ® 20 to pesticide molar ratio 5 : 1) was placed in the 3 Results and discussion spectrophoto meter, and the light trans mission was co mpared with that measured through a disk loaded with surfactant 3.1 P hotolysis of c hlori nated p he noxyacetic acid herbicides alo ne. T he di ff erence was used to approxi mate the absorbance i n solve nts of pesticides in the solid for m. Further details on the Direct photolysis of the six chlorinated phenoxyacetic acid measure ment of absorbance on quartz surface are provided in her bici des was  rst i nvestigate d i n solve nts s pa n ni ng a ra nge of Text S 4. † T he molar exti nctio n coe ffi cie nts of pestici des o n wax polarities: water, aceto nitrile, met ha nol, 2-propa nol, a nd n - cannot be deter mined using this method due to the negligible hepta ne. Due to solubility co nstrai nts, 2,4- D a nd 2,4,5- T were

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not eval uate d i n n -heptane. Preli minary experi ments sho wed met hyl esters i n 2-propa nol was 6 – 9 ti mes faster t ha n t hat i n t hat t he ester co mpou nds hydrolyzed rapidly i n water: less t ha n met ha nol a n d at least 35 ti mes faster t ha n t hat i n aceto nitrile. 10 % of the initial 2,4- D ME and 2,4,5-T ME mass re mained in In the sa me solvent, the rate constants across di ff ere nt p he- water a  er 6 h i n t he dark, w hic h is co nsiste nt wit h previous noxyacetic acid herbicides were within the sa me order of reports of facile hydrolysis of t hese esters. 4 3 T herefore, t he mag nit u de. photolysis of the four ester co mpounds was not evaluated in Give n t hat t he polarity of t he solve nts follo we d t he or der n - water. Additionally, we observed that signi  ca nt tra ns- heptane < 2-propanol < methanol z aceto nitrile, t he res ults est eri  cation occurred bet ween butoxyethyl esters and meth- suggested t hat faster p hotolysis occurred i n solve nts of lo wer a nol i n t he dark; less t ha n 30 % of t he i nitial 2,4,5- T B E E mass polarity. T he correlatio n bet wee n p hotolysis rate co nsta nts a n d re mained in methanol a  er 48 h i n t he dark ( Text S3 † ). T h u s, solve nt polarity ( measured by t he dielectric co nsta nt) is s ho w n photolysis experi ments of butoxyethyl esters were not per- i n Fig. S3. † T h e i n  ue nce of orga nic solve nt polarity o n t he for med in methanol. direct p hotolysis of c hlori nate d p he noxyacetic aci d her bici des is Upon irradiation, all six co mpounds decayed follo wing consistent with that observed for the persistent organic appare nt  rst-or der ki netics ( Fig. S1 A – F † ). No degradatio n was polluta nt octac hlorodibe nzo- p -dioxin and azoxystrobin fungi- observed in dark control sa mples (data not sho wn). The cide, w hic h p hotodegraded faster i n orga nic solve nts of lo wer appare nt  rst-order photolysis rate constants in di ff ere nt p ol arit y. 8, 4 4, 4 5 Ho wever, the opposite trend was reported for solve nts are s ho w n i n Fig. 1 (le  pa nel) a n d Table S3. † A mong polycyclic aro matic hydrocarbons and the vita min ribo  a vi n, t he orga nic solve nts teste d, 2- pro pa nol feat ure d t he fastest 2,4- both of which photodegraded faster in solvents of higher D and 2,4,5-T photolysis. Methanol and acetonitrile yielded p ol arity ( e. g. water, aceto nitrile, a n d met ha nol) t ha n i n solve nts si milar rate co nsta nts, 6 – 11 ti mes lo wer than 2-propanol. For of lo wer polarity ( e. g. ethyl acetate, cyclohexane, and t he four ester derivatives (2,4- D ME, 2,4,5-T ME, 2,4- D BEE, a nd hexa ne). 4 6 – 4 8 It is worth noting that the trend observed for 2,4,5- T B E E), a si milar tre n d was o bserve d: p hotolysis was fastest organic solvents does not extend to water, the most polar i n n -heptane, follo wed by 2-propanol, methanol, and acetoni- solve nt teste d i n t his st u dy. For exa mple, 2,4,5- T p hotolysis i n trile. T he p hotolysis rate co nsta nts for 2,4- D M E, 2,4,5- T M E, 2,4- water (p H 7.0) was t hree ti mes faster t ha n t hat i n met ha nol a n d D BEE, a nd 2,4,5-T BEE i n n - hepta ne were 2 – 4 ti mes hig her t ha n aceto nitrile, alt houg h still 3 ti mes slo wer t ha n i n 2-propa nol. t he correspo ndi ng rate co nsta nts i n 2-propa nol. P hotolysis of This may be attributed to the di ff erent reaction path ways do minating in water co mpared to those occurring in organic solve nts (see f urt her disc ussio n i n Sectio n 3.5). T o di ff ere ntiate w het her solve nts i n  ue nced p hotolysis by altering light absorption of herbicides or by altering their reactio n pat h ways, t he molar exti nctio n coe ffi cie nts i n di ff ere nt solvents were deter mined (Fig. 2 and S2 † ). As solve nt polarity i ncreases i n t he or der of n -heptane < 2-propanol < methanol z aceto nitrile, t he absorptio n spectra of all c hlori nate d p he noxyacetic aci d her bici des ex hi bite d a slig ht bl ue s hi  , b ut t he s hi  Published on 28 June 2018. Do wnloaded by T O WS O N ST ATE U NI VE RSIT Y on 7/5/2018 4:29:29 P M.

Fi g. 1 Apparent fi rst- or der p h ot olysis rate c o nsta nts f or 2, 4- D, 2, 4,5- T, and their methyl and but oxyethyl esters in di ff erent s olvents ( open and blue shaded bars), and on quartz and para ffi n wax surfaces ( hatc he d bars). Err or bars re prese nt t he sta n dar d deviati o n of d u plicate experi ments. Hep ¼ n - he ptane, I P A ¼ 2-propanol, Me O H ¼ m et h a n ol, A C N ¼ acet o nitrile, q uartz ¼ quartz surface, Wax ¼ p ar a ffi n wax surface. The values of the apparent fi rst-order photolysis rate c o nsta nts are liste d i n Ta ble S3. † C onditi ons: the sunlight si mulat or intensity was set to 0.68 W m 2 at 3 4 0 n m, wit h t otal irra diati o n of 320 W m 2 as deter mined by the actino meter 2- N B; initial concen- trati o n: 2 0 m M for 2,4- D and 2,4,5-T, and 5 m M for esters; 5 m M phosphate bu ff er (p H 7.0) for experi ments in water; 26 C. I niti al surface pesticide c oncentrati on: 3 1 0 9 m ol c m 2 on quartz or Fi g. 2 M olar exti ncti o n c oe ffi cie nts of 2, 4- D i n s olve nts a n d o n q uartz p ar a ffi n wax surface; surfactant (T ween ® 20) concentration: 3 s urface. I P A ¼ 2-propanol, Me O H ¼ methanol, A C N ¼ a c et o nitril e, 1 0 9 m ol c m 2 o n q uartz s urface, a n d 3 1 0 8 m ol c m 2 o n para ffi n q uartz ¼ quartz surface. Absorbance was measured using a 20 m M wax surface. 2,4- D and 2,4,5-T were not evaluated in n -heptane; aqueous sa mple (5 m M phosphate bu ff er, p H 7. 0), a 5 m M orga nic esters were n ot evaluated in water; 2,4- D BEE and 2,4,5-T BEE were n ot s olvent sa mples, and a 6.17 1 0 8 m ol c m 2 quartz surface sa mple, evaluated in methan ol. r es p ectively.

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was less than 1 n m at the absorption peaks near 290 n m. acco u nt for t he faster p hotolysis o n q uartz base d o n t wo li nes of Considering the absorption spectra of the chlorinated phe- evi de nce:  rst, T ween ® 20 has negligible sunlight absorption noxyacetic acid herbicides and the sunlight spectru m, the (Fig. S4 † ); second, the photolysis rate constants were not Ð 0 a mount of light absorbed can be calculated by E p (l )3 l d l , si g ni  ca ntly di ff erent bet ween experi ments in which the molar integrated across wavelength 290 – 315 n m (Table S4 † ). T h e ratios of 2,4,5- T B E E to T wee n ® 20 was 1 : 1 (0.22 0.0088 h 1 ) variatio n of lig ht a bsor ptio n by pestici des i n di ff ere nt solve nts co mpare d to 1 : 10 (0.22 0.0077 h 1 ). Neit her ca n t he di ff er- was wit hi n 41 %, s u bsta ntially s maller t ha n t he variatio n of t heir e nce be attrib ute d to hig her e ff ective lig ht i nte nsity o n q uartz, p hotolysis rate co nsta nts. because the che mical actino meter 2- NB, which photodegrades The quantu m yields for the six chlorinated phenoxyacetic vi a intra molecular rearrange ment, decayed at si milar rates on acid herbicides were calculated usi ng eq n (6) a nd are s ho w n i n quartz and on wax surfaces (<12 % di ff ere nce, Fig. S1I † ). Pr e vi- Fig. 3 a n d Ta ble S3. † T he relative mag nitude of qua ntu m yields ously, herbicide isoproturo n a nd pla nt activator acibe nzolar- S - i n di ff ere nt orga nic solve nts was si milar to t hat of t he appare nt met hyl were also reported to p hotodegrade faster o n glass t ha n  rst- or der rate c o nsta nts ( n -heptane > 2-propanol > methanol z o n para ffi n wax a n d leaf-extracte d c uti n s urface by a factor of 1 – aceto nitrile), as wo ul d be expecte d fro m t he s mall di ff ere nce i n 3 upo n U V a nd su nlig ht irradiatio n. 4 9, 5 0 lig ht absorptio n i n di ff erent solvents. The quantu m yields of When co mpared with photolysis in solvents, photolysis on 2,4- D ME and 2,4- DBEE in n - hepta ne were 2.4 1 0 2 a n d 1.7 q uartz was m uc h faster. T he rate co nsta nts of 2,4- D a n d 2,4,5- T 1 0 2 , res pectively, w hic h is lo wer t ha n t hose previo usly re porte d p hotolysis o n quartz were 34 a nd 8 ti mes hig her, respectively, i n n -hexane and n - hexadeca ne (0.13 – 0. 1 7). 4 3 T he reaso n for t his than the corresponding rate constants in 2-propanol, where discrepancy is unkno wn. t heir p hotolysis was t he fastest a mo ng t he solve nts teste d. 2,4- D BEE and 2,4,5-T BEE photolysis on quartz were 2 and 3 ti mes faster t ha n i n n - hepta ne. I n co mpariso n, p hotolysis o n para ffi n 3.2 P hotolysis of c hlori nated p he noxyacetic acid herbicides wax procee de d at si milar rates as t hat i n solve nts of lo w polarity o n quartz a nd para ffi n wax s urfaces (i. e. , n -heptane and 2-propanol). The di ff ere nce i n p hotolysis T he direct p hotolysis of 2,4- D, 2,4,5-T, 2,4- D BEE, a nd 2,4,5- rates a mong the four co mpounds on the sa me surface was TBEE was exa mined on quartz and para ffi n wax surfaces. T he wit hi n a fact or 7. t wo met hyl esters were not eval uate d d ue to t heir hig h volatility. T he observatio n t hat c hlori nated p he noxyacetic acid herbi- Si milar to t hat i n t he b ulk solve nt p hase, p hotolysis o n s urfaces cides photodegraded faster on quartz surface than in organic also follo wed appare nt  rst-or der ki netics (Fig. S1 G a n d S1 H † ). s olve nts ( i. e. , 2-propa nol, met ha nol a nd aceto nitrile) a nd water No degradatio n was observed i n dark co ntrol sa mples (data not is opposite to t he tre nds observed for herbicides clet hodi m a nd s ho w n). sethoxydi m 1 5 and fungicide benzothiostrobin, 2 9 b ut is c o nsis- T he appare nt  rst-or der rate co nsta nts for pestici de p hotol- te nt wit h t hose for fu ngicide guazati ne, a nd herbicides be nta- ysis o n s urfaces are s ho w n i n Fig. 1 (rig ht pa nel) a n d Ta ble S3. † zo n, clopyralid, sulcotrio ne, a nd mesotrio ne. 6, 1 8, 2 6, 2 8 I n t h e  r st For all four co mpounds, photolysis was 3 – 9 ti mes faster o n t wo st u dies, 1 5, 2 9 pestici de sol utio ns were directly pipette d o nto q uartz t ha n o n para ffi n wax. Lo wer surfacta nt loadi ng was used the glass surface without surfactant, while the other on quartz surface due to its lo wer hydrophobicity than wax. st u dies 6, 1 8, 2 6, 2 8 used surfactants to load pesticides or used

Published on 28 June 2018. DoNevert wnloaded by T O WS heless, O N ST ATE U NI VE di RSITﬀ Y on ere7/5/2018 4:29:29 nces P M. i n surfacta nt loadi ng are u nlikely to co m mercial for mula t hat co ntai ned surfacta nts. T herefore, t he

Fi g. 3 Q ua nt u m yiel ds f or t he direct p h ot olysis of 2, 4- D, 2, 4,5- T, a n d t heir esters i n di ﬀ ere nt s olve nts. Err or bars re prese nt t he sta n dar d deviati o n of duplicate experi ments. Hep ¼ n -heptane, I P A ¼ 2-propanol, Me O H ¼ methanol, A C N ¼ acet onitrile. The values of the quantu m yields are list e d i n Ta bl e S 3. † 2,4- D and 2,4,5-T were n ot evaluated in n -heptane; esters were not evaluated in water; 2,4- D BEE and 2,4,5-T BEE were not evaluated in methan ol.

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slo w p hotolysis o n surface observed i n t he  rst t wo st u dies was 3.3 Photoproducts of chlorinated phenoxyacetic acid likely attributable to t he i nterfere nce of lig ht absorptio n by herbici des i n orga nic solve nts pestici de aggregates. Previous researc h suggested t hat 2,4- D a nd its esters u ndergo Out of the four organic solvents and t wo surface syste ms p h ot olysis vi a three path ways in water: (I) photoreductive tested i n t his study, para ffi n wax surface best si mulates the dec hlori natio n, (II) p hotos ubstit utio n of c hlori ne by a hy droxyl reaction environ ment on leaves. Using the photolysis rate gro u p, a n d (III) cleavage of t he et her bo n d (Fig. 4 s ho wi ng 2,4,5- constants sho wn in Fig. 1 (320 W m 2 s u nlig ht i nte nsity) a n d T a n d its esters). 4 3, 5 4, 5 5 Because photoreductive dechlorination accounting for the diurnal change in sunlight intensity for was previo usly reporte d as a major pat h way for t he p hotolysis of 00 a midsu m mer day at 40 N latit u de (sea level) u n der clear 2,4- D ME and 2,4- DBEE in non-polar solvents hexane and hex- s ki e s, 3, 5 1 t he half-lives of 2,4- D, 2,4,5- T, 2,4- D B E E, a nd 2,4,5- a deca ne, 4 3 the corresponding products were  r st t ar g et e d i n t h e T B E E were esti mated to be 72, 42, 159, a nd 27 h, respectively. orga nic solve nt sa mples of 2,4,5-T BEE a nd 2,4,5-T ME. Wit h t he possible exceptio n of 2,4- D B E E, t hese p hotolysis half- For 2,4,5-T BEE, sa mples i n n -heptane and 2-propanol were lives are on the sa me order of magnitude as the half-lives a nalyze d b ut not t hose i n aceto nitrile d ue to i nsig ni  c a nt 2, 4, 5- previously reported for biodegradation (30 – 4 0 h), 3 3 w hic h is T BEE decay (<10 % a  er 72 h). P hotore d uctive dec hlori natio n of note wort hy give n t hat bio degra datio n is c urre ntly co nsi dere d as 2,4,5-TBEE can for m three iso meric butoxyethyl dichlor- 5 2 t he major dissipatio n pat h way for t hese herbicides. ophenoxyacetates ( DBEEs, i. e. , 2,4- DBEE, 2,5- DBEE, and 3,4- Asshownin Fig. 2 and S2, † the experi mentally deter mined D B E E), b ut a n a ut he ntic sta n dar d is o nly availa ble for 2,4- D B E E. molar exti nctio n coe ffi cients of chlorinated phenoxyacetic Because of t he si milar octa nol – water partitio n coe ffi cie nts of acid herbicides on quartz surface, despite being the upper t he t hree iso mers, 5 6 – 5 8 it was hypot hesized t hat t hey would have bo u n d (Sectio n 2.4), is sig ni  ca ntly lo wer t ha n t hose i n t he very si milar retention ti mes in HPL C. We observed only one solve nt p hase. T he qua ntu m yields o n quartz calculated usi ng peak at the retention ti me corresponding to 2,4- DBEE, sug- these molar extinction coe ffi cie nts are t herefore t he lo wer gesti ng t hat eit her all t hree iso mers co-eluted or 2,4- D B E E was bo u n d of t he tr ue val ues, b ut t hey are still greater t ha n t hose t he do mi na nt product. Additio nally, it was esti mated t hat t he in the solvent phase. For exa mp le, the calculated quantu m three iso mers would feature si milar molar extinction coe ffi - yields for 2,4- D and 2,4,5-T on quartz (0.12 and 0.070, cie nts at 230 n m (t he D A D detectio n wavele ngt h) wit h di ff ere nce respectively) are 38 and 6.4 ti mes higher than those in 2- within 5 %, which was the di ff erence bet ween the molar propa nol, a nd t he calculated qua ntu m yields for 2,4- D BEE a nd exti nctio n coe ffi cie nts for 2,4- D BEE a nd 2,4,5-T BEE i n met ha nol 2,4,5- T BEE o n quartz (0.074 a nd 0.028, respectively) are 4.3 at 230 n m ( Table S5 † ). Accordi ngly, t he total a mou nt of D BEEs a nd 1.2 ti mes hig her t ha n t hose i n n - hepta ne. O ur res ults are were esti mated using the 2,4- D BEE calibration curve. Fig. 5A a mong the very  rst atte mpts to qua ntify qua ntu m yields for sho ws the evolution of DBEEs in n -heptane and 2-propanol pestici de p hotolysis o n s urfaces. T he o nly ot her s uc h report 5 3 fro m 2,4,5-T BEE p hotolysis. I n t he early p hase of t he experi- was on the quantu m yield of i midacloprid, a neonicotinoid m e nt s ( i. e. , <40 % 2,4,5-TBEE decay), DBEE for mation accounted insecticide, on the top surface of a clean ger maniu m AT R for all of t he 2,4,5- T B E E decay. Wit h more exte nsive 2,4,5- T B E E crystal ( 1. 6 1 0 3 at 305 n m), and sho wed that it was 3 – 9 decay, t he accu mulatio n of D B E Es deviated fro m t he 1 : 1 li ne, ti mes s maller t ha n t hat i n water. Because t he molar exti nctio n which may be attributed to their further photolysis. Indeed,

Published on 28 June 2018. Doc wnloaded o e by Tﬃ O WScients O N ST ATE U NIon VE RSIT Y waxon 7/5/2018 4:29:29cannot P M. be deter mined using the absor- a ne w HPL C peak, with a shorter retention ti me than the D BEE bance method (Section 2.4), the corresponding quantu m peak, appeared in the n -heptane sa mple when >83 % of the yiel ds were not calc ulate d. i nitial 2,4,5- T B E E had decayed.

Fi g. 4 Putative ph ot olysis path ways of chl orinated phen oxyacetic acid herbicides: (I) ph ot oreductive dechl orinati on, (II) ph ot osubstituti on of chl orine by a hydr oxyl gr oup, and (III) cleavage of the ether b ond.

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Usi ng a si milar met hod, t he met hyl dic hlorop he noxyacetates ( D M E s, i. e. , 2,4- D ME, 2,5- D ME, and 3,4- D ME) were quanti  e d i n t he 2,4,5-T ME sa mples i n 2-propa nol a nd met ha nol (Fig. 5 B). I n bot h solve nts, t he for matio n of D M Es accou nted for 68 – 100 % of 2,4,5-T ME decay throughout the experi ment. The n - hepta ne sa mples for 2,4,5-T ME photolysis were not analyzed by HPL C due to the extensive loss (presu mably due to volatilization) during solvent exchange for reverse-phase HPL C analysis. Ho wever, the for mation of the three D MEs fro m 2,4,5-T ME p h ot olysis i n n - hepta ne, as well as i n 2-propa nol a nd met ha nol, was c o n  r med by G C- MS using che mical ionization and the full scan mode of MS. The most distinct product peaks for the irradiated 2,4,5-T ME sa mples are t hree co nsecutive peaks wit h rete ntio n ti mes of less t ha n 0.2 mi n apart (Fig. 6). T hese peaks sho w identical mass spectra featuring m /z 235, 237, a nd 239 wit h a rati o 9 : 6 : 1 ( Fi g. S 7 B – D † ), w hic h s uggest t he prese nce of t wo chlorine ato ms in the molecule. The second peak was c o n  r med as 2,4- D ME (298.3 2 5. 0 C) 5 7 usi ng a n aut he ntic sta n dar d. Base d o n t he boili ng poi nts of t he t hree iso mers, 5 6, 5 8 it was suspected that the  rst and third peaks were 2,5- D ME ( 2 9 6. 1 2 5. 0 C), a nd 3,4- D M E (302.9 2 7. 0 C), res pectively. T he sig nal i nte nsity for all t hree iso mers i ncrease d as 2,4,5- T M E degra de d (Fig. S6 † ). For the 2-propanol and methanol sa mples (up to 24 % 2,4,5-T ME decayed), t he G C- MS spectra did not s ho w ot her pro d uct peaks (c uto ﬀ sig nal t o n oise rati o of 5). F or t he n - heptane sa mple in which 86 % of 2,4,5-T ME decayed, an additio nal product peak was detected wit h a rete ntio n ti me s horter than 2,4- D ME and a mass spectru m consistent with the presence of one chlorine ato m ( m /z 201 a nd 203, ratio 3 : 1, Fig. S 7 E † ). T his suggests t hat t he D MEs ca n furt her degrade to for m mo noc hlori nated products. T his observatio n also suggests t hat t he u nide nti  ed HPL C peak observed in the irradiated 2, 4, 5- T B E E n -heptane sa mples could be a butoxyethyl chlor- ophenoxyacetate. 2,4,5-TBEE sa mples cannot be analyzed by G C- MS because of the lo w ther mal stability of butoxyethyl

Published on 28 June 2018. Do wnloaded by T O WS O N ST ATE U NI VE RSIT Y on 7/5/2018 4:29:29 P M. group, w hic h ca n i ntroduce G C artifacts. T he for matio n of 2,4,5-tric hlorop he nol, t he product a ntici- pated fro m the cleavage of the ether bond of 2,4,5-TBEE and 2,4,5- T ( pat h way III), 4 3, 5 4, 5 9 was monitored using HPL C, but its concentration was belo w the detection li mit in all sa mples, corresponding to a yield less than 1.0 % fro m parent co mpound p h ot olysis.

3.4 Photoproducts of chlorinated phenoxyacetic acid herbici des o n s urfaces The 2,4,5-T and 2,4,5-TBEE surface sa mples were analyzed using HPL C and UPL C-ESI( )-qT OF. Fig. 5 C and D sho w the evolutio n of D B E Es a nd dic hlorop he noxyacetic acids fro m 2,4,5- TBEE and 2,4,5-T photolysis on surfaces, respectively, as deter mined by HPL C. Concentrations of the dechlorination Fi g. 5 For mation of reductive dechlorination products fro m the products of 2,4,5-TBEE (su m of 2,4- DBEE, 2,5- DBEE, and 3,4- p h ot olysis of ( A) 2, 4,5- T BEE a n d ( B) 2, 4,5- T ME i n orga nic s olve nts, a n d D BEE) were quanti  ed by HPL C as described i n Sectio n 3.3. A ( C) 2,4,5-T BEE and ( D) 2,4,5-T on surfaces. Hep ¼ n - he ptane, I P A ¼ 2 - propanol, Me O H ¼ met ha n ol, q uartz ¼ q uartz s urface, wax ¼ p ar a ﬃ n P P D BEEs ¼ (2,4- DBEE, 2,5- DBEE, 3,4- DBEE), D MEs ¼ ( 2, 4 - D M E, wax surface. C onditi ons: the sunlight si mulat or intensity was set t o P 2,5- D ME, 3,4- D ME), and Ds ¼ (2,4- D, 2,5- D, 3,4- D). Dash lines 0.68 W m 2 at 34 0 n m; initial c oncentrati on of parent c o mp ound: 1 0 correspond to 1 : 1 stoichio metric conversion fro m the parent m M in organic s olvents and 3 1 0 9 m ol c m 2 on surfaces; 2 6 C. co mpound to the product.

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Fi g. 6 Gas chr o matogra m f or 2,4,5-T ME and its degradati on pr oducts after 12 h irradiati on in n -heptane. G C- MS conditions: initial colu mn te mperature was set at 9 0 C and held f or 1 min, and then increased t o 27 0 C at a rat e of 1 0 0 C mi n 1 a n d hel d f or 1 0 mi n. MS f ull sca n wit h che mical ionization using methanol. The mass range of the MS scan was 50 – 4 0 0 m /z .

si milar met hod was also used to qua ntify t he co nce ntratio ns of co mpound decay ( tm ), a nd t he ti me correspo ndi ng to approxi-

Published on 28 June 2018. Dodec wnloaded by T hlori O WS O N ST natio ATE U NI VEn RSITpro Y on 7/5/2018 d 4:29:29 ucts P M. of 2,4,5- T, for w hic h t he s u m of 2,4- D, mately 90 % parent co mpound decay or 48 h whichever is

2,5- D, and 3,4- D was esti mated using the HPL C calibration s h orter ( tf). Table 2 su m marizes t he results regardi ng t he t hree

c urve of 2,4- D. groups of putative photoproducts sho wn in Fig. 4 for the tf

I n co ntrast to t hat observed i n orga nic solve nts, t he a mou nt sa mples. No ne of t hese putative products were detected i n t he t0 of photoreductive dechlorination products accounted for less or tm sa mples with su ﬃ cie nt co n  dence. Dichlorophenoxy- than 60 % of the parent co mpound decay on surfaces on a molar acetic acids were detected fro m 2,4,5- T p hotolysis o n surfaces, basis. On wax surface, DBEEs accounted for 51 – 60 % of 2,4,5- while DBEEs were not detected fro m 2,4,5-TBEE photolysis. TBEE decay, while dichlorophenoxyacetic acids accounted for Ho wever, monochlorinated products were detected in both 2 8 – 43 % of 2,4,5- T decay; o n quartz surface, t he perce ntage was 2,4,5- T a nd 2,4,5- T B E E sa mples, suggesti ng furt her decay of t he eve n lo wer (10 – 42 %). Si milar to t hat observed i n solve nts, t he dic hlori nated products. T he putative products correspo ndi ng to yield of 2,4,5-trichlorophenol fro m 2,4,5-T and 2,4,5-TBEE p hotosubstitutio n of c hlori ne by a hydroxyl group (II) a nd t he p hotolysis o n surfaces was also less t ha n 1.0 %. A rece nt study cleavage of t he et her bo nd (III) were not detected. T he abse nce o n t he p hotoproducts of 2,4- D et hyl ester o n glass surface a nd of DBEE signals in the UPL C-ESI( )-q T OF a nalysis for 2,4,5- leaf c uti n s urface propose d t hat 2,4- D et hyl ester ca n hy drolyze T B E E is i n co ntrast to t heir detectio n by HPL C (Fig. 5 C). It may to for m the corresponding acid (2,4- D), 6 0 but 2,4,5- T was not be attri b ute d to t he s urfacta nt prese nt i n t he extracts of s urface detected in the 2,4,5-T BEE surface sa mples, corresponding to sa mples, w hic h ca n sig ni  ca ntly s uppress io nizatio n d uri ng t he a yield less than 1.5 % fro m parent co mpound decay. These UPL C- ESI( )-qT OF analyses. Overall, the UPL C-ESI( )- q T O F  ndings suggest that ester hydrolysis of 2,4,5-T BEE did not a nalysis of 2,4,5- T a nd 2,4,5- T B E E p hotoproducts co n  r m e d t h e appreciably occur i n t he exa mi ned syste ms. prevalence of the photoreductive dechlorination path way (I), For each surface experi ment, additional sa mples were but did not identify ne w products that can close the mass collected for UPL C- ESI( )-q T OF a nalysis at t hree ti me poi nts: balance for phenoxyacetic acid herbicide photolysis on

ti me zero ( t0 ), the ti me corresponding to 25 % parent surfaces. T he a nalysis of 2,4- D a nd 2,4- D BEE p hotoproducts o n

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Ta bl e 2 Photoproducts of 2,4,5-T and 2,4,5-T BEE analyzed by UPL C-ESI( )-qT OF. a T h e t hr e e gr o u p s of p ut ati v e p h ot o pr o d u ct s (I), (II), a n d (III)

c orres p o n d t o t he str uct ures i n Fig. 4. tf ¼ reaction ti me corresponding to approxi mately 90 % parent co mpound decay or 48 h, whichever is s h orter; R T ¼ r et e nti o n ti m e; m /z ¼ measured mass-t o-charge rati o; D m /z ¼ ( measured m /z – calc ulate d m /z )/calc ulate d m /z 1 0 6 ; N. D. ¼ n ot detected

P utative pro d ucts

(I) b

c d Parent co mpound Surface tf (h) Parent decay Dichlorinated Monochlorinated (II) (III)

2, 4, 5- T Quartz 6 87% Detecte d e Detecte d e N. D. N. D. R T ¼ 3.73 min RT ¼ 1.62 mi n m /z ¼ 218.9620 m /z ¼ 185.0004 D m /z ¼ 1.8 pp m D m /z ¼ 0.5 pp m Wax 48 90% Detecte d e Detecte d ( weak sig nal) e N. D. N. D. R T ¼ 3.69 min RT ¼ 1.62 mi n m /z ¼ 218.9624 m /z ¼ 185.0020 D m /z ¼ 3.7 pp m D m /z ¼ 8.1 pp m 2, 4, 5- T B E E Quartz 6 73% N. D Detecte d e N. D. N. D. R T ¼ 7.74 mi n m /z ¼ 285.0834 D m /z ¼ 21 pp m Wax 48 98% N. D Detecte d e N. D. N. D. R T ¼ 7.71 mi n m /z ¼ 285.0899 D m /z ¼ 1.8 pp m a Pro d uct i de nti  cation was based on measure ments of exact masses; authentic standards were not used. b Reductive dec hlori natio n products: dichlorinated and monochlorianted phenoxyacetic acids or butoxyethyl phenoxyacetates, corresponding to path way (I) in Fig. 4. c Pro d ucts for med t hroug h p hotosubstitutio n of c hlori ne by a hydroxyl group, correspo ndi ng to pat h way (II) i n Fig. 4. d Et her bo nd cleavage product 2,4,5- tric hlorop he nol, correspo ndi ng to pat h way (III) i n Fig. 4. e It is u nk no w n w hic h iso mer(s) is/are represe nted by t his peak.

quartz a nd wax surfaces by UPL C- ESI( )- q T OF also di d not yiel d p hotosubstitutio n of c hlori ne by a hydroxyl group a nd cleavage additio nal i nfor matio n ( Table S6 † ). of t he et her bo n d (II a n d III i n Fig. 4), rat her t ha n re d uctive dec hlori natio n. 5 4, 5 9 T his di ﬀ ere nce i n pat h ways may accou nt for 3.5 Direct photolysis mechanis m of chlorinated the higher quantu m yields of 2,4- D and 2,4,5-T in water than p he noxyacetic acid herbicides t hose i n met ha nol a n d aceto nitrile, despite t hat water is a more polar solvent. Nevertheless, the quantu m yields of 2,4- D and We observed that reductive dechlorination is the do minant 2,4,5- T i n water is 3 – 9 ti mes lo wer t ha n t hose i n 2-propa nol. path way for the direct photolysis of trichlorinated phenoxy- I n a d ditio n to polarity, H- do nati ng ability also varies a mo ng

Published on 28 June 2018. Doacetic wnloaded by T O WSacid O N ST ATE herbicides U NI VE RSIT Y on 7/5/2018 4:29:29 (2,4,5-P M. T B E E a nd 2,4,5- T M E) i n met ha nol, aceto nitrile, met ha nol, 2-propa nol, a n d n - hepta ne. Because t he 2-propa nol, a n d n - he pta ne (Fig. 5 A a n d B). Previo usly, t he b utyl, p hotoreductive dec hlori natio n process i nvolves t he ge neratio n methyl, and butoxyethyl esters of 2,4- D were also sho wn to of a ra dical vi a ho molysis of the C – Cl bond in an excited undergo photoreductive dechlorination in hexane and hex- molecule and subsequent abstraction of a hydrogen ato m fro m a deca ne. 4 3, 5 5 Our results suggest t hat p hotoreductive dec hlori- s ol v e nt, 9 t he H-do nati ng ability of solve nt molecules may also nation can be i mportant in the more polar solvents methanol i n  uence photolysis. Table S7 † co mpares the dissociation a nd 2-propa nol. Additio nally, t he results also suggest t hat t he energy of the weakest C – H bond in the four solvent mole- additio nal c hlori ne substitutio n o n t he aro matic ri ng does not c ul e s. 6 1 – 6 4 n - Heptane has the highest C – H bond dissociation si g ni  ca ntly i mpact t he reactio n pat h ways of c hlori nated p he- energy, follo wed by methanol and acetonitrile, and then 2- noxyacetic acid herbicides. The si milar photolysis behavior propa nol. Si nce t he poorest H-do nor ( n - hepta ne) ex hibited t he bet ween the esters of 2,4,5-T and 2,4- D in organic solvents fastest p hotolysis, it ca n be co ncl u de d t hat H-a bstractio n is not ec hoes t he si milarity i n degradatio n pat h ways bet wee n 2,4,5-T rate-li miti ng a n d t hat t he H- do nati ng ability of orga nic solve nts and 2,4- D in water. 4 3, 5 4, 5 9 Faster photolysis was observed in does not appreciably i n  ue nce p hotolysis rate. organic solvents of lo wer polarity, where the quantu m yields On quartz and para ﬃ n wax surfaces, t he c hlori nated p he- were hig her (Fig. 1 a n d 3). It was previo usly pro pose d t hat polar noxyacetic acid herbicides degraded faster than or at co mpa- solve nts are more likely to i nteract wit h a nd to que nc h polar- ra ble rates as i n orga nic solve nts of lo w polarity ( Fig. 1). T he fast iza ble excite d states relative to less polar solve nts; t his ratio nale p hotolysis o n quartz was due to t he hig h qua ntu m yields rat her was used to explain the decrease in dioxin direct photolysis t ha n stro ng lig ht absorptio n. I n fact, t he lig ht absorptio n of rates wit h i ncreasi ng solve nt polarity. 8 This mechanis m may p he noxyacetic acid herbicides o n quartz is lo wer t ha n t hat i n also a p ply to o ur syste ms. orga nic solve nts (Fig. 2 a nd S2 † ). Although the presence of Previous studies have i nvestigated p hotolysis of 2,4- D a nd surfacta nt i n surface sa mples likely hi ndered product a nalysis 2,4,5-T in water and reported that the major path ways are

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by UPL C- ESI( )-qT OF, dechlorination was sho wn to be a prev- References ale nt pat h way for 2,4- D, 2,4,5- T, a nd t heir butoxyet hyl esters. Ho wever, the apparent yields of the dechlorination products 1 T. Katagi, P hotodegradatio n of pesticides o n pla nt a nd soil (i. e. , dichlorophenoxyacetic acids and DBEEs fro m 2,4,5-T and s urfaces, Rev. Environ. Conta m. Toxicol. , 2004, 1 8 2 , 1 – 1 8 9. 2,4,5-TBEE, respectively) on surfaces was much lo wer than 2 C. K. Re mucal, The role of indirect photoche mical t hose i n orga nic solve nts (Fig. 5). T here are at least t wo possi ble degradation in the environ mental fate of pesticides: reasons: (1) photoreductive dechlorination on surfaces is so a r e vi e w, Environ. Sci.: Processes I mpacts , 2014, 1 6 , 6 2 8 – 6 5 3. facile t hat t he p hotoproducts rapidly u ndergo furt her dec hlo- 3 R. P. Sch warzenbach, P. M. Gsch wend and D. M. I mboden, ri natio n; a n d/or (2) alter native reactio n pat h ways are sig ni  c a nt Environ mental Organic Che mistry , Wiley, Hoboke n, N.J., 2 nd o n surfaces. Assu mi ng t he  rst reaso n to be vali d, a preli mi nary e d n, 2003. kinetic model was developed for the accu mulation of DBEEs 4 T. Zeng and W. A. Arnold, Pesticide photolysis in prairie fro m 2,4,5-T BEE photolysis on surfaces (detailed description potholes: probing photosensitized processes, E n vir o n. Sci. provi de d i n Text S5 † ). As s ho w n i n Fig. S8, † t he ti me pro  l e of T ec h n ol. , 2012, 4 7 , 6735 – 6 7 4 5. DBEE concentrations predicted by the model does not agree 5 M. W. La m, K. Tantuco and S. A. Mabury, PhotoFate: a ne w with the experi mental results. This suggests that the lo wer approach in accounting for the contribution of indirect molar ratio of dechlorination products to parent co mpound photolysis of pesticides and phar maceuticals in surface decay o n surfaces s hould be attributed to alter native reactio n w aters, E nviro n. Sci. Tec h nol. , 2003, 3 7 , 8 9 9 – 9 0 7. path ways in parallel with reductive dechlorination, and that 6 B. Eyheraguibel, A. ter Halle and C. Richard, t hese pat h ways are speci  c to s urfaces. F urt her st u dy is nee de d P hotodegradatio n of be ntazo n, clopyralid, a nd triclopyr o n to i de ntify t hese pat h ways. model leaves: i mportance of a syste matic evaluation of pesticide p hotostability o n crops, J. Agric. Food Che m. , 4 Conclusion 2 0 0 9, 5 7 , 1960 – 1 9 6 6. 7 K. Koch, B. Bhushan and W. Barthlott, Multifunctional The direct photolysis of six chlorinated phenoxyacetic acid surface structures of plants: an inspiration for herbicides (2,4- D, 2,4,5-T and their methyl and butoxyethyl bio mi metics, Prog. Mater. Sci. , 2009, 5 4 , 1 3 7 – 1 7 8. esters) was i nvestigated i n organic solve nts and o n quartz and 8 J. C hoi, W. C hoi a nd B. J. M hi n, Solve nt-speci  c p h ot olytic p ar a ﬃ n wax surfaces under si mulated su nlight. P hotolysis was behavior of octachlorodibenzo- p - di oxi n, E n vir o n. Sci. e nhanced i n solvents of lo w polarity, and the e nhance me nt was T ec h n ol. , 2004, 3 8 , 2082 – 2 0 8 8. due to higher quantu m yields rather than stronger light 9 J. Dolinova, A. Klanova, P. Klan and I. Holoubek, a bsor ptio n i n t hese solve nts. P hotolysis o n para ﬃ n wax, a s urface Photodegradation of organic p olluta nts o n t he spruce needle closely rese m bli ng leaf c uticle, 9 was slo wer t ha n t hat o n q uartz, wax surface u nder laboratory co nditio ns, Che mosphere , 2004, but was co mparable to t hat i n organic solvents of lo w polarity 5 7 , 1399 – 1 4 0 7. (i. e. , n -heptane and 2-propanol). Under the environ mentally- 10 V. Ra ma murthy, Organic photoche mistry in organized relevant conditio ns e mployed i n t his study, most of t he exa m- m e di a, Tetrahedron , 1986, 4 2 , 5753 – 5 8 3 9. i ne d p he noxyacetic acid herbici des u n der we nt p hotolysis at rates 11 W. Schwack and H. Fl o¨ ß er- M uller,¨ Fungicides and

Published on 28 June 2018. Doon wnloaded by T par O WS O N ST with ATE U NI VE rates RSIT Y on 7/5/2018 4:29:29 of P M. biodegradation, the major dissipation photoche mistry. Photodehalogenation of captan, path way currently considered for these herbicides. 5 2 T hese Che mosphere , 1990, 2 1 , 9 0 5 – 9 1 2. res ults suggest t hat p hotolysis o n leaf s urfaces ca n appreciably 12 W. Sch wack, W. Andlauer and W. Ar mbruster, Photoche mistry i n  uence the environ mental fate of phenoxyacetic acid herbi- of parat hio n i n t he pla nt cuticle e nviro n me nt: model reactio ns cides i n t he e nviro n me nt. in the presence of 2-propanol and methyl 12-hydroxystearate, Product analysis sho ws that photoreductive dechlorination Pest Manage. S ci. , 1 9 9 4, 4 0 , 2 7 9 – 2 8 4. is the do minant path way in organic solvents. While photo- 13 W. Sch wack, Photoinduced additions of pesticides to reductive dec hlori natio n also plays a n i mporta nt role o n quartz bio molecules. 2 Model reactions of D DT and methoxychlor a n d para ﬃ n wax s urfaces, it ca n not acco u nt for all of t he p ho- wit h met hyl oleate, J. Agric. Food Che m. , 1988, 3 6 , 6 4 5 – 6 4 8. todegradation as quanti  ed by loss of parent co mpounds. 14 D. E. Breithaupt and W. Sch wack, Photoinduced addition of Photolysis on surface features unique path ways that requires the fungicide anilazine to cyclohexene and methyl oleate as f urt her i nvestigatio n. model co mpou nds of pla nt cuticle co nstitue nts, Che mosphere , 2 0 0 0, 4 1 , 1401 – 1 4 0 6. C o n ﬂ icts of i nt er est 15 P. Sandin-Espana, B. Sevilla- Moran, C. Lopez- Goti, M. M. Mateo- Miranda and J. L. Alonso-Prados, Rapid The authors have no con  ict of i nterest t o declare. photodegradation of clethodi m and sethoxydi m herbicides in soil and plant surface model syste ms, Arabian J. Che m. , Ackno wledge ments 2 0 1 6, 9 , 6 9 4 – 7 0 3. 16 Z. C he n, M. J. Zabik a nd R. A. Leavitt, Co mparative study of This research was supported by the National Science Founda- t hi n  l m p hotodegradative rates for 36 pesticides, I n d. E n g. tion (#1610807; and #1531562 for the UPLC-ESI( )- q T O F Che m. Prod. Res. Dev. , 1984, 2 3 , 5 – 1 1. a nalyses).

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