Sunlight Photolysis of 2,4-D Herbicides in Systems Simulating Leaf Surfaces
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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 This j ournal is © The R oyal S ociety of Che mistry 2 018 Envir on. Sci.: Pr ocesses I mpacts Vi e w Arti cl e O nli n e Environ mental Science: Processes & I mpacts Pa per 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.