Evolution Of Acute Toxicity Upon Hydrolysis And Photolysis Of Fenoxaprop-p-ethyl And Fenchlorazole-ethyl

Assessing the hazard of a pesticide requires an understanding of its degradation and toxicity of the products.Fenoxaprop-p-ethyl(FE),an aryloxyphenoxypropionate herbicide,is used to selectively control annual grass weeds in broadleaf crops.Fenchlorazole-ethyl(FCE) is the safener of FE,and they have been widely used in many countries.The purpose of this study was to elucidate the hydrolysis and photolysis behaviors of FE and FCE.The effects of different influential factors on the rate constants and the pathways of hydrolysis and photolysis of them were studied.Besides,evolution of acute toxicity upon hydrolysis and photolysis of FE was assessed using Daphnia magna.The hydrolysis of FE and FCE follow first-order kinetics at different pH levels and temperatures.FE and FCE hydrolysis rates decreased with the increasing of the ratio of acetonitrile and were greatly increased at elevated temperatures at pH=9.0.FE was relatively stable at pH=6.0,whereas it degraded rapidly with decreasing or increasing pH values.In acidic conditions(pH=4.0,5.1),the benzoxazolyl-oxy-phenyl ether linkage of FE was cleaved to form ethyl 2-(4-hydroxyphenoxy) propanoate(EHPP) and 6-chloro-2,3-dihydrobenzoxazol -2-one(CDHB).While in basic conditions(pH=8.0,9.0,10.0), fenoxaprop-p was formed via breakdown of the ester bond of the herbicide.Both the two pathways were concurrent in near neutral conditions(pH=6.0,7.0).For FCE,half-lives were greatly reduced at elevated pH levels at 30℃.In basic conditions(pH=8.0,9.0,10.0), fenchlorazole was formed via the breakdown of the ester bond of the safener.The hydrolysis rate constants of FE and FCE as a function of temperature and pH were mathematically combined to predict the hydrolytic dissipation of them.The equations suggested that the hydrolysis half-lives of FE ranged from 390 d to 24.5 d and that of FCE ranged from 192 d to 1.7 d when temperature between 10℃and 15℃and pH between 6 and 8.The maximum contribution of buffer catalysis to the hydrolysis of FE and FCE was assessed based on application of the Bronsted equations for general acid-base catalysis.The results suggested that the buffer solutions played an obvious catalysis role in hydrolysis of FE and FCE and the hydrolysis rate of them was quickened by the buffer solutions.The existence of FCE did not have significant effects on FE hydrolysis at pH 4.0 and 9.0.And FE did not impact the hydrolysis rates of FCE too. Under the irradiation atλ＞200 nm andλ＞290 nm,the photolysis of FE and FCE followed the first-order kinetics,and photodegraded rapidly atλ＞200 nm.Under solar irradiation,FE can undergo photodegradation and acetone enhanced the photolysis rates significantly.The same photoproducts of FE were formed under both high and low wavelengths irradiation experiments but relative proportions of the photoproducts were different.FE was phototransformed to at least 15 products through de-esterification,loss of carboxyl after de-esterification,rearrangement,loss of ethanol after rearrangement, dechlorination,photohydrolysis,and the breakdown of the ether linkages.The main photolysis products were photodegraded under irradiation atλ＞200 nm.4-[(6-chloro-2-benzoxazolyl)oxy] phenol(CBOP) as one of the photolysis products of FE,had a slower photolysis rate than FE atλ＞200 nm irradiation and was almost resistant to photolysis atλ＞290 nm irradiation.Therefore,photolysis of FE resulted in a more persistent product.The major degradation pathways of FCE under irradiation atλ＞200 nm were hydroxylated process and loss of trichloromethyl fragment.The trichloromethyl fragment substituted by -OH,C-N cleavage and triazole ring-opening also occurred.Under irradiation atλ＞290 nm, increasing nitrate concentrations increased the half-lives of FE and FCE due to screen of the incident light.The photolysis rates of FE and FCE increased with increasing concentrations of bicarbonate due to the degradation by both photolysis and hydrolysis,and photolysis played a more important role than hydrolysis.Under irradiation atλ＞290 nm,FCE did not have significant effects on FE degradation rate,and FE neither has significant effects on FCE photolysis.It was not possible to obtain EC₅₀ value of FCE since it is higher than its solubility in water.FE transformed products,1,4-dihydroxybenzene and CBOP were more toxic to Daphnia magna than the parent FE.Other tested products of FE were less toxic to Daphnia magna than the parent,which suggested toxicity of FE to Daphnia magna decreased due to its hydrolysis but increased due to its photodegradation.Since CBOP was resistant to photolysis atλ＞290 nm irradiation,FE resulted in a product more photochemically stable and toxic than itself.The binary mixture toxicity of CDHB and EHPP,which were both hydrolysis and photolysis products of FE,mixed in the ratio of their individual EC₅₀ values can be predicted by independent action model.