Mechanism Of Hydrolysis And Photolysis Transformation Of The Amisulbrom In Aqueous Environment

Once released into the environment,pesticides can form a great number of transformation products by chemical degradation and biodegradation,etc.In here,both hydrolysis and photolysis are important processes in the environmental transformation of pesticides.Many studies have shown that the transformation products containing the several activity moieties of the parent compounds,have a greater bioconcentration factor or toxicity than the parent.Amisulbrom is a fungicide newly developed by Nissan Chemical Company in 2002,and its transformation products in environment were unclear.In order to clarify the degradation behavior and transformation mechanism of amisulbrom in the environment,the hydrolysis and photolysis kinetics of amisulbrom in water under laboratory conditions were investigated in this paper.Important transformation products(TPs:hydrolysis or photolysis products of amisulbrom)were identified,and the hydrolysis and photolysis mechanisms of amisulbrom were proposed.The main research results are as follows:1.Hydrolysis kinetics and mechanism of amisulbrom(1)Hydrolysis kinetics in buffer solutions,effects of pH and temperature:Amisulbrom was hydrolytic stable in acidic and neutral aqueous solutions at 25?,while quickly hydrolyzed with a half-life of 4.5 d(25?)at pH 9.0.The kinetic rate equation was determined as k=1.0234×1010 exp(-61.3760/R·T)(R2=0.9642)for amisulbrom hydrolysis at pH 9.0.(2)Hydrolysis kinetics in natural water samples:The hydrolysis of amisulbrom was performed in natural water samples such as distilled water(DW),rain water(RW),lake water(LW),paddy water(PW)and tap water(TW).The results showed that amisulbrom had the fastest hydrolysis rate in PW with a half-life of 3.7 d and the slowest in DW with half-life of 57.8 d.The DOM(dissolved organic matter),and metal and non-metal ions(ionic strength)that were ubiquitous in natural waters affected the water solubility and hydrolysis rate of amisulbrom.The content of DOM was the largest in LW(44.38 mg/L)and PW(33.82 mg/L),and the solubility of amisulbrom was also large(1.12 mg/L and 2.36 mg/L),and the hydrolysis half-life was 5.2 d and 3.7 d,respectively.(3)Hydrolysis product identification and hydrolysis mechanism:Three major hydrolysis products(HP-?,HP-? and HP-?)were identified using HPLC-DAD and UPLC-Q-TOF-MS/MS techniques.Based on the identified hydrolysis products,DFT analysis and hydrolysis profiles,possible hydrolysis pathways of amisulbrom were proposed.DFT research results showed that the hydrolysis reaction of amisulbrom was mainly the nucleophilic substitution reaction by OH radicals or H2O molecules in two sulfonamide functional group.(4)Toxicity prediction:The QSAR model was used to predict the toxicity of the hydrolysis products.The results showed that the toxicity of HP-? and HP-?was greater than that of the parent.Therefore,the toxicity of the hydrolysis products(HP-? and HP-?)is worthy of attention.2.Photolysis kinetics and mechanism of amisulbrom(1)Direct photolysis:Direct photolysis of amisulbrom was performed under low pressure mercury lamp(<390 nm)at pH=4.0,7.0 and 9.0.The result shown that the direct photolysis rates of amisulbrom were 2.5-2.8 times faster in the basic solutions than in the acidic(pH 4.0)or neutral(pH=7.0)solutions(The direct photolysis rate constants were 0.0557,0.0609,and 0.1549 min-1,and the half-lives were 12.4,11.4,and 4.5 min in buffer solutions of pH 4.0,7.0 and 9.0,respectively).(2)Quantum yield:The quantum yield of amisulbrom was determined in the wavelength range of 190 to 400 nm.Meanwhile the photolysis rate constant of amisulbrom was calculated at each wavelength.The results showed that the direct photolysis rate of amisulbrom was inversely proportional to the light wavelength and directly proportional to the molar absorbance and light intensity.(3)Indirect photolysis:Fe(?),NOx and H2O2 existing in environment and TiO2 and g-C3N4,which were artificial synthetic photocatalysts,were used as the photosensitizers.Fe(?),NOx and H2O2 accelerated the indirect photolysis rate of amisulbrom,and that with H2O2 was the fastest.The artificial synthetic photocatalysts TiO2 and g-C3N4 also accelerate the photocatalysis rate of amisulbrom.Under UV light irradiation,the photocatalysis rate of amisulbrom was 3-5 times faster than under the simulated sun light irradiation.Under low-pressure mercury lamps,the indirect photolysis rates of Fe(III),NOx,and H2O2,TiO2 and g-C3N4 were 0.00730-0.0116,0.0340-0.0402,0.0265-0.0315,0.0067-0.0098 and 0.0074-0.0207 min-1,respectively.Under simulated sunlight,the indirect photolysis rates of Fe(?),NOx,H2O2,TiO2 and g-C3N4 were 0.00064-0.00072,0.0010-0.0013,0.0026-0.0032,0.0067-0.0098,0.0017-0.0022,0.0016-0.0019 min-1,respectively.(4)Photolysis product identification and photolysis mechanism:Eight main photolysis products were identified using HPLC-DAD and UPLC-Q-TOF-MS/MS analysis techniques.Among them,the three photolysis products of TP-?,TP-?,and TP-? were consistent with the hydrolysis products of HP-?,HP-?,and HP-?.Based on the identified photolysis products,DFT analysis and photolysis profiles,possible photolysis pathways of amisulbrom were proposed.DFT research results shown that the photolysis reaction of amisulbrom were mainly the nucleophilic substitution reaction by OH radicals and H2O molecules and the substitution reaction by hydrogen atoms on two sulfonamide functional group and triazole ring.(5)Toxicity prediction:The QSAR model was used to predict the toxicity of amisulbrom and its TPs.The results showed that TP-?,TP-?,TP-? and TP-?were more toxic than the parent,so the toxicity of these transformants was worthy of attention.