Effect Of Oxidant On Phenylurea Herbicide And N-nitrosodimethylamine Formation

Nowadays,The pre-oxidation and disinfection methods commonly used in drinking water treatment mainly include chlorine,chlorine dioxide,ozone and chloramine.Although these treatments remove contaminants and pathogenic microorganisms,they may be converted to more toxic substances that affect human health.N-nitrosodimethylamine（NDMA）can be produced under variety of oxidative conditions,and more widely attention has been paid to the transformation mechanism of its precursors.Herbicides are widely used due to their broad spectrum,low toxicity and high efficiency,and cause different degrees of pollution to the water environment because of their extensive use and accompanied by rainfall and runoff into the water body.At present,the research on the conversion of herbicides into NDMA in water is not systematic,so it is particularly important to understand the situation and transformation pathway of NDMA produced by herbicides under various oxidant.In this study,four kinds of phenylurea herbicides,diuron,fluometuron,chlorotoluron and soproturon,were selected according to their structural characteristics,and study the degradation of herbicides and the generation of NDMA under the various oxidants（Cl₂,O₃,ClO₂）.The changes of degradation and NDMA formation potential（NDMA-FP）after addition of monochloramine（NH₂Cl）.Then,the degradation and formation of four herbicides under O₃ were further studied.Finally,the transformation pathways of herbicides under different oxidants were preliminary study.The main conclusions are as follows:（1）Diuron,fluometuron,chlorotoluron and isoproturon decreased with the increase of oxidant concentration under O₃,Cl₂ and ClO₂.For diuron,chlorotoluron and isoproturon,the degradation effect was ClO₂>O₃>Cl₂;and the degradation effect of fluometuron under oxidant was O₃>ClO₂>Cl₂.Among the three oxidants,four herbicides were converted to NDMA during ozonation,and the conversion rate increased with the increase of oxidant concentration.After the addition of chloramine,the amount of NDMA increased with the increase of oxidant concentration.The formation of NDMA by diuron,fluometuron,chlorotoluron and isoproturon under 2.5 mg/L O₃ was increased by 27 times,30 times,68times and 23 times,respectively.The comparison between the actual value of NDMA and the theoretical value of the theoretical value of the herbicide at oxidative and chlorination reveals the correlation between the degradation product and NDAM-FP.（2）During ozonation,the degradation rate of the four herbicides decreased with the increase of initial concentration.However,the degradation amount per unit ozone herbicide increases with the increase of initial concentration.The degradation rate of isoproturon was the fastest,and the degradation rate was over 97%within 15 min.There was a significant correlation between NDMA and initial concentration of herbicide（R²>0.9）.First-order kinetics was used to fit the relationship between the concentration of herbicide[ln（[D]/[D]₀）]and time during ozonation The results showed that the degradation of O₃ of four herbicides accorded with first-order kinetics.（3）Degradation pathways indicated that diuron,fluometuron,chlorotoluron and isoproturon produced similar intermediates（compounds containing N=O bond）during ozonation,and their DMA functional groups were not degraded during the degradation process.They with the unreacted herbicides were converted to NDMA during chloramination.Under chloration,the four herbicides contain two methyl branched chains linked to nitrogen atom,the N-C bond is cleaved to form dealkylation product and destroy the DMA functional group,which may be the main reason for the decrease of NDMA-FP under Cl₂oxidation.The degradation pathway under ClO₂ oxidation is similar to Cl₂,however,the intermediates of chlorotoluron and isoproturon don’t destroy the DMA structure under a certain dose of ClO₂,which made higher contributed to NDMA-FP.