Control Mechanism Of Chloroacetanilide Herbicide Contaminations By Sodium Bisulfite And Cyclodextrins

Chloroacetanilide herbicides are among the most commonly used pesticides. They have been frequently detected in soil,groundwater and surface waters. Chloroacetanilide herbicides have toxicity to some animals, plants and microorganisms.Alachlor and acetochlor are listed as class B2 human carcinogens by the U.S.Environmental Protection Agency. Therefore, the studies on control technologies and mechanisms of chloroacetanilide herbicide contaminations are of great importance.In the present study, the kinetics, transformation products and mechanisms of the reactions of chloroacetanilide herbicides with sodium bisulfite (NaHSO3) and the changes of toxicity of transformation products following the degradation of the parent herbicides were investigated. The effects of cyclodextrins (CDs) on environmental behavior (i.e.,solubility, adsorption, desorption and the reactions of herbicides with glutathione (GSH)) of chloroacetanilide herbicides were also determined. These results provided the supporting information for new approach of control of chloroacetanilide herbicide contaminations by NaHSO3 and CDs.The reaction of three chloroacetanilide herbicides (alachlor, acetochlor and S-metolachlor) with NaHSO3 were investigated. These herbicides were rapidly dechlorniated by NaHSO3 in neutral conditions.At the same condition, the reactivity of the herbicides decreased in the following order:alachlor≈actochlor>S-metolachlor. The products were identified as chloroacetanilide ethane sulfonic acids (ESA) by HPLC-MS.The dechlorination was accelerated with increasing pH values, temperatures and NaHSO3 concentrations. The dissipation kinetics of the herbicides at different pH values indicates that SO32- is the predominating reactive species.Kinetic analysis and products identification revealed that the reaction followed SN2 nucleophilic substitution. The differences in reactivity of the three herbicides stemmed primarily from entropic factors;the steric hindrance of herbicide molecules stemming from variations in N-alkyl (or N-alkoxyalkyl) substituents or phenyl ring substituents could cause the observed difference in reactivity. The electrophilicity indices were calculated by employing hybrid density functional theory. Electrophilicity indices of the three chloroacetanilide herbicides decreased in the following order:alachlor≈acetochlor> S-metolachlor, which is in accordance with the reactivity of the herbicides. Electrophilicity indices can be used to estimate the reactivity of chloroacetanilide herbicides when they are treated by sulfur nucleophile. Acute toxicity of chloroacetanilide herbicides and ESAs to green alga (Scenedesmus obliquus) was investigated. Three chloroacetanilide herbicides have high toxicity to green alga Scenedesmus obliquus.EC50 values of the major degradation products (ESA) increased significantly compared with those of the chloroacetanilides, suggesting that ESA were less toxic than the parent herbicides. The facilitated dechlorination by NaHSO3 provides a selective route for formation of chloroacetanilide ESAs, results in the detoxification of chloroacetanilide herbicides and would be an environmentally beneficial process.Formation of a 1:1 stoichiometric inclusion complex between acetochlor, metolachlor and butachlor and P-cyclodextrin(β-CD), hydroxypropyl-β-cyclodextrin (HPCD) and 2-o-methyl-β-cyclodextrin (2-o-MCD) was confirmed by phase-solubility determination and mass spectrum in the solution. The solubility of three chloroacetanilide herbicides in water increased linearly with the increase of three CDs concentration. The solid complexes were prepared by the kneading and sealed-heating method. Fourier transform infrared spectroscopy, differential scanning calorimetry analysis and X-ray diffraction characterized the formation of solid inclusion complexes.The effects of CDs on solubility and soil adsorption of butachlor, desorption of the herbicide butachlor from soil and the reactions of acetochlor, metolachlor and butachlor with GSH were investigated. Significant enhancing dissolution of butachlor in the inclusion complex occurred in comparison to the free herbicide.The adsorption of butachlor on soil was reduced with an increase ofβ-CD concentration because of the formation of the inclusion complex with low adsorption potency. Three CDs solutions can both improve desorption of butachlor from soil. The reaction rates of acetochlor and metolachlor with GSH were decreased in the present of three CDs. P-CD didn't influence on the reaction rate of butachlor and GSH, while HPCD and 2-o-MCD decrease the reaction rate of them. The reactivity of complexed herbicides is not the same as that of free herbicides.The reaction rate variation of the guest induced by the inclusion of cyclodextrin is dependent on both the inclusive ability of the cyclodextrin to the guest and the reactivity of the inclusion complex.