The Hydrolysis Behavior Of Typical Organophosphorus Pesticides

To meet the food demand of the increasing population, agriculture practices revealed an increase in use of pesticides to control insects pests and weeds, the majority of which were residued in soil and water, seriously contaminated the social environment. Organophosphorous pesticides （OPPs）, a group of highly toxic agrochemicals utilized in crop protection, were considered to be the largest consumed pesticides worldwide. Methyl parathion （MP）, one of the most widely applied OPPs, had significantly exerted adverse effects on non-target species including human beings, especially children, by the irreversible inhibitory effect on acetylcholinesterase （AChE）, a key enzyme for nerve transmission. Due to the frequent and immoderate utilization of MP, it was frequently found as a residue in environmental matrices, such as in soil, water, atmosphere, fruit, honey, and even human adipose tissues. Consequently, the fate of MP in natural environmental system aroused extensive attention. And hydrolysis was the prevalent attenuation process in natural system for pesticides, so comprehensively studying the hydrolysis of MP, could help to understand its migration, transformation and the final fate in natural environment, simultaneously to provide a scientific basis for controlling it.This paper studied the hydrolysis behavior and mechanism of dissolved and adsorbed （goethite, Al₂O₃, montmorillonite and kaolin） methyl parathion. The hydrolysis behaviors of methyl parathion were completed by the batch laboratory experiments, meanwhile the influence factors on the hydrolysis, such as pH, temperature, the initial concentration, cationic(Cu²⁺, Fe²⁺, Mn²⁺, Na⁺), anionic （HCO3-、Cl-、HPO42-）, organic matter（HA, EDTA） and different structures of organophosphorus pesticide （dichlorvos and methamidophos） also had been studied. In addition, the hydrolysis products of methyl parathion were detected by liquid chromatography-high resolution mass spectrometry （LC-HRMS） and two different hydrolysis pathways of methyl parathion were illuminated.Through study and analysis, a series of significant conclusions were drawn as followings.（1） The hydrolysis kinetics of methyl parathion, dichlorvos and methamidophos were studied first. The results showed that:a.With the influence of pH, the hydrolysis rate of methyl parathion, dichlorvos and methamidophos all increased. Because hydroxyl ions catalyzed hydrolysis efficiency of pesticide was much higher than hydrogen ions. And when in alkaline conditions, the hydrolysis half-lives of the three pesticides followed:dichlorvos> methamidophos> methyl parathion; while in neutral and acidic conditions they followed:dichlorvos （31.99 h）> methyl parathion （18.15d）> methamidophos（22.91d）, especially in acid solutions, methamidophos was hard to hydrolysis, its half-life was 191.48 d. This could be related to the polarity of P-S, and P-P-O, P-N and the the stability of the groups connected with P atoms.b.The hydrolysis rate of methyl parathion, dichlorvos and methamidophos all increased significantly with the increase of temperature. And the hydrolysis rate constant value would increase 2-4 times when the reaction temperature raised 10 ℃. The hydrolysis activation energy of methyl parathion, dichlorvos and methamidophos were 81.61 kJ·mol-1,78.87 kJ·mol"1, 83.01kJ·mol-1 respectively, and according to the compound hydrolysis classification standard, the hydrolysis ability of the three pesticides all belong to medium.c.In addition, the hydrolysis rate of different initial concentration of methyl parathion was in order of 20 mg/L>10 mg/L>5 mg/L. The hydrolysis rate of methyl parathion increased with initial concentration of methyl parathion enhanced, but its hydrolysis half-life was not affected.（2） The hydrolysis mechanism of methyl parathion, dichlorvos and methamidophos were studied. The results showed that:a. When in at high pH （e.g., pH≥11）, SN2@P pathway dominated the hydrolysis process of methyl parathion while SN2@C was the main behavior at low pH （e.g., pH< 9） of it. The main hydrolysis product of SN2@P pathway was p-nitrophenol, while the other one was methanol. b.The hydrolysis of dichlorvos in alkaline environment was SN2@P reaction, one of the main product was 2 chloroacetaldehyde; while in neutral and acidic environments, it was the same as methyl parathion. c.The hydrolysis of methamidophos in alkaline solutions was SN2@P reaction, while the absence of methoxy was the main reaction, the absence of methylthio was the secondary reaction; and in acid condition, the SN2@N reaction dominated and generated NH3.（3）The hydrolysis behaviors on different mineral surface of methyl parathion in adsorption state were studied, and the results showed that:a.These four selected minerals all had effectively improved the hydrolysis rate of methyl parathion, and its catalytic efficiency order was:FeOOH> Al₂O₃> montmorillonite> kaolinite, which meant that the catalytic effect of metal oxide to the hydrolysis of methyl parathion was better than clay minerals. This was mainly related to their specific surface area and catalytic mechanism. b.The surface-catalyzed hydrolysis mechanisms of FeOOH and Al₂O₃ were as the followings:the combination of metal elements and S on the surface of the particles led to the density decrease of P atomic electron cloud, which strengthened its electrophilic activity, leading a more conducive attack by nucleophilic reagent （H2O and OH"）; Surface chelation; The hydroxyl combinating with the particle surface attacks the pesticides absorbing to the oxide as a nucleophilic reagent, which promotes the hydrolysis. c.There were two mechanisms of catalytic hydrolysis of montmorillonite and kaolinite of methyl parathion:surface acidification and surface chelation.（4） The influences of different factors on hydrolysis of methyl parathion on the montmorillonite surface had been studied,and the results showed that:a. Cu²⁺could effectively promote the hydrolysis of methyl parathion, but Fe3+、Mn²⁺、Na⁺ did not have obvious effect. b. Cl- and HPO42- restrained the hydrolysis of methyl parathion on the surface of Cu-montmorillonite, but HCO3- was just on the opposite. The Cu-montmorillonite-chelating ability of Cl- and HPO42- were stronger than HCO3- that they occupied most effective adsorption sites;The promotion of the surface catalytic hydrolysis by HCO3- was mainly due to its ionization which strengthened the surface acidity of montmorillonite. c. Methyl parathion was mainly adsorbed on dissolved EATD and HA in a short time.While HA absorbing on the surface of montmorillonite could promote the hydrolysis of methyl parathion, but EDTA was just on the opposite.EDTA was haxadentate ligand that it could chelate with montmorillonite and occupy most effective adsorption sites; The HA-modified montmorillonite could expand its interfloor distance that the affinity of methyl parathion was advanced, in favour of the adsorption of methyl parathion.（5）The hydrolysis behaviors of organophosphorus pesticides in different structures on the surface of Cu- montmorillonite were studid, and the results showed that:Cu-montmorillonite could effectively promote the hydrolysis of methamidophos and methyl parathion, but it did not work to dichlorvos. Because both of methamidophos and methyl parathion had an appropriate ligand that their reactions included surface acidification and surface chelation;Whatever, there was no ligand in dichlorvos that only surface acidity could work in its reaction. There were two catalytic mechanisms of the hydrolysis of organophosphorus pesticide in the clay surface, surface acidity and surface chelation; But surface chelation was only suitable to the pollutants with appropriate ligands that it was not essential to surface catalytic hydrolysis.