Theoretical Study On Oxidative Degradation Mechanisms Of The Typical Benzene Organochlorine Pesticides

Organochlorine pesticides are known as one class of environment priority pollutants. They have the characteristics of refractory, semi-volatile, bioaccumulation and high toxic, which could bring great harm to human health and environment. Therefore, it is a great significance for controlling and treating pollutants to study the transform and degradation of organochlorine pesticides in the environment.The degradation mechanisms of the organochlorine pesticides are ambiguity, the initial reaction step of degradation these pesticides are controversial, and the mechanism comparisons for degradating pesticides with similar structures are lack. To understand above-mentioned problems, we employ high-accuracy density functional theory to study the transformation and degradation mechanisms of the typical organochlorine pesticides (dicofol, chlorine-substituted dimethyl pesiticide and chlorinated phenoxy acids) in this paper. The aims are to obtain the detail degradation mechanisms of the typical organochlorine pesticides, expatiate the mechanism defferences between molecular and anionic forms of one organochlorine pesticide, and reveal the effects of different structures on the reaction mechanisms. Then, these results could provide theoretic guiding for how to effectively remove these organochlorine pesticides. The main contents and conclusions are as follows:1. The oxidative degradation mechanisms of dicofolDicofol (2,2,2-trichloro-1,1-bis(4-chlorophenyl)ethanol) is one nonsystemic organochlorine acaricide, and it could remain in the environment. To dicuss detailedly the OH induced degradation mechanisms of dicofol, B3LYP/6-311++G(d, p) is employed to study the degradation mechasim s of OH and dicofol reactions. First, the frontier electron densities and APT are calculated to predict the position vulnerable to be attacked by OH, and the result indicates that C8atom is the most vulnerable site to be attacked by OH. Then bond dissociation energy calculations show that the C7-C8bond dissociation energy is lowest, meaning that the C7-C8bond is easiest to be attacked. Therefore, the OH-induced reation pathways of the vulnerable sites are analyzed, and4pathways are obtained. The results show that the activation energy for the pathway attacking C7-C8bond (R1) is the lowest with0.10kcal·mol-1; then the next are pathway R4with3.14kcal·mol-1, pathway R2with19.19kcal·mol-1and pathway R3with33.70kcal·mol-1. It can be seen that the pathway Rl has the lowest activation energy, which is the initial reaction for photocatalytic degradation of dicofol, and the dominant product is (C6H4CO2CO, which is in good agreement with the experimental observation result. However, the pathway R3is also for OH attacking C8site, but with the highest activation energy of33.70kcal·mol-1. This result shows that static parameters calculations are not enough to understand the degradation mechanisms, and the dynamic study is necessary. Furthermore, the results with the PCM calculations show that the energy barriers in acetone solvent are about slightly higher than that in gas, but the reaction mechanisams are not changed. Thus, it is believed that the real reaction system can be depicted simply employing the gas phase model.2. The oxidative degradation mechanisms of diuronDiuron (N-(3,4-dichlorophenyl)-N,N-dimethylurea) is a representative of most commonly used phenylurea herbicides. Many degradation methods have been developed, and some degradation pathways have been proposed according to the results of photocatalytic and electrochemical degradation diuron. However, the detailed reaction mechanisms including exhaustive transformation processes from reactants to intermediates, activation energies and kinetic property are understood too limited. Furthermore, some reaction intermediates are difficult to be monitored with the experiments. Therefore, the theoretical method MPWB1K/6-311+G(3df,2p)//MPWBlK/6-31+G(d,p) has been used to studty the OH-induced degradation mechanisms of diuron in water, and11degradation pathways have been obtained in total, including four H-atom abstractions, six addtion pathways and one subsitution pathway. For H-atom abstraction reactions, the calculation results show that the reaction abstracting H atom from the methyl group has the lowest energy barrier, and H-atom abstraction from-CH3is probable the initial reaction; the potential barrier of ortho-H(H1’) abstraction is higher than the meta-H abstraction, and the reason is possibly that part of the potential energy is to overcome the side chain torsion for the HI’abstraction reaction. For addition pathways, the C(2) atom is the most favorable site that·OH may first attack; the potential barriers for OH additions to the ortho-sites (pathways R7and R8) and the chloro-substituted para-site (R10) are lower than other sites, indicating the ortho-and para-sites are more favorable to be attacked, matching well with that the-NHCO-group is an ortho-para directing group. For the only found substitution reaction, that OH substituting H4’atom attached to the Nl atom has the highest energy barrier of68.85kcal·mol-1, meaning that this reaction is not expected to be important.3. The oxidative degradation mechanisms of4-chloro-2-methylphenoxyacetic acid (MCPA)4-chloro-2-methylphenoxyacetic acid (MCPA) is a chlorinated herbicide. There exists controversy concerning initial step in the photocatalytic oxidation of MCPA. Additionally, the photocatalytic decomposition of anionic MCPA has only been studied in one available literature, and the difference of the OH-induced mechanisms between molecular MCPA and anionic MCPA in water have not been discussed. Therefore, we employ the theorectical method to recalculate the acidity coefficient to discuss the two existing forms of MCPA in water, and then investigate possible pathways of H-atom abstraction and OH addition to different sites of MCPA using quantum chemistry to determine the initial reaction step and discuss the mechanism differences between molecular and anionic MCPA. The calculation results show that the reaction mechanisms for OH and two forms of MCPA are different, and most reactions for anionic MCPA are easier than those for molecular MCPA. For molecular MCPA, the energy barrier of H-atom abstraction from-CH3is the lowest, indicating that this reaction is the initial reaction; additionally, OH adding to the Cl site also has the lowest enegy barrier among the addition reactions and the primary product is4-chloro-2-methylphenol; the H-atom abstraction from-CH2-could occur feasibly. The pathways R2and R3are the initial steps for the above studies by the two researchers, respectively, and our results show that the enegy barriers of the two pathways are both lower, and they can easily occur. For anionic MCPA, The C4site is the most reactive position, and the main product is A-P4, the hydroxylation of the aromatic ring, indicating that this pathway is the intial reaction for OH and anionic MCPA reactions; the energy barriers of OH addition to the Cl site and H-atom abstraction from-CH2-are not high, meaning that the two reactions can occur easily. Additionally, the PCM calculations indicate that most reactions for anionic MCPA are easier than those for molecular MCPA and most reactions are generally easier to occur in water phase than in gas phase.4. The effects of structures on the degradation mechanisms of chlorined phenoxy acidsChlorinated phenoxy acids are one class of the most used herbicides, and the representatives are4-chlorophenoxyacetic acid (4-CPA),2,4-dichlorophenoxyacetic acid (2,4-D),4-chloro-2-methylphenoxyacetic acid (MCPA),2-(2,4-dichlorophenoxy) propanoic acid (DCPP) and2-(4-chloro-2-methylphenoxy)propanoic acid (MCPP). Little information has reported on the differences of the degradation mechanisms of the five herbicides having the similar structures. Therefore, the theoretical study for the OH-induced degradation mechanisms for the five herbicides is necessary. According to the experiment results, the the active sites in these herbicides people particularly concern, are ipso-C, para-C sites and the H atom in-CHR-group of the side chain. Furthermore, our results from the theoretical calculations show that reactions related to the above sites for MCPA are all low energy barriers, favorably to occur. Therefore, OH attacks the above three active sites with DFT are considered to compare different reaction mechanisms of the five herbicides. Comparison results show that the mechanisms of the same type reactions are affected by the steric hindrance and the electronegativities of the-CH3and-Cl groups. For each herbicide, OH adding to the Cl atom, which is the nexus between the benzene ring and the side group, has the lowest energy barrier among the three kinds of reactions, indicating the OH addition-substitution of the side chain is most energetically and kinetically favorable. For the addition of OH to the Cl sites of the five herbicides, the activation energy for the OH and DCPP reaction is the lowest, while the energy barrier for the OH and4-CPA reaction is the highest. For OH addition to the C4sites of the five herbicides, the energy barriers among the three kinds of the active position reactions are highest, indicating that the para-Cl is difficult to be broken down. For H-atom abstraction reactions of the five herbicides, the H atoms in the-CH2-group of2,4-D are easiest to be abstracted by OH and those of DCPP and MCPP are more difficult to be abstracted due to the steric hindrance of the-CH3group. Additionally, the results with the PCM calculations reveal that most reactions are easier to occur in water than in gas.