| Today, a variety of organic pollutants have been found in varied environmental medias worldwide, such as atmosphere, water and organism. Many kinds of organic pollutants can be accumulated in creature for a long time, causing immune system, endocrine system, reproduction and development diseases. Especially for persistent organic pollutants (POPs), the effect on environment and creature is more serious. So how to find a efficient degradation method has been the focus of researchs in the world.As three kinds of degradation techniques which have strong oxidation ability and high degradation efficiency, dielectric barrier discharge (DBD), electrochemical oxidation and Fenton method, have been used in the degradation of organic pollutants frequently. In this paper, three typical organic pollutants, octachlorinated dibenzo-p-dioxin (OCDD), perfluorooctanoic acid (PFOA) and Sulfamethoxazole (SMX) have been chosen to study. Using high level quantum chemical density functional theoretical method, their oxidation degradation mechanism in the corresponding environment has been studied at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The related rate constants of the elementary reactions are calculated by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and CVT/SCT method. The study of mechanism leads to some meaningful conclusions:1. Theoretical study on the degradation of OCDD with atomic oxygen O (3P) by dielectric barrier dischargeDielectric barrier discharges (DBD) have been used in degradation of dioxins due to the large number of excimers and free radicals produced in discharge process. In this paper, the density functional theory (DFT) is used to study the degradation mechanism of octachlorinated dibenzo-p-dioxin (OCDD) with the atomic oxygen O (3P) in DBD reactor. The reactants, intermediates, transition states and products are optimized at the MPWB1K/6-31+G(d,p) level. The vibrational frequencies have been calculated at the same level. The reaction pathways and mechanisms are analyzed in detail, which can partly explain the degradation mechanism of dioxins by DBD. What is more, the effect of the removing chlorine atom on environment also has been discussed.2. The electrochemical oxidation degradation mechanism and rate constant study of perfluorooctanoic acidThe·OH radical, O2and H2O take part in the degradation process and play different roles. The OH-initiated process was found to be the main degradation pathway, and the existence of O2obviously accelerated the degradation of PFOA in aqueous solution. On the basis of the DFT calculation, an optimal electrochemical oxidation mechanism of PFOA is proposed, which involve the electronic migration, decarboxylation, radical reaction, hydrogen abstraction reaction and radical fragmentation reaction. The mechanisms have been verified by dynamics and intermediate determination experiments. All the rate constants of the elementary reactions are calculated, and the quantitative structure-activity relationship analysis has done to describe the relationship between the rate constants of decarboxylation reactions and the structures of PFCAs radicals (～C8-C2).3. The Fenton method degradation mechanism and rate constant study of sulfamethoxazoleWe studied the mechanisms of addition, abstraction and bond-broken reactions for SMX and OH radicals, and the corresponding rate constants have been calculated. Among the three mechanisms, addition and abstraction reactions are all important, while the bond-broken reactions are hard to occur due to their low rate constant. But SMX can be degraded directly by bond-broken reactions, so they can not be neglected. There are four subsequent reactions of addition reactions that also can degrade SMX by bond-broken reaction, we have analyzed in detail in this paper. |
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