Study On Treatment Of Refractory Organic Pollutants In Wastewater By Using Nonthermal Plasma

With the acceleration of global industrialization, industrial production is developing rapidly. However, the great improvement of production efficiency is accompanied with a large number of organic compounds discharge into the surrounding environment. The environmental behavior of organic compound is highly dependence on its structure. From the molecular structure point of view, organic compound with higher average bond enthalpy and shorter average bond length shows better chemical stability and lower reactivity, and is considered as refractory. Thus, the environmental organic pollutants can be divided into three categories according to their molecular structures, i.e., heterocyclic organic pollutants, benzene-containing environmental organic pollutants and chain-type environmental organic pollutants. The use of conventional water treatment technology to treat these pollutants could not meet the requirements of the improving state environmental quality standards for organic pollutants.Plasma is a neutral particle aggregate, which is composed by positive and negative ions, electrons, free radicals and a variety of active groups. Although the electron temperature of nonthermal plasma is very high (> 104K), its ion temperature could be as low as 300K, and the reaction system can be kept at low temperature and even at room temperature, which is the characteristic of conventional chemical reaction. Since nonthermal plasma is rich in electrons, ions, free radicals and excited state molecules with high reactivity, it can realized the reaction which was normally difficult to conduct under the normal conditions.In this paper, thiamethoxam (TIHA),2,4-dichlorophenol (2,4-DCP) and perfluorooctanoic acid (PFOA) were screened as typical heterocyclic, benzene-containing and chain-type organic pollutants, respectively, from the aspects of molecular structure, chemical stability and toxicity. These environmental organic pollutants were treated by nonthermal plasma technology, and the degradation kinetics and degradation mechanism of the pollutants were investigated.The main conclusions of this paper are as follows:1) A novel dielectric barrier discharge nonthermal plasma reactor with intermittent liquid circulation was developed according to the characteristics of nonthermal plasma technology. The reactor used a radial flow device, and water was pumped from the bottom and flowed back through the edge. Electrode was located above the water surface to form a stable plasma area perpendicular to the surface of water. Thus, plasma could bomb the water surface directly, and react with the refractory organic pollutants in the gas-liquid interface.2) The reaction parameters for the nonthermal plasma treatment of different environmental organic pollutants were optimized. Results showed that the degradation efficiency of THIA solution was 86.67% when the initial concentration was 100 mg / L, the discharge voltage was 80 V and the initial pH was 6.86. The degradation efficiency of benzene-containing organic pollutant 2,4-DCP solution was 85.32% when the initial concentration was 100 mg / L, the discharge voltage was 80 V and the initial pH was 5.45. The degradation efficiency of chain-type organic pollutant PFOA solution was 77.08% when the initial concentration was 100 mg / L, the discharge voltage was 80 V and the initial pH was 3.5.3) The degradation mechanisms of different environmental organic pollutants were investigated. THIA was mainly oxidized to small molecular substances, including some weak acid products as intermediate, which resulted in the gradually decrease in pH value of the solution. Hydroxyl radical (? OH) played a major role in the decomposition of 2, 4-DCP, since only a small fraction of 2, 4-DCP was decomposed after the addition of ? OH inhibitor. And although the degradation efficiency of PFOA was decreased after the addition of ethylene glycol, a part of PFOA was degraded, indicating that ? OH was dominant but not the only oxidizing substance in the degradation reaction. A certain degree of synergistic oxidative degradation existed in the solution.4) The reaction kinetics equation for the treatment of refractory organic pollutants in water by nonthermal plasma was studied. The kinetic equation was established, which lays a theoretical foundation for further optimization of reaction parameters and improvement of degradation efficiency. It also provided certain technical guidance for the engineering application of nonthermal plasma technology.The degradation of heterocyclic organic pollutants THIA and benzene-containing organic pollutants 2,4-DCP mainly follows first-order kinetic reaction. The degradation of chain-type organic pollutant PFOA is consisted with first-order kinetic reaction when pH value of the solution was less than 7, while followed the second-order kinetic reaction when pH value was 9.5. This was mainly because the alkaline substance in the chain-type organic pollutant PFOA solution could affect the content of active substance. The degradation of PFOA was not only related to the PFOA content, but also related to the active substance content.5) The degradation products of refractory organic pollutants in water were analyzed by HPLC-MS, to reveal the degradation process of heterocyclic organic pollutant TIHA, benzene-containing organic pollutant 2,4-DCP and chain-type organic pollutant PFOA, which provided theoretical support for the application of nonthermal plasma technology in wastewater treatment area.