Studies Of The Glow Discharge Plasma For Aqueous Organic Pollutants Removal And One-step Phenol Synthesis From Benzene

Water treatment by electrical discharges is a novel kind of advanced oxidation processes that is developed in recent 20 years. Many types of discharges have been or are being developed, such as pulsed corona, gliding are or glow discharge. Compared with other electrical processes, the glow discharge plasma has the advantage of lower voltage needed and being able to operate at high conducting liquids. At the same time, it has the problem of electrode erosion. Based on the analysis of the principles of plasma generation, a new diaphragm glow discharge reactor for water treatment is developed for the first time in this thesis, where the issue of electrode erosion has been completely resolved.The organic synthesis in electrical discharges is an interesting topic for many researchers, because many organic compounds that are difficult to synthesize by conventional thermal reactions can be easily implemented by plasma. However, at present this technology has the disadvantages of poor yield and selectivity. We first utilize the glow discharge plasma in combination with catalysts to direct synthesize phenol from benzene, a very important industrial raw material, where the phenol yield and selectivity of as high as 8.3% and 81% have been reached.The present paper includes 5 chapters:Chapter 1 reviews the advanced oxidation processes especially the application of low temperature plasma technology in the treatment of wastewater and the organic syntheses in electrical discharges.Chapter 2 describes the degradation of nitrobenzene in a normal glow discharge plasma reactor. It is found that the nitrobenzene degradation follows a first order reaction kinetics. The degradation products were detected by high performance liquid chromatography and ionic chromatography, respectively. The major aromatic intermediates were nitrophenols, phenol and 1, 3-dinitrobenzene. The distribution of nitrophenols follows the order o-＞p-＞m-nitrophenol. Oxalic acid, formic acid and acetic acid were found to be major carboxylic intermediates. The eventual products were shown to be nitrate ion and carbon dioxide. During the degradation, a large amount of hydrogen peroxide was produced. Addition of ferrous or ferric ion into the system greatly enhanced the degradation rate due to Fenton's reaction. The energy efficiency of NB degradation and simultaneous hydrogen peroxide formation were compared with pulsed corona discharges. Based on the intermediate analysis, a possible reaction pathway was proposed.In Chapter 3, various influencing factors for 4-chlorophenol removal and simultaneous hydrogen peroxide production in a normal glow discharge reactor are examined. It is found that enhancing pH and applied voltage favors the 4-chlorophenol removal and hydrogen peroxide production. In addition, the major intermediate products in the presence of catalysts are found to be different from that observed in the absence of the catalysts. The major aromatic intermediate products were 4-chlorocatechol, benzoquinone and hydroquinone in the presence of iron ions.In Chapter 4, a non-pulsed DC diaphragm glow discharge process was first developed for phenol degradation in aqueous solution. The discharge was generated by a small hole in a dielectric diaphragm interposed between two submersed graphite electrodes. The experimental results revealed that the supplied voltage, initial pH, iron salts and radical scavengers impact the phenol degradation significantly. Enhancing the applied voltage, lowering the solution pH and adding appropriate amounts of Fe²⁺ or Fe³⁺ to the solution have shown to be favorable for phenol degradation. Carbonate ions or n-butanol in the solution can decelerate the phenol removal. With increasing time, the pH value of the solution decreased, leading to a synergistic effect on phenol elimination. The main intermediates such as hydroquinone, pyrocatechol, p-benzoquinone and organic acids produced during the treatment process were determined by HPLC and IC analysis. Compared with the high-voltage corona discharge plasma in distilled water, this process bears the advantages of experimental simplification, higher energy efficiency, easier scale-up and applicability to salt-containing wastewater with no electrode erosion and electrical radiation. The results of this chapter have been published by EST. salt-containing wastewater with no electrode erosion and electrical radiation. The results of this chapter have been published in EST.In Chapter 5, direct phenol synthesis from benzene in aqueous solution where water was used as the oxidant through glow discharge plasma process is described for the first time. The effec of pH, ferrous and cupric ions on the phenol yield and selectivity was examined. Phenol yield of 8.3% and selectivity of 81% has been achieved. The results of this chapter are to be published in Plasma Chem. Plasma process.