氮气对电晕放电等离子体降解甲基蓝的影响

doi:10.3964/j.issn.1000-0593(2018)11-3354-08

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Abstract The corona discharge plasma technology is a new type of advanced oxidation processes (AOPs) which has developed in recent years. Corona discharge plasma technology, which has characteristics such as high degradation efficiency, simple operation and less occupied area, has been widely used in the field of printing and dyeing wastewater treatment. At present, little is known about most of the organic pollutants degradation mechanism, hence this technology in the field of printing and dyeing wastewater treatment is still in the exploratory stage. Therefore, in order to apply the corona discharge plasma technology to the treatment of industry printing and dyeing wastewater as soon as possible, the exploration of different pollutants degradation mechanism is of great significance to the industrialization and industrial application of this technology. Up to now, corona discharge plasma technology has good degradation efficiency on the dyes which had been studied. However, the suitability of this technique for the degradation of all dyes remains to be further researched. In this paper, choosing methyl blue which is of triphenylmethane dye as a target contaminant, corona discharge plasma technology was used to degrade it. The impact of the initial concentration of methyl blue solution on the degradation rate of aromatic ring (314 nm) and the change of chromophore group (603 nm) absorbance in the ultraviolet visible spectra (UV-Vis) were investigated. The changes of the solution concentration, total organic carbon (TOC), total nitrogen (TN) and the pH values of the methyl blue solution were measured with the discharge time increased, and the correlation between them were analyzed. In this paper, three kinds of spectroscopy methods, based on ultraviolet visible spectra (UV-Vis), three dimensional fluorescence spectrum (3-D fluorescence) and Fourier transform infrared spectroscopy (FTIR), were used to analyze the changes of color, varieties of fluorescent substance and functional groups of methyl blue solution during the degradation process by corona discharge plasma. And the intermediate products generated after the methyl blue was degraded by corona discharge for 30 minutes were analyzed. The experimental results showed that the concentration of methyl blue in the solution decreased gradually with the increase of discharge time in the degradation process of methyl blue by corona discharge plasma, which indicated that the technology has a certain ability in the degradation of methyl blue solution. In the degradation process of the methyl blue solution by corona discharge plasma technology, turn on the high-voltage power supply, and the high voltage electrodes penetrate the air containing a large amount of nitrogen between the high voltage electrode and liquid surface to produce N, NO·, N⁺₂ and other nitrogen-containing high activity particles, these particles migrate to the liquid phase through diffusion, which leads to the TN content in the solution to gradually increase throughout the degradation process. In addition, another part of the nitrogen-containing high-activity particles and the C element dissolved from tungsten needle electrode bonding to generate CN double bound, which is of chromophore group in organic matter, so that total organic carbon in the solution increased at the discharge time of 5 minutes. More highly active particles generated by extending the discharge time reacted with the residual organic matter (methyl blue and intermediate products) in the solution, and some organisms were mineralized to form CO₂ with the discharge time increasing, which caused a drop in the TOC content of the solution. The parameters of the corona discharge plasma reactor without any change throughout the degradation process, hence the number of active species generated within the same time frame was identical in the degradation process of methyl blue by corona discharge. Increasing the initial concentration of methyl blue solution is, the more methyl blue molecules are not degraded, which induces the reduction of the degradation rate of methyl blue. The chromophoric group absorbance got the maximum value when the discharge time was 5 minutes, which resulted from the polymerization between methyl blue molecules in the process of corona discharge and the formation of chromophore group of CN double bonds. In addition, the higher the initial concentration of methyl blue solution, the more the absorbance increases between the discharge time of 5 minutes and 0 minutes (A₅-A₀). It can also be observed from the experimental phenomenon that the methyl blue sample in discharge time of 5 minutes had the deepest color in all the samples and then gradually became lighter, further confirming the formation of the CN bound during the discharging process. In general, nitrogen in the air has an important effect on the degradation of methyl blue by corona discharge plasma, which is mainly attributed to the CN chromophore group in chemical structure of methyl blue, and the main reason of the solution color deepens and then lighter in the degradation process of methyl blue by corona discharge is that the existence of CN chromophore group. Moreover, the consumption of the hydroxyl free radicals, which is a kind of active species, generated hydroxyl ions, resulting in the increase of the pH value at the discharge time of 5 minutes. With the progress of the degradation reaction, nitric acid and small molecule acids generated in the solution enhance the acidity of the solution, resulting in a decrease in pH value of the methyl blue solution. It can be seen from the three-dimensional fluorescence spectrum of dissolved organic compounds (DOM) that there exists three kinds of obvious fluorescence peaks in degradation process of methyl blue solution at different times by corona discharge plasma technology. These fluorescence peaks, which are located at E_X/E_M=310~320/430~450，E_X/E_M=240~250/320~340 and E_X/E_M=280/340，representing humic acid, aromatic proteins and soluble microbial metabolites, respectively. Three-dimensional fluorescence spectrum results showed that the fluorescent substances in methyl blue solution before degradation are mainly humic acids, with the prolongation of the degradation time, the humic acid was first degraded to aromatic protein, which could further degrade to the soluble microbial metabolic by-product. After the corona discharge time of 30 minutes, compared the infrared spectra and the fitting curves of infrared spectra (1 750~1 540 cm^-1) of the methyl blue solution before and after corona discharge, the FTIR Spectra of the degraded samples changed obviously. The asymmetric stretching vibration peak at 3 432.8 cm^-1 of N—H bond is red-shifted by 0.3 cm^-1. The stretching vibration peak at 2 975.9 cm^-1 of C—H bond on the olefin and benzene ring is shifted by 0.5 cm^-1, the stretching vibration of the double bond of RCHCHR at 1 638.7 cm^-1 shifts blue-shifted by 3.2 cm^-1. The C—N stretching vibration peak of aromatic amine 1 341.6 cm^-1 shifts to 1.3 cm^-1. The stretching vibration peaks at 1 121.1 and 1 034.3 cm^-1 of SO are red shifted by 3.8 and 13 cm^-1, respectively. Furthermore, the absorption peaks situate at 1 657.9 and 1 676.9 cm^-1 of CC and the CN outside the ring in the chemical structure of methyl blue all disappear after the corona discharge for 30 minutes. It showed that the corona discharge plasma discharge effectively destroys these two bonds, and stretching vibration absorption peaks of CO and NO appear at 1 692.4 and 1 400.4 cm^-1, respectively. Additionally, its degradation products might be 2,5-cyclohexadiene-1,4-diketone, sodium p-nitrobenzene sulfonate and ketones and other intermediates. The results showed that the research has important theoretical significance and practical value in Methyl Blue treatment by Corona discharge plasma technology.

Key words：Corona discharge plasma; Methyl Blue; Nitrogen free radicals; FTIR; FTIR fitting; Three-dimensional fluorescence

Received: 2017-10-09 Accepted: 2018-02-26

ZTFLH:

X661.1

Corresponding Authors: DONG Fa-qin E-mail: fqdong@swust.edu.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2018)11-3354-08 OR https://www.gpxygpfx.com/EN/Y2018/V38/I11/3354