Effects of Nitrogen on the Degradation of Methyl Blue by Corona Discharge Plasma
电晕放电等离子体技术是近年发展起来的一种新型高级氧化工艺, 因其处理效果好、 操作简单、 占地面积小的特点在印染废水处理领域得到了广泛应用。 目前因大部分有机污染物的降解机理不详, 该技术尚处于探索阶段。 因此, 为了尽早将电晕放电等离子体技术应用于工业印染废水的处理, 不同污染物降解机理的研究对该技术的工业化和产业化应用具有重要意义。 至今, 电晕放电等离子体技术对研究较多的染料的降解效果均较好, 然而, 是否适合所有染料的降解有待进一步研究。 采用电晕放电等离子体技术处理三苯甲烷类染料甲基蓝, 研究了溶液的初始浓度对甲基蓝紫外-可见光谱中芳香环的降解率、 发色基团吸光度变化的影响, 测定了溶液的浓度、 总有机碳(TOC)、 总氮(TN)、 pH值等指标随着放电时间的变化, 并对其相关性进行了分析。 结合紫外-可见光谱(UV-Vis)、 三维荧光光谱(3D-fluorescence)和傅里叶变换红外光谱(FTIR)三种光谱学手段分析了电晕放电降解甲基蓝过程溶液的颜色、 荧光物质和官能团变化, 分析了电晕放电降解甲基蓝30 min后生成的中间产物。 结果表明: 电晕放电等离子体降解甲基蓝过程, 溶液的浓度随着放电时间的延长逐渐减小, 表明该技术对甲基蓝溶液有一定的降解能力; 降解过程高压电极放电击穿含有大量氮气的空气产生N, NO·, N+2等含氮高活性粒子, 这些粒子通过扩散作用迁移至液相, 使得溶液中TN含量在整个降解过程逐渐升高; 另有部分含氮高活性粒子与钨钢针电极溶出的C元素键合生成发色的CN双键, 使得溶液中的总有机碳在放电5 min时有所升高。 延长反应时间产生的高活性粒子与溶液中的有机物(甲基蓝及中间产物)继续作用, 部分有机物矿化生成CO2, 引起溶液中TOC含量的下降。 电晕放电相同时间内产生的活性粒子数量相当, 增大甲基蓝浓度, 未被降解的甲基蓝分子越多, 导致甲基蓝降解率的减小。 电晕放电过程甲基蓝分子之间的聚合与发色CN双键的生成共同促使甲基蓝发色基团吸光度在放电5 min时达到最大; 且甲基蓝溶液的初始浓度越高, 吸光度(A5-A0)升高的越多。 概括来说, 甲基蓝结构中发色CN双键的存在是电晕放电等离子体降解甲基蓝过程溶液颜色加深再变浅的主要原因。 反应过程羟基自由基的消耗导致放电5 min时溶液的pH值升高; 随着反应的进行溶液中生成的硝酸及小分子酸增强了溶液的酸性, 导致pH值降低。 三维荧光光谱结果表明, 甲基蓝降解过程出现了三类明显的荧光峰, 位于EX/EM=310~320/430~450, EX/EM=240~250/320~340和EX/EM=280/340, 分别代表腐殖酸类物质、 芳香族蛋白质和溶解性微生物代谢副产物。 甲基蓝溶液降解前的荧光物质主要为腐殖酸类, 随着降解时间的延长, 腐殖酸类物质首先降解生成了芳香族蛋白质, 进一步降解产生可溶性微生物代谢副产物。 比较电晕放电前后甲基蓝溶液的红外光谱图和红外分峰图发现, 甲基蓝结构中N—H键3 432.8 cm-1处不对称伸缩振动峰红移了0.3 cm-1, 烯烃和苯环上C—H键2 975.9 cm-1处的伸缩振动峰向高波数偏移了0.5 cm-1, 1 638.7 cm-1处RCHCHR的双键伸缩振动位置蓝移了3.2 cm-1, 芳仲胺的C—N伸缩振动峰1 341.6 cm-1向高波数偏移了1.3 cm-1, 磺酸基SO的伸缩振动峰1 121.1和1 034.3 cm-1分别红移了3.8和13 cm-1, 甲基蓝结构中的环外CC双键与CN双键吸收峰消失, 在1 692.4和1 400.4 cm-1处分别出现了CO和NO的伸缩振动吸收峰, 产生了2,5-环己二烯-1,4-二酮、 对硝基苯磺酸钠和芳香酮类等中间产物。 该结果对于利用电晕放电等离子体技术处理甲基蓝废水具有重要的理论意义和实用价值。
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 EX/EM=310~320/430~450, EX/EM=240~250/320~340 and EX/EM=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.
董发勤：固体废物处理与资源化教育部重点实验室, 四川 绵阳 621010
霍婷婷：固体废物处理与资源化教育部重点实验室, 四川 绵阳 621010西南科技大学环境与资源学院, 四川 绵阳 621010
周 磊：固体废物处理与资源化教育部重点实验室, 四川 绵阳 621010西南科技大学环境与资源学院, 四川 绵阳 621010
李 刚：固体废物处理与资源化教育部重点实验室, 四川 绵阳 621010西南科技大学环境与资源学院, 四川 绵阳 621010
周世平：西南科技大学环境与资源学院, 四川 绵阳 621010国家城市污水处理及资源化工程技术研究中心, 四川 绵阳 621000
王 彬：固体废物处理与资源化教育部重点实验室, 四川 绵阳 621010西南科技大学环境与资源学院, 四川 绵阳 621010
何 平：西南科技大学材料科学与工程学院, 四川 绵阳 621010
备注：李 苗, 1992年生, 西南科技大学材料科学与工程学院硕士研究生
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LI Miao,DONG Fa-qin,HUO Ting-ting,ZHOU Lei,LI Gang,ZHOU Shi-ping,WANG Bin,HE Ping. Effects of Nitrogen on the Degradation of Methyl Blue by Corona Discharge Plasma[J]. Spectroscopy and Spectral Analysis, 2018, 38(11): 3354-3361
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