摘要
氮杂环化合物是一种重要的化工原料,广泛存在于焦化废水、制药废水、农药废水及印染废水中,严重污染水体环境。氮杂环化合物是一类难生物降解的有机物,具有致癌、致畸、致突变的性质,对人类健康构成巨大威胁,因此开发一种高效、经济、绿色的氮杂环化合降解技术具有十分重要的意义。
论文提出含氮杂环化合物废水的介质阻挡放电(DBD)处理法,并对介质阻挡放电用于水中难降解有机物的去除作用机理进行了初步探讨。在此基础上,提出以待处理废水作为接地极的一种新型介质阻挡放电水处理反应器结构。通过实验,考察了该反应器在不同电源频率、极板间距及极板电压作用下的放电特性。以模拟含氮杂环化合物废水为处理对象,研究了放电电压、电源频率、初始浓度、初始pH值等因素对去除效果的影响,采用液质联用技术对放电处理过程中的中间产物进行分析,初步探讨了此类化合物的降解机理。
本论文的研究成果如下:
(1)研制新型介质阻挡放电水处理反应器,以待处理废水作为接地极,使介质阻挡放电过程中产生的各种化学活性粒子(·OH、·O、H_2O_2、O_3等)与废水充分接触反应,能够提高废水中有机污染物的降解率。
(2)研究介质阻挡放电反应器特性发现,电源频率与极板间距对极板电压影响较大。在相同极板间距下,随着电源频率的增大,极板电压先增大后减少,极板电压最大时的电源频率为9kHz,当极板电压达到放电电压后其值基本保持不变。相同电源频率下,随着极板间距的增大,放电电压也随之升高。
(3)降解喹啉的实验中发现放电电压、电源频率、初始浓度、初始pH值都会对喹啉的降解造成影响。不同实验条件下,发现喹啉水样的吸光度都出现先降低后升高的现象,说明有大量中间产物的产生,导致吸光度叠加。实验中发现,放电电压为10kV,电源频率为10kHz,初始浓度为20ppm,在酸性条件下都能取得较好的处理效果。通过液质联用技术对喹啉降解中间产物进行研究,发现介质阻挡放电过程中产生的·OH、O_3主要进攻喹啉分子的5,8位碳原子,进而改变整个分子的π电荷密度分布,使喹啉易于降解。
(4)降解吡啶的实验发现吡啶水样的吸光度一直处于上升趋势,说明有大量中间产物的产生。本实验中放电电压为10kV,电源频率为9kHz,初始浓度为60ppm,在碱性条件下都能得到较好处理效果。对吡啶降解机理进行初步推断,放电过程中3号位碳原子先羟基化,进而改变整个吡啶环π电荷密度分布,使吡啶降解容易进行。
(5)降解吲哚的实验发现吲哚水样由无色变成红棕色,推断放电过程中2,3号位碳原子发生羟基化反应,产物为吲哚衍生物靛红。
(6)介质阻挡放电对氮杂环化合物有机废水的确具有降解效果,因此可作为一种预处理手段,降低后续水处理工艺的处理难度。
N-heterocyclic compounds are one of the important chemical raw material and have great harm of the environment, which exist in many kinds of industrial wastewater, for example , coal coking wastewater; pharmacy wastewater, pesticide wastewater, printing and dyeing wastewater. N-heterocyclic compounds are included in the persistent organic pollutants, and very dangerous to human health, because of theirs carcinogenicity, teratogenicity, mutagenicity. Therefore it is great significant to search an efficient, economic and clean technology on the degradation of N-heterocyclic compounds.
In this paper, dielectric barrier discharge(DBD) using on degrading N-heterocyclic compounds wastewater was studied. The mechanism on DBD for the persistent organic pollutants was preliminary researched. By these studies, a new type of DBD reactor was manufactured with wastewater as the earthing electrode. Then the influences of the frequency and the gas gap distance between two electrodes were studied, and the characteristics of the DBD reactor were determined. The influences of discharge voltage, power frequency, initial concentration and initial pH value were researched .The intermediate products in the process of discharge were analyzed by LC-MS, and the mechanism of degradation was also discussed.
The main conclusions are drawn as follows:
(1) The new type of DBD reactor was manufactured, in which the wastewater was the earthing explode. Many active substances (O·, H·,·OH, O_3, et al) generated in DBD reacted with the wastewater directly, which can improve the rate of degradation.
(2) The study on the characteristics of DBD reactor indicates that the frequency or the gas gap distance influenced the voltage between the two plates greatly. At the same gas gap distance, the voltage between the two plates increased with the increase of frequency at first. But after the frequency went up to 9kHz, the voltage declined with the increase of frequency. After the voltage between the two plates reached some value, named discharge voltage, it almost kept still. At the same frequency, with increase of the gas gap distance, the discharge voltage increased.
(3) The degradation of quinoline with DBD is influenced by the discharge voltage, power frequency, initial concentration, initial pH. Under different experimental conditions, the absorbance of water samples went up firstly, then reduced, which indicated that a large number of intermediate products were produced. In the experiment, it was found that the discharge voltage of 10kV, the frequency of 10kHz, the initial concentration of 20ppm and under the acidic conditions, we can achieved better treatment effect. The study of the intermediate products by LC-MS indicated that·OH, O_3, which were generated in the process of DBD, attacked 5,8-C of quinoline molecule, and then changed the % charge density, which made quinoline to be degraded easily.
(4) In the experiment of pyridine, it was found that the absorbance of pyridine has been rising from the very beginning, which indicated a large number of intermediate products were produced. It was found that the discharge voltage of 10kV, the frequency of 9kHz, the initial concentration of 60ppm, in alkaline conditions, pyridine was degraded better. It was inferred that 3-C may be attacked early, then changed theπcharge density in the molecule.
(5) In the experiment of indole, it was found that the color of the water sample changed from colorless to red. It was inferred that during the discharge, 2,3-C were hydroxylated, and indole changed into isatin.
(6) It is effective that DBD is used to degrade N-heterocyclic compounds wastewater. DBD can reduce the difficulty of the following process as a pretreatment.
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