Experimental Investigation and Mechanism Exploration on the Degradation of N-heterocyclic Compounds with Dielectric Barrier Discharge
|Keywords||N-heterocyclic DBD Quinoline Plasma Micro-discharge|
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, O3, 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, O3, 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.