Research On Characteristics Of Radio Frequency Plasma Discharge Underwater And Its Application Of Azo Dyes Degradation

In all of the dyes which are used currently,80~85percent are azo dyes. About10~15percent are released into water during use, causing serious environmental pollution. Howto remove such kind of organic pollutants efficiently and quickly is an urgent problem tohuman beings. As a novel advanced oxidation technology (AOT), low temperature plasmadischarge underwater has been paid much attention recently. It has gradually become aneffective method for a preliminary practical degradation of organic pollutants throughcontinuous research in recent decade.In this study, we present a new radio frequency (RF) underwater discharge systemwhich can be used for organic wastewater treatment. The system mainly consists of areactor, a matching box and a RF source. The reactor contains a tungsten electrode whichis inserted into a proper quartz tube, a grounded metal electrode, an open glass container,and a grounded shielding box with a view port. The matching box composes of twoadjustable vacuum capacitors and a fixed inductance. The driving frequency can be13.56MHz,27.12MHz,40.68MHz or60MHz by using different inductances.The plasma is generated within bubbles at the electrode surface formed by RF andplasma heating. In order to feature the parameters of the discharge, characteristics of thedischarge were studied. Optical emission spectroscopy (OES) was also employed fordiagnosing radical emissions. Congo Red, a well known class of azo dyes was selected as amodel dyeing pollutant to evaluate the degradation efficiency in different dischargeconditions. The products after the plasma treatment were analyzed and one possibledegradation process was proposed. The results shown that the size of the bubbles andplasmas increased as the driving frequency and the input power went up. The breakdown voltage decreased rapidly with the increase of the water conductivity and driving frequency.A lot of radicals such as·OH,·O,·H, etc. were produced during the discharge which can berevealed by optical emission spectroscopy. These radicals were essential to the degradationprocess. For the plasma treatment, higher exciting frequency and lower conductivity of thesolution were more effective for organic degradation. The main products after the plasmatreatment were4,4-diaminobipheny,4,4-dinitrosobipheny,4,4-dinitrobipheny,1,8-diamino-3-napht-alene-sulfonic acid sodium salt,1,8-dinitroso-3-naphtalene-sulfonic acid sodiumsalt and1,8-dinitro-3-naphtalene-sulfonic acid sodium salt.