Microwave-Driven In-liquid Plasma in Chemical and Environmental Applications. III. Examination of Optimum Microwave Pulse Conditions for Prolongation of Electrode Lifetime, and Application to Dye-Contaminated Wastewater

Abstract

Generating in-liquid plasma using continuous microwave radiation has proven problematic as the surface of the electrode undergoes significant deterioration because of the generated plasma. This article describes a method by which this problem can be resolved by the utilization of pulsed microwave radiation from a magnetron microwave generator and presents results in the search for optimal pulsed microwave irradiation conditions; these would avoid damage to the electrode and would afford reduced power consumption. Results show that continuous generation of in-liquid plasma that avoids electrode (antenna) damage requires strict and very limited pulsed oscillation conditions. Evaluation of this device was investigated by the discoloration of a rhodamine-B (RhB) dye-contaminated wastewater, for which it was shown that higher treatment efficiency can be obtained compared to more traditional methods such as the UV photolysis (UV), the UV-assisted photocatalytic TiO2 method (UV/TiO2), and the NaClO methodology (NaClO). The energy consumed during the 3 min needed to discolor 50 mL of a 0.10 mM aqueous RhB dye solution was 6.3 × 10−3 kWh per mg of RhB; complete mineralization of the dye solution by the in-liquid plasma occurred within 15 min (loss of TOC).