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
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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).
KeywordsIn-liquid plasma Semiconductor microwave generator Pulsed microwaves Rhodamine-B dye Wastewater treatment
We are grateful to the Japan Society for the Promotion of Science (JSPS) for financial support to SH through a Grant-in-aid for Scientific Research (No. C-25420820). Financial support from the Sophia University-wide Collaborative Research Fund to SH is also appreciated. One of us (NS) thanks Professor Angelo Albini of the University of Pavia (Italy) for his continued hospitality during the many winter semesters in the PhotoGreen Laboratory at the University of Pavia, Italy.
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