Decomposition of Persistent Organics in Water Using a Gas–Liquid Two-Phase Flow Plasma Reactor

Pulsed dielectric barrier discharges (DBDs) generated inside inert gas bubbles were studied to decompose persistent organic materials in water. Two glass tubes were coaxially mounted, and metal electrodes were attached at the inner and outer surfaces of the tubes that acted as dielectric barriers. A 10-ppm acetic acid solution filled the gap between the tubes and acted as the persistent organic material. Flow-controlled inert gases, such as argon and neon, were fed through a bubbler into the solution and formed a gas-liquid two-phase flow. Fast-rising pulsed voltages at repetitive frequencies up to 10 kHz generated pulsed DBDs inside the gas bubbles that bridged the 1-mm gap between the tubes. Bubble images were captured with a high-speed video camera, while an intensified charge-coupled device camera captured temporal discharge images at an exposure time of 5 ns. The decomposition rate and energy efficiency were measured by the decreased amount of acetic acid concentration; it was found that higher efficiency was obtained using neon gas. The hydroxyl (OH) radical emission intensity from the neon discharge was lower than that from an argon discharge. Laser-induced fluorescence measurements revealed that the number density of OH radicals in the ground state was higher in neon discharge experiments using a pin-plane-type electrode. The OH radicals in the ground state are more important for the decomposition of acetic acid.