Detection of Free Radicals Produced by a Pulsed Streamer Corona Discharge in Solution Using Electron Spin Resonance

Experiments are carried out on the detection of free radicals in the plasma induced by a pulsed electrohydrodynamic discharge using electron spin resonance (ESR) measurement. In addition, decolorization of reactive dye (C. I. Reactive RED 106) solution is investigated as a related topic. The plasma reactor is a barrier-type and consists of a stainless needle electrode inside the solution and quartz glass solution container (200 mL) around which the aluminum film grounded electrode is wrapped. A positive pulse high voltage of +35 ∼ 65 kV is applied between the electrodes of the plasma reactor. Streamers in solution are induced and the onset voltage of the streamer increases with increase in the electrical conductivity of the solution. In the ESR measurements, a radical trapping agent of DMPO (5, 5-Dimethyl-1-pyrroline N-oxide) is used. When the plasma is solely applied, hydroxyl radical is scarcely detected with the ESR and the decoloration of the reactive dye does not occur because the lifetime of hydroxyl radical is too short to be detected although the generation of hydrogen peroxide H2O2 is confirmed. However, after adding iron (II) sulfate (FeSO4•7H2O) including Fe before the plasma application, the ESR spectra for hydroxyl radical is clearly detected and the dye solution is decolorized dramatically. It is found that hydrogen peroxide is initially generated by the plasma and is decomposed by Fe (Fenton reaction) or under higher pH (∼10) condition to form hydroxyl radical in relatively slow chemical reactions. Furthermore, both hydroxyl and hydrogen radical can be detected when the plasma is applied for the solution including DMPO which is added before the plasma application.