Hydroxyl radical production in DC streamer discharge

Plasma-induced advanced oxidation processes do not suffer from the drawbacks, such as carcinogenic byproducts, associated with conventional water treatment, and enable the removal of micro-pollutants [1]. The high oxidation strength of hydroxyl radicals enables degradation of resistant contaminants. Many reactions are known to occur at the plasma-water interface; however, the mechanisms of hydroxyl radical production are still not clear. To understand the physical and chemical processes occurring at the plasma-water interface, this research involved investigation of the hydroxyl radicals produced during d.c. streamer discharges. A needle-plate electrode configuration in atmospheric air was used, with the treated solution used as the ground electrode. To understand the effects of polarity and gas types on hydroxyl radical production, both positive- and negative-polarity energization in air, nitrogen and helium were investigated. Plasma filaments were developed from the needle electrode, which was in contact with the solution. Terephthalic acid (TA) was used as a scavenger of hydroxyl (OH) radicals, with OH production subsequently being quantified by fluorescence emission from 2-hydroxyterephthalic acid (HTA), which is formed through specific reaction between TA and OH [2]. The power inputs in positive pulsed streamer discharges were 0.125 W, 0.18 W and 0.26 W in air, nitrogen and helium, respectively; the corresponding hydroxyl radical production efficiencies were 0.56 mmol/kWh, 1.1 mmol/kWh and 5.94 mmol/kWh, respectively. For negative pulsed streamer discharges in air, the power input was 0.063 W and the efficiency was 1 mmol/kWh. The hydroxyl radical production rates were 2.6 × 10^-7 Ms^-1 in negative air discharge, and 2.7 × 10^-7 Ms^-1, 1.8 × 10^-6 Ms^-1, and 2.2 × 10^-6 Ms^-1 in positive air, nitrogen and helium discharges, respectively.