Active Species Generated by a Pulsed Arc Electrohydraulic Discharge Plasma Channel in Contaminated Water Treatments

Pulsed arc electrohydraulic discharge (PAED) direct plasma technique was applied for various types of contaminated water treatment. The experimental system consists of a spark-gap switch type pulse power supply (0.5 kJ/pulse) and a 3 L stainless steel reactor with eccentrically configured rod-to-rod electrodes. The current and voltage waveforms are fundamentally different for different conductivity water. Double pulse current discharges were observed for pond water with relatively higher conductivity (637 mS/m) while a single pulse current discharge was observed for lake water with relatively lower conductivity (78 mS/m). From the optical emission spectrum and UV dosimeters, UV-A, UV-B, excited molecules and radicals including N2*, O, H, OH, O3 etc. were observed during the discharge period. Both optical emission and UV intensities in pond water are slightly lower than lake water. The decay time of the UV-A, N2*, OH, H and O radicals were around 0.6 ms, where the discharge period ended around 0.4 ms. The results indicate that the radicals existed longer than the discharge period. The pH, dissolved oxygen and conductivity were changed during the course of PAED treatment. The ions and radicals such as H·, O·, H+, OH· etc. generated by PAED may cause alterations density decay time and the active species in water were present for a longer period. The reduction of total organic carbon (TOC) in pond water reached 80% after 5 min of PAED treatment. Based on local thermal plasma equilibrium (LTE) model, LTE thermal plasma chemical composition model for 1 mol water vapour was used to compare to present experiments. PAED discharge in contaminated pond water generated peak concentration of OH = 28% and O2H = 0.012% mol in gas-phase and migrated to water-phase via gas–liquid interfaces generated by arc and micro-bubbles to form more stable O3, H2O2, H2 and O2. The model suggested that the reduction of TOC occurred in gas/plasma phases or liquid-phase side of gas liquid interfaces.