Plasmachemical oxidation processes in a hybrid gas–liquid electrical discharge reactor

Oxidation processes induced in water by pulsed electrical discharges generated simultaneously in the gas phase in close proximity to the water surface and directly in the liquid were investigated in a hybrid series gas?liquid electrical discharge reactor. The mechanism of phenol degradation was studied through its dependence on the gas phase and liquid phase compositions using pure argon and oxygen atmospheres above the liquid and different initial pH values in the aqueous solution. Phenol degradation was significantly enhanced in the hybrid-series reactor compared with the phenol removal by the single-liquid phase discharge reactor. Under an argon atmosphere the mechanism of phenol degradation was mainly caused by the electrophilic attack of OH? radicals produced by the liquid phase discharge directly in water and OH? radicals produced by the gas phase discharge at the gas?liquid interface. Under an oxygen atmosphere the formation of gaseous ozone dominated over the formation of OH? radicals, and the contribution of the gas phase discharge in this case was determined mainly by the dissolution of gaseous ozone into the water and its subsequent interaction with phenol. At high pH phenol was degraded, in addition to the direct attack by ozone, also through indirect reactions of OH? radicals formed via a peroxone process by the decomposition of dissolved ozone by hydrogen peroxide produced by the liquid phase discharge. Such a mechanism was proved by the detection of cis,cis-muconic acid and pH-dependent degradation of phenol, which resulted in significantly higher removal of phenol from alkaline solution observed under oxygen atmosphere than in argon.