Oxidative decoloration of dyes by pulsed discharge plasma in water

Degradation of organic dyes by the pulsed discharge plasma between needle-to-plane electrodes in contaminated water has been investigated in three discharge modes: (i) streamer, (ii) spark, (iii) spark–streamer mixed mode. The process of the decoloration has been found to be most effective if the discharge operates in the spark–streamer mixed mode in dye solutions. The decoloration rate during the pulsed discharge plasma treatment was dependent on the initial pH values. The decoloration rate was increased when more acidic condition was used, especially in the case of streamer discharge mode. The decoloration rate at the pH value of 3.5 was found to be approximately three times higher than that at a pH value of 10.3. A small effect of initial pH during the decoloration process by spark and spark–streamer discharge mode means that the physical effects, such as shock-wave and ultraviolet radiation, may play an important role in the oxidation process. It was found that the decoloration rates in the case of spark and spark–streamer mixed discharge modes, which are characterized by high intensity ultraviolet radiation, were found to be much higher than that in the case of streamer discharge that is characterized by low intensity ultraviolet radiation. In addition, the considerable increase in the decoloration efficiency of H2O2 containing solutions can be attributed to the increase in hydroxyl radicals’ concentration. These are produced by ultraviolet light photo-dissociation of H2O2 molecules in water surrounding the plasma channel.