Chemical processes in aqueous phase pulsed electrical discharges: Fundamental mechanisms and applications to organic compound degradation.

摘要

A pulsed electrical discharge in water produces conductive channels called streamers or "streamer-like plasma channels" and high energy electrons which are capable of dissociating and/or ionizing water molecules. The primary species formed when a water molecule is dissociated in the channel are hydroxyl and hydrogen radicals. After being formed in a discharge channel, these and other radical species either react with each other to form product molecules, such as molecular hydrogen and hydrogen peroxide, or diffuse away to react with solutes. Hydroxyl radical is one of the most powerful oxidants capable of destroying almost any organic compound.; The experiments conducted in the present study have two specific goals: (1) to develop an understanding of the basic mechanism of water dissociation and active species formation within a plasma channel and (2) to evaluate the efficiency of plasma reactors for the destruction of environmental pollutants.; As a first goal stated, a mathematical model which accounts for the pulsed nature of the discharge was developed. The model predicted, for the first time, how molecular oxygen is formed in an underwater discharge and confirmed the mechanism for molecular hydrogen and oxygen formation. In order to gain more insight in the primary reactions taking place in the discharge channel, pulsed electrical discharge in light (H2O) and heavy (D2O) water was investigated.; Further experiments are focused on explaining the platinum behavior as the high voltage electrode as compared to the nickel-chromium. Platinum high voltage electrode causes the pH dependent decomposition of hydrogen peroxide in the bulk solution as well as the reduction of ferric ions to ferrous ions. Both processes take place on the surface of the platinum particles ejected from the platinum high voltage electrode. A mathematical model developed suggests that the platinum catalyzed decomposition of hydrogen peroxide is caused by the adsorption of molecular hydrogen produced by the discharge and hydroxyl ions on the platinum surface. Ferric ions are reduced to ferrous ions by the hydrogen adsorbed on the platinum surface.; As the second goal stated, the efficacy of electrical discharges in water has been demonstrated by performing experiments with two organic contaminants. The model compounds included herbicide atrazine and s-triazine and their degradation kinetics in a pulsed electrical discharge was evaluated. The effects of initial pH of the solution, electrolyte, electrode materials, additives such as zeolites and platinum particles on the degradation kinetics of these organic contaminants were investigated with the goal of understanding degradation mechanisms and the parameters leading to maximum degradation.