Analysis of Time-Resolved Plasma Jet Emissions That Drive Methylene Blue Dye Decomposition

Abstract

Plasma-based water purification uses energetic electrons to induce chemical reactions that break down harmful contaminants into benign components. The chemical reactions needed to decompose organic chemicals and bacteria in water are driven by the production of the hydroxyl radical, OH. In this article, an atmospheric pressure plasma jet was used to produce OH and decompose methylene blue (MB) dye in water samples. The behavior of the plasma and the emission of excited OH* near the plasma–liquid interface were analyzed. Nanosecond-resolved measurements near the plasma–water interface were obtained using ICCD imaging and emissions spectroscopy synchronized to the pulsed dc power that drives plasma formation in the plasma jet. The plasma was observed to form along the front of ionization waves as “bullets” that locally produce reactive species. When the bullet hits the water surface, it rebounds and creates a secondary excitation of species at the water surface. The bouncing phenomenon increases the plasma interactions above the water surface and increases OH* excitation up to 192%. However, this increase in excitation and emission does not necessarily mean increased total OH. Higher discharge frequencies produced more OH* emission but did not change the rate of MB dye removal. Higher voltages do increase dye removal rate but with decreasing effectiveness. The results indicate that in steady state, most of the water in the gas channel has been dissociated to form OH, and each bullet merely re-excites the ground state OH (X) into OH* (A) state. A minimum operational frequency of 1 kHz was thus found to provide the best efficiency while maintaining OH production.