Contrasting Characteristics of Gas–Liquid Reactive Species Induced by Pulse-Modulated RF and kHz Sinusoidal Plasma Jets

The pulse-modulated radio frequency (RF) and kHz sinusoidal voltages are used for the excitation of two plasma jets, for which the reactive species in the plasma plumes and in the treated water are comparatively studied. With the same discharge power, it is found that concentrations of aqueous reactive species generated by the kHz sinusoidal plasma jet are higher than that of the pulse-modulated RF plasma jet, especially for the short-lived species OH. In addition, the comparison between the bacterial inactivation results is in accordance with that for the concentrations of aqueous reactive species for the two types of plasma jets. On the contrary, the emission spectra indicate that the gaseous reactive species such as OH(A) and O(3p5P) have much higher densities for the pulse-modulated RF plasma jet, which should ascribe to its higher electron density and electron temperature. Moreover, the concentrations of the aqueous reactive species and the emission intensities of the gaseous reactive species have similar variation trends for each of the plasma jets, as a function of the discharge power and the gap width. These results imply that the emission intensity could not reflect the relative concentration of an aqueous reactive species between these two plasma jets, but it could be used as an indicator of the concentration variation of an aqueous reactive species for each of the plasma jets. In addition, the phenomenon of electron trapping effect also indicates that the aqueous reactive species can be effectively generated only when more electrons act on the water sample.