Generation mechanism of reactive oxygen-nitrogen species in plasma-treated water and evaluation of its biological effect

Abstract

Atmospheric pressure plasma has been used in various fields such as ozone generators, sterilizers, and dust collectors since the 1800s because of its ability to produce various active oxygen and nitrogen species (RONS). Entering the 1990s, many cases have been reported to replace conventional processing or maximize efficiency by applying atmospheric pressure plasma to green industries such as medical, agriculture, and food. Considering that RONS plays a significant role in the application of plasma technology, there is a growing interest in controlling of RONS and efficient methods of plasma application. In the food and medical field, the RONS is delivered to the treated subject through the liquid, since the plasma-treated subject is mostly in aqueous phase or surrounded by liquid. Accordingly, researches on generating plasma-treated water (PTW) that can store RONS for a long time, research of RONS controlling in PTW, and utilizing it has been actively conducted in recent years. Therefore, it is time to identify RONS generation mechanisms in PTW and how to control them, and how to apply it that can be effectively used in industrial. This paper deals with the analysis of plasma sources used in the manufacture of PTW, the identification of production mechanisms and control methods for various RONSs in PTW, and the analysis of biofilm removal for PTW. A planar patterned dielectric barrier discharge (DBD) plasma source was used to produce PTW. The operation conditions of the plasma were tested at the low power consumption of 10-15 W at low frequency (LF, 20-50 kHz), and fused silica and alumina were used as the dielectric with 1 mm of thickness. In addition, the in-situ measurement system was established by various methods such as absorption spectrum, scavenger usage, and colorimetry to accurately measure RONS. In addition to measuring methods, zero-dimensional simulations have been developed and built to track RONS which is difficult to measure. Through the experiments and simulations, the characteristics of the DBD plasma source were analyzed, and the difference of RONS generation according to the material difference was identified in terms of temperature and electric field. In order to control the produced RONS, the increase and decrease of the production were verified as by terms of UV irradiation, temperature control, power control, pH control, and dielectric material selection. In particular, the intensive control method for ozone ($O_{3}$), hydroxyl radical (OH), nitrite ($NO_{2}^{-}$), and nitrate ($NO_{3}^{-}$), which are widely used for sterilization and nitrogen fixation, etc, have been sought. Control mechanisms have also been identified. $O_{3}$ was decomposed through high temperature and UV irradiation, and it was confirmed that heat generation due to power rise was also involved in $O_{3}$ decomposition. The pH control allows you to control the rate of $NO_{2}^{-}$ reduction, which helps to extend the useful life of PTW. It was also confirmed that pH control can increase the amount of OH produced during the PTW storage period. Since each of the control methods is driven independently, this paper suggests the possibility of selectively increasing or suppressing production through the appropriate combination. In order to determine the influence of RONS in PTW, biofilm was selected as a subject. Because biofilm can be easily accessible around us, such as kitchen knives, water pipes, and food processing facilities, and it can grow very quickly and cause problems such as secondary infection or contamination. According to the results of the present study, when E. coli, Salmonella, and Listeria were formed on the surfaces of stainless steel, glass, PET, PE, and PVDC, respectively, it was confirmed that PTW shows effective removal efficiency. $H_{2}O_{2}$ (1 $mM$), $O_{3}$ (10 $\mu$M), OH (0.1 $\mu$M) and $NO_{2}^{-}$ (5 $mM$) showed 25%, 14%, 10%, and 9% biofilm removal in the order. Although $H_{2}O_{2}$ showed the best efficiency for biofilm removal, the order can be changed according to the type of treatment subject. This paper shows great scientific information about that PTW can show high efficacy if the appropriate RONS for each treatment subject is controlled through the aforementioned control method.