Key Chemical Pathways of Hydroxyl Radicals in the Plasma-Liquid System

Abstract

In the last decades, active attempts have been made to apply atmospheric-pressure plasma technology toe various industrial fields such as biomedicine, agriculture, and food processing. For such applications, plasma-treated-water (PTW) has attracted great attention because it facilitates the storage of reactive oxygen and nitrogen species (RONS) produced by plasma. Among the various RONS, hydroxyl radical (OH*), which is one of the most powerful oxidants and the most important intermediate in PTW, becomes increasingly significant, however, generation mechanism of OH* in plasma-liquid system still remain unclear. Here, we report that N(III) [i.e., nitrite anion (NO2-) and nitrous acid (HONO)] and hydrogen peroxide (H2O2) are a largely unconsidered and potential sin of OH* in plasma-liquid system where an argon atmospheric-pressure plasma jet was used. The contribution of N(III) and H2 O2 photolysis to OH* production is estimated by numerical and experimental approaches. The result of model experiments reveals that the concentration of OH* generated by the UV photolysis of N(III) duet to plasma emission is approximately 20% of its total concentration of in the PTW. This means that a significant amount of OH* is produced via photolysis of N(III) comparable with other generative reactions such as direct dissociation of H2O at the plasma-solution interface. Furthermore, the concentration of OH* is almost doubled by the photolysis of N(III)and H2O2 in the presence of an external UV light source. We therefore suggest that an additional light exposure is the way to effectively enhance the OH* production in plasma-liquid systems.