Hydroxyl Radicals and Hydrogen Peroxide Formation at Nonthermal Plasma–Water Interface

Abstract

This work investigated hydroxyl radicals and hydrogen peroxide formation under a needle-plate electrode configuration using positive-polarity d.c. discharges generated in air, nitrogen and helium. The discharge mode in air and nitrogen was found to change above ultrapure water, initially a nanosecond pulse discharge was observed, transitioning to a diffuse discharge due to the increasing conductivity of the water. The discharge in helium was a nanosecond pulse discharge and the repetition rate increased with increasing water conductivity. It was found that hydroxyl radicals contribute to 7%, 78% and 70% of hydrogen peroxide formation when using ultrapure water in air, nitrogen and helium, respectively. It is suggested that hydroxyl radicals are formed by water reactions with energetic positive ions and neutral particles such as N 2 + , He + , O, H and HO 2. Part of the hydrogen peroxide is formed directly from atoms and radical reactions with water in nitrogen and helium, while oxygen reactions are heavily involved for hydrogen peroxide formation in air. A fluorophotometry method, using terephthalic acid, was used to directly quantify the formation of hydroxyl radicals and compared with the tert-butanol method.

Non-touching plasma–liquid interaction – where is aqueous nitric oxide generated?

“…Knowledge of the importance of the latter is growing and several recent publications emphasize its role over that of the bulk liquid. [24][25][26][27][28][29] However, as discussed by Bruggeman et al, transport processes, chemical reactions in the bulk gas, the liquid phase, and at the interface occur simultaneously. 30 This makes the segregation of processes challenging, especially with the leak in gas-liquid interface diagnostic approaches.…”

“…Furthermore, possible destruction reactions could occur also via reactions of NO with H (ESI, † reaction (S9)). Moreover, also O 3 , O 2 , or other nitrogen species can be involved in the destruction of NO (reactions (21)- (29)). In the long-term view, without spin trapping agents or other organic molecules present in the solution, NO will result in NO…”

Insecticidal Effects of Plasma Treated Water

“…When applying a plasma discharge to a water surface the occurring hydroxyl radicals are offering a possible source for hydrogen peroxide. The effectivity of hydrogen peroxide formation through OH radical dimerization is depending on the applied parameter setting of each plasma source, as for example shown by Porter et al [ 45 ] as well as Zhao et al [ 46 ]. Examining the hydrogen peroxide concentration as depicted in Figure 3 is giving an idea of the responsible parameters.…”

“…This factor could very well promote the formation of OH and H 2 O 2 within the volume of the humidified air at a much larger scale, compared to the standard liquid-Gas interface as apparent in lesser humid air. OH radical dimerization is acting as the major pathway for the formation of H 2 O 2 in nitrogen and helium gases, as shown by Zhao et al [ 46 ].…”

Low-Pressure Discharge Plasma Treatment of Aqueous Solutions with Mn, Cr and Fe

“…For examples, sometimes the measured rate of formation of ОН • radicals was much lower than the rate of formation of H 2 O 2 [18]. As found in [21] (12) When accumulating in the course of chemical transformations, hydrogen peroxide tends to polymerise forming hydrogen polyoxides [22]. In general, this process can be described as follows:…”