Optimization study of the chemical composition of plasma-activated water through plasma positioning

Plasma-activated water (PAW) is gaining prominence in agro-food applications due to its versatile and safe reactive oxygen and nitrogen species (RONS), such as hydroxyl radicals, hydrogen peroxide, nitrites, and nitrates [1], [2]. The key components responsible for PAW’s efficacy in various applications are the RONS generated in water, necessitating a deeper understanding of their chemistry to tailor PAW for specific responses [3]. This study investigated the impact of varying plasma treatment distances on the physicochemical parameters of PAW, generated using a pin-to-plate dielectric barrier discharge plasma source at fixed treatment voltage of 7 kV, time 6 min, volume 35 ml, frequency 20 kHz and different distances of $5,10,15$, and 20 mm. Physicochemical parameters, including temperature, TDS, pH, conductivity, and concentrations of hydrogen peroxide and nitrate were measured to discern the chemical composition of RONS in water. Notably, nitrate and hydrogen peroxide concentrations underwent significant changes, decreasing from $181.7905 \mathrm{mg} / \mathrm{l}$ and $13.95188 \mathrm{mg} / \mathrm{l}$ at 5 mm to $4.7 \mathrm{mg} / \mathrm{l}$ and $0 \mathrm{mg} / \mathrm{l}$ at 20 mm. A treatment distance of 5 mm resulted in the generation of more RONS due to a higher discharge current that hindered the radial propagation of streamers on liquid surface, resulting in a chaotic and unstable plasma discharge accompanied by spark formation. In contrast, at 20 mm, the electric field intensity decreased, leading to a small discharge current and the formation of a few streamers in a fountain shape. This led to a decrease in RONS concentration, diminishing interactions among themselves and with the liquid interface. Increasing the treatment distance can potentially lead to a uniform electric field, but it necessitates a higher breakdown voltage for discharge initiation. This limitation sets a practical upper limit on the treatment distance that can be used to achieve a uniform electric field while maintaining a manageable breakdown voltage.