Effect of Process Parameters on HNO _y=2,3 and NO _y ^- Formation in Plasma-Treated Water

Here, we investigate the effects of hydrodynamics and temperature on the formation of nitrous and nitric acids (HNO _y=2,3) and their conjugate bases (NO _y ^-) in plasma-treated water (PTW) in a surface dielectric barrier discharge (sDBD) reactor. The stirring rate and bulk water temperature were systematically varied from 200 to 1000 rpm and 20 to 80 °C, respectively, while the plasma was held constant. Species in both the gas and liquid phases were tracked in real time using synchronized dual optical absorption spectroscopy, allowing correlation of the gas-phase O₃/NO _x=1-3 dynamics with the evolving aqueous species of NO₂ ^-, HNO₂, and NO₃ ^-. Nitrate production monotonically accelerated with increasing agitation and dominated above 800 rpm, whereas the nitrite pool, NO₂ ^- + HNO₂, peaked at a moderate speed of 600 rpm and declined at higher speeds, probably due to volatilization losses. Elevated water temperature suppressed NO and NO₂ formation, extended the O₃-rich regime, and shifted selectivity toward NO₃ ^-; conversely, cooler conditions favored nitrite accumulation. These trends reveal the temperature-dependent kinetics and mass-transfer limits that govern plasma-driven nitrogen fixation. Our findings offer practical guidelines for tuning the composition of PTW, thereby advancing its efficient, application-specific production at scale.