Optimizing dissolved gas composition in a double-bath-type sonoreactor for efficient production of ultrasonic-activated water with stable oxygen and nitrogen reactive species

Abstract

Plasma-activated water (PAW), which mainly contains reactive oxygen and nitrogen species, has recently gained considerable attention due to its potential applications in disinfection of food and sustainable agriculture. However, the development and up-scaling of a lower-cost PAW production system for its commercial applications has yet to be achieved. This is where ultrasonic-activated water (UAW) presents itself as a viable alternative since it has similar physico-chemical properties to PAW but its production is more technically and economically feasible. In this work, a novel double-bath-type sonoreactor was successfully demonstrated to produce UAW efficiently. The composition and physico-chemical properties of as-produced UAW and formation efficiency of H₂O₂ and nitrate are greatly influenced by the dissolved gases and acoustic density. At high acoustic density, oxygen exhibited a higher sonochemical efficiency in terms of H₂O₂ production compared to that obtained in argon and vice versa. Interestingly, a fixed amount of nitrogen is needed not only to produce RNS but also to enhance the H₂O₂ production due to the scavenging effect of nitrate ions. The optimal composition of dissolved gases was found to be a mixture of oxygen, nitrogen, and argon with the volume ratio of 6 : 4 : 10, respectively. The concentration of nitrate in UAW remained stable while that of H₂O₂ slightly decreased up to 25% for 90 days of storage at tropical room temperature (37–41 °C) and under normal indoor light. This low-cost double-bath-type sonoreactor exhibited a comparable efficiency of H₂O₂ production (1.24 × 10⁻⁹ mol J⁻¹) compared to other expensive sonoreactors as well as some plasma devices.