Formation of inorganic nitrogenous byproducts in aqueous solution under ultrasound irradiation

The effects of ultrasonic frequency, power intensity, temperature and sparged gas on the generation of nitrogenous by-products NO₂^- and NO₃^- have been investigated, and the new kinetics model of NO₂^- and NO₃^- generation was also explored. The results show that the highest primary generation rate of NO₂^- and NO₃^- by direct sonolysis in the cavitation bubbles (represented by k₁' and k₂', respectively) was obtained at 600 kHz and 200 kHz, respectively, in the applied ultrasonic frequency range of 200 to 800 kHz. The primary generation rate of NO₂^- (represented by k₁') increased with the increasing ultrasonic intensity while the primary generation rate of NO₃^- (represented by k₂') decreased. The lower temperature is beneficial to the primary generation of both NO₂^- and NO₃^- in the cavitation bubbles. The optimal overall yields of both NO₂^- and NO₃^- were obtained at the N₂/O₂ volume (in the sparged gas) ratio of 3:1 which is near to the ratio of N₂/O₂ in air. The dissolved O₂ is the dominant oxygen element source for both NO and NO₂, compared with water vapor. Ultrasonic irradiation can significant enhance the recovery rates of dissolved N₂ and O₂ and thus keep the N₂ fixation reaction going even without aeration.