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.