Abstract
The sonochemical generation of NO2- and NO3- is considered to be one of the reasons for the low sonochemical oxidation activity in the presence of N2 in the liquid phase. In this study, the generation characteristics of NO2- and NO3- were investigated using the same 28 kHz sonoreactor and the 12 gas conditions used in Part I of this study. Three gas modes, saturation/closed, saturation/open, and sparging/closed, were applied. N2:Ar (25:75), N2:Ar (50:50), and O2:N2 (25:75) in the saturation/closed mode generated the three highest values of NO2- and NO3-. Ar and O2 were vital for generating relatively large concentrations of NO2- and NO3-. The absorption of N2 from the air resulted in high generation of NO2- and NO3- for Ar 100 % and Ar/O2 mixtures under the saturation/open mode. In addition, gas sparging enhanced the generation of NO2- and NO3- for N2:Ar (25:75), O2:N2 (25:75), and N2 significantly because of the change in the sonochemically active zone and the increase in the mixing intensity in the liquid phase, as discussed in Part I. The ratio of NO3- to NO2- was calculated using their final concentrations, and a ratio higher than 1 was obtained for the condition of Ar 100 %, Ar/O2 mixtures, and O2 100 %, wherein a relatively high oxidation activity was detected. From a summary of the results and findings of previous studies, it was revealed that the observations of NO2- + NO3- could be more appropriate for investigating the NO2- and NO3- generation characteristics. In addition, H2O2/NO2-/NO3- related activity rather than H2O2 activity was suggested to quantify the OH radical activity more appropriately in the presence of N2.