Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems, part II: NO2-/NO3- generation and a brief critical review

초록

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 N-2 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. N-2:Ar (25:75), N-2:Ar (50:50), and O-2:N-2 (25:75) in the saturation/closed mode generated the three highest values of NO2- and NO3-. Ar and O-2 were vital for generating relatively large concentrations of NO2- and NO3-. The absorption of N-2 from the air resulted in high generation of NO2- and NO3- for Ar 100 % and Ar/O-2 mixtures under the saturation/open mode. In addition, gas sparging enhanced the generation of NO2- and NO3- for N-2:Ar (25:75), O-2:N-2 (25:75), and N-2 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/O-2 mixtures, and O-2 100 %, 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 N-2.

키워드