Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems, part II: NO(2)(−)/NO(3)(−) generation and a brief critical review

The sonochemical generation of NO(2)(−) and NO(3)(−) 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 NO(2)(−) and NO(3)(−) 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 NO(2)(−) and NO(3)(−). Ar and O(2) were vital for generating relatively large concentrations of NO(2)(−) and NO(3)(−). The absorption of N(2) from the air resulted in high generation of NO(2)(−) and NO(3)(−) for Ar 100 % and Ar/O(2) mixtures under the saturation/open mode. In addition, gas sparging enhanced the generation of NO(2)(−) and NO(3)(−) 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 NO(3)(−) to NO(2)(−) 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 NO(2)(−) + NO(3)(−) could be more appropriate for investigating the NO(2)(−) and NO(3)(−) generation characteristics. In addition, H(2)O(2)/NO(2)(−)/NO(3)(−) related activity rather than H(2)O(2) activity was suggested to quantify the OH radical activity more appropriately in the presence of N(2).