OH attack to the azo group in chemical structure of the dye molecule might yield nitroso- or nitro-aromatic compounds such as acetic acid, succinic acid, benzoquinone and dicarboxylic acids substituted with nitro groups. Other intermediate products were also identified such as nitrobenzene, dinitrobenzene, nitrophenol, nitrocatechol, dinitrophenol, and nitroso-N,N-dimethylaniline. The following mineralization reaction for azobenzene (C
12H
10N
2) can be generally represented as the complete oxidation process. Wang et al.
[46],
[59] reported that sulfate anions (
SO42−), nitrite anions (
NO2−) and nitrate anion (
NO3−) were produced after 40 to 120 min of sonocatalytic degradation due to the presence of nitrogen and sulfur atoms in organic dye molecule. All three peaks of anions shown in ion chromatograms gradually increased along with the time of ultrasonic irradiation. Wang et al.
[21],
[61] reported that the small or even total disappearance of peaks in ion chromatograms was due to the formation of TiO
2+ cations in the solution under ultrasonic irradiation which could easily react with
SO42− to form insoluble TiOSO
4 deposits.
SO42− in the solution were easily adsorbed by TiO
2 powder compared with other anions. Besides, the appearance of
NO2− and
NO3− indicated that volatile nitrogen oxides might escape from the reaction system after the cleavage of C―N bond between nitro group and benzene ring in a parathion molecule during sonocatalytic degradation.