Sonocatalytic ozonation, with nano-TiO₂ as catalyst, for degradation of 4-chloronitrobenzene in aqueous solution

The results for the degradation efficiency of 4-chloronitrobenzene (4-CNB) by various advanced oxidation processes in a batch mode using nano-structured titanium dioxide demonstrated the order of O₃/US/TiO₂ > US/O₃ > O₃/TiO₂ > O₃ > US/TiO₂ > US. The utilized TiO₂ nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Moreover, all processes obeyed pseudo-first order kinetics. Then, the effect of the main operational conditions including pH, ozone, initial 4-CNB concentration, TiO₂ dosage and ultrasonic power on 4-CNB degradation was studied in the coupled O₃/US/TiO₂ process as the most significant treatment method with a high synergistic effect. The optimal pH and TiO₂ dosage were selected to be 9 and 50 mg/L owing to the efficient indirect attack of hydroxyl radicals and less screening effects of TiO₂ particles on ultrasonic waves, respectively. By increasing the amount of O₃ concentration and ultrasonic power, the degradation of 4-CNB was enhanced due to the more production of reactive oxygen species. However, the opposite trend was observed for 4-CNB concentration because the same amounts of generated oxidizing species under the identical experimental conditions had to degrade more organic pollutants and its degradation intermediates. Eventually, the total organic carbon (TOC) was utilized to monitor the mineralization of 4-CNB in the above-mentioned processes and the same results were obtained with lower rate constants compared to degradation rate constants.