Degradation of nitrobenzene wastewater in an acidic environment by Ti(IV)/H2O2/O3 in a rotating packed bed

The rotating packed bed (RPB) as a continuous flow reactor performs very well in degradation of nitrobenzene wastewater. In this study, acidic nitrobenzene wastewater was degraded using ozone (O3) combined with hydrogen peroxide and titanium ions (Ti(IV)/H2O2/O3) or using only H2O2/O3 in a RPB. The degradation efficiency of nitrobenzene by Ti(IV)/H2O2/O3 is roughly 16.84% higher than that by H2O2/O3, and it reaches as high as 94.64% in 30 min at a H2O2/O3 molar ratio of 0.48. It is also found that the degradation efficiency of nitrobenzene is significantly affected by the high gravity factor, H2O2/O3 molar ratio, and Ti(IV) concentration, and it reaches a maximum at a high gravity factor of 40, a Ti(IV) concentration of 0.50 mmol/L, a pH of 4.0, a H2O2/O3 molar ratio of 0.48, a liquid flow rate of 120 L/h, and an initial nitrobenzene concentration of 1.22 mmol/L. Both direct ozonation and indirect ozonation are involved in the reaction of O3 with organic pollutants. The indirect ozonation due to the addition of different amounts of tert-butanol (·OH scavenger) in the system accounts for 84.31% of the degradation efficiency of nitrobenzene, indicating that the nitrobenzene is dominantly oxidized by ·OH generated in the RPB-Ti(IV)/H2O2/O3 process. Furthermore, the possible oxidative degradation mechanisms are also proposed to better understand the role of RPB in the removal of pollutants.