Insight into synergies between ozone and in-situ regenerated granular activated carbon particle electrodes in a three-dimensional electrochemical reactor for highly efficient nitrobenzene degradation

This study compared the removal and mineralization of nitrobenzene (NB) by electrolysis using granular activated carbon (GAC) as three-dimensional (3D) electrodes, ozonation, and the combination of electrolysis, GAC, and ozone (E-GAC-O3). A highly synergetic effect was demonstrated by combining electrolysis, ozone, and GAC, and able to achieve 95.58% of TOC removal within 120 min due to abundant production of OH in the E-GAC-O3 process. Interestingly, further study revealed 92.30% of NB removal was due to the oxidation of OH, and the E-GAC-O3 process could achieve a much higher energy efficient ratio for OH production compared with other processes. Besides, the mechanism of OH generation was explored through quantitatively estimating the contribution of different reaction paths involved in E-GAC-O3 process. Results demonstrated that electrochemical oxidation of hydroxyl ion, peroxone reaction, GAC catalyzed ozone reaction, and electro-reduction of ozone reactions were responsible for 12.50%, 37.50%, 8.75%, and 31.25% of OH generation, respectively. Moreover, the durability of GAC in E-GAC-O3 process was systematically investigated by reusing GAC for 50 times. It is worth noting that GAC possessed a very stable activity for transforming ozone into OH with almost unchanged functional groups and pore texture during long consecutive recycles in E-GAC-O3 process, while the cathode insulation experiment revealed that replenishing of free electrons from both cathode and inside of GAC was critical for maintaining the stability of GAC. These findings should be widely considered in the combination of electrolysis using 3D electrodes and ozone technologies to obtain further improvement of their potential and applicability in industrial practice. Finally, the removal efficiency of other ozone-refractory organics, Ibuprofen (IBP), Benzotriazole (BTA), and N,N-Dimethylformamide (DMF) were also investigated while the effects of different water matrices on NB removal in E-GAC-O3 process was studied. All the results suggest that the E-GAC-O3 process was efficient and sustainable for refractory organic wastewater treatment.