Degradation of P-Nitrobenzoic Acid and 4-Chlorobenzoic Acid by Catalytic Ozonation with Modified Birnessite-Type MnO2 as Catalyst

Bir-MnO2 was synthesized through a hydrothermal method and modified into H-Bir by nitric acid acidification. The mechanism of its action in ozonation was explored by characterization. H-Bir possessed more oxygen vacancies and lower average oxidation state (AOS) than Bir-MnO2. The oxygen vacancies and surface hydroxyl groups are the main active sites of this catalyst. Under the reaction conditions of initial pH=7, ozone dosage of 7 mg/L, initial pollutant concentration of 10 mg/L, and catalyst dosage of 0.5 g/L for 40 min, the degradation rates of H-Bir catalyzed ozonation of p-nitrobenzoic acid (PNBA) and 4-chlorobenzoic acid (PCBA) reached 82.68% and 85.83%, respectively. Differences in the degradation factors affecting PCBA and PNBA were found in the performance study of H-Bir catalyzed ozonation. The combination of density functional theory (DFT) and catalytic ozonation experiment results revealed the difference between the degradation of PNBA and PCBA in the H-Bir/O3 system. It was deduced that the nitro, chlorine and carboxyl groups were the main reaction sites. Possible degradation pathways for the two pollutants were also proposed.