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Effect of pH on COD Removal During Ozone Oxidation of Pollutants by Activated Carbon

Common catalysts in the heterogeneous ozone oxidation process include metal oxide catalysts, carbon-based material catalysts, and the like. So far, various metal-based catalysts (e.g., MnO2, CuO, ZnO, etc.) have shown good performance in heterogeneously catalyzed ozonation, but metal dissolution limits their practical application in water treatment. Carbon-based materials-catalyzed ozone oxidation has attracted much attention as an efficient advanced oxidation water treatment method. Among them, activated carbon is a black porous material with rich pore structure, large specific surface area and good adsorption performance, which can effectively promote the adsorption of ozone and organic matter to the catalyst surface and improve the degradation rate of organic matter. Activated carbon has the advantages of low price, high catalytic activity and good stability, and it has the function of catalyzing ozone decomposition to generate hydroxyl radicals. It is often used as a catalyst for ozone catalytic oxidation and has good application prospects in the field of wastewater treatment. ozone sterilizer

The research group of Harbin Institute of Technology and others grinded commercial granular activated carbon (GAC), screened out 2-3mm GAC, then washed and dried, and explored its activity and performance in the catalytic ozone oxidation degradation of p-nitrophenol (PNP). reaction mechanism.

It can be seen from Figure 2-2-1a that under acidic conditions, the rate of PNP oxidation by ozone alone is limited, and the rate of PNP oxidation by ozone alone increases with the increase of pH. This phenomenon shows that the oxidation efficiency of ozone molecules directly attacking PNP is low under acidic conditions; under alkaline conditions, OH¯ promotes the decomposition of O3 to generate OH, thereby increasing the degradation rate. It can be seen from Figure 2-2-1b that the efficiency of GAC adsorption of pollutants decreases with the increase of pH. Under acidic conditions, GAC adsorption of pollutants plays a dominant role, while the catalytic ozone oxidation process (Fig. 2-2-1c) mainly works under alkaline conditions. Due to the different reaction mechanisms, the trend of solution COD degradation curves under alkaline conditions is different from that under acidic conditions. Adsorption plays a dominant role under acidic conditions, and GAC rapidly adsorbs pollutants leading to COD removal. Therefore, under acidic conditions, the removal rate of organics is faster in the initial stage of the reaction. Under alkaline conditions, the reaction is based on a series of free radical reaction processes such as OH oxidation. Due to the electrophilic attack of OH, PNP is rapidly converted into an intermediate. Therefore, organic matter is removed at the initial stage under alkaline conditions. The rate is slower and gradually becomes faster in the later stage of the reaction.

Effect of pH on COD Removal During Ozone Oxidation of Pollutants by Activated Carbon

This work examines the change in liquid-phase ozone concentration as a function of the reaction. In Figure 2-2-2a, the dissolved oxygen concentration increased rapidly in the first 10~15min, and then gradually stabilized. The equilibrium ozone concentration at pH 10.0 of the solution is much lower than that at pH 4.0, indicating that the increase in OH¯ concentration under alkaline conditions promotes ozonolysis. In the ozonation process, hydrogen peroxide (H₂O₂) is usually produced by ozonolysis or by direct ozone attack on aromatic rings. As shown in Figure 2-2-2a, the H₂O₂ concentration increased rapidly within the first 10 minutes, and then gradually decreased. In a given time, the concentration of H₂O₂ at pH ≥ 10.0 was lower than that at pH ≥ 4.0, which indicated that the decomposition of peroxide gas was more favorable to generate free radicals under alkaline conditions, thereby promoting the oxidative removal of PNP.

Furthermore, in order to explore the effect of OH on the degradation of pollutants at different pH, a free radical suppression experiment using t-BAt-BA as OH quencher was carried out in this work. As shown in Figure 2-2-2b, under acidic conditions, t-BA has little effect on the reaction process, indicating that in acidic solution, OH oxidation does not play a dominant role. When the solution H is 10.0, adding t-BA to the system will significantly reduce the reaction rate, indicating that under destructive conditions, the main reaction is OH .

{Ozone dose is 2.53g/h, GAC dose is 10g/L; reaction temperature is 25℃, [initial COD]=1560mg/L}

Dissolved O₃ and H₂O, concentration

Schematic representation of the main reactions on the GAC surface

Under acidic conditions, the adsorption of GAC led to a rapid decrease in COD concentration. Under alkaline conditions, the activated carbon showed a strong synergistic effect with the ozone oxidation process, and the activated carbon catalyzed the catalytic decomposition of ozone and hydrogen peroxide to generate OH, thereby improving the pollutant removal rate. The main reaction process is shown in formula (2-2-1) to formula (2-2-8):
Organic+O₃→Intermediate+H₂O, (2-2-1)
H₂O₂→HO₂¯+H+ (2-2-2)
HO₂¯+O₃, →HO₂·+O₃·¯ (2-2-3)
O₃+H₂O+2e¯→O₂·¯+2OH¯ (2-2-4)
O₃+OH¯→O₂·¯+HO₂· (2-2-5)
O₃+HO₂·→2O₂·+HO· (2-2-6)
GAC+2H₂O→GAC-H₃O+ +OH¯ (2-2-7)
O₃+GAC-H₃O→O₃-GAC+HO (2-2-8)