3.1. Mn/[email protected] Catalyst Preparation Optimization
As shown in
Figure 1a, under ozone-free conditions, the blank RM catalyst has a fast adsorption rate within 0–60 min and reaches the adsorption saturation state at 80 min, and the COD removal rate is 7.89%. In the ozonation reaction system alone, the COD removal rate remains stable after 80 min, which is 31.14%. Compared with ozonation alone, the COD removal rate increases after adding the blank RM catalyst and finally reaches 36.58%. As shown in
Figure 1b, compared with the RM catalyst with a single active component, the incorporation of Mn and Ce considerably improves the degradation performance of the catalyst. In addition, the Mn/
[email protected] catalysts prepared with different Mn:Ce doping ratios show distinct catalytic activities. When Mn:Ce = 1:4, the COD removal rate is low. With the increase in the doping ratio of Mn, the COD removal rate continues to rise. When Mn:Ce = 2:1, the COD removal rate reaches the maximum value of 59.07% in the experiment. However, when the doping ratio of Mn continues to increase, the degradation of the Mn/
[email protected] catalyst on the biochemical tail water gradually worsens. When Mn:Ce = 4:1, the COD removal rate drops to 55.01%.
Figure 1c shows that when the calcination temperature is 350–550 °C, with the increase in calcination temperature, the performance of the Mn/
[email protected] catalyst is significantly improved, and the COD removal rate increased from 47.38% to 64.38%. Nonetheless, when the temperature is increased to 600 °C, the performance of the Mn/
[email protected] catalyst decreases slightly, and the COD removal rate drops to 57.24%. As shown in
Figure 1d, when the calcination time is 150–240 min, the performance of the Mn/
[email protected] catalyst shows a significant improvement with time, and the COD removal rate increases from 51.38% to 66.93%. The degradation performance of the catalyst shows a slight decrease when calcination time over 240 min. When the calcination time is 300 min, the COD removal rate drops to 63.31%.
3.2. Characterization of the Mn/[email protected] Catalyst
As shown in
Figure 2a, the morphology of the blank RM is mainly granular and massive, the surface is rough and uneven. A few uneven pores exist in the blank RM and the pore size is not uniform. According to
Figure 2b, after fly ash float beads are added, the pore structure of the Mn/
[email protected] catalyst is improved significantly, the number of pores increases, the particle distribution is uniform, and a large amount of metal oxides are attached. At the same time, the components are tightly combined without agglomeration.
Figure 2c shows the SEM characterization of the Mn/
[email protected] catalyst reused 25 times. The pore connectivity of the catalyst is significantly reduced after repeated use. This may be due to the clogging of the pores by pollutants or the collapse of the pores caused by aeration during the degradation [
23]. As shown in
Table 2 and
Figure S3, the specific surface area of the optimal Mn/
[email protected] catalyst is 12.1829 m
2/g, which is 1.97 times the specific surface area of the blank RM (6.1959 m
2/g). The expansion of the specific surface area can improve the adsorption performance of the catalyst, prolong the residence time of ozone in the wastewater, and thus improve the catalytic efficiency [
24]. The specific surface area, specific pore volume, and average pore diameter of the Mn/
[email protected] catalyst are reduced after reusing 25 times, which are 11.5218 m
2/g, 0.0684 cm
3/g, and 3.5716 nm, respectively.
Figure 3a demonstrates that the main constituent elements of RM include Al (16.61 wt%), Si (10.12 wt%), Fe (37.41 wt%), Mn (0.07 wt%), and Ce is not detected.
Figure 3b shows that the content of Al, Si, and Fe in the catalyst prepared using the doping–calcination method is reduced to 15.21, 7.59, and 35.46 wt%, respectively. By contrast, the content of Mn and Ce is significantly increased to 1.52 and 0.73 wt%, respectively.
Figure 3c presents that after repeated use, the active components in catalyst fall off slightly [
25]. The possible reasons for the loss of active components include the collision of the Mn/
[email protected] catalyst and the continuous erosion of the catalyst surface by aeration.
Table 3 shows that Fe
2O
3, SiO
2, and Al
2O
3 are the main components in the blank sample of RM, and their contents are 39.32, 12.23, and 25.37 wt%, respectively. The remaining components include CaO, Na
2O, TiO
2, and a trace amount of MnO
2. Compared with the blank sample of RM, the content of MnO
2 and CeO
2 in the optimal sample of Mn/
[email protected] catalyst increased to 1.69 and 0.75 wt%, respectively. After the Mn/
[email protected] catalyst is used 25 times, the content of MnO
2 and CeO
2 decreased by 0.11 and 0.07 wt%, respectively. In addition, the active components, such as Fe and Ti, contained in the RM will form Fe
2O
3-MnO
2, TiO
2-MnO
2, Fe
2O
3-CeO
2, TiO
2-CeO
2, and other multicomponent synergistic catalytic degradation systems with the oxides of Mn and Ce [
18]. Such formation will further improve the performance of the Mn/
[email protected] catalyst to degrade biochemical tail water.
As shown in
Figure 4a, the blank sample of RM has evident characteristic peaks at 2θ = 27.4°, 33.1°, 37.1°, which are Fe
2O
3, TiO
2, and Al
2O
3, respectively. Comparison of the diffraction peaks of each catalyst and the blank RM shows that the diffraction peaks of other catalyst samples at 2θ = 28.1°, 38.1°, 43.2°, 56.7° are consistent with MnO
2, and the diffraction peaks at 2θ = 29.2° and 45.5° are consistent with CeO
2. This finding indicates that the Mn and Ce in the Mn/
[email protected] catalyst mainly exist in the crystalline phase of MnO
2 and CeO
2. Compared with the Mn/
[email protected] catalyst prepared at 500 and 550 °C, the diffraction peaks of the catalyst obtained at 400 and 450 °C have lower intensity at 2θ = 28.1°, 29.2°, 38.1°, 45.5°. This result indicates that the calcination temperatures of 400 and 450 °C are inconducive to the formation of MnO
2 and CeO
2 crystal phases. With the increase in calcination temperature, the intensity of each characteristic diffraction peak also gradually increases. The crystallinity of Mn and Ce is further improved and reaches the best when the calcination temperature is 550 °C.
As shown in
Figure 4b and
Figure S4, the characteristic peaks of Al, Si, Ca, Fe, Na, O, and other elements are detected in the blank sample of RM, but Mn and Ce are not detected. Relatively sharp characteristic peaks exist for Mn and Ce, indicating that the active components have been successfully supported on the catalyst in the form of MnO
2 and CeO
2. The XPS full spectrum of the Mn/
[email protected] catalyst after 25 times of repeated use shows that the characteristic peaks of Mn and Ce do not appear to be significantly weakened, indicating that the loss of active components of the catalyst after repeated use is minimal.
3.3. Study on the Best Operating Conditions for Catalytic Ozonation and Catalyst Stability
Figure 5a depicts that the COD removal rate rises rapidly from 0% to 72.33% at 0–80 min. It rises slowly after 80 min, and the COD removal rate at 160 min is 73.53%, which is only 1.20% higher than the removal rate at 80 min. The possible reason is that the concentration of organic matter at the beginning of the reaction is relatively high. Active oxygen-containing groups, such as •OH, produced by catalytic ozonation can rapidly degrade organic matter, such that the removal rate of COD increases significantly. As the reaction time is prolonged, a large amount of organic matter has been degraded in the early stage, and the concentration of organic matter is lower, which may lead to a slow increase in the COD removal rate in subsequent experiments. Therefore, from the experiments, the optimal reaction time is 80 min.
As shown in
Figure 5b, when the ozone dosage is increased from 1.2 to 2.0 g/h, the COD removal rate at 80 min rises from 57.38% to 75.68%, an increase of 18.3%. However, when the ozone dosage continues to be increased from 2.0 to 2.8 g/h, the increase is only 2.43%. When the intake air volume is low, the amount of ozone dissolved in the water can be increased by increasing the intake air volume. The Mn and Ce in the Mn/
[email protected] catalyst can fully contact the liquid phase ozone, thereby improving the degradation efficiency of the biochemical tail water [
26]. Nevertheless, when the dosage of ozone > 2.0 g/h, the hydraulic residence time of ozone in the water increases correspondingly, the utilization rate of ozone decreases, and the rate of OH generation is slowed down, resulting in a small increase in COD removal rate [
27]. In addition, excessive ozone dosage will cause ozone to be directly discharged from the reaction system. Considering the efficiency and economic benefits of the reaction system, 2.0 g/h is selected as the optimal ozone dosage.
In
Figure 5c, when the catalyst dosage is increased from 25 to 62.5 g/L, the COD removal rate at 80 min increases from 55.38% to 79.95%. Nonetheless, when the catalyst dosage is increased to 75 g/L, the COD removal rate drops to 78.32% at 80 min. When the dosage of the Mn/
[email protected] catalyst increases, the contact of ozone, biochemical tail water, and Mn/
[email protected] catalyst in the reactor increase, which improves the mass transfer efficiency of ozone and the generation of •OH [
28,
29]. However, when the dosage of the Mn/
[email protected] catalyst exceeds 62.5 g/L, the excess catalyst may overlap, which reduces the active sites that can contact ozone [
30]. Consequently, the degradation performance of the system decreases. Moreover, due to the constant dosage of ozone in the system, the excess Mn/
[email protected] catalyst has more active sites than required for ozone decomposition, resulting in a certain number of catalysts failing to exert their effect in the reaction and increasing operating costs [
20]. In summary, 62.5 g/L is selected as the optimal dosage of the Mn/
[email protected] catalyst.
Figure 5d illustrates that when the initial pH value increases from 3 to 9, the COD removal rate in 80 min increases from 54.28% to 84.96%, an increase of 30.68%. Nonetheless, when the initial pH continues to increase to 11, the removal rate only increases by 1.38% compared with that when pH = 9. The decomposition of ozone under acidic conditions is strongly inhibited, resulting in low degradation efficiency. The degradation efficiency is higher under alkaline conditions because Fe-OH and other complexes are more easily formed on the catalyst surface, which contributes to the generation of hydroxyl radicals [
31,
32]. Ozone is also easier to decompose under alkaline conditions, and the generation rate of OH is increased, which promotes the degradation of pollutants [
33,
34]. In summary, the best initial pH = 9.
As shown in
Figure 5e, as the number of uses increases, the degradation of the Mn/
[email protected] catalyst on the biochemical tail water decreases slightly. After 25 times of repeated use, the COD removal rate is 72.72%, which is only 11.21% lower than the COD removal rate at the initial stage of use. The decrease in activity may be due to the collision and friction among the Mn/
[email protected] catalyst particles during long-term use, which reduces and separates the surface active components. It may also be because in the process of degrading the biochemical tail water, certain substances in the environment remain and accumulate in the catalyst for a long time. As a result, the pore connectivity of the Mn/
[email protected] catalyst deteriorates, and its performance decreases.