# Removal of Methylene Blue by Metal Oxides Supported by Oily Sludge Pyrolysis Residues
## 3 Results and Discussion
### Characterization of Composite Materials  
#### 3.2.1 Morphology Characterization of Composite Materials  
The morphology and composition of the pyrolysis residue, AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\) are shown in Figure 3. It can be seen from Figure 3a that the pyrolysis residue had a relatively smooth outer surface and a blocky structure. Compared with Figure 3b-d, it can be seen that the structures of AC-CuO and AC-ZnO were not much different from that of the pyrolysis residues. However, compared with that of the pyrolysis residue, the surface roughness and particle size of AC-TiO\({}_{2}\) increased.  
From the EDS diagram in Figure 3, it can be seen that there were no energy spectrum peaks of Cu and Zn elements in the pyrolysis residue, but there were energy spectrum peaks of trace Ti elements. However, compared with those of AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\), it was found that energy spectrum peaks of Cu, Zn, and Ti appeared on the energy spectrum, respectively [30], indicating that CuO, ZnO, and TiO\({}_{2}\) were successfully loaded on the pyrolysis residue.  
Figure 2: Effect of (**a**) CuO; (**b**) ZnO; (**c**) TiO\({}_{2}\) composite materials’ impregnation ratio on methylene blue removal (impregnation time (CuO) = 70 min, impregnation time (ZnO) = 50 min, impregnation time (TiO\({}_{2}\)) = 50 min, initial methylene blue concentration = 5 mg/L, composite materials dosage = 1 g/L, contact time = 60 min, temperature = 25 \(\pm\) 0.5 \({}^{\circ}\)C, initial pH = 7.00).  
Figure 3: SEM and EDS images (**a**) oily sludge residue; (**b**) AC-CuO; (**c**) AC-ZnO; (**d**) AC-TiO\({}_{2}\).  
BET  
Table 2 illustrates the specific surface area, pore volume, and pore size of AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\). The specific surface areas of AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\) were 5.9082, 4.9138, and 7.8951 m\({}^{2}\)/g, respectively. It can be seen that the specific surface areas of AC-TiO\({}_{2}\) > AC-CuO > AC-ZnO were consistent with their respective removal efficiencies of methylene blue, which indicates that the pore structure of the composite material was involved in the removal of methylene blue. This conclusion is consistent with previous research findings [31; 32].  
In addition, by analyzing the ratio of mesopores to total pores, the mesoporous of AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\) was 93.69%, 94.44%, and 99.64%, respectively, indicating that the pore size distribution of AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\) was dominated by the mesopores.  
Figure 4 shows the N\({}_{2}\) adsorption-desorption curves of AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\), respectively. All the curves did not overlap, which may be due to the small specific surface area of them and the underdeveloped pores. In addition, the N\({}_{2}\) adsorption-desorption curves were all closed, and the classification of the hysteresis loop belonged to the IV type, indicating AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\) were mostly composed of mesopores [33].  
Figure 5 shows the pore size distribution curves of AC-CuO, AC-ZnO, and AC-TiO\({}_{2}\), respectively. It can be seen that the pore size distributions were relatively concentrated, and the pore structure was dominated by mesopores between 2 and 20 nm, with almost no macropores. This result is consistent with that of N\({}_{2}\) adsorption-desorption analysis. The concentrated pore size distribution facilitated the methylene blue removal process [34].  
Figure 4: N\({}_{2}\) adsorption-desorption curves of (**a**) AC-CuO; (**b**) AC-ZnO; and (**c**) AC-TiO\({}_{2}\).