3.1.1. XRD analysis
In order to analyze the structural differences between SMO and LSMO, the samples were characterized by XRD. It can be seen from Fig. 1 that SMO has obvious diffraction peaks at 2θ = 38.1, 42.6, 62.1, 74.2 and 78.3°. According to the literature and the standard card (PDF #45–0946), the above diffraction peaks belong to the (111), (200), (220), (311) and (222) lattice planes of magnesium oxide, respectively, So the formation of magnesium oxide was confirmed (Sutradhar et al. 2011). However, in the XRD spectrum of 15LSMO, the diffraction peaks of 2θ = 38.1 and 74.2°. are not obvious, which belong to (111) and (311) crystal planes respectively This can be explained by the successful loading of lanthanum on the surface of magnesium oxide. The three main diffraction peaks of 15LSMO are well matched with SMO, and no new diffraction peaks are generated, indicating that the lanthanum oxide formed by impregnation and calcination of lanthanum nitrate is uniformly distributed on the surface of SMO.
3.1.2 Microscopic study
In order to observe the morphology of the formed SMO and LSMO, the product was characterized by SEM. According to Fig. 2(a), we found that the magnesia precursor powder was calcined to form a spheroidal oxide packaged by flake magnesia. Figure 2(b) is the scan image of 15LSMO. After loading, there is no change in the size of the morphology. And the Fig. 2(c) and Fig. 2(d) after scanning by EDS prove that lanthanum was successfully loaded on the surface of SMO. And Fig. 2(d) is the scanning distribution diagram of 15LSMO lanthanum, which can prove that lanthanum is evenly distributed on the surface of SMO.
In order to further study the morphological characteristics of SMO, the SMO was characterized by TEM, and the characterization results are shown in Fig. 3. Figure 3(a) is a TEM image of scattered flake magnesium oxide from the assembled spherical magnesium oxide, it can be found that there are many void sites on the surface of the flaky magnesia, which are caused by the overflow of carbon dioxide and water during the calcination process. Of course, this void site provides void sites for the adsorption of fluorine and the loading of lanthanum. The inset in Fig. 3(a) is the SAED image corresponding to flaky magnesium oxide, showing that the flaky magnesium oxide has a polycrystalline and crystal structure, the crystal plane corresponds to the XRD pattern. Figure 3(b) is the edge TEM image of SMO. It can be seen that the morphology of the flake-shaped magnesium oxide matches the SEM image of Fig. 2(a).
3.1.3 BET analysis
The microstructure of SMO and 15LSMO is further analyzed by BET characterization. Figure 4 shows the N2 adsorption and desorption isotherms of SMO and 15LSMO, and the corresponding pore size distribution curves. According to the classification of the article, the isotherm of SMO belongs to type IV, H3 type hysteresis loop, the isotherm of LSMO also belongs to type IV, H3 type hysteresis loop (Sing 1985). The hysteresis ring of type IV isotherm is its remarkable feature, which represents the condensation of the mesoporous capillary, which is a typical mesoporous adsorbent; the H3 type hysteresis ring represents the abundant fissure-type pores in it. After comparing the isotherms of a and c in Fig. 4, it is found that they belong to the same type, because most of the pores are composed of SMO, while the comparison of b and d shows that the pore peak is at the same 4nm, while d contains more pores at 17nm. The peak is formed due to the shrinkage of the gaps between the flake magnesium oxide due to the loading of lanthanum, which is consistent with the SEM observation. The detailed BET data is shown in Table 1. The specific surface area and pore diameter of 15LSMO have been improved compared to SMO, which also provides a basis for the subsequent enhancement of fluorine removal capacity.
3.1.4 FTIR analysis
In order to explain the fluorine removal mechanism of the materials, the two materials were characterized by infrared before and after the fluorine removal. The results are shown in Fig. 5. Figure 5a is the infrared spectrum of SMO before and after fluorine adsorption. The peaks at 3430cm− 1 and 1640cm− 1 before adsorption belong to the stretching vibration of the OH band and the bending vibration of the H-OH band formed by the adsorbed water in the air (Zhang et al. 2015). The strong peak near 3699cm− 1 after fluorine adsorption is attributed to the A2u(OH) lattice vibration caused by the formation of Mg(OH)2 (Niu et al. 2006). Moreover, after adsorption, the peak intensity at 3430 cm− 1 is greatly reduced, which proves that the substitution of OH is part of the mechanism of fluoride ion adsorption. The peaks at 1470cm− 1, 1129cm− 1 and 868cm− 1 before adsorption belong to the antisymmetric stretching vibration peak, the symmetric stretching vibration peak and the out-of-plane bending vibration absorption peak of CO32−, respectively (Xu and Zeng 2003). After fluorine adsorption, the disappearance of the 1129cm− 1 peak indicates that carbonate also plays a role in replacing fluoride ions. The range of 400cm− 1-800cm− 1 basically belongs to the infrared characteristics of the metal bond. The peaks at 640cm− 1 and 438cm− 1 before adsorption are the significant infrared characteristics of MgO. The peak at 575cm− 1 after adsorption belongs to the tensile vibration of Mg-F (Jin et al. 2015). It is proved that the fluorine removal mechanism of magnesium oxide is the result of the combined action of hydroxyl, carbonate and metal bond adsorption.
Figure 5.b shows the infrared spectra of 15LSMO before and after fluorine adsorption. We can find that the continuous double peak at 1463cm− 1 is the typical infrared characteristic of lanthanum carbonate (Sun et al. 2020). Compared with the one before the adsorption in Fig. 5.a,the extra peak, t 1069cm− 1 before the adsorption in Fig. 5.b is the infrared characteristic of lanthanum carbonate, The split with the peak near 1100cm− 1 and 1463cm− 1 indicates the presence of non-equivalent carbonate ions, which verifies the successful loading of lanthanum on the other hand, and the peak near 1100cm-1 disappears after adsorption, which is similar to the carbonate ion in Fig. 5.a. The peak at 3697 cm− 1 after adsorption is attributed to the A2u(OH) lattice vibration caused by the formation of Mg(OH)2 and La(OH)3. Before adsorption, 418cm− 1 is the infrared characteristic of MgO, while the peaks near 609cm− 1 belong to the infrared characteristics of MgO and La2O3 (Chowdhury et al. 2016, Khalil et al. 2021), while the peak at 455cm− 1 after adsorption is the infrared characteristic of MgO covered with F, and the peak at 566cm− 1 is the formation of the bond between metal and F (Mg-F and La-F). In summary, it can be found that the fluorine removal mechanism of SMO and 15LSMO is basically similar.
3.1.5 XPS analysis
In order to further study the fluorine removal mechanism of the material, XPS characterization was performed on the material before and after fluorine adsorption. Figure 6 and Fig. 7 are the relevant XPS spectra of SMO and 15LSMO, respectively. From Fig. 5.a and Fig. 6.a, we can find that a new peak of 684.9eV belonging to F1s appears after adsorption, which clearly proves the effective adsorption of fluorine by the material (Sugama et al. 1998). Figure 6.b and Fig. 7.b are the C1s spectra before and after the adsorption of SMO and 15LSMO, the peak values are 284.8 eV (after calibration) and 289.6 eV, 284.8 eV and 289.6 eV are organic carbon and carbonate, respectively peak (Jia et al. 2013). Carbonate is the thermal decomposition of urea in the raw material and the supply of air atmosphere. After adsorption, the peak of carbonate is reduced to a certain extent, which means that carbonate is consumed in fluorine adsorption. Figure 6.c and Fig. 7.c are the O1s spectra before and after the adsorption of SMO and 15LSMO, with a total of 529.7 eV, 531.5 eV, 532.3 eV and 533.5 eV peaks. 529.7 eV belongs to the lattice oxygen atom combined with metal (Wuttke et al. 2008). The peaks at 531.5 and 533.5 eV belong to the metal hydroxide and the hydroxyl group of adsorbed water, respectively (Benedetti et al. 2011, Newberg et al. 2011). The peak at 532.3 eV is due to the presence of oxygen in carbonate (Descostes et al. 2000, Wang et al. 2017). In Fig. 6.c and Fig. 7.c, we can find that the metal-bound lattice oxygen atoms are transformed into metal hydroxides and hydroxyl groups of adsorbed water after adsorption, which is consistent with the results of the FT-IR spectrum. The peak of the oxygen atom in the carbonate is also reduced, which is consistent with the C1s spectrum. In Fig. 7.c, the hydroxyl groups that have adsorbed water before adsorption may be caused by the lanthanum impregnation process. Figure 6.d and Fig. 7.d are the F1s spectra of SMO and 15LSMO after adsorption, of which 684.8, 686.5 and 689.4eV belong to metal complex MF, metal hydroxide fluoride M-FOH and metal fluoride cover, respectively (Wang et al. 2017, Li et al. 2016). Figure 6.e and Fig. 7.e are the Mg2p spectra before and after the adsorption of SMO and 15LSMO. Both have peaks near 49.1eV and 50.4eV before adsorption, of which 49.1eV is the Mg-OH peak, and near 50.3eV is Mg-O Peak (Wuttke et al. 2008, Li et al. 2014). After adsorption, the position of the peak shifts to the left, and the binding energy increases. This is due to the adsorption of fluorine, and the Mg-OH peak increases significantly, while the Mg-O peak decreases, which proves that the formed Mg-FOH is the main adsorption mechanism, and Mg-F is auxiliary adsorption mechanism. Figure 7.f shows La3d spectrum before and after 15LSMO adsorption. The main peaks of La3d5/2 and La3d3/2 before adsorption are located at 835.7eV and 852.5eV, respectively, and the corresponding satellite peaks are located at 839.0eV and 855.8eV, respectively. These peaks confirm the presence of La2O3. After adsorption, the binding energy of La3d5/2 and La3d3/2 becomes higher, which represents the formation of new lanthanum compounds (Wu et al. 2017).
3.2. Effect of La impregnation ratio
Fig.S1 shows the effect of different lanthanum impregnation ratios on the removal of fluoride from the material. In the range of impregnation ratio from 10–20%, the fluorine removal balance tends to be balanced, and the fluorine removal rate reaches more than 90%. Among them, the material with 15% impregnation ratio (15LSMO) has the largest fluorine removal rate, which is 93.1%. Therefore, 15LSMO is used as the analysis model in the experiment. The corresponding SEM images are shown in Fig.S2.
3.3 Kinetic and thermodynamic study
The Langmuir adsorption isotherm is assumed to be a homogeneous adsorption process, while the Freundlich adsorption isotherm is suitable for the equation of a heterogeneous system. As shown in Fig. 8, two models are used to fit the adsorption process of SMO and 15LSMO. The relevant parameters are shown in Table 2. The experimental results of the two materials are closer to the Langmuir adsorption isotherm, indicating that the adsorption on the surface of the adsorbent is homogeneous adsorption. However, theoretically, the maximum adsorption capacity of SMO is greater than 15LSMO, indicating that lanthanum-supported materials are more advantageous for low-concentration fluorine removal.
In order to further study the changes of the adsorption capacity of the two materials with time, the pseudo-first-order kinetic equation and the pseudo-second-order kinetic equation were used for kinetic simulation. The experimental diagram of the fitted adsorption kinetics is shown in Fig. 9. Table 3 is the relevant data of the fitting results of the kinetic model. The correlation coefficient of the pseudo-second-order model of the two materials is higher, and the adsorption capacity value calculated by the pseudo-second-order kinetic model is closer to the actual value, which means that the fluorine removal mechanism on the surface of the material is mainly chemical adsorption.
3.4 Effect of pH and interfering anions
To investigate the effect of pH on the adsorption performance of the material, set the pH value from 2 to 12, a constant adsorbent dosage of 1g/L, an initial fluorine concentration of 10mg/L, and a contact time of 180min. The results are shown in Fig. 10. The results show that under acidic conditions, the material exhibits extremely high stability. Under alkaline conditions, even if the presence of OH- competes with F− for OH− active sites, However, the material has a significant effect on fluorine removal, when pH is 12. The fluorine removal performance does not change much in the pH range of 2–11. According to the comparison of the results of SMO and 15LSMO, it is speculated that the acid and alkali resistance of the material is derived from the presence of magnesium oxide.
To investigate the influence of interference ions on the adsorption performance of the material, set a constant adsorbent dosage of 1g/L, initial fluorine concentration of 10mg/L, interference ion concentration of 100mg/L, and contact time of 180min. The results are shown in Fig. 11. The results show that the interference ion has little effect on the fluorine removal efficiency of the material (< 4%).