The consumption of Al³⁺ and the substitution amount of Mg²⁺ and Si⁴⁺ in Sep were calculated by Eq. (1), shown in Table 2. For Al-0.00-Sep, traces of Mg²⁺ and Si⁴⁺ exist in the solution “S,” indicating that traces of Mg²⁺ and Si⁴⁺ elute from the Sep during the hydrothermal reaction even without AlCl₃. After introducing AlCl₃, the substitution amount of Mg²⁺ and Si⁴⁺ (i.e., the amounts of Mg and Si dissolved from Sep) increases, demonstrating that Al³⁺ can promote the substitution of Mg²⁺ and Si⁴⁺ in Sep. The consumption of Al³⁺ and the substitution amount of Mg²⁺ increase, while the substitution amount of Si has no obvious change with the increase in dosage of AlCl₃. As the dosage of AlCl₃ reaches 70.0 mmol, the substitution amounts of Al³⁺, Mg²⁺, and Si⁴⁺ are 1.5831 mmol/g, 1.6756 mmol/g, and 0.1017 mmol/g, revealing that the substitution of Al³⁺ mainly occurs with Mg²⁺.

Table 3 shows the element content of Al-Sep modified by different dosages of AlCl₃ characterized by XRF. As the dosage of AlCl₃ increases, the content of Al in Sep increases, the content of Mg gradually decreases, while the content of Si has no significant change, which is consistent with the substitution amount result exhibited in Table 2. The content of Cl in Al-Sep is less than 0.10%, which is significantly lower than the content of Al in the samples, indicating that Cl⁻ adsorbed on the surface of Al-Sep during the modification process is effectively removed.

According to the calculation of the ion substitution amount and characterization of XRF, the composition of Al-Sep is mainly changed via the substitution of Al³⁺ for Mg²⁺ in Sep during the Al modification process. Since the radius of Al³⁺ is smaller than that of Mg²⁺ and the charge of Al ³⁺ is larger than that of Mg²⁺, the substitution of Al³⁺ for Mg²⁺ can reduce the energy and stabilize the system (Liao and Xia, 2013).

Figure 2 presents the XRD patterns of Sep and Al-Sep. The typical reflection peaks of Sep are located at 2θ = 7.45°, 20.53°, and 34.92°corresponding to (110), (131), and (371) planes, according to JCPDS card no. 13-0595 (Figure 1), without other impurity peaks. The peaks of Al-Sep are identical with that of Sep (Figure 2A), suggesting that the fiber structure of sepiolite has no significant change, and no impurity crystal is formed during the modification. The reflection peaks at a small angle of Al-Sep shift to a low angle slightly compared with that of Sep, and as the dosage of AlCl₃ increases, the shift angle increases (Figure 2B). The diffraction angles at (110) plane in Sep, Al-3.50-Sep, and Al-70.0-Sep are 7.45°, 7.43°, and 7.34°, respectively, corresponding to the interplanar spacing of 11.85 Å, 11.89 Å, and 12.03 Å, revealing that the interplanar space of (110) plane increases as the dosage of Al increases. The change in the interplanar space and the difference between the radii of Al³⁺ and Mg²⁺are in the same order of magnitude; therefore, the shift of diffraction angle at (110) plane is attributed to a slight distortion of the sepiolite during the modification process.

Figure 3 provides the TEM images of Sep, Al-3.50-Sep, and Al-70.0-Sep. The length of Sep is about 0.5–6 μm, the width is about 20 nm, and Sep mainly exists as single nanofibers (Figure 3A, D). As illustrated in Figure 3B, C, the fibrous structure of sepiolite has not been destroyed after being modified by AlCl₃ while the single fibers obviously decrease, indicating that Al modification will reduce the dispersibility of sepiolite. From the results of EDS-point (Table 4) on the sepiolite fibers marked in Figure 3D–F, the main elements on the fiber in Sep are O, Si, and Mg, and the elements Al and Cl appear on the fiber of Al-3.50-Sep after introducing AlCl₃. The content of Al in Al-70.0-Sep is significantly increased, demonstrating that the element Al exists on the fibers of sepiolite during the hydrothermal reaction with AlCl₃, and the content increases with the increase of the dosage of AlCl₃.

The substitution position of Al³⁺ was distinguished by solid-state ²⁷Al NMR spectra, shown in Figure 4. The signals are observed at ∼7 and ∼70 ppm, attributed to octahedral [AlO₆] (Al_T) and tetrahedral [AlO₄] (Al_O) in the structure of sepiolite, respectively (Caillerie and Fripiat, 1992; Komarneni et al., 1986). The Al_T and Al_O in Sep occur due to amorphous impurity and the substitution of Al³⁺ for Mg²⁺ located at [MgO₆] and Si⁴⁺ located at [SiO₄] in sepiolite lattice. The intensity of ²⁷Al NMR signal reflects the content of Al, and the peak area of the signal can be integrated to calculate the content of Al at different positions, shown in Figure 4C, D. The width of the signal reflects the symmetry of Al complex, and the narrower spectrum reflexes the higher symmetry of the complex (Wang et al., 2000).

For Sep, two bands of Al_O at 7.5 ± 0.5 ppm and −2 ± 1 ppm, deriving from the Al at the edge (Al_O2) and the section (Al_O1) of the octahedron in sepiolite, and two signals of Al_T at 65 ± 1 ppm and 74 ± 1 ppm, corresponding to the Al at the center (Al_T2) and edge (Al_T1) of tetrahedron in sepiolite, are very close to the previous literature, marked in Figure 4B (Caillerie and Fripiat, 1992; Wang et al., 2000; Aslya et al., 2016). The signals at 86 ± 5 ppm are broad, revealing poor symmetry, which might be from the Al in the tetrahedron in the amorphous impurity (Wang et al., 2000). The ²⁷Al signals in Sep come from the impurity and the substitution of Mg²⁺ in the octahedron and Si⁴⁺ in tetrahedron by Al³⁺ during the mineralization process (Zhuang et al., 2019).

The intensity of ²⁷Al signals of Al-3.50-Sep and Al-70.0-Sep is higher than that of Sep, illustrating the content of Al increases after modification, which is consistent with the result of ICP and XRF. Al-3.50-Sep and Al-70.0-Sep have similar chemical shifts to Sep, revealing that Al³⁺ can enter the tetrahedron and octahedron in sepiolite, rather than just forming oxides or adsorbing on the surface of sepiolite. Al-Sep has a signal at 2 ± 1 ppm that is absent in Sep, which belongs to the chemical shift for amorphous aluminum silicate formed during the hydrothermal reaction. The signal of Al-0.05-Sep at 86 ± 5 ppm is divided into two signals, which are attributed to the change of the impurity in sepiolite and then the formation of two kinds of Al_T with different chemical environments.

The content of Al at different positions in sepiolite of samples is calculated and shown in Table 5. The Al content at −2 ± 1 ppm and 7.5 ± 0.5 ppm increased significantly as the dosage of AlCl₃ increases, indicating that Al³⁺ mainly substitutes the Mg²⁺ at the edge and section of the octahedron in sepiolite. Al³⁺ can substitute Si⁴⁺ in tetrahedron as well, but the substitution amount is obviously lower than that occurs in the octahedron. In addition, the signals at 108 and 127 ppm are the chemical shift produced by the tetrahedral coordination formed by Al³⁺, Cl⁻, and F⁻ adsorbed on the surface of sepiolite (Wang et al., 2000).

The N₂ adsorption-desorption isotherms and the pore size distribution plots of Sep, Al-3.50-Sep, and Al-70.0-Sep are shown in Figure 5. As illustrated in Figure 6A, the N₂ amounts adsorbed on Al-3.50-Sep and Al-70.0-Sep are smaller than that adsorbed on Sep, indicating they have a smaller specific surface area, as the dispersibility of sepiolite rapidly reduces after Al modification (exhibited in Figure 3). The micropores ranging from 1 to 2 nm represent the channels of sepiolite formed by the periodic reversal of [SiO₄] (Chen et al., 2019). As the dosage of AlCl₃ increases, the distribution of channel size becomes wide, since the substitution of Al³⁺ for Mg²⁺ at the edge and section of the octahedron, i.e., the edge of sepiolite channel, makes the distribution of sepiolite channel size becoming uneven.

Table 6 gives the zeta potential of Al-Sep. The following possibilities will occur during the Al modification: 1) the substitution of Al³⁺ for Mg²⁺ in the octahedron will reduce the negative surface charges; 2) the substitution of Al³⁺ for Si⁴⁺ in tetrahedron will increase the negative surface charges; 3) the formation of vacancy defects in the octahedron and tetrahedron will increase the negative surface charges; and 4) the adsorption of Al³⁺ on sepiolite surface will reduce the negative charges. These actions conduct at the same time and affect the final zeta potential of the sepiolite. The result shows that the absolute value of the zeta potential decreases with the increase of dosage of AlCl₃, since the substitution of Al³⁺ mainly occurs on the octahedron in sepiolite and reduces the negative charge on the sepiolite surface.

NH₃-TPD has been verified to characterize the solid acidity of clay mineral quantitatively by calculating relative areas of resolved bands. The acid strength is positively correlated with the desorption temperature of NH₃ (Liu et al., 2013b). The acidity of montmorillonite is formed by the water polarized by cations, Si-OH and H₃O⁺ adsorbed on the negative charges, unsaturated Al³⁺ in the octahedron, and adsorbed water on unsaturated Al³⁺ (marked as Al-OH₂) (Heller-Kallai, 2006; Liu et al., 2013b; Liu et al., 2011). Si-OH, H₃O⁺, and Al-OH₂ behave as weak-strength acid sites (Hair and Hertl, 1970). The polarized water and unsaturated Al³⁺ behave as medium-strength acid site. The water polarized by adsorbed cations, e.g., Al³⁺, Ca²⁺, and Fe³⁺, will be transformed into exposed hydroxy and then behaves as strong-strength acid site after heating at 160°C (Liu et al., 2013b; Kaufhold et al., 2011). The acid sites of sepiolite are similar to montmorillonite; therefore, the bands of NH₃-TPD curves of samples (shown in Figure 6A–E) at 100–200°C, 200–400°C, and 400–600°C correspond to weak-strength, medium-strength, and strong-strength acid sites, respectively. In addition, the desorption temperature occurs at ∼100°C attributed to physical adsorbed and hydrogen-bound NH₃ (Liu et al., 2011; Liu et al., 2013b). The total acid amounts of Sep, Al-0.00-Sep, Al-3.50-Sep, Al-7.00-Sep, and Al-70.0-Sep are 1.137, 1.273, 1.177, 1.440, and 2.220 mmol/g. The amounts of weak-strength and strong-strength acid sites increase while the amount of medium-strength acid sites firstly decreases and then increases with the increase of the dosage of AlCl₃ (Figure 6F).

Compared with Sep, the acid amount in Al-0.00-Sep increases, since Mg²⁺ and Si⁴⁺ in sepiolite lattice are dissolved out during the hydrothermal reaction, resulting in the increase of negative charge and the adsorbed H₃O⁺. The grinding process during the preparation of Al-0.00-Sep can also increase acid sites by increasing the unsaturated cations and then enhancing the polarization of adsorbed water.

The weak-strength and strong-strength acid sites increase while the medium-strength acid sites reduce in Al-3.50-Sep, since the Al-OH₂ at the edge of the octahedron and exposed octahedral Al-OH increase and the unsaturated Al³⁺ decreases (Figure 4C) after being modified by 3.50 mmol AlCl₃. With the increase in dosage, the Al-OH₂, unsaturated Al³⁺, and exposed Al-OH increase, and thus, the weak-, medium-, and strong-strength acid amounts are improved. TPD results illustrate that Al modification will increase the acid sites of sepiolite.