2.1 Chemical reagents

Magnesium fluoride (MgF₂, >99.5 %), magnesium chloride (MgCl₂, >99.5 %), aluminum wire (Al, 5 N), isopropanol (CH₃CH(OH)CH₃, >99.7 %) were purchased from Sinopharm Chemical Reagent Corporation (Shanghai, China). Magnesium powder (Mg, 100–200 mesh), iodine (I₂, >99.8 %) and Congo red (C₃₂H₂₂N₆Na₂O₆S₂, Ind) was obtained from Aladdin Industrial Corporation (Shanghai, China). The above reagents were used without further purification.

2.2 Samples preparation

The MgAl₂O₄ sol was prepared by the NHSG method and MgAl₂O₄/C adsorbent were obtained by a subsequent calcination process. Briefly, 2.160 g aluminum wire and 1.000 g iodine were carefully added to 100 mL isopropanol, followed by refluxed under magnetic stirring at 80 °C for 6 h. As the aluminum wire dissolved, a grey transparent solution gradually formed. Different magnesium source (2.492 g MgF₂, 3.818 g MgCl₂ or 0.097 g Mg powder) was added into the solution above with molar ratio n(Mg)/n(Al) = 1:2 for samples prepared with different magnesium sources, respectively. Keeping refluxing at 110 °C for 24 h, MgAl₂O₄ wet gel was obtained. It was further evaporated to form xerogel and dried at 100 °C for 12 h. Finally, the MgAl₂O₄/C was achieved after calcining at different temperatures (200–1300 °C) for 1 h with a 5 °C /min heating rate. The samples were prepared based on the steps above, as shown in Fig. 1. Fixing Al wire as aluminum source, samples of M1, M2, and M3 were obtained by using MgF₂, MgCl₂ and Mg powder as magnesium source, respectively.

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Fig. 1

Samples preparation process.

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2.3 Characterization

The phases present in the samples were examined by XRD analysis (D8 Advance, Bruker, Germany) using copper target Kα radiation at 30 mA and 40 kV. The sol-gel transition process, along with MgAl₂O₄/C before and after adsorbed CR were characterized by Fourier transform infrared (FT-IR) spectroscopy (Nicolet 5700, Thermo, America) in the wavenumber range 400–4000 cm⁻¹. The ²⁷Al solid-state NMR spectrum of MgAl₂O₄/C was analysed (400 M, Bruker, Switzerland) at a Larmor frequency of 10 KHz. The zeta potential (ζ-potential) of MgAl₂O₄/C before and after adsorbed CR was studied by a zeta potential analyzer (Zetasizer Nano ZS90, Malvern, England). DTA-TG-MS analysis was conducted by a DTA-TG (STA449C, Netzsch, Germany) coupled with a mass spectrometer (GMBH QMS403C, Netzsch, Germany). The element distribution of as-prepared MgAl₂O₄/C composite were collected by energy dispersive X-ray spectroscopy (Model 500i, IXRF, America). Transmission electron micrograph (TEM) images were obtained with a JEOL 2010 microscope operated at 200 kV. The surface chemical state of the as-synthesized MgAl₂O₄/C adsorbent before and after adsorption were collected by X-ray photoelectron spectroscopy (K-Alpha + XPS, Thermo Fisher Scientific, America) with the Al Kα radiation. All spectra were calibrated using the binding energy of C 1 s (284.8 eV) to compensate for the surface charging effects. The avantage software was used to fit the XPS spectra of each relevant element into subcomponents. The nitrogen adsorption experiments of sample calcined at different temperature were conducted on a nitrogen adsorption apparatus (ASAP2020M, Micromeritics, America).

2.4 Adsorption performance experiments

The adsorption kinetic experiments of MgAl₂O₄/C were obtained on the adsorption of Congo red (CR) in water using batch experiments. Firstly, the MgAl₂O₄/C adsorbent was added into a glass beaker containing CR solution. The dosage of adsorbent was kept at 1 g/L, and solution pH value was fixed at 7. The beaker was then magnetically stirred with a speed of 500 rpm/min. Subsequently, a certain amount of sample (3 mL) was taken from the solution and centrifuged at different time intervals, and the concentration of the supernatant solution was measured with UV–vis spectrophotometer (Lambda 850, PerkinElmer, America). The adsorption capacity (q_t, mg/g) of CR adsorption at time t was calculated by Eq. (1). The isotherm experiments were proceeded in the similar way to the adsorption kinetic experiments. The as-prepared adsorbent was placed into CR solution (500–6000 mg/L), and the adsorbent dosage was kept at 1g/L. Followed by magnetic stirring till reaching the adsorption equilibrium. Subsequently, the mixture was taken out and centrifuged to determine the residual CR concentration. The equilibrium adsorption capacity (q_e, mg/g) was calculated according to Eq. (2).

Where C₀ (mg/L) is initial concentration of CR; C_t & C_e (mg/L) are CR concentration at time t and equilibrium concentrations; V (L) is the volume of the CR solution; m (g) is mass of the adsorbent.

3.1 Effects of types of magnesium source on the synthesis of MgAl₂O₄

Fig. 2 shows the XRD patterns of samples series M1-M3 calcined at 400–1300 °C to study the effects of magnesium source. Samples series M1, M2 and M3 shown in Fig. 2(a), (b) and (c) are samples prepared with MgF₂, MgCl₂ and Mg powders as magnesium source calcined at different temperatures, respectively.

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Fig. 2

XRD patterns of samples M1(a), M2(b), M3(c) calcined at different temperatures.

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As displayed in Fig. 2(a), most MgF₂ has not reacted with aluminium source or participated in non-hydrolytic polycondensation reaction in the sol-gel process. This is because MgF₂ is dfficult to dissolve in isopropanol, which makes it can not react with aluminium source. After annealing at 900 °C, α-Al₂O₃ (JCPDS 10–0173) appears in the sample, which is due to the decomposition of aluminium isopropoxide obtained in the reaction between aluminiume and isopropanol (shown in eq. (4)). MgAl₂O₄ spinel (JCPDS 21–1152) forms at 1000 °C. α-Al₂O₃ and MgF₂ disappear with the temperature further increasing to 1300 °C. Correspondingly, the intensity of the peaks belonging to MgAl₂O₄ spinel phase increases, which confirms the MgAl₂O₄ is produced through the reaction between α-Al₂O₃ and MgF₂ (shown in eq.(5)). The presence of diffraction peaks of α-Al₂O₃ and MgF₂ at 900–1200 °C also demonstrates that pure MgAl₂O₄ phase can not be synthesized before 1300 °C. In summary, the formation of MgAl₂O₄ in M1 sample mainly due to the solid phase reaction between MgF₂ and Al₂O₃ when using MgF₂ as magnesium source, instead of through NHSG route.

When MgCl₂ is used as magnesium source, after heat-treatment at 700 °C, the diffraction peaks of MgAl₂O₄ spinel (JCPDS 21–1152) appear, indicating that the formation of MgAl₂O₄ spinel crystalline phase. However, the impurity phase of MgO (JCPDS 45–0946) is also observed at 500–1300 °C. The presence of MgO is due to the ionic character of MgCl₂. The electrongativity of magnesium and chlorine element in MgCl₂ is 1.2 and 3.0, respectively. According to Eq.(6), the ionic character percentage of Mg-Cl bond is calculated to be 55.5 %, which demonstrates that MgCl₂ possess relative ionic character. Consequently, the nonhydrolytic polycondensation reaction is difficult to be adequate (Feng et al., 2018). Part of MgCl₂ exists in solvent isopropanol in the form of Mg²⁺ and Cl^-, which ultimately lead to the formation of MgO.

In sharp contrast to the XRD results of M1 and M2, only one phase of MgAl₂O₄(JCPDS 21–1152) can be detected in the sample M3, no impurity peak is observed during the whole temperatures range.

To further study the NHSG process when using Mg powder and Al wire as raw material, Fig. 3 presents the FT-IR spectra of samples of M3 at different reaction stages: (a) isopropanol; (b) mixtures of Mg powder and Al wire with isopropanol (after refluxing at 110 °C for 24 h); (c) xerogel; (d) powder calcined at 700 °C for 1 h.

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Fig. 3

FT-IR spectra of (a) (CH₃)₂CHOH, (b) precursors mixture, (c) xerogel, (d) powder calcined at 700 °C.

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The FT-IR spectrum of (a) isopropanol shows absorption peaks at 817 & 952 & 1129 &1162 cm⁻¹, 1310 & 1378 & 1467 cm⁻¹, 2886 & 2973 cm⁻¹, 663 & 3382 cm⁻¹, corresponding to the stretching vibration of C—O bond, the asymmetric stretching vibration of C—H, the asymmetry stretching mode of CH₃ groups, the bending vibration and stretching vibration of O—H bond in the isopropanol, respectively (Ermini et al., 2000). These demonstrate a characteristic FT-IR spectrum of isopropanol.

In FT-IR spectrum of (b)mixture of Mg + Al+(CH₃)₂CHOH (after refluxing at 110 °C for 24 h), the bending vibration at 663 cm⁻¹ ascribing to O—H bond of isopropanol disappears, and the broad bond at 3382 cm⁻¹ assigned to O—H bond becomes weaker. It can be concluded that isopropanol has already reacted. In addition, it is worth mention that the absorption peak located at 817 cm⁻¹ which associated to the C—O stretching vibration belonging to isopropanol shift to a lower wavenumber of 813 cm⁻¹ and became weaker. These indicate the formation of C-(O-Mg) bond (Thanabodeekij et al., 2003). Furthermore, the absorption peaks of 457 and 613 cm⁻¹ are related to the stretching vibration of Al—O and C—(O—Al) (Alinejad et al., 2008). The peak at 613 cm⁻¹ is significantly broader than that of the typical C—(O—Al) bonds (Li et al., 2017), which indicates the existence of Al—O and Mg—O—Al bond. Based on the analysis above, the chemical reaction between Mg, Al and isopropanol is displayed in Eq. (7) (Liu et al., 2013):

Fig. 3(c) illustrates the FT-IR spectrum of xerogel, it can be seen that the bond associated with the C-(O-Al) at 613 cm⁻¹ become stronger, whereas the peak at 457 cm⁻¹ (Al-O) disappears, demonstrating the further reaction of Al-O bond to form Mg-O-Al bond. It is mainly due to the polycondensation reaction happens between MgAl₂(OiPr)₈, which further increases the Mg-O-Al bond. The reaction is shown in Eq. (8) (Wu et al., 2022, Turova et al., 2002).

After calcination at 700 °C (Fig. 3(d)), two broad absorption bands appear at 528 and 707 cm⁻¹, corresponding to the vibration of Mg-O-Al network in the MgAl₂O₄ (Sanjabi et al., 2015). These indicate the formation of MgAl₂O₄ spinel.

The high-resolution ²⁷Al solid state NMR spectrum of sample M3 obtained at 300 °C is shown in Fig. 4. The peaks located at 66, 37, and 8 ppm can be associated to Al³⁺ ions in tetrahedral [AlO₄], hexahedral [AlO₅] and octahedral [AlO₆] coordination, respectively (Düvel et al., 2011). These coordination unsaturated Al-species (AlO₄ and AlO₅) are derived from polycondensation reaction between MgAl₂(OⁱPr)₈. It is generally believed that the coordination unsaturated Al-species exhibit high activity (Sarou et al., 2013). Notably, the coordinatively unsaturated Al-species accounted for a large proportion in the MgAl₂O₄/C composite adsorbent.

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Fig. 4

²⁷Al NMR of M3-300 adsorbent.

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Fig. 5 shows the effect of contact time on the adsorption process of sample M1, M2 and M3 calcined at 300 °C. The initial concentration of CR is 1000 mg/L. The samples obtained at 300 °C are labeled as M1-300, M2-300 and M3-300, respectively. Fig. 6 shows the schematic diagram of the products of gel powder prepared from three magnesium sources after heat treatment at 300 ℃.

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Fig. 5

Adsorption capacity of the sample M1-300, M2-300 and M3-300 for CR removal with the change of contact time.

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Fig. 6

Schematic diagram for the effect of magnesium source on the CR adsorption.

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It can be seen from Fig. 5 that the adsorbents obtained from the three different magnesium sources exhibit distinct different adsorption performance for CR solution. Among them, the MgAl2O4 sample prepared with magnesium powder as magnesium source shows the highest adsorption capacity (98.89 mg/g) for CR, followed by the MgCl₂ (543.95 mg/g) and the MgF₂ (19.51 mg/g). Therefore, it can be speculated that the sample M3 which possesses the bimetallic alkoxides structure is favorable for the adsorption of CR solution. XRD patterns shown in Fig. 2 demonstrate that the three different magnesium sources have different phase transition process. These correspond to the different proceed degree of nonhydrolytic heterogeneous polycondensation reaction, and ultimately lead to their difference in adsorption performance. Sample M1-300 is composed of amorphous aluminium isopropoxide and MgF₂, because MgF₂ cannot dissolve in isopropoxide. The saturated Al³⁺ and Mg²⁺ ions in aluminium isopropoxide and MgF₂ lead to poor adsorption performance. However, when using Mg powder as magnesium source, tetracolecular association structure (Wu et al., 2022, Turova et al., 2002) shown in the right part of equation (8) formed in sample M3-300 through the polycondensation between MgAl₂(OⁱPr)₈. The Al³⁺ and Mg²⁺ irons in tetracolecular association structure are mostly unsaturated, which guarantees the excellent adsorption performances of M3-300. Sample M2-300 contains tetracolecular association structure, amorphous aluminium isopropoxide and MgCl₂. The tetracolecular association structure can effectively adsorb CR, while amorphous aluminium isopropoxide and MgCl₂ cannot. Therefore, the adsorption performance of M2-300 is at the value between M1-300 and M3-300.

3.2 Effect of calcination temperature on the adsorption performance

Fig. 7(a) illustrates the relationship between adsorption capacity and initial concentration when using sample M3 prepared at different temperatures as adsorbents. The maximal adsorption capacities variation with calcination temperature are shown in Fig. 7(b). The samples calcined at 200–900 °C are labelled as M3-200, M3-300, M3-400, M3-500, M3-600, M3-200, M3-800 and M3-900, respectively.

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Fig. 7

(a) adsorption capacity of the sample M3 calcined at different temperatures for CR removal with the change of initial concentration, (b) maximal adsorption capacity of sample M3 calcined different temperatures.

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The results reveal that calcination temperature has a significant influence on the adsorption capacity toward CR solution. It can be seen from Fig. 7(b) that the maximal adsorption capacity is calculated to be 5723.15, 5710.12, and 5612.12, 5564.51 mg/g for M3-300, M3-400, M3-500 and M3-600, respectively. Thereby, M3-300 adsorbent displays the optimum adsorption efficiency among all these samples, which is slightly higher than the adsorption capacity of M3-400, M3-500 and M3-600. Taking M3-300 adsorbent as an example, it can be seen from Fig. 7 (a) that the adsorption capacities q_e increase linearly with the increase of initial concentration C₀ (at the range of 600–5000 mg/L), the linear growth of adsorption capacities q_e is retarded under further increase of initial concentration, and then the adsorption capacities q_e gradually become a fixed value (at the range of 5000–6500 mg/L). It reveals that sample M3-300 exhibits excellent adsorption capacity toward CR solution.

However, the maximum adsorption capacity of M3-700, M3-800 and M3-900 decrease significantly with the elevating of temperature. Samples M3-700, M3-800 and M3-900 only have the adsorption capacities of 934.72, 611.99 and 425.76 mg/g. This significant decreasing of adsorption capacities of M3-700, M3-800 and M3-900 is mainly due to the crystallization of MgAl₂O₄ spinel at 700 °C. There are abundant coordinative-unsaturated Al³⁺ and Mg²⁺ in the tetracolecular association structure at the temperatures below 700 °C. In the process of MgAl₂O₄ crystallization, the unsaturated Al³⁺ and Mg²⁺ irons transfer to the saturated Al³⁺ and Mg²⁺ in MgAl₂O₄, which leads to the reduction of adsorption performance.

Meanwhile, it should also be noted that sample M3-200, which is also amorphous, exhibits a lower adsorption capacity than M3-300. In order determine the reason for causing this difference, Fig. 8 shows the DTA-TG-MS curves of the xerogel of M3. There are three stages for the decomposition of xerogel in the air atmosphere. The first stage locates in the temperature ranges from room temperature to 126 °C, which corresponds to a weight loss of 4.79 %. This endothermic peak centered at 76 °C is associated with the further removal of solvent isopropanol and polycondensation by-product isopropyl ether. The second stage involves a wide-broad endothermic peak accompanied by a mass loss of 31.42 %, and the endothermic peak is in the range of 160–600 °C centered at 227 °C. This endothermic peak can be ascribed to the carbonization of the skeleton of residual organic groups in the xerogel and the emission with burning of residual organic groups. After the temperature of 600 °C, the third weight loss stage in the thermogravimetric curve is observed, and it is attributed to the slowly oxide of the carbon-containing group, which lays the foundation for the preparation of MgAl₂O₄/C composite. These are the reasons that sample M3-300 with the carbonization exhibits the higher adsorption capacity than that of sample M3-200. Besides, thermogravimetry (TG) curve of > 300 °C is shown in Fig. 8(b). The total weight loss varies in the range of 74.34–54.88 wt%. According to the mass of the sample tested and the final weight loss in the test process (Zhao et al., 2020b), the carbon-containing residual content of M3-300 is calculated to be 26.15 wt%.

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Fig. 8

(a) DTA-TG curves of the xerogel, (b) TG curve of the xerogel from 300 °C to 1100 °C, (c-d)MS curves of various products during thermal decompositon.

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In addition, the emissions of the MS curves of xerogel shown in Fig. 8(c)(d) present high concentration emissions and low concentration emissions, respectively. These emissions include C₃H⁺ (225 °C), C₃H₂⁺ (225 °C), C₃H₃⁺ (225 °C), C₃H₄⁺ (225 °C),C₃H₅⁺ (225 °C), C₃H₆⁺ (225 °C), H₂O⁺ (260 °C), OH^–(252 °C）and CO₂⁺ (188 °C & 301 °C & 455 °C), respectively·H₂O⁺ (260 °C) shows the strongest emission in the temperature ranges from 120 to 600 °C, and OH^– (252 °C) also exhibit a high emission in the range of 120–500 °C. These two points suggest that a great number of O and H originated from residual organic group releasing during the heat treatment of the xerogel, which ensures in situ C residue to form MgAl₂O₄/C composite material.

The formation mechanism of OH^– from the residual organic group can be explained as follows. In contrast to MgAl₂O₄ crystal phase, M3 sample calcined at the temperatures < 600 °C has an amorphous structure, the amorphous MgAl₂O₄ are composed of abundant coordinative-unsaturated Mg²⁺ and Al³⁺. Both Mg²⁺ and Al³⁺ are strong Lewis acid, they can coordinate strongly with oxygen-containing groups. This process makes oxygen-containing groups undergo strong polarization during calcination and decompose into various free radicals or radical association compounds (Martra et al., 2000, Suh et al., 1992). The emission of H₂O⁺ can be attributed to the H contained in isopropyl combined with OH^–, then ultimately caused the carbonization of residual organic group. In addition, as can be seen from Fig. 8(c), the group of C₃H₇⁺ emission occurs at 189 °C. It is worth mention that a series group of C₃H⁺, C₃H₂⁺, C₃H₃⁺, C₃H₄⁺, C₃H₅⁺ and C₃H₆⁺ emit at 225 °C, which are the isopropyl group (-C₃H₇) that have lost hydrogen with a higher emitting concentration. These are the specific reasons ensure the residual of carbon in the system.

As presented in Fig. 8(d), the continuous escape of C₃H₄⁺ after 300 °C demonstrated that in-situ carbon in the MgAl₂O₄/C may exist in the form of C₃H₄⁺. During the adsorption process, the existence of C₃H₄⁺ in the MgAl₂O₄ adsorbent is benefit to improve the wetting process between organic pollutants and adsorbent, leading to the promoting of the adsorption, and carbon itself is also a commonly used adsorbent. Based on the analysis above, the groups emission-carbonization process in the heating process of xerogel is shown in Fig. 9.

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Fig. 9

Schematic illustration of the whole transition process of xerogel during heating process.

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3.3 Adsorption kinetics

The contact time dependence of the adsorption capacity at different initial concentrations is shown in Fig. 10. The adsorbent shows a very fast adsorption rate, when the initial concentration of CR solution is 500, 1000 and 2000 mg/L. Its time required is only 20, 120 and 270 min, respectively. The quick adsorption is ascribed to the abundant sorption sites on the surface of MgAl₂O₄/C adsorbent material.

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Fig. 10

Contact time dependence of the adsorption capacity at different initial concentrations.

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To further analyze the adsorption mechanism, the kinetics of CR adsorption on the MgAl₂O₄/C composite were investigated by pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetics models. The corresponding linear equations of the two kinetic models are shown in Eq. (9) and (10), respectively. Fig. 11 illustrates the fitted straight lines, and Table 1 displays the values of kinetic parameters calculated according to the two equations.

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Fig. 11

(a)linearly fitted kinetic curves for CR adsorption according to the linear equations for pseudo-first-order, (b) pseudo-second-order kinetics.

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Where k₁ (min⁻¹) and k₂ (g/mg·min) are rate constants of pseudo-first and second-order models, respectively.

As observed, the straight lines in Fig. 11(b) show a better match with the experimental data points compared with those in Fig. 11(a). According to Table 1, it can be seen that experimental results agree well with the PSO kinetic model. Because the R² (correlation coefficients) values for this model are closer to 1 (0.993–0.997), and the R² calculated from PFO model is much lower (0.728–0.821). In addition, the as-measured experimental adsorption capacity data [q_e(exp)] can match better to the calculated adsorption capacity values [q_e(cal)] obtained by PSO model than that from PFO kinetic model. Therefore, it can be seen that the adsorption process of MgAl₂O₄/C fitted better with the PSO model, and it is reasonable to conclude that chemical adsorption is probably-one of the rate-limiting steps.

3.4 Adsorption isotherms

The adsorption isotherms were used to reveal the relationship between the amount of CR adsorbed and the constant equilibrium concentration of CR in the liquid phase. The adsorption isotherms of MgAl₂O₄/C composite are shown in Fig. 12. The equilibrium adsorption quantity increases with the increase of equilibrium concentrations and then reaches adsorption saturation.

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Fig. 12

Equilibrium isotherms of MgAl₂O₄/C for CR adsorption. (Initial CR concentration = 2000–6000 mg/L, adsorbent dose = 1 g/L, T = 30 °C).

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To attain profound information about the adsorption behavior and mechanism, two well-known adsorption isotherms have been selected to analyze the experimental adsorption results. They are Langmuir and Freundlich isotherm models. The linearized forms of the two models can be expressed by Eqs. (11) and (12), respectively. The Langmuir model is used to described monolayer adsorption, and it assumes the adsorption occurs on the homogenous surface of adsorbent. The Freundlich model stands for the heterogeneous adsorption and multilayer adsorption patterns.

Where K_L (L/mg) is the Langmuir adsorption constant, K_F is the Freundlich constant, and n is the adsorption intensity factor.

Fig. 13 illustrates the fitted results using isotherm equations, and the determined parameters of both isotherm models are presented in Table 2. As shown in Fig. 13 and Table 2, the Langmuir model fitted better to the adsorption data. Because the Langmuir model gave higher correlation coefficients (R² = 0.9996) than that of the Freundlich equations (R² = 8057), so the CR is adsorbed on the surface of MgAl₂O₄/C composite in a monolayer coverage.

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Fig. 13

Corresponding linearly fitted isotherms: Langmuir isotherm (a) and Freundlich isotherm (b).

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The as-prepared MgAl₂O₄/C with outstanding adsorption capability is superior to other previously reported magnesium oxide based and alumina based metal (hydro)oxides adsorbents shown in Table 3.

3.5 Adsorption mechanisms analysis

FT-IR was used to further investigate the interaction between MgAl₂O₄/C composite adsorbent and CR in the adsorption process. The spectra of MgAl₂O₄/C, CR and CR-adsorbed MgAl₂O₄/C are performed and displayed in Fig. 14. For MgAl₂O₄/C before adsorption (curve (a) in Fig. 14), two strong vibration bonds centered at around 528 and 707 cm⁻¹ appear. These bonds correspond to the characteristic bond of Al-O-Mg vibrations from MgAl₂O₄/C (Dash et al., 2017).

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Fig. 14

(a)FT-IR spectra of MgAl₂O₄/C, (b) MgAl₂O₄/C after CR adsorption and (c) pure CR.

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In the IR spectrum of CR (curve (c) in Fig. 14), the adsorption peak appears at 3459 cm⁻¹ is ascribed to the stretching vibration of N—H bond (Cao et al., 2021). The peak at 1548 cm⁻¹ can be attributed to the stretching vibration of azo group (—N⚌N—) (Cao et al., 2021). The peak at 1616 cm⁻¹ is associated with C⚌C stretching vibration in benzene (Wang et al., 2013), and the peaks at 1226, 1178 and 1062 cm⁻¹ are ascribed to the stretching vibration of S⚌O in sulfonate group (—SO₃^2-) (Cao et al., 2021). In comparison with the FT-IR spectrum of the pure MgAl₂O₄/C and CR, it is noticed that several obvious changes appear after adsorption. The —NH₂ group peak (at 3459 cm⁻¹) shifts to a lower position (3425 cm⁻¹). It merges with the OH stretching vibration bond of the MgAl₂O₄/C (3384 cm⁻¹). It can be explained by the Lewis acid-base interaction between Mg²⁺ and Al³⁺ on the MgAl₂O₄ and —NH₂ on CR molecules. According to Pearson‘ s hard-soft acid-base (HSAB) principle, Mg²⁺ and Al³⁺ are both hard Lewis acid site, and —NH₂ is belong to a hard Lewis base site. They tend to combine with each other to form more stable bonds (Wang et al., 2020). Furthermore, after adsorption on the MgAl₂O₄/C, it is evident that the sharp bond at 1062 cm⁻¹ which relate to S⚌O stretching vibration occurs at a lower wavenumber of 1048 cm⁻¹. The shift of S⚌O bond, which ascribe to the characteristic peak of sulfonate group (—SO₃^2-) in CR molecules, indicates that the stability of S⚌O bond on CR decreases. The results above verify that Congo red molecules have successfully adsorbed onto the surface of MgAl₂O₄/C.

XPS measurement was performed to further study the surface chemical states and surface composition of MgAl₂O₄/C adsorbent before and after adsorption of CR. The XPS test results are shown in Fig. 15. The XPS survey profile of MgAl₂O₄/C reveals four major sets of peaks corresponding to Mg 1 s, Al 2p, O 1 s and C 1 s. In addition, no other impurity peaks are detected. The carbon peak is attributed to the residual carbon in the sample and hydrocarbons from the XPS instrument (Ansari et al., 2019).

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Fig. 15

XPS spectra of samples.

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As shown in Fig. 15, CR-adsorbed MgAl₂O₄/C exhibits obvious Na, N and S peaks besides to MgAl₂O₄/C relative peaks. This confirms the existence of CR onto MgAl₂O₄/C. No other significant changes are observed in the spectra after CR adsorption, which suggests that no secondary pollutant is generated during the adsorption process (Beheshti et al., 2018).

For more specific insight comparisons before and after CR adsorption, high-resolution XPS spectra of Mg 1 s, Al 2p, N 1 s and S 2p, collected from adsorbents, pure CR powders and CR-adsorbed adsorbent were carefully recorded. The results are shown in Fig. 16. As shown in Fig. 16(a), for the Mg 1 s and Al 2p spectra, the peaks located at 1303.73 eV and 74.43 eV are shown in the MgAl₂O₄/C, and both of them shift to higher binding energy positions (1304.38 eV and 74.73 eV) after CR adsorption, indicating some charge transfer to the Mg and Al during the process of adsorption (Carley et al., 1996).The N 1s peak of CR can be divided into two peaks, and they are 399.45 eV (C⚌N) and 401.88 eV (—NH₂) (Xiong et al., 2021).After the adsorption of CR onto MgAl₂O₄/C, the peak position of-NH₂ shifts to a higher binding energy position, suggesting that the N containing groups are responsible for strong chemical bonding which can lead to electron density change (Aoopngan et al., 2019).

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Fig. 16

XPS spectra of (a) Mg 1s, (b) Al 2p on the surface of MgAl₂O₄/C before and after removal of CR, (c) N 1s, and (d) S 2p on the surface of CR and MgAl₂O₄/C after CR adsorption.

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Furthermore, S 2p spectra of CR (Fig. 16d) can be divided into two peaks. Peaks of S1 (168.45 eV) and S2(169.74 eV) are related to S 2p_1/2 and S 2p_3/2 of sulfonic group (-SO₃^2-) of CR molecules. After CR adsorption onto the surface of MgAl₂O₄/C, the S 2p peaks shift to lower binding energies (168.34 and 169.60 eV) slightly, further suggesting that electrostatic attraction occurs via the -SO₃^2- groups of Congo red (Aoopngan et al., 2019).

Here zeta potential analysis is applied to study the surface charge of the adsorbent in aqueous solution at pH = 7. Fig. 17 illustrates the change of zeta potential value before and after CR adsorption. It is clear that the adsorbent obtained at 700 °C possesses electropositive surface, with the zeta potential values of 7.62 mV. After adsorption of CR, the zeta potential of MgAl₂O₄/C adsorbent changed from positive to negative. As known, Congo red is polar molecules, it exists in the form of anions and cations in both alkaline and acidic aqueous. Its isoelectronic point is about 3. Therefore, when adsorbent MgAl₂O₄/C is dispersed in CR aqueous at pH = 7, the negatively charged CR can be adsorbed by the positive charged MgAl₂O₄/C due to the electrostatic attraction.

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Fig. 17

Zeta potentials of sample prepared at 600 °C before (black) and after CR adsorption (red).

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The N₂ adsorption–desorption isotherms of samples calcined at different temperatures are presented in Fig. 18. Table 4 lists corresponding characterized data. The samples prepared at 200, 300, 600 and 700 °C are M3-200, M3-300, M3-600 and M3-700, respectively.

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Fig. 18

N₂ adsorption–desorption isotherms of samples calcined at different temperatures: (a) 200 °C (b) 300 °C (c) 600 °C (d) 700 °C.

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As shown in Fig. 18(a), sample M3-200 has the smallest amount of adsorption of N₂, which is about 2 cm³/g, and the BET of M3-200 is only 0.79 m²/g. However, in Fig. 18(b-d), the amount of N₂ adsorption and BET specific surface area of M3-300, M3-600 and M3-700 are evidently higher than M3-200. Besides, all the three isotherms (Fig. 18b-d) show the hysteresis loop when the P/P₀ in the range of 0.3–0.8. Based on the IUPAC classification (Zhang et al., 2016a), this kind of isotherms could be classified as the type IV curve, suggesting the presence of mesopores in their structure. Moreover, the hysteresis loop obtained in M3-300 and M3-600 are evidently broader than M3-700 within the range of P/P₀ = 0.3–0.8. It is well-known that the phenomenon of capillary condensation will occur in the middle-pressure sections (P/P₀ = 0.3–0.8) when the sample has a mesoporous structure, and this is the root cause for the hysteresis of N₂ desorption curve. Therefore, it can be concluded that higher quantity of mesoporous exist in M3-300 and M3-600 than M3-700. Besides, it can be seen in Fig. 18(d) that M3-700 has large amount of adsorption in the high-pressure sections (0.8–1.0), which is nearly 30 cm³/g. Essentially, the quantity of N₂ adsorption belong to M3-600 shows a flatlining growth, which is only 11 cm³/g. This flatlining growth indicates that less large pores exist in the sample calcined at 600 °C. These results indicate that the sample M3-300 and M3-600 shows a mesoporous structure, which is favorite to the adsorption of CR solution.

Fig. 19 illustrates the TEM images of M3-600. It is clearly seen that numerous mesopores distributed in the sample. Fig. 19 (b) show the pore diameter is about 6.56 nm, which is in good agreement with the result determined via Barrett-Joyner-Halenda (BJH) method shown in Table.4.

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Fig. 19

(a)(b) TEM images of sample M3-600.

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Fig. 20 shows the SEM (a) and EDS mapping (b-e) of adsorbent M3-300. The images displayed in Fig. 20 demonstrated that the signal of C is appeared, verifying the existence of C in the adsorbent. Moreover, the C element distributes homogeneously, and matches with the dispersion of Mg, Al and O elements. These findings indicate that the in-situ carbon-containing composite can be prepared by the NHSG method, through forming a carbon-containing precursor. The element composition of M3-300 was studies by EDS as shown in Fig.S1, the atomic fractions of C is 13.496 %.

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Fig. 20

SEM image of (a) MgAl₂O₄/C adsorbent, EDS mapping of (b-e) Al, Mg, O and C.

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Based on the MgAl₂O₄/C before and after adsorbing CR identified by FT-IR spectra, XPS spectra, zeta potential, TEM and N₂ adsorption–desorption results, along with the adsorption kinetics and isotherms, a possible adsorption mechanism of MgAl₂O₄/C composite to CR is proposed and shown in Fig. 21.

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Fig. 21

Schematic illustration of the CR adsorption process on to MgAl₂O₄/C.

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According to the zeta potential analysis, electrostatic attraction involves in the adsorption process. The negatively charged sulfate group (-SO₃^2-) on CR molecules are attracted by the unsaturated Mg²⁺ and Al³⁺ on the MgAl₂O₄/C composite surfaces. Furthermore, according to the pseudo-second-order kinetics evaluation, chemical adsorption dominates the adsorption process. It is confirmed that such chemical adsorption is Lewis acid-base interaction between unsaturated Mg²⁺/Al³⁺ on the MgAl₂O₄/C nanocomposite as the hard Lewis acid site and —NH₂ on CR as the hard Lewis base site (Ansari et al., 2019). Meanwhile, the presence of in-situ carbon is benefit to improve the wetting process between organic pollutants and adsorbent, leading to the promoting of the adsorption. Besides, the adsorption process is also driven by the pore filling, this was confirmed by the mesoporous structure of as-prepared MgAl₂O₄/C nanocomposite observed from TEM and N₂ adsorption–desorption analysis.

Overall, these mechanisms might take effect synergistically to endow the MgAl₂O₄/C composite with such a high adsorption capacity. Specifically, it is the uniformly dispersed microstructure, electrostatic attractions, mesoporous structure, homogenous mixing of unsaturated Mg, Al and C elements, and more exposure of Lewis acid sites (Guo et al., 2020) that have enabled such high adsorption properties.