2.5.1. Langmuir isotherm

The Langmuir model [21] is based on the assumption that the maximum adsorption capacity corresponds to the complete coverage of a monolayer of molecules on the surface of the adsorbent without any interaction between the adsorbed molecules. The Langmuir equation for a homogeneous surface is expressed as

2.5.2. Freundlich isotherm

The Freundlich isotherm model [23] describes adsorption on heterogeneous surfaces in multilayers. The linear form of the Freundlich equation is given by the following equation:

2.5.3. Dubinin–Radushkevich isotherm

The D–R model [24] assumes that the surface of the material is heterogeneous. This model is more general than the Langmuir and Freundlich models and is used to describe the adsorption on both homogeneous and heterogeneous surfaces [25].

Equation (6) gives the linear form of the D–R equation:

3.1.1. X-ray diffraction

The XRD patterns of the synthesized material are illustrated in Figure 1. The observed patterns show excellent crystallinity as evidenced by the prominent peaks, confirming the efficacy of the hydrothermal synthesis process.

Moreover, the obtained spectrum is consistent with the reference code number 00-043-0439, confirming the ZSM-12 type phase in accordance with the existing literature [30, 31]. The peaks at 7.9°, 8.8°, 20.8°, and 23° were successfully indexed as characteristic diffraction peaks of ZSM-12 zeolite [32]. Furthermore, the peaks at 11.89°, 13.24°, 13.89°, 14.60°, and 15.90° provide evidence for the presence of TPA⁺ cations [33] with an organized arrangement of the organic template used to synthesize ZSM-12 zeolite within the crystal framework [34].

3.1.2. Fourier transform infrared spectrum

In order to identify the functional groups on the surface of the prepared material (ZSM-12), FTIR spectroscopy analysis was conducted in the wavenumber range of 400–4000 cm⁻¹, and the spectrum obtained is shown in Figure 2. The absorption band at 3297 cm⁻¹ could be attributed to the elongation of the O–H bond of hydroxyl groups (alcohols or phenols) [35]. The small band at 1627 cm⁻¹ indicates the C=O stretching vibration in carbonyl groups (aldehydes or ketones) [36]. The bands located at 1227 and 1100 cm⁻¹ can be assigned to the axial deformation of C–O bonds in carboxylic acids, alcohols, esters, and ethers [37]. The bands observed at 798 cm⁻¹ and 548 cm⁻¹ could be attributed to the external symmetric Si–O stretching vibration and the internal vibration mode of tetrahedra (SiO₄, AlO₄), respectively [38]. Finally, the absorption peak observed at 445 cm⁻¹ corresponds to Si–O bending vibration [38].

3.1.3. Point of zero charge (PZC)

The acid–base properties of ZSM-12 zeolites were investigated by means of the zero charge point pH (pH_PZC) at which the net charge of the particle is zero; the amounts of positive and negative charges at this point are equal [39]. Figure 3 shows the point where the initial and final pH values intersect. The pH_PZC value of the sorbent was observed to be 2.19. This value reflects the acidity of this zeolite due to the predominance of acidic groups on its surface [28]. At the point of PZC, the net charge of the sorbent is zero and below this point, the sorption of U(VI) is low. The organic functional groups present in the cells of the sorbent material are charged in a positive or negative way depending on the pH of the solution. Due to the negative charge of the sorbent surface, the sorption process is favored when the pH is above 2.19 [40].

3.1.4. Nitrogen adsorption–desorption isotherm

Figure 4 shows the nitrogen adsorption–desorption isotherm of the synthesized ZSM-12 zeolite, which is type IV according to the IUPAC classification [41]. A hysteresis loop is observed during the desorption process in the range of 0.4 < P/P0 < 0.95, indicating the coexistence of micro- and mesopores [31, 42]. The large branch of hysteresis indicates that the synthesized ZSM-12 is mainly composed of mesopores accompanied by a few micropores. The mesoporosity might be the result of inter/intracrystalline mesopores corresponding to the zeolitic material as well as the intrinsic mesoporosity of the amorphous material [43]. Furthermore, the synthesized ZSM-12 exhibits a high BET surface area (343.69 m²⋅g⁻¹), low micropore volume (0.1067 cm³⋅g⁻¹), and a small mesopore size (3.0 nm) calculated by BJH models. The narrow mesopore size distribution may be attributed to the nanocrystals that constitute the agglomerates [44]. The high BET specific surface area of ZSM-12 offers abundant adsorption sites, which favors the adsorption of uranium from aqueous solution.

3.2.1. Initial pH effect

The effect of initial pH on the adsorption of uranium(VI) by the synthesized zeolitic material ZSM-12 was studied in the pH range of 1.5 to 9.0 using initial concentrations of 10 and 150 mg/L. As shown in Figure 5, the adsorption efficiency of uranyl ions increases in the pH range of 1.5 to 3 and then decreases with increasing pH. The highest levels of adsorption efficiency, namely 93% and 50% for initial concentrations of 10 and 150 mg/L, respectively, are observed at a pH of 3. These results can be explained by considering the concentration of hydrogen ions (H⁺), the pH of the charge zero point (pH_PZC), and the speciation of uranium(VI) [45]. At low pH values (pH < pH_PZC = 2.19), the active surface functional groups of ZSM-12 zeolite, which are mainly the silanol (Si–OH) and aluminol (Al–OH) groups, and the adsorption of H⁺(a q) on the zeolite surface cause an increase in the solution pH. Under such conditions, the electrostatic repulsion between ${\mathrm{UO}}_2^{2+}$ and (${\mathrm{SiOH}}_2^{+}$, ${\mathrm{AlOH}}_2^{+}$) leads to a decrease in the adsorption rate of uranyl ions [46, 47]. In the pH range 2.19 < pH < 3, the surface of ZSM-12 becomes negatively charged with increasing pH. The predominance of the ${\mathrm{UO}}_2^{2+}$ species in this pH zone leads to an increased adsorption efficiency of uranium(VI) via electrostatic attraction between the positively charged uranium species (${\mathrm{UO}}_2^{2+}$) and the negatively charged functional groups on the surface of ZSM-12. Finally, at pH values higher than 3.0, the surface charge of ZSM-12 remains negative, but complex uranium species such as UO₂(OH)⁺ and $({\mathrm{UO}}_2{)}_2(\mathrm{OH}{)}_2^{2+}$ appear [48]. The low adsorption percentage observed in this zone can be explained by the lack of affinity between the material and hydroxylated species.

3.2.2. Effect of contact time

The impact of contact time on the adsorption efficiency of uranium(VI) by the ZSM-12 zeolite adsorbent was explored in a time range of 5–300 min. As shown in Figure 6, the adsorption efficiency of uranyl ions by the synthesized material rises with increasing contact time until it reaches a plateau [48]. The uptake of uranium by ZSM-12 can be delineated into two distinct stages: an initial rapid process occurring within the first 60 min followed by a subsequent very slow phase. Approximately 95% and 50% of the total uranyl ions are adsorbed from uranyl nitrate solutions of 10 and 150 mg⋅L⁻¹, respectively, during the first hour. After this period, most of the active sites on the adsorbent become saturated, marking the end of sorption. It is noteworthy that in the case of the 150 mg⋅L⁻¹ uranyl nitrate solution, the kinetic curve shows a slight decrease in the adsorption rate due to material saturation. A contact time of 1 h is chosen for further investigation.

The mechanism of the surface reaction between uranyl ions (${\mathrm{UO}}_2^{2+}$) and the active sites of ZSM-12 zeolites was tested using the two kinetic equations, namely, pseudo-first-order and pseudo-second-order (Figure 7). The kinetic parameters calculated from the two theoretical models and the values of the regression coefficients are given in Table 1. From Figure 7, we can see that our experimental data follow very well the two kinetic models applied to our system (R² > 0.96) but with better regression coefficients (R² > 0.999) if pseudo-second-order reactions are allowed. Therefore, it can be assumed that the adsorption process is controlled by pseudo-second-order kinetics. These results are in agreement with previous research studies, which mentioned that the pseudo-second-order kinetic model described data better than the pseudo-first-order model when investigating U(VI) adsorption by different adsorbents such as cross-linked magnetic chitosan beads [49], magnetic Schiff base [50], chitosan tripolyphosphate beads [51], and cross-linked chitosan [52].

3.2.3. Solid/Liquid ratio effect

Batch experiments were conducted using different solid/liquid ratios to examine the impact of this parameter on the adsorption of uranyl ions by the ZSM-12 zeolite. The results are shown in Figure 8. It is evident that the uranium adsorption efficiency increases with rise in solid/liquid ratio, reaching a maximum at 5 g⋅L⁻¹ and 8 g⋅L⁻¹ for initial concentrations of 10 mg⋅L⁻¹ and 150 mg⋅L⁻¹, respectively. Correspondingly, the adsorption efficiency of U(VI) increases from 54.9% to 89% for the 5 g⋅L⁻¹ ratio and from 11.32% to 50.58% for the 8 g/L ratio, after which a slight decrease is observed. This increasing trend may be attributed to the growing number of adsorption sites and the enhanced contact probability between uranyl ions and the available adsorption sites [53]. Conversely, the decreasing trend could be due to the presence of a large number of active particles, causing overlapping and aggregation, thereby reducing the total surface area available for uranium ion adsorption [54].

3.2.4. Initial uranium concentration effect

The effect of the initial concentration of uranium on its adsorption by the synthesized ZSM-12 was investigated by varying the concentration of uranium from 5 to 200 mg/L. The experiments were conducted for two solid/liquid ratios of 5 g⋅L⁻¹ and 8 g⋅L⁻¹. The graphs depicting the uranium uptake efficiency in relation to the initial concentration are presented in Figure 9.

The curves in Figure 9 show that the adsorption efficiency of the synthesized ZSM-12 is inversely proportional to the increase in initial uranium concentration. It is known that at low concentration, the mobility of uranium ions is high, which favors their uptake by the adsorbent. However, their mobility decreases as their concentration increases, causing a decrease in adsorption efficiency. In another way, increasing the initial concentration of uranium ions increases the charges of the uranium species in the solution, which increases the Coulomb repulsions, leading to a decrease in the adsorption efficiency. A similar trend was observed by Monika Jail et al. in their studies on the removal of Cr(VI), Cu(II), and Cd(II) using iron oxide/activated carbon nanoparticles [55]. The experimental uranium adsorption capacities (Q_e) of the synthesized material were determined by plotting the adsorption capacity as a function of a steady-state uranium concentration as shown in Figure 9. The maximum experimental adsorption capacities of the prepared ZSM-12 zeolite were found to be equal to 14 and 12 mg⋅g⁻¹ for the two solid–liquid ratios of 5 and 8 g⋅L⁻¹, respectively. Table 2 highlights the superior adsorption capacities of ZSM-12 zeolite, especially at different solid–liquid ratios, in comparison with various zeolite-based adsorbents reported in the literature.

The Langmuir, Freundlich, and D–R models are used to fit the data of the adsorption process of uranium(VI) on ZSM-12 material. The results are shown in Figure 10, and the obtained parameters are listed in Table 3.

The results show that for a solid–liquid ratio of 5 g⋅L⁻¹, the three models fit the experimental data well with high correlation coefficients (R² > 0.98). However, the calculated APEs for the Freundlich and D–R models are very high (82.21% and 255.06%, respectively). Furthermore, the corresponding adsorption capacity of U(VI) predicted by the Freundlich model is very low (2.49 mg⋅g⁻¹) and that obtained with the D–R model is very high (49.708 mg⋅g⁻¹) compared to the experimental value (14 mg⋅g⁻¹). For a solid–liquid ratio of 8 g⋅L⁻¹, the correlation coefficient (R²) of the Langmuir model is higher than that of the Freundlich and D–R models. In addition, the Q_max value (12.315 mg⋅g⁻¹) for the Langmuir model is more similar to the experimental value (12 mg⋅g⁻¹). Based on the above results, it can be concluded that the Langmuir model is the most suitable for representing the adsorption equilibrium isotherm of uranium(VI) by the ZSM-12 material. This suggests that ZSM-12 provides specific homogeneous sites and sorption of U(VI) ions on the monolayer generated by the ZSM-12 material [57, 60]. The Langmuir separation parameter (K_l) is between 0 and 1, indicating that the adsorption of U(VI) on ZSM-12 is favorable. The estimation of the thermodynamic parameters from the Langmuir model requires using the value of the Langmuir isotherm constant (K_l) in the van’t Hoff equation by multiplying it with the molar weight of uranium(VI) [61]. The free energy is evaluated to be −24.93 and −24.20 kJ⋅mol⁻¹ for the two studied solid–liquid ratios. According to the negative values of energy ΔG°, the adsorption process is both feasible and spontaneous. The values of energy between 0 and −20 kJ⋅mol⁻¹ are generally indicative of physisorption while the values between −80 and −400 kJ⋅mol⁻¹ are indicative of chemisorption [62]. The negative ΔG° values indicate that both chemisorption and physisorption occur simultaneously during the adsorption process.

3.2.5. Temperature effect and thermodynamics

The temperature effect was studied by varying the temperature of the uranyl nitrate solution from 20 °C to 60 °C for the two initial uranium concentrations of 10 mg⋅L⁻¹ and 150 mg⋅L⁻¹. The experiments were carried out with a thermostatic bath. The curves in Figure 11 clearly show that the increase in temperature was inversely proportional to the uranium uptake by the studied material ZSM-12. This indicates that the adsorption process was exothermic in nature and that the adsorption of uranium(VI) ions on ZSM-12 was favored at low temperatures. This result can be attributed to fact that the interaction between the active groups on the surface of the adsorbent and the ${\mathrm{UO}}_2^{2+}$ ions was weaker at higher temperature [48, 63].

Thermodynamic parameters, such as the enthalpy ΔH° and the entropy ΔS° of the adsorption reaction, are obtained from the slope and intercept of Van’t Hoff plots (Figure 12) and are shown in Table 4.

The standard enthalpy of the adsorption reaction of uranyl ions by the ZSM-12 material was found to be equal to −33.289 kJ⋅mol⁻¹ for the 10 mg⋅L⁻¹ solution and −67.09 kJ⋅mol⁻¹ for the 150 mg⋅L⁻¹ solution. These results indicate that the adsorption process for the two initial concentrations of uranyl nitrate solutions is exothermic.

Negative values of the adsorption entropy (ΔS°) indicate that the transition of uranyl ions from the aqueous phase to the solid phase (adsorption) results in a reduction in the degree of freedom of the adsorbate, hence the favorable nature of the adsorption and the affinity of the adsorbent toward uranyl ions [64, 65]. Negative values of free energy (ΔG°_ads), in the case of an initial uranium concentration of 10 mg⋅L⁻¹, suggest the feasibility and spontaneity of the adsorption reaction, especially at low temperatures. The increase in ΔG°_ads with increasing temperature signifies a rise in disorder during adsorption. This phenomenon can be attributed to the redistribution of energy between the adsorbent and the adsorbate [66]. The positive free enthalpy observed at temperatures of 50 °C and 60 °C indicates that an external energy supply is required for the process to occur. For an initial uranium concentration of 150 mg⋅L⁻¹, ΔG° values of the sorbent are obtained as positive, which concludes the reaction to be nonspontaneous, but it shows a range of spontaneity in the reverse direction [67]. In addition, the spontaneous reaction decreases after the initial concentration becomes more significant, and the spontaneous reactivity is negatively correlated with the initial concentration of U(VI) [68].