3.1. Characterization of Prepared Adsorbent before and after Phenol Uptake
The SBAC-MgAlFe-LDH clear FTIR spectrum peaks that are presented in
Figure 1a indicate the abundance of surface functional groups of the ternary LDH spectrum before and after phenol uptake. The FTIR on the fresh composite exhibited peak of interlayers at 1370 cm
−1 and 3404 cm
−1 assigned nitrate anions (
) and hydroxyl groups (OH) binding onto the metal ions of the LDH (Al, Mg, or Fe) [
47]. However, for the loaded SBAC-MgAlFe-LDH that was evidently indicated by the reduced
) anions in the composite, the high and clearer intensity of the -OH groups was attributed to the existence of the higher content of the SBAC-MgAlFe-LDH in the resulting composite. Meanwhile, mixed metal oxides are indicated by the peaks located below 600 cm
−1; the C-O-C group’s presence is attributed to the peak observed at 1023 cm
−1 [
48]. The existence of these identified surface functional groups indicates the potential of the new adsorbent as a candidate material for organic compounds uptake from water. As phenol was removed from the aqueous phase, there was an obvious transformation of the fresh SBAC-MgAlFe-LDH prior to the absorption, suggesting a possible contribution of surface functionalities for the phenol uptake [
49,
50].
The SBAC-MgAlFe-LDH diffraction peaks are displayed in
Figure 1b. The peaks at 2θ values of 62.18°, 57.42°, 51.77°, 43.04°, 38.39°, 35.28°, and 30.63° are mainly attributed to the patterns of the nanoparticles [
17]. The weak yet broad peak located at 11.23° corresponds to the graphitic carbon of the SBAC index plan. The XRD results suggests effective integration of the MgAlFe-LDH with the SBAC, resulting in the excellent crystalline structure of the composite. The measured BET specific surface area of 320.58 m
2/g, pore volume 0.278 cm
3/g, and pore radius 17.32 nm (based on BJH) for the composite SBAC-MgAlFe-LDH are a significant improvement over the values obtained for the SBAC alone as 254.3 m
2/g, 0.14cm
3/g, and 117.59 nm. The N
2 isotherm trend suggests a type-IV hysteresis loop (
Figure 1c), while the pore size distribution (
Figure 1d) establishes that the SBAC-MgAlFe-LDH composites are characterized as mesoporous.
The SEM photo depicted in
Figure 2c implies a heterogeneous, highly porous, and rough surface morphology, agreeing with the XRD patterns (
Figure 1b). Moreover, the TEM analysis in
Figure 2a suggests that the SBAC-MgAlFe-LDH nanoparticles were uniformly and homogenously dispersed and framed within the interlayers of the MgAlFe-LDH with no indication of aggregation in the resulting composite, as required for LDH composites for enhancing required adsorptive characteristics of adsorbents. The EDS chemical composition analyses (
Figure 2d) provided indicated the dominance of the chemical elements, Fe, Mg, and Al, which forms the basis of the SBAC-MgAlFe-LDH. Thus, the observed improved physicochemical characteristics could be attributed to the uniform distribution of the SBAC nanoparticles into MgAlFe-LDH layers.
3.2. Development and Validation of RSM Model for Phenol Uptake
The RSM regression model in Equation (1) was employed to fit in the obtained SBAC-MgAlFe-LDH phenol uptake data from which Equation (2) was obtained as the best-fitted model. The lower residuals, as provided in
Table 1 and
Figure 3a, indicate the closeness between the actual and model’s predicted values which suggests the high prediction ability of the developed phenol uptake RSM model. This is further corroborated via considerations of the significance of the model’s term and the insignificance of lack-of-fit (LOF);
p-values < 0.0001 and 0.5066 were established at 5% (
p-value < 0.05) (
Table 2), respectively [
34]. The analysis of variance (ANOVA) presented in
Table 2 also shows that the influence of all the investigated parameters was established as all their
p-values are <0.05 [
34,
37]. The developed model implies that the coded model’s terms A, B, C, C², A²B, and AB² are the primary significant model terms. Thus, the best phenol uptake model was a reduced cubic model that included only terms that significantly influence the model directly or indirectly. This means that some of the included terms, even though they are insignificant on their own, must be included as they are hierarchical terms that are indirectly reflected in higher terms that were found to significantly influence the model. In this case of the presented phenol uptake model, AB and A
2 are insignificant; however, A²B and AB² are highly significant terms that greatly contributed to the final model quality. Accordingly, terms AB and A
2 must be associated with the model as they contributed to the higher cubic terms that are significant; otherwise, if not included, the non-hierarchical coded model’s predictions are more unlikely to match the actual model predictions [
34].
In addition, the high predictive ability of the model is manifested in high values and closeness of the different coefficients of determination R
2 (0.998), R
2-predicted (0.993), and R
2-adjusted (0.959) (as provided in
Table 2). Thus, this implies that the values of both the biased and non-bias R
2 and well the LOF are in conformity with one another, thereby meeting the requirements for RSM predictive models [
51]. Meanwhile, the normal probability plot depicted in
Figure 3b implies that the model satisfied the assumption of normality. On the other hand, the linear model’s normal probability plot shown in
Figure 3b implies meeting the normality distribution assumption of the phenol uptake experimental data [
34,
52,
53]. Similarly, the adequate precision of measures 51.149 (>4) fulfilled the signal-to-noise ratio requirements, and the CV = 7.56% indicates the suitability and adequacy of the employability of the model for navigating the design space. Collectively, these checks imply that the developed phenol uptake model can adequately represent the experimental data obtained for evaluation, assessments, and drawing meaningful conclusions on phenol uptake by the SBAC-MgAlFe-LDH adsorbent.
3.3. Influence of Operating Parameters on SBAC-MgAlFe-LDH Phenol Uptake
The influence of SBAC-MgAlFe-LDH dosage and adsorption time is depicted in
Figure 2e and
Figure 3a, respectively. These results (10–25 mg/L) indicated that as the dosage was increased, the adsorption capacity decreased. Meanwhile, at 10 mg dosage, increasing the time yielded higher capacity for phenol uptake, which equilibrated at 180 mg/L. As a result, the dosage of 10 mg and reaction time of 180 min were used for all RSM experiments.
The influences of the operational conditions (temperature, initial phenol concentration, and pH) on the performance of SBAC-MgAlFe-LDH for phenol uptake are presented in the Pareto chart (
Figure 3b), and the contours and 3D plots are presented in
Figure 4a–c. The Pareto chart depicts visual comparative hierarchical contributions of the single, binary and higher interactions of the RSM model’s parameters on the phenol uptake capacity. The higher the Pareto chart’s bar, the higher the relative contribution of an investigated parameter on the model’s performance, [
34,
52]. Hence, parameter B (initial concentration) has the strongest influence, which is followed by factor C (initial pH).
Meanwhile, the influence of the interaction effects follows the order BC > AB > AB. This shows that parameters B and C are the major influencing factors outweighing temperature (A), whose influence was less, comparatively. Moreover, the Pareto chart shows that factors A, B, and AB (orange bar) and factors C, BC, and AC (blue bars) have an antagonistic and synergetic influence on the SBAC-MgAlFe-LDH adsorption capacity, respectively, which dependently corroborates the ANOVA analyses as well as the developed model’s terms coefficients (Equation (1)). The Pareto chart t-test cut line indicated that besides the normal t-test (t = 1.367), the more conservative Bonferroni correction limit (t = 2.92) further establishes the stronger influences of initial concentration and initial pH on phenol uptake by the SBAC-MgAlFe-LDH [
34,
52].
The 3D curves and two-counter plots provided in
Figure 4a–c depict visual influences of changing of the two parameters investigated on q
e while the other factor was fixed. At the fixed lowest initial pH 2 and when the initial concentration was at the lowest level, the obtainable q
e was the lowest (1.56–6.00 mg/g) and was not significantly affected by temperature changes (
Table 2). However, at pH 6 (
Figure 4a), temperature (A) and initial concentration (B) greatly influenced the q
e value, which resulted in the highest obtainable capacity of 98.7 mg/g at the highest phenol concentration of 125 mg/L phenol and mid-level temperature of 35 °C.
Figure 4a implies that the best performances of the adsorbent at the lowest investigated phenol concentration (q
e = 50 mg/g) was achievable at pH 6 and 25 °C (or 45 °C), while at the mid-value temperature of 35 °C, the q
e drastically dwindled to the low value. Meanwhile,
Figure 4b depicts the stronger influence of interaction between factors B and C, which clearly shows the direct positive direct dependency of the q
e on initial phenol concentration with the best performance attached at 125 mg/L phenol concentration, confirming the earlier observation. On the other hand, as the pH was raised from 2, the performance climax was achieved when it reached the value of 6 before it started decreasing and returning to the initial lower value at pH 10. Moreover, at a fixed initial phenol concentration of 73.5 mg/L,
Figure 4c further reaffirms the stronger curvature influence of initial pH (the higher model’s C
2 coefficient) as a result of interaction with the temperature at a fixed initial concentration.
Figure 4c also revealed the stronger influence of initial pH compared to the temperature, which exerted less effect, and it also showed that the optimal pH was pH 6. These trends clearly corroborated the Pareto chart (
Figure 3c), which suggests higher relative contributions of factors B and C on the adsorptive performance of the SBAC-MgAlFe-LDH compared to factor A. Thus, jointly,
Figure 3b,c and
Figure 4, and ANOVA (
Table 2) show that temperature variation evidently exerted a lower influence on the LDH phenol adsorptive performance compared to initial pH and initial phenol concentrations. For the best performance of the SBAC-MgAlFe-LDH for phenol uptake, the values of factors C, B, and A should be preferably at 6, 125 mg/L, and 35 °C, respectively.
The q
e enhancement as a result of an increase in initial phenol concentration was ascribed to an improved induction of more contacts among phenol molecules and the active SBAC-MgAlFe-LDH sites possessing abundant functional groups as more phenol particles were introduced in the aqueous phase solution [
54].
The dependencies of the SBAC-MgAlFe-LDH phenol uptake capacity changes with initial pH can be deduced based on the phenol speciation and surface charge of the SBAC-MgAlFe-LD. Thus, the results of the drift method employed to determine the point of zero charge (pH
pzc) of the SBAC-MgAlFe-LDH suggest a pH
pzc = 7.09 (
Figure 2e). Accordingly, this result implies that when the pH was below the neutral point, the charges on the SBAC-MgAlFe-LDH surface becomes positive due to protonation, which implies stronger electrostatic repulsion between phenol molecules and the surface of the SBAC-MgAlFe-LDH composite, which resulted in lower q
e [
17,
25]. However, at pH 6, which is closer to the pH
pzc, a significant reduction in the positive charge on the surface of the adsorbent could be attributed to the higher performance of the SBAC-MgAlFe-LDH at the mid-value pH, as observed earlier.
Meanwhile, as the pH was increased beyond 6, the gradual transformation of the active SBAC-MgAlFe-LDH sites to negative charge might have induced repulsive electrostatic forces between phenol molecules and the LDH sites, thus leading to the observed decreased in the q
e. Similar behavior for phenol uptake dependency on initial pH has been reported earlier [
17]. The results indicated that electrostatic attraction is not associated to phenol uptake onto an SBAC-MgAlFe LDH composite. Interestingly, higher phenol uptake at pH 6 is mainly associated to a lower protonated surface, which allows phenol molecules to easily interact with composite surface functional groups (OH, MMO, and C-O-C) [
20,
55]. Generally, the good q
e at the best operational conditions was attributed to the successful intercalation of SBAC onto the interlayers of MgALFe-LDH, thereby improving its adsorptive characteristics.
3.4. RSM Optimization
Numerical optimization has been an indispensable tool for multivariate problems [
34,
56]. This is due to the intricacies associated with the identification of optimal operational points, which necessitated the simultaneous examination of all independent and dependent variables data. Consequently, optimization for SBAC-MgAlFe-LDH phenol uptake was performed under different operational conditions using the Design-Expert
® software numerical optimization function, which is called the “desirability function”. Thus, the desirability function capabilities were implemented under five (5) different scenarios, as presented in
Table 3. According to the respective scenarios’ target goals and constraints (collectively), the well-defined and strong desirability algorithm that navigates the data finds the best solution(s) and ranks them according to the value of the “desirability” parameter that is between 1 and 0, which are designated as best to worst solutions in satisfying the optimization criteria and target goals. Accordingly, the maximum phenol uptake was set as the targeted objectives, while a variety of objectives and constraints for operating are targeted for the five (5) different scenarios 1 to 5 (
Table 3). The results for the different scenarios suggest that higher initial phenol concentration and mid-value pH provide a higher desirability of the optimal solution and thus the best uptake capacity. Meanwhile, for lower initial phenol concentrations, the temperature has minimal influence on the achievable phenol uptake capacity. Thus, the highest uptake capacity obtained is inconsistent with the developed RSM model analyses presented earlier with the condition of optimality selected as A = 35 °C, B = 125 mg/L, and C = 6, which is employed for understanding mechanisms of phenol uptake onto SBAC-MgAlFe-LDH via equilibrium and kinetics studies.
3.5. Adsorption Kinetics and Equilibrium Studies
Kinetics of phenol uptake onto the SBAC-MgAlFe-LDH was studied using five (5) non-linear forms of popular kinetic models [
39,
40,
41,
57]. The kinetic models’ fittings against the experimental values of the four (4) best fitted models are displayed in
Figure 5, while the respective models’ parameters are provided in
Table 4. Considering the higher R
2 = 0.98 and lower RMSE = 2.706 for the pseudo-first-order model implies that the kinetics of phenol uptake onto the SBAC-MgAlFe-LDH can be explained by the pseudo-first-order model. Interestingly, the Avrami model performance and model’s parameters match that of the first order (
Table 4), reaffirming the first-order model’s better representation of the experimental data and its suitability for providing insight into the phenol uptake kinetics.
To further elucidate the phenol uptake mechanism under equilibrium conditions and determine the maximum uptake capacity of phenol onto the SBAC-MgAlFe-LDH, non-linear forms of the five (5) different popular equilibrium models were employed in this study [
35,
58]. The fittings of these isotherm models are depicted in
Figure 6, while their corresponding models’ parameters obtained are presented in
Table 5. In terms of R
2 and RMSE, the predictive performance of these models follows the order: Liu > Redlich–Peterson > Langmuir > Freundlich > Tempkin. The Liu and Redlich–Peterson models represent a combination of Langmuir and Freundlich models distinctly at either higher or lower concentration. This analysis implies that SBAC-MgAlFe-LDH phenol uptake can be satisfactorily explained by Liu (R
2 = 0.996 and RMSE = 1.67) [
46], Langmuir (R
2 = 0.995 and RMSE = 1.91), and Redlich–Peterson (R
2 = 0.995 and RMSE = 1.91) [
44]. The better fittings for the Liu [
46] and Redlich–Peterson [
44] models are in line with earlier findings for phenolic compounds uptake by SBAC [
22]. The maximum achievable monolayer phenol uptake capacity (q
max) for SBAC-MgAlFe-LDH as per the Langmuir model was estimated at 216.76 (
Table 5), even though
Figure 6 indicates that the adsorption isotherms are not complete and were just reaching the maximum adsorption capacity.
A comparison of SBAC-MgAlFe-LDH phenol uptake capacity with other sewage sludge-based adsorbents produced using different activation methods is provided in
Table 6. The ternary SBAC-MgAlFe-LDH reported herein possessing 216.76 mg/g capacity for phenol sorption implies an effective composite material for the effective removal of phenol from water. This is a significant improvement over SBAC reported capacities using conventional heating = 34.36 mg/g and microwave [
22] and activation using CO
2 = 32.4 mg/g [
23]; chemical agents such as ZnCl
2 = 20.95–81.6 mg/g [
24,
25], citric acid–ZnCl
2 mixture [
26] = 2.01 mmol/g, NaOH = 17.82–96.15 mg/g [
25,
27], H
2SO
4 = 26.16 mg/g [
28], polymer flocculants = 132.33 [
29], and ZnCl
2-activated SBAC-MgFe-LDH = 138.69 mg/g [
17]. Thus, the high uptake capacity exhibited by the NaOH SBAC-LDH reported in this study was ascribed to the successful synergistic influence of SBAC and the MgAlFe-based LDH composite, which yielded improved and abundant surface functional groups that supported the adsorption of more phenol molecules from water.
3.6. Thermodynamics and Regeneration Studies of SBAC-MgAlFe-LDH Composite
At different experiment temperatures, 25, 35, and 45 °C, and fixed other conditions as per the optimal RSM conditions of initial phenol concentration of 125 mg/L and pH 6, the thermodynamics of SBAC-MgAlFe-LDH composite phenol uptake were undertaken. At the respective different temperatures, the Gibbs free ΔG° values are close to each other with values of −5.33, −5.53 and −5.77 kJ/mol, respectively, yielding an energy enthalpy ΔH = 16.52 (kJ/mol) and entropy change ΔS = −0.105 (kJ/mol) obtained based on plotting the ΔG° values against the respective temperatures. The obtained parameters are given in
Figure 7a. In general, the ΔG° value that is in the range of 0 to −20 kJ/mol corresponds to physical adsorption [
59,
60]. Moreover, the value of the ΔH is within the range of 2.1–20.9 kJ mol
−1, which further establishes the physical nature of phenol uptake by SBAC-MgAlFe-LDH. All the ΔG° values are negative, indicating the spontaneous and favorable nature of the phenol uptake by the SBAC-MgAlFe-LDH composite [
59,
60]. Additionally, as the temperature was raised, there was an observed drop in the ΔG° values. Meanwhile, there was also positive +ΔH and absolute lower ΔG° (<20 kJ/mol), implying the endothermic nature of the phenol uptake.
The regeneration of the SBAC-MgAlFe-LDH for phenol uptake was conducted for consecutive recycles adsorption onto the SBAC-MgAlFe-LDH and subsequent desorption using ethanol (95% solution). The detailed procedure adopted to achieve this part has been reported elsewhere [
17]. The result presented in
Figure 7b shows that the initial phenol uptake capacity for the fresh adsorbent of 66.12 mg/g decreased to 60.81, 52.88, and 45.88 mg/g after the first, second, and third regeneration and recycling. This is about 8.03%, 20.02%, and 30.611% of the original capacity, indicating the higher reusability potential of the SBAC-MgAlFe-LDH compared with the original capacities of similar adsorbents (
Table 6).
3.7. Possible Mechanisms of Phenol Uptake
The synergetic effects of SBAC and the MgAlFe-based LDH resulted in improved and abundant surface functional groups that favored a high uptake of phenol molecules from the aqueous phase through multiple mechanisms involving surface adsorption and π–π interactions [
20,
22,
55]. To further elaborate the possible adsorption mechanism of phenol, the energy of adsorption (E) was estimated from the linear form of the Dubinin–Radushkevich (DR) isotherm model (R
2 = 0.986) and characterization of SBAC-MgAlFe-LDH before and after phenol adsorption (
Figure 1 and
Figure 2). The value of E at optimized adsorption conditions (pH 6, time 120 min, and temperature 35 °C) is found to be 40.82 (kJ/mol), suggesting that the possible main phenol adsorption mechanism onto SBAC-MgAlFe-LDH is involved in physisorption (π–π interactions) as a result of the phenol aromatic ring interaction with that of the SBAC-MgAlFe-LDH via charge transfer, dispersive-force, and polar attractions [
17,
22,
29,
57]. The -OH groups on the SBAC-MgAlFe-LDH surface (FTIR in
Figure 1a) act as electron donors are susceptible to enhancing the aromatic-ring π-donating intensity, thereby increasing the phenol attraction onto the SBAC-MgAlFe-LDH [
22,
61]. Additionally, the well-established hydrophobicity of phenol provides another dimension of phenol uptake onto the SBAC-MgAlFe-LDH, as this property enhances the affinity between phenol and SBAC-MgAlFe-LDH [
22,
61]. Similar mechanisms for uptake phenol by sludge-based adsorbents have been postulated by other authors [
22,
29].