Examination of the MgFe2O4/chitosan/Schiff base compositeX-ray diffraction (XRD)X-ray diffraction analysis (XRD) was utilized to reveal the structural characteristics and crystalline size of chitosan, MnFe2O4 nanoparticles, and MnFe2O4/chitosan/Schiff base composite, as displayed in Fig. 2A–C, respectively. The sorption capacity of a sorbent is significantly influenced by its crystal assembly. The XRD diffraction shape of pure chitosan flakes exhibited prominent peaks at 2Ɵ = 9.9° and 22.5°, signifying the establishment of inter- and intramolecular hydrogen bonds facilitated by the existence of amino (NH2) groups, which is consistent with the known crystal structure of chitosan. Peaks at 2Ɵ = 18.17°, 30.03°, 35.49°, 37.07°, 43.16°, 53.54°, 57.11°, 62.87°, 71.27°, 74.22°, 75.15°, and 79.26° corresponded to the (111), (220), (311), (222), (400), (442), (511), (440), (620), (533), (622), and (444) Miller planes of MnFe2O4 nanoparticles, respectively, as identified by JCPDS No. 38-043029. Similarly, the distinctive diffraction peaks of MnFe2O4 nanoparticles were observed in the MnFe2O4/Schiff base/chitosan composite pattern. This investigation found that chitosan modified with Schiff base exhibited various aromatic side groups in its chemical composition, indicating that altering replacement groups in the chitosan molecule could modulate crystallinity. As a result, the modified chitosan demonstrated a less crystalline arrangement compared to pure chitosan.Figure 2XRD analysis of (A) chitosan, (B) MnFe2O4 nanoparticles, and (C) MnFe2O4/chitosan/Schiff base composite.Fourier transform infrared spectroscopy (FTIR)Figure 3A–C displays the FTIR spectra of chitosan, chitosan/Schiff base, and MnFe2O4/chitosan/Schiff base composite, respectively. The typical peaks of chitosan are positioned at 3570, 3259, 2971, 1680, 1464, 1304, 1237, 1045, and 795/cm. In addition, the absorption peak positioned at 3570/cm is owing to the asymmetric stretching vibration of the NH2 group and/or the stretching vibration of the OH group. The absorption peak positioned at 3259/cm is owing to the symmetric stretching vibration of the NH2 group. The absorption peak positioned at 2971/cm can be attributed to the stretching vibration of the aliphatic CH groups. The absorption peak positioned at 1680/cm is due to the bending vibration of the OH group. Besides, the absorption peak positioned at 1464/cm is attributed to the bending vibration of the CH group. The absorption peak positioned at 1304/cm is assigned to the stretching vibration of the C–N group. The absorption peak positioned at 1237/cm is attributed to the stretching vibration of the C–O group. The absorption peak positioned at 1045/cm is attributed to the stretching vibration of the C–C group. The absorption peak positioned at 795/cm is assigned to the glycosidic linkage between chitosan units. The obvious displacements of these characteristic peaks to the higher or lower wavenumbers reveal chemical associations of the chitosan/Schiff base with the MnFe2O4 nanoparticles. Besides, the peaks positioned at 1610 and 1605/cm is attributed to the stretching vibrations of the C=N group21,22,23. The peaks positioned at 535 and 434/cm can be assigned to the stretching vibration of the Fe–O and Mn–O groups of MnFe2O4 nanoparticles, respectively29.Figure 3FTIR absorption spectra of (A) chitosan, (B) chitosan/Schiff base, and (C) MnFe2O4/chitosan/Schiff base composite.Scanning electron microscopy (SEM)Figure 4A–C displays the SEM images of pure chitosan, MnFe2O4 nanoparticles, and MnFe2O4/chitosan/Schiff base composite, respectively. Pure chitosan displays an irregular surface marked by a unique crystalline structure. The MnFe2O4 nanoparticles exhibit a spherical shape. Conversely, the MnFe2O4/chitosan/Schiff base composite reveals a notably distinct morphological structure, showcasing a rough, non-uniform, and irregular arrangement of the sorbent surface.Figure 4SEM images of (A) chitosan, (B) MnFe2O4 nanoparticles, and (C) MnFe2O4/chitosan/Schiff base composite.Vibrating sample magnetometry (VSM)Figure 5A, B displays the magnetization curves of MnFe2O4 nanoparticles in addition to the MnFe2O4/chitosan/Schiff base composite, respectively. Hence, the magnetization of the MnFe2O4 nanoparticles as well as the MnFe2O4/chitosan/Schiff base composite is 85 and 30 emu/g, respectively. The decrease in the magnetization of the MnFe2O4/chitosan/Schiff base composite may be due to the crosslinking of chitosan/Schiff base and MnFe2O4 nanoparticles.Figure 5Magnetization curves of (A) MnFe2O4 nanoparticles and (B) MnFe2O4/chitosan/Schiff base composite.Transmission electron microscopy (TEM)The TEM images of the MnFe2O4/chitosan/Schiff base nanocomposite are presented in Fig. 6. The images reveal that the composite particles are predominantly spherical and well-dispersed with an average diameter of approximately 9.54 nm. This confirms the nanoscale size of the synthesized material, thereby justifying its classification as a nanocomposite.Figure 6TEM images of MnFe2O4/chitosan/Schiff base composite.BET surface area analysisThe BET surface area of the MnFe2O4/chitosan/Schiff base nanocomposite was found to be 70.32 m2/g, with a total pore volume of 0.1235 cm3/g and an average pore size of 3.58 nm. These values indicate a substantial surface area and porosity, which are crucial factors for enhancing adsorption performance. A higher surface area provides more active sites for the adsorption of Zn(II) ions, thereby improving the overall efficiency of the nanocomposite as an adsorbent.Removal of Zn(II) ions from aqueous mediaEffect of pHAs shown in Fig. 7A, the Zn(II) ion removal percentage increased from 6.45 to 76.65% as the pH was raised from 2.5 to 7.5. This variation can be attributed to the point of zero charge (pHPZC) of the MnFe2O4/chitosan/Schiff base composite, which is 4.09, as illustrated in Fig. 7B. MnFe2O4 nanoparticles exhibit unique magnetic and surface properties that facilitate the attraction of H+ and OH− ions. This attraction can be explained through the interaction between the magnetic nanoparticles and the charged species in the solution. The MnFe2O4 surface can become charged depending on the pH of the solution, influencing its interaction with H+ and OH− ions. At lower pH values, the surface of MnFe2O4 can become positively charged, attracting OH- ions due to electrostatic forces. Conversely, at higher pH values, the surface can acquire a negative charge, attracting H+ ions. Additionally, the presence of functional groups such as hydroxyl (–OH) on the Schiff base-chitosan matrix further enhances the interaction with H+ and OH- ions. If the pH was below 4.09, a decrease in the removal percentage of Zn(II) ions occurred due to the electrostatic repulsion between the positively charged composite surface and Zn(II) ions, as depicted in Fig. 8. Conversely, at pH values above 4.09, an increase in the removal percentage of Zn(II) ions was observed due to electrostatic attraction between the negatively charged composite surface and Zn(II) ions, along with complexation, as indicated in Fig. 8.Figure 7(A) Impact of pH on the Zn(II) ion removal efficiency by the synthesized nanocomposite. (B) Point of zero charge of the synthesized nanocomposite.Figure 8Removal mechanism of Zn(II) ions using the synthesized nanocomposite.Effect of timeThe data in Fig. 9A indicates that the Zn(II) ion elimination percentage rose from 25.15 to 76.79% as the elimination time progressed from 20 to 100 min. Additionally, the elimination percentage of Zn(II) ions showed a nearly constant trend when the time extended from 100 to 140 min, attributed to the complete incorporation of active centers.Figure 9(A) Effect of time on the elimination percentage of Zn(II) ions by the synthesized nanocomposite. (B) The pseudo-first-order and (C) pseudo-second-order kinetic models.To comprehend sorption kinetics, an analysis of the experimental sorption data was conducted employing two kinetic models: the pseudo-first-order model (represented by Eq. 3) and the pseudo-second-order model (expressed by Eq. 4)22,29.$$\log \left( {{\text{Q}}_{{\text{e}}} – {\text{Q}}_{{\text{t}}} } \right) = {\text{logQ}}_{{\text{e}}} – \frac{{{\text{K}}_{1} }}{2.303}{\text{t }}$$
(3)
$$\frac{{\text{t}}}{{{\text{Q}}_{{\text{t}}} }} = \frac{1}{{{\text{K}}_{2} {\text{Q}}_{{\text{e}}}^{2} }} + \frac{1}{{{\text{Q}}_{{\text{e}}} }}{\text{t }}$$
(4)
Qt demonstrates the quantity of Zn(II) ions eliminated at time t (mg/g) whereas Qe indicates the quantity of Zn(II) ions eliminated at equilibrium (mg/g). In addition, K1 describes the rate constant of the pseudo-first-order (1/min) whereas K2 describes the rate constant of the pseudo-second-order (g/mg·min). In addition, Fig. 9B, C shows the pseudo-first-order in addition to the pseudo-second-order models, respectively. Besides, Table 1 provides the constant values for the two kinetic models. Analysis of Table 1 demonstrates that the R2 value regarding the pseudo-second-order kinetic model is higher than that of the pseudo-first-order kinetic model. Moreover, in the pseudo-second-order model, the calculated sorption capacity is more closely aligned with the experimental sorption capacity (QExp) than in the pseudo-first-order model. Therefore, the sorption of Zn(II) ions is better described by the pseudo-second-order kinetic model.
Table 1 Kinetic parameters for Zn(II) ion elimination by the synthesized nanocomposite.Effect of temperatureAs demonstrated in Fig. 10A, the Zn(II) ion elimination percentage elevated from 76.79 to 93.81% as the temperature expanded from 298 to 328 K. The sorption process of Zn(II) ions is temperature-dependent and involves various thermodynamic constants. Thermodynamic parameters, including enthalpy change (ΔHo), Gibbs free energy change (ΔGo), and entropy change (ΔSo), were utilized to evaluate the thermodynamic feasibility of the sorption process. These constants were estimated through Eqs. (5, 6, and 7)22,29.$${\text{lnK}}_{{\text{d}}} = \frac{{\Delta {\text{S}}^{{\text{o}}} }}{{\text{R}}} – \frac{{\Delta {\text{H}}^{{\text{o}}} }}{{{\text{RT}}}}$$
(5)
$$\vartriangle {\text{G}}^{{\text{o}}} = \Delta {\text{H}}^{{\text{o}}} – {\text{T}}\Delta {\text{S}}^{{\text{o}}} { }$$
(6)
$${\text{K}}_{{\text{d}}} = \frac{{{\text{Q}}_{{\text{e}}} }}{{{\text{C}}_{{{\text{eq}}}} }}$$
(7)
Figure 10(A) Influence of temperature on the Zn(II) ion elimination percentage by the synthesized nanocomposite. (B) Plot of ln Kd versus 1/T (B).T denotes the elimination temperature (K), and R is the universal gas constant (KJ/molK). Besides, the distribution coefficient (L/g) is denoted as Kd. Figure 10B illustrates the graph of lnKd versus 1/T, with the thermodynamic constants detailed in Table 2. Examining Table 2 reveals that the negative values of ΔGo signify the spontaneity of the Zn(II) sorption onto the MnFe2O4/chitosan/Schiff base composite. Furthermore, the positive sign and magnitude of ΔHo suggest a chemisorptive and endothermic elimination process. In addition, the positive ΔSo value indicates an increased level of irregularity at the interface between the solution and the adsorbent.
Table 2 Thermodynamic parameters for Zn(II) ion elimination using the synthesized nanocomposite.Effect of concentrationAs demonstrated in Fig. 11A, the percentage of Zn(II) ion elimination decreased from 97.50 to 56.89% as the concentration increased from 50 to 250 mg/L. To comprehend sorption equilibrium, an analysis of the experimental sorption data was conducted employing two equilibrium isotherms: the Langmuir isotherm (represented by Eq. 8) and the Freundlich isotherm (expressed by Eq. 9)22,29.$$\frac{{{\text{C}}_{{\text{e}}} }}{{{\text{Q}}_{{\text{e}}} }} = \frac{1}{{{\text{K}}_{{\text{L}}} {\text{Q}}_{{{\text{max}}}} }} + \frac{{{\text{C}}_{{\text{e}}} }}{{{\text{Q}}_{{{\text{max}}}} }}$$
(8)
$${\text{lnQ}}_{{\text{e}}} = {\text{lnK}}_{{\text{F}}} + \frac{1}{{\text{n}}}{\text{lnC}}_{{\text{e}}} { }$$
(9)
where, 1/n denotes the non-uniformity constant while KL denotes the Langmuir equilibrium constant (L/mg). In addition, KF denotes the Freundlich equilibrium constant (mg/g)(L/mg)1/n while Qmax denotes the Langmuir uppermost adsorption capacity (mg/g). Besides, Eq. (10) can be utilized to compute Qmax through the application of the Freundlich isotherm22,29.$${\text{Q}}_{{{\text{max}}}} = {\text{K}}_{{\text{F}}} \left( {{\text{C}}_{{\text{i}}}^{{1/{\text{n}}}} } \right){ }$$
(10)
Figure 11(A) Impact of concentration on the Zn(II) ion elimination percentage by the synthesized nanocomposite. (B) The Langmuir and (C) Freundlich isotherm.Figure 11B, C illustrates the Langmuir in addition to the Freundlich isotherms, respectively. Also, the constant values for the two equilibrium isotherms were recorded in Table 3. As indicated in Table 3, the R2 value regarding the Langmuir equilibrium isotherm outperforms that of the Freundlich equilibrium isotherm. Thus, it can be concluded that the sorption of Zn(II) ions adheres to the Langmuir isotherm. This confirmation suggests that the adsorbent surface is homogeneous, with all adsorption sites being identical.
Table 3 Constants of equilibrium for Zn(II) ion elimination using the synthesized nanocomposite.The sorption capacity of the MnFe2O4/chitosan/Schiff base composite in this study is contrasted with the values reported in Table 4 from the literature35,36,37,38,39. Notably, the uptake capacity of the synthesized nanocomposite exceeds that of the mainstream of the adsorbents mentioned in the literature. Consequently, this cost-effective and easily preparable composite can serve as a suitable alternative to more expensive adsorbents for the effective removal of Zn(II) ions.
Table 4 Comparison of the uptake capacity of different sorbents with that of MnFe2O4/chitosan/Schiff base composite towards Zn(II) ions.
Efficient removal of Zn(II) ions from aqueous media using a facilely synthesized nanocomposite based on chitosan Schiff base
