3.1. Characterization of nanosized MnFe2O4
The XRD pattern of nanosized MFe2O4 is presented in Fig. 1. The principal XRD peaks which are ascribed to (4 4 0), (3 3 3), (4 2 2), (4 0 0), (2 2 2), (3 1 1), (2 2 0), and (1 1 1) crystal planes, prove the cubic assembly of nanosized manganese ferrite (JCPDS No. 742403) [34.35]. The average crystallite size of the synthesized manganese ferrite sample estimated via the subsequent Scherrer equation is 18.14 nm:
Crystallite size (nm) = Kλ/βcosθ
Where, K is the Scherrer constant, β is the full width at halfmaximum of XRD peaks, λ is the wavelength of KαCu radiation, and θ is the diffraction angle. Figure 2 shows the EDS analysis of nanosized MnFe2O4 sample. The results show that the % Wt of Mn, Fe, and O is 24.12, 48.04, and 28.15, respectively. The surface morphology using FESEM for nanosized MnFe2O4 is displayed in Fig. 3. The results confirmed the cubic nature of the surface of MnFe2O4 nanoparticles which have an average size of 110 nm. Figure 4 shows the HRTEM image of nanosized MnFe2O4 sample. The results confirmed that the sample consist of sphere and irregular shapes with an average diameter of 15.93 nm. Construction of the spinel nanosized MnFe2O4 was further confirmed using FTIR analysis. Figure 5 presents the FTIR spectrum of nanosized MnFe2O4. The spectra display two principal bands below 1000 cm− 1 which is a familiar characteristic of ferrites. These characteristic bands at 575 and 450 cm− 1 attributed to essential stretching vibrations of the oxygenmetal at the tetrahedral and octahedral sites, respectively. The bands which were observed at 1635 and 3370 cm− 1 are due to stretching and bending vibration of adsorbed water, respectively [36–45]. The magnetization curve of the nanosized MnFe2O4 is displayed in Fig. 6. The results prove the superparamagnetic performance of the synthesized sample. Also, the saturation magnetization for nanosized MnFe2O4 is 65 emu/g which is considered more than 16.3 emu/g (Minimum required saturation for magnetic separation from aqueous media with an external magnet).
3.2. Adsorption of Zn(II) ions from aqueous media
The relation between adsorption pH and % removal of Zn(II) ions using nanosized MnFe2O4 is displayed in Fig. 7A. Also, the relation between adsorption pH and the quantity of adsorbed Zn(II) ions with nanosized MnFe2O4 is displayed in Fig. 7B. The results confirmed that the % removal of Zn(II) ions and the quantity of adsorbed Zn(II) ions improved via increasing pH until equals 78.5 % and 314 mg/g, respectively at the optimum pH which equals 6.5.
The relation between adsorption time and % removal of Zn(II) ions using nanosized MnFe2O4 is displayed in Fig. 8A. Also, the relation between adsorption time and the quantity of adsorbed Zn(II) ions with nanosized MnFe2O4 is displayed in Fig. 8B. The results confirmed that the % removal of Zn(II) ions and the quantity of adsorbed Zn(II) ions improved via increasing time until equals 77.5 % and 310 mg/g, respectively at the optimum time which equals 60 min.
The adsorption kinetics was examined via pseudofirstorder (Fig. 9A) and, pseudosecondorder (Fig. 9B) models, which are donated as Eqs. (3) and (4), respectively [36–45]:
log (Qe  Qt) = log QeK1t/2.303 (3)
t/Qt = (1/K2Qe2) + (1/Qe) t (4)
where, Qt (mg/g) is the amount of adsorbed Zn(II) ions at time t whereas Qe (mg/g) is the amount of adsorbed Zn(II) ions at equilibrium. Also, K2 (g/mg.min) is the pseudosecondorder rate constant whereas K1 (1/min) term is the pseudofirstorder rate constant. The data of pseudofirstorder and pseudosecondorder kinetics models are displayed in Table 1. Owing to the greater correlation coefficient (R2) of pseudosecondorder and the small alterations between the calculated and experimental adsorption capacity by pseudosecondorder, the adsorption of Zn(II) ions follows the pseudosecondorder kinetic model.
Table 1
Pseudo first order

Pseudo second order

Q (mg/g)

KF (1/min)

R2

Q (mg/g)

KS (g/mg.min)

R2

260.68

0.034

0.994

364.96

0.0001

0.999

The relation between primary concentration and % removal of Zn(II) ions using nanosized MnFe2O4 is displayed in Fig. 10A. Also, the relation between primary concentration and the quantity of adsorbed Zn(II) ions with nanosized MnFe2O4 is displayed in Fig. 10B. The results confirmed that the % removal of Zn(II) ions decreased while the quantity of adsorbed Zn(II) ions improved via increasing primary concentration. The adsorption isotherm is an appreciated relation that explains the behavior of adsorption equilibrium between a solidphase and adsorbate at a constant pH and temperature. The Langmuir (singlelayer adsorption) (Fig. 11A) and Freundlich (multilayer adsorption) (Fig. 11B) isotherms were estimated in this work as denoted in Eqs. (4) and (5), respectively [36–45].
lnQe = lnKF + (1/n) lnCe (4)
Ce/Qe = (1/KLQm) + (Ce/Qm) (5)
where, Qm (mg/g) is the maximum Langmuir adsorption capacity while KL (L/mg) is the Langmuir constant. Further, KF (mg/g)(L/mg)1/n is the Freundlich constant whereas 1/n is the heterogeneity factor. Furthermore, the Qm of Freundlich isotherm was estimated via Eq. (6).
Qm = KF (Co) 1/n (6)
The data of Langmuir and Freundlich kinetics models are displayed in Table 2.
Table 2
Langmuir

Freundlich

Q (mg/g)

KL (L/mg)

R2

Q (mg/g)

KF (mg/g)(L/mg)1/n

R2

330.03

0.423

0.999

323.92

226.19

0.971

Owing to the elevated correlation coefficient of Langmuir isotherm (RL2), the adsorption happens for MnFe2O4 and the maximum adsorption capacity equals 330.03 mg/g.
The relation between adsorption temperature and % removal of Zn(II) ions using nanosized MnFe2O4 is displayed in Fig. 12A. Also, the relation between adsorption temperature and the quantity of adsorbed Zn(II) ions with nanosized MnFe2O4 is displayed in Fig. 12B. The results confirmed that the % removal of Zn(II) ions and the quantity of adsorbed Zn(II) ions decreased via increasing temperature. The thermodynamics findings were carried out to examine the spontaneous and feasibility of the adsorption as denoted in Eqs. (7) and (8), respectively [36–45].
lnKd = (∆So/R)  (∆Ho/RT) (7)
∆Go= ∆Ho  T∆So (8)
where, R (KJ/mol K) is the gas constant while T (Kelvin) is the temperature. Also, ΔGo is the Gibbs free energy change while ∆Ho is the enthalpy change. Further, ∆So is the entropy change. Distribution coefficient (Kd, L/g) can be estimated via Eq. (9)
Kd = [% R/ (100% R)] V/m (9)
Figure 12C represents the plot of ln Kd vs. 1/T. The data of thermodynamic parameters are listed in Table 3.
Table 3
∆Go (KJ/mol)

∆So (KJ/molK)

∆Ho (KJ/mol)

Temperature (Kelvin)

298

308

318

328

27.90

28.29

28.68

29.07

0.039

16.344

The negativity of enthalpy elucidates the exothermic nature of adsorption of Zn(II) ions while the negativity of Gibbs free energy displays the spontaneous and feasibility adsorption. The Gibbs free energy values reduce by growing the temperature from 298 K to 328 K, which show the beneficial adsorption of Zn(II) ions using MnFe2O4 at room temperature, as compared with elevated temperatures. The physical adsorption was the prevailing as the ΔHo value is less than 40 kJ/mol.
Lastly, as seen in Table 4, via comparing the maximum uptake capacity of nanosized MnFe2O4 with other adsorbents, it can be decided that the current adsorbent is the most operative adsorbent utilized for the removal of Zn(II) ions from aqueous media [31, 32, 46–48].
Table 4
comparison between adsorption capacity of MnFe2O4 and other adsorbents
Adsorbent

Adsorption capacity (mg g− 1)

Ref

Clinoptilolite

8.70

[46]

Magnetite

107.70

[31]

Fe3O4@SiO2

169.50

[32]

MnO2

54.50

[33]

Hydroxyapatite

37.50

[47]

MnFe2O4

330.03

This study
