Facile Co-precipitation Synthesis of Nanosized MnFe2O4 for Effective Removal of Zn(II) Ions From Aqueous Media

In this work, the spinel nanosized MnFe 2 O 4 (18.14 nm) was facilely synthesized through the co-precipitation method to study the removal of Zn(II) ions from aqueous media. The fabricated MnFe 2 O 4 sample was characterized using VSM, XRD, HR-TEM, EDS, FE-SEM, and FT-IR analyses. 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 as shown from JCPDS No. 74-2403. The EDS pattern conrmed that the % Wt of Mn, Fe, and O is 24.12, 48.04, and 28.15, respectively. The FE-SEM image conrmed the cubic nature of the surface of MnFe 2 O 4 nanoparticles which have an average size of 110 nm. The saturation magnetization was 65 emu/g. The impacts of initial pH, concentration of Zn(II) ions, contact time, and temperature on the uptake of Zn(II) ions were accurately investigated. The removal of Zn(II) ions is spontaneous, exothermic, and followed the pseudo-second-order model and the Langmuir isotherm. The maximum adsorption capacity equals 330.03 mg/g.


Introduction
The expression "heavy metal" is a description for an assembly of metalloids and metals with a density above 5 g/cm 3 [1]. Also, with the fast expansion of manufacturing like melting, mining, food industry, pigment manufacture, cosmetics, pharmaceuticals, fertilizer productions, galvanizing, and insecticides.
The access level of these metals into the water sources has raised and according to the carcinogenicity and toxicity of these metals, the underground and surface water contamination become a worldwide concern. Besides, zinc, as an example of heavy metal such as copper, and iron is essential for our health however an elevated concentration of it is extremely poisonous and risky for health and produce speci c diseases as skin irritations, stomach cramps, fever, nausea, anemia, vomiting, and even rapid death [2][3][4][5][6].
Consequently, elimination of elevated concentration of zinc similar additional heavy metals is mandatory and principal, and due to the aforementioned causes, the world health organization (WHO) with an elevated warmth informed an allowed concentration of 300 µg/L for zinc in water [7]. Recently, numerous traditional approaches have been exploited for heavy metals eliminations, for example, ion exchange, electrochemical precipitation, chemical precipitation, adsorption, and reverse osmosis [8][9][10][11][12]. Amongst all of the exploited procedures, due to its operation easiness, low-priced, eco-friendly and elevated e ciency, adsorption is informed as the greatest appropriate approach [13,14]. So far, there have been exploiting various adsorbents for heavy metals elimination similar zeolites, resins, clay minerals, biosorbents, and several industrial wastes such as y ash, however, all of them have public characteristics of separation di culties and little uptake capacities [15][16][17][18][19][20]. Newly, with the expansion of nanoscience, nanosized substances (typically with the size < 100 nm) due to their distinctive properties such as elevated surface/volume ratio, rich active centers, and elevated uptake capacity, have been respected and have additional application in the recognition and removal of inorganic and organic pollutants. Amongst all nanosized substances, magnetic nanoparticles, besides having the beyond properties, owing to their magnetic characteristics and simple separation with the service of an outer magnet, have been respected more [21][22][23][24]. Furthermore, spinel ferrites (Chemical formula: MFe 2 O 4 ) (M = Mn(II), Fe(II), Zn(II), Co(II), etc.) is additionally a cluster of magnetic nanosized substances which are broadly used in numerous elds such as catalysts, electrical devices and biomedicine, and their distinct properties such as outstanding magnetic characteristics, chemical constancy, elevated particular surface area and fast uptake kinetics have moreover been exploited in the uptake and elimination of the contamination from wastewater [25][26][27][28]. The chemical and physical characteristics of spinel ferrites such as magnetic performance depend on the nature of divalent positive ions in addition to the distribution of divalent cation and Fe(III) ions between octahedral and tetrahedral crystallographic locations. In this view, the spinel ferrites categorized into three classes: inverse, normal, and mixed. Also

Fabrication of nanosized MnFe 2 O 4
For the fabrication of nanosized MnFe 2 O 4 particles, a directly co-precipitation approach was utilized as the following; a particular quantity of MnCl 2 ·4H 2 O and FeCl 3 ·6H 2 O with a Fe/Mn molar ratio equals 2:1 was dissolved in 130 mL of deionized water under strong stirring and presence of nitrogen gas. Then, the solution was warmed up to 75°C and afterward 7.50 M KOH (was warmed up 75°C before) was added to beyond solution drop by drop with constant stirring, till pH equals 11.22. The mixture kept in that condition for 120 min. Then, the black produced precipitate was magnetically separated at room temperature. Besides, the last precipitate was dried for one day at room temperature after being cleaned with deionized water and acetone numerous times.

Adsorption tests
The adsorption reactions were accomplished via adding 0. Crystallite size (nm) = Kλ/βcosθ Where, K is the Scherrer constant, β is the full width at half-maximum of XRD peaks, λ is the wavelength of KαCu radiation, and θ is the diffraction angle. Figure 2 Figure 5 presents the FT-IR spectrum of nanosized MnFe 2 O 4 . 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 oxygen-metal 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][37][38][39][40][41][42][43][44][45]. The magnetization curve of the nanosized MnFe 2 O 4 is displayed in Fig. 6.
The results prove the superparamagnetic performance of the synthesized sample. Also, the saturation magnetization for nanosized MnFe 2 O 4 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).

Adsorption of Zn(II) ions from aqueous media
The relation between adsorption pH and % removal of Zn(II) ions using nanosized MnFe 2 O 4 is displayed in Fig. 7A. Also, the relation between adsorption pH and the quantity of adsorbed Zn(II) ions with nanosized MnFe 2 O 4 is displayed in Fig. 7B. The results con rmed 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 MnFe 2 O 4 is displayed in Fig. 8A. Also, the relation between adsorption time and the quantity of adsorbed Zn(II) ions with nanosized MnFe 2 O 4 is displayed in Fig. 8B. The results con rmed 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.
lnQ e = lnK F + (1/n) lnC e (4) C e /Q e = (1/K L Q m ) + (C e /Q m ) (5) where, Q m (mg/g) is the maximum Langmuir adsorption capacity while K L (L/mg) is the Langmuir constant. Further, K F (mg/g)(L/mg) 1/n is the Freundlich constant whereas 1/n is the heterogeneity factor.
Furthermore, the Q m of Freundlich isotherm was estimated via Eq. (6).
The data of Langmuir and Freundlich kinetics models are displayed in Table 2.  Fig. 12A. Also, the relation between adsorption temperature and the quantity of adsorbed Zn(II) ions with nanosized MnFe 2 O 4 is displayed in Fig. 12B. The results con rmed that the % removal of Zn(II) ions and the quantity of adsorbed Zn(II) ions decreased via increasing temperature. The thermodynamics ndings were carried out to examine the spontaneous and feasibility of the adsorption as denoted in Eqs. (7) and (8), respectively [36][37][38][39][40][41][42][43][44][45].
where, R (KJ/mol K) is the gas constant while T (Kelvin) is the temperature. Also, ΔG o is the Gibbs free energy change while ∆H o is the enthalpy change. Further, ∆S o is the entropy change. Distribution coe cient (K d , L/g) can be estimated via Eq. (9) K d = [% 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. 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 bene cial adsorption of Zn(II) ions using MnFe 2 O 4 at room temperature, as compared with elevated temperatures.
The physical adsorption was the prevailing as the ΔH o value is less than 40 kJ/mol.
Lastly, as seen in Table 4, via comparing the maximum uptake capacity of nanosized MnFe 2 O 4 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][47][48].

Conclusions
The cubical nanosized manganese ferrite (18.14 nm) was facilely synthesized via co-precipitation method. The nanosized MnFe 2 O 4 sample could uptake Zn(II) ions with adsorption capacity equals 330.03 mg/g. The removal of Zn(II) ions was tted with the Langmuir isotherm and pseudo-second-order kinetic model. Besides, the removal was exothermic, physical, and spontaneous.

Con ict of Interest
The authors declare that there is no con ict of interest for this paper.