Structural, morphological and magnetic properties of hexaferrite BaCo2Fe16O27 nanoparticles and their efficient lead removal from water

W-hexaferrite BaCo2Fe16O27 was prepared using the citrate nitrate combustion method. The sample was characterized using XRD, SEM, EDX and elemental mapping. XRD confirmed that the sample was synthesized in a single phase hexagonal structure with an average crystallite size 37.39 nm. SEM images of the sample show a spongy morphology with the agglomerated grain owing to dipole interaction between the crystallites. The magnetic properties of BaCo2Fe16O27 were studied using H-M hysteresis loop and the DC magnetic susceptibility. The sample has a ferrimagnetic behavior with saturation magnetization 64.133 emu/g. The magnetic properties of BaCo2Fe16O27 are originated from the Fe3+–O–Fe3+ superexchange. The synthesized sample is used as an adsorbent to remove the heavy metal Pb2+ from water. BaCo2Fe16O27 has Pb2+ removal efficiency 99% and 28% at pH 8 and 7 respectively. The Langmuir and Freundlich isotherms were used to analyze the experimental data. The Freundlich adsorption isotherm fitted the experimental data well.


Introduction
The hexaferrites have six main types (M-, U-, W-, X-, Y-and Z-types) according to the chemical formulas and stacking sequences of these building blocks. W-type hexaferrites have a general formula AMe 2 Fe 16 O 27 , where A is an alkaline earth ion (i.e., Ba and Sr) and Me is a divalent transition metal ion such as Mg, Zn, Co, Ni, Fe. They are characterized by ferrimagnetic behavior with high Curie temperatures. The magnetic properties of Ba(Sr)M 2+ Fe 16 O 27 vary with the type divalent cation. The hexagonal ferrites are used in a wide range of devices, such as magnetic recording media, highperformance microwave-absorbing, permanent magnets, shielding materials in electronic devices operating at GHz frequencies and bubble memories and microwave devices [1][2][3][4].
To unravel the verities of application, various divalent and trivalent cations are frequently inserted in the sub lattices of W-type barium hexagonal ferrites (BHF). Several methods of preparation were used to prepare BHF compounds with different nanocrystal sizes, shapes, and characteristics [5]. The synthesize routes such as the conventional solid-state reaction method [6], the sol-gel method [7], the microemulsion route [8], the ball milling [9], auto-combustion [10] and co-precipitation method [11] were used for preparation nanoparticles of BHF systems. The citrate combustion method is a fast, simple and easy technique for the synthesis of a variety of advanced nanomaterials and ceramics [12]. In this method, the thermal redox reaction occurs between an oxidant and a fuel (as citric acid) [13]. The monophasic nanopowders were prepared with a homogeneous microstructure in short reaction times using a citrate combustion method [14,15].
The structural, morphological and magnetic properties of Ba W-type hexaferrite (BHFs) were strongly influenced by the divalent cations and rare-earth substitution. The enhancement of zinc in Ba 1 Cu 2-x Zn x Fe 16 O 27 (x = 0.0 and 0.4) has been studied by R. Sagayaraj [5]. The pure and doped samples have a semiconductor like behavior.
The SEM images reveal that the Ba hexaferrites gained the hexagonal structure with spongy morphology. Ba 1 Cu 2-x Zn x Fe 16 O 27 nanoparticles may be utilized for treating bacterial infection in the clinical sector. Kai Huang et al. [16] studied the effect of Ca ions on the structure and magnetic properties of BHFs. The samples Ba 1-x Ca x Co 2 Fe 16 O 27 (x = 0, 0.1, 0.3, 0.4 and 0.5) were prepared using a sol-gel method. The coercive field and the saturation magnetization were increased by increasing the amount of Ca substitution. The Ca 2+ doped W-type hexaferrite improved microwave absorbency. M.A. Ahmed et al. [17] studied the effect of the influence of rare-earth ions on the magnetic properties of barium hexaferrite Ba 0.95 R 0.05 Mg 0.5 Zn 0.5 CoFe 16 O 27 (R = Y, Er, Ho, Sm, Nd, Gd, and Ce ions). The curie temperature (T C ) and effective magnetic moment increased for the Sm doped sample. Jin Tang et al. [18] were prepared Ba 1−x La x Fe 2+ 2 Fe 3+ 16 O 27 using the standard ceramic method. The values of the lattice constant c decreased with the increase of the doped La 3+ in the samples. There is a monotonic dependence of the coercivity (H c ) and the magnetic anisotropy field (H a ) on the La 3+ amount.
Heavy metals in aqueous solutions, such as Cr(VI), Pb(II), and Cd(II), are poisonous even in low amounts and have caused serious health effects on humans. As a result, it is critical to remove these heavy metals from the aqueous environment to protect biodiversity, hydrosphere ecosystems, and humans. To remove these harmful heavy metals from wastewater, several techniques such as chemical precipitation, electrolytic separation, membrane separation, ion exchange, and adsorption have been used [19].
For the removal of heavy metals, the adsorption technique has been widely used. Nanomaterials have been considered an excellent adsorbent for the removal of heavy metal ions from wastewater [20][21][22]. Nanomaterials were used for removal of heavy metals from water using adsorption techniques based on the physical interaction between metal ions and nanomaterials [23].
In this present work, the author synthesized BaCo 2 Fe 16 O 27 nanoparticles using the citrate combustion method as a fast, simple and easy technique. The aim of this study is understanding the crystallite structure, morphology and magnetic properties of W-type hexaferrites, additionally, studying the heavy metal removal efficiency using BaCo 2 Fe 16 O 27 nanoparticles. To identify the best conditions for heavy metal Pb 2+ ion removal from water, the removal efficiency of Pb 2+ ion was studied as a function of contact time and pH.

Preparation of nanoparticles
The hexaferrite BaCo 2 Fe 16 O 27 sample was prepared by citrate nitrate combustion method as illustrated in Fig. 1. The metal nitrates (purity 99.9%, Sigma-Aldrich) were mixed in stoichiometric ratios.

Measurements
The X-ray diffraction (XRD) was carried out using X-ray diffraction (XRD, Bruker advance D8 diffractometer, λ = 1.5418 Å). The sample was characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to study the surface morphology and the chemical structure using OXFORD INCA PentaFETX3-England. The magnetic properties of the hexaferrite BaCo 2 Fe 16 O 27 sample were assessed using two techniques: the first is the H-M hysteresis loop using the vibrating sample magnetometer (VSM; 9600-1 LDJ, USA). While the other is the measurement of the DC magnetic susceptibility as a function of absolute temperature using the Faraday method [24].

Effect of pH value
The heavy metal Pb 2+ ion removal efficiency of BaCo 2 Fe 16 O 27 sample was studied. Pb 2+ ions solution of concentration (50 ppm) was prepared as an initial concentration. 0.02 g of BaCo 2 Fe 16 O 27 nanoparticles was added to five beakers (250-mL) containing the above solution. The pH values of the solutions were adjusted from 3 to 8 and they were stirred using an electric shaker (ORBITAL SHAKER SO1) at 200 rpm for 1 h. A 0.2-m syringe filter was used to filter 10 ml of the supernatant solutions at regular intervals. Inductively coupled plasma (ICP) spectrometry (Prodigy7) was used to determine the concentration of heavy metals in the filtrate.

Effect of contact time
The 50 ppm Pb 2+ solution was pipetted into a beaker containing 0.10 g of BaCo 2 Fe 16 O 27 , with the pH value adjusted to its optimal value. After varying contact times, the concentration of Pb 2+ in the solution was determined. The following equations were used to calculate the metal ion removal efficiency ( ) and the equilibrium adsorption capacity (q) [25]: where C i and C e are the initial and final concentrations (mg/L) of metal ion solution, respectively. While m is the mass of adsorbent and V is the volume of Pb 2+ solution. Figure 2 illustrates the XRD pattern of BaCo 2 Fe 16 O 27 nanoparticles. The sample has a single phase hexagonal structure with the space group P6 3 / mmc [5]. The data were indexed with ICDD card number 019-0098. The average crystallite size was calculated using the well-known Debye-Scherer equation formula [26] where D is the average crystallite size, λ is the wavelength of X-ray radiation, θ is the Bragg angle, and β is the full-width at half-maximum intensity of the powder pattern peak. The average crystallite size is 37.39 nm. The lattice parameters

Results and discussion
were calculated based on the hexagonal symmetry according to Eq. (4). The value of theoretical density was calculated from Eq. (5) and reported in Table 1 where Z = 2 is the number of molecules per unit cell, N is Avogadro's number, M is the molecular weight and V is the unit cell volume [27].
The crystal axis ratio c/a is normally 4.0 for W type Ba hexaferrites. While the value of c/a ratio is 4.6733 for the investigated sample, owing to the migration of divalent cations in the voids causes the John-Teller effect [5].
The morphology and the grain size of the investigated sample were studied using SEM [28]. Fig. 3 illustrates SEM micrographs of BaCo 2 Fe 16 O 27 nanoparticles. SEM images show a spongy morphology with the agglomerated grains. The average grain size of the sample is 73.7 nm. A reason for agglomeration is dipole interaction between the crystallites [29]. The grains have an irregular distribution with a hexagonal shape, which is the basic crystal cell of W-type ferrites. clearly illustrate a small variation between the experimental and theoretical values due to oxygen deficiency, which can be advantageous for heavy metal removal from water. Figure 5 illustrates the elemental mapping of BaCo 2 Fe 16 O 27 sample. The elements barium, cobalt, iron and oxygen are present in a homogenous distribution.
The magnetic properties of the investigated sample were studied using a vibrating sample magnetometer (VSM).  Table 2. The saturation magnetization (M s ) was determined also by the law of approach to saturation (LAS) [30,31]: where M s is the saturation magnetization of the domains per unit volume, A is a constant associated with microstress, B is a constant representing the magneto-crystalline  anisotropy contribution, and χH is the forced magnetization term. Figure 6 (b) illustrates the plotting of M versus 1/H 2 in the high field range given a straight line. The saturation magnetization M s of BaCo 2 Fe 16 O 27 nanoparticle can also be detected by extrapolating the plot of magnetization versus 1/H 2 to approach zero [32][33][34]. In this method, the M s value is 67.28 emu/g. The obtained value is very comparable to the experimental value, signifying that an applied field of ± 20 kOe is appropriate to saturate the investigated sample. The factors affected on the magnetization of W-type hexaferrite are the composition of ferrite, the distance of the Fe-O bond and the superexchange interaction through Fe 3+ -O-Fe 3+ [35]. Moreover, the coercivity of W-type hexaferrite is affected by many factors such as magneto-crystalline anisotropy, shape anisotropy, and saturation magnetization [36]. The magneto-crystalline anisotropy can be determined by the Stoner-Wohlfarth equation as follows [37]: where K is the magneto-crystalline anisotropy constant, M s is the saturation magnetization and H c is the coercive field. The saturation magnetization and the net magnetic moment of the sample depend on the presence of the magnetic ions such as Co 2+ (3.7 μB) and Fe 3+ (5 μB) [38].
The squareness ratio R of the remanence to the saturation magnetization (M r /M s ) indicates the domain structure of the investigated sample. The squareness ratio R value = 0.5 is indicative of a single domain and the lower value is associated with a multidomain structure and the particles interact by magneto-static interactions [39]. In the present work, the sample BaCo 2 Fe 16 O 27 has R > 0.5, indicating the exchange coupled interaction between the domains. Figure 7 illustrates the dependence of the molar magnetic susceptibility on absolute temperature as a function of the magnetic field intensities. As shown in the figure, three distinct regions were obtained. With increasing temperature, χ M increases steadily in the first region, rapidly increased in the second region, and decreases rapidly in the third region.
A deeper examination of the first region reveals the sample to be pure ferrimagnetic material in which the thermal energy was insufficient to disturb the aligned moments of the spins. In the second region, the thermal energy is enough to make the dipoles freely align in the direction of the applied field. In the last region, the sample transfers from ferrimagnetic to paramagnetic behavior after the Curie temperature where χ M decreases drastically. Figure 8 shows the relation between the reciprocal of magnetic susceptibility χ M −1 and the absolute temperature at different magnetic field intensities in the paramagnetic region. The magnetic parameters, such as the Curie constant (C) and the Curie-Weiss constant (θ), were calculated from the extrapolation of the linear part χ M −1 in the paramagnetic region. The Curie constant (C) equal to the reciprocal of the slope of a straight line in the paramagnetic region. The values of effective magnetic moments (μ eff ) were calculated from the following relationship where C is the Curie constant. These magnetic parameters were reported in Table 3. The data obey the well-known Curie-Weiss law [40].
where χ M is the molar magnetic susceptibility, T is the absolute temperature and θ is the Curie-Weiss constant. The magnetic properties of the sample BaCo 2 Fe 16 O 27 are originated from the Fe 3+ -O-Fe 3+ superexchange interactions, which are mutually antiparallel, and also the Fe 3+ -Fe 3+ direct exchange interactions. Table 4 illustrates the comparative study between the present results and those reported earlier. The author noticed that my sample is the largest in the remanence magnetization and the coercive field, compared with the results obtained by other authors [16,[41][42][43][44]. The saturation magnetization (M s ) of the present sample is smaller than the sample obtained by Mohammad K. Dmour [42]. The variation in the values of M s , M r and H c for the samples is due to the different preparation methods and ionic radii.
The investigated sample was prepared in nano scale (crystallite size = 37.39 nm) with a spongy morphology so the surface to volume ratio is large, which increases the number of active sites to trap Pb 2+ ions and increases the removal efficiency. One of the main advantages of using Baco 2 Fe 16 O 27 for Pb 2+ adsorption is the easy separation from the solution using an external magnetic field due to its large magnetization.
The heavy metal Pb 2+ ion removal from the wastewater was studied with different parameters such as pH value and contact time. Figure 9 illustrates the effect of pH solution on the heavy metal adsorption process. The adsorption of Pb 2+ ion increased by increasing the pH value. It is clear that at lower values of pH, the adsorption of heavy metal ions is low. This is due to competition between H + and Pb 2+ on the active sites of adsorbent [45]. At pH = 7, the amount of H + decreases in the solution and Pb 2+ can be easily adsorbed on the active sites. Otherwise, at pH = 8, the solution contains oH − . Consequently, the heavy metal Pb 2+ can be precipitated as lead hydroxide [46]. The precipitation of Pb 2+ at pH 8 is not only a result of the adsorption of Pb 2+ on BaCo 2 Fe 16 O 27 , but also the formation of lead hydroxide. So, the optimum pH value is 7.
The effect of contact time on the Pb 2+ ion efficiency illustrates in Fig. 10 over a range  min. The adsorption of heavy metal Pb 2+ increases by increasing the contact time. At the beginning of adsorption, a large number of active sites are available [47]. Finally, the optimum conditions for Pb 2+  The mechanism of Pb 2+ adsorption process can be studied using the adsorption isotherm. In the present work, two isotherm models that have been studied are the Langmuir and Freundlich models.
The multilayer adsorption on a heterogeneous surface can be described by Freundlich isotherm, which is commonly used to describe heavy metal adsorption on various adsorbents. The empirical model was proven to be consistent with a heterogeneous surface's exponential distribution of active centers. The relation between the amount of solute adsorbed (q e ) and the equilibrium concentration of solute in solution (C e ) is given by the following equation: This equation can be linearized to give the following expression: Where, K f is a constant for the Freundlich system, depends on the quantity of metal ion adsorbed onto adsorbent at an equilibrium concentration. Figure 11 illustrates the dependence of ln q e on ln C e . The values of K f and 1/n are calculated from the intercept and slope of the best fit line in Fig. 11. The Langmuir isotherm model assumes that maximal adsorption occurs in a saturated monolayer of adsorbate molecules on the adsorbent surface. Many ground water effluent treatment procedures employ the Langmuir adsorption isotherm, which has also been used to explain the adsorption of heavy metals by various adsorbents. The Langmuir isotherm is considered that once a metal ion occupies an active site, no further adsorption occurs, and the adsorbed layer is unimolecular. The following equation describes the Langmuir isotherm model: where q m is the adsorption capacity at maximum adsorption. Figure 12 shows the relation between (C e /q e ) and (C e ) for heavy metal Pb 2+ to determine the Langmuir constants.

Conclusion
The barium hexaferrites was synthesized using citrate nitrate combustion method. XRD revealed that the sample BaCo 2 Fe 16 O 27 was crystallized in a single phase hexagonal structure with space group P6 3 /mmc. The average crystallite size is 37.39 nm. The morphology and the grain size of the investigated sample were studied using SEM. The sample has a spongy morphology with the agglomerated grains. The average grain size of the sample is 73.7 nm. The magnetic properties of BaCo 2 Fe 16 O 27 were studied using two techniques: H-M hysteresis loop and the DC magnetic susceptibility. The sample has antiferromagnetic properties. The values of the saturation magnetization (M s ) and remanence magnetization (M r ) are 64.133 and 33.099 emu.g −1 respectively. The squareness ratio (M r /M s ) is greater than 0.5, indicating the exchange coupled interaction between the magnetic domains. The heavy metal Pb 2+ ion removal from the wastewater was studied with different parameters such as pH value and contact time. BaCo 2 Fe 16 O 27 has Pb 2+ removal efficiency 99% and 28% at pH 8 and 7 respectively. The Langmuir and Freundlich isotherms were used to analyze the experimental data. The Freundlich adsorption isotherm fitted the experimental data well.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Conflict of interest
The author declares that he has no conflict of interest.
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