1 Introduction

One of the ferromagnetic materials which have unique magnetic properties is BaFe12O19 belonging to M-hexaferrite. It has a crystal structure of P63/mmc space group, which consists of SRS*R* alternative layers of hexagonal (R) and spinel (S) layers. The S* and R* layers are the same as S and R but rotated at 180° along the c-axis. One unit cell of M-hexaferrite is containing 38 O2− ions, two Ba2+ ions as well as 24 Fe3+ ions as shown in Fig. 1 [1]. The large spontaneous polarization of hexaferrite is related to 5 polyhedral sites occupied by Fe3+. Three of them are octahedral sites having spin up related to 2a, 2b, and 12 k sites. The other two sites have spin down, one of them 4f1 tetrahedral and the other is 4f2 trigonal bipyramidal. Cobalt doped BHF enhances its magnetic properties as well as the adsorption of heavy metals [2].

Fig. 1
Fig. 1
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a, b Schematic hexagonal nanoparticles and crystal structure of the M-type BaFe12O19. The polyhedral with different color depict the different Fe3+ sites with their surrounding O2− ions. The yellow spheres depict the Ba2+ ions

The magneto plumbite unit cell (MFe12O19) of the hexaferrite system contains two formula units, consisting of 10 hexagonally close-packed oxygen layers stacked along the hexagonal basal plane (c-axis). The divalent ion, M2+, is substituted for an oxygen atom in every fifth layer.

Barium hexaferrite is one of the important classes of materials in the family of permanent magnets due to their potential applications in numerous electronic and microwave devices. It has the broadest commercial attention due to its high magneto crystalline anisotropy, high coercivity, remanent magnetization, and large spontaneous polarization [3].

Recently, numerous methods have been built up to remove heavy metals from industrial wastewater. However, most of these techniques have some demerits as high waste treatment equipment cost, large consumption of reagents, and incompetent recovery of treated metals for reprocessing [4]. In this regard, the new developments in utilizing the adsorption process with the aid of magnetic nano particles have solved some of the problems due to their multiple and unique characteristics: high surface area, magnetic response, easy preparation, fast adsorption kinetics, low price, and high efficiency [5].

In the present research, we aimed to prepare BaFe12O19 (BHF), and BaFe11.5Co0.5O19 (BCHF) nanoparticles using the citrate auto combustion technique. This method was successful to obtain single- phase ultrafine barium hexaferrite with nano size, and excellent homogeneity. On the other hand, the obtained magnetic properties, structural analyses and morphology are comparable with those previously reported.

2 Methodology

2.1 Material Used

All chemicals Ba (NO3)3·6H2O (99%), Fe (NO3)3·6H2O (98%), Co (NO3) 0.6H2O and citric acid [C4H8O7], and ammonia solution (33%) were obtained from (LOBA, India).

2.2 Experimental Technique

Nanoparticles of barium and cobalt doped Barium hexaferrite were synthesized by the citrate auto-combustion method as mentioned in the previous work [6, 7]. The proper amounts of the precursors corresponded to the stoichiometric ratios from nitrates were added. These precursors were mixed with citric acid by adding dropwise of distilled water under vigorous stirring. After that, the pH value was adjusted to 7, and the temperature was raised to 200 °C until all fumes were ended as illustrated in Fig. 2.

Fig. 2
Fig. 2
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Schematic of preparation of BHF, and BCHF nanoparticles

2.3 Characterizations and Measurements

The phase formation was determined by X-ray diffraction (analytical-x' pertpro, Cu Kα1radiation, λ = 1.5404 Å, 45 kV, 40 mA, Netherlands). Fourier transformed infrared (FT-IR) spectra were measured in the wavenumber range of 4000–400 cm−1 by FT-IR spectrometer (Perkin-Elmer 2000). HRTEM (HRTEM, JEOL/JME 2100) (TEM, JEOL, JSM 7001F) microscope was used for morphology study. X-ray photoelectron spectroscopy (XPS) was used to examine the chemical states of numerous elements in the prepared samples via monochromatic X-ray Al K-alpha radiation. The magnetic susceptibility of the sample was determined by the Faraday technique. A very small amount of powdered sample was placed into a cylindrical glass tube at the point of a maximum gradient in different magnetic field intensities of 1340, 1660, and 1990 Oe [8].

3 Results and Discussion

3.1 Structure Analyses

3.1.1 XRD Diffraction

XRD patterns of BHF and BCHF hexaferrite are illustrated in Fig. 3. All indexed peaks of the studied samples are similar to ICDD card number [00-043-0002] of BHF, indicating that the obtained samples are prepared successfully in a single-phase with the space group of P63/mmc. It is observed that Co2+ ions completely diffused into magneto plumbite structure for samples BHF.

Fig. 3
Fig. 3
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XRD Diffraction pattern of BaFe12O19 and BaCo0.5Fe11.5O19

The diffraction pattern for BCHF has eleven broad peaks at 30.4°, 32.3°, 34.2°, 37.2°, 40.4°, 42.6°, 55.2°, 56.6° and 63.2°, corresponding to (110), (107), (114), (203), (205), (206), (217), (2011) and (220) of pure BFO and BFCO phases. The crystallite sizes D for BHF and BCHF are calculated using the Scherrer equation [9] and tabulated in Table 1. The lattice parameters (a,c), and the volume (V) of the unit cell are calculated from the following equations [10] and presented in Table 1

Table 1 Lattice parameters (a and c), volume, the crystallite size (D), Dislocation density (δ), and theoretical density (dx) for BHF and BCHF
Full size table
$$\frac{1}{{d}^{2}}=\frac{4}{3}\left(\frac{{h}^{2}{+hk+k}^{2}}{{a}^{2}}\right)+\frac{{l}^{2}}{{c}^{2}}$$
(1)
$${\mathrm{V}}_{\mathrm{cell}}=\frac{\sqrt{3}}{2}{\mathrm{a}}^{2}\mathrm{c}$$
(2)

The changes in the lattice parameters can be attributed to the differences in ionic radii of Co2+ (0.745A°), Fe3+ (0.645 A°) as well as the changes of the c-axis anisotropy with respect to the c-plane [11]. In the present study, the c/a ratio equals to 3.938, and 3.937 for pure and doped samples respectively. Additionally, the formation of oxygen vacancies is expected. The charge imbalance between trivalent ions (Fe3+) is being replaced by divalent ions (Co2+) altering the values of the lattice parameter as well as the unit cell volume [12].

Additionally, the theoretical density (dx) is determined as mentioned earlier [13, 14] and the obtained data are tabulated in Table 1. As revealed in the Table, the presence of Co2+ increases the (dx). This increase is due to the higher molar mass of Co2+ (58.933 amu) compare to that of Fe (55.845 amu) [15].

The number of dislocations in a unit volume can be measured by the dislocation density δ. The samples with fewer δ exhibit high mechanical strength. In addition, as the δ increases, the strength of the samples decreases. The dislocation density δ is calculated from the following relation [16] and the results are illustrated in Table 1.

$$\delta = { 1}/D^{{2}}$$
(3)

The obtained low δ values are indications of a good crystallinity in BHF and BCHF nanoparticles.

3.1.2 FT-IR Spectra

Figure 4. reveals FT-IR spectra of BHF and BCHF hexaferrite samples in wave numbers range of 400–4000 cm−1. Two significant absorption bands of M-type hexaferrite, υ1 and υ2, are detected in the spectra. The intrinsic stretching vibrations of cation at site A–O bond are detected around 584 cm−1. While the υ2 is observed around 432 cm−1, which is corresponding to metal ions at site B–O bond [17, 18]. Because of the short bond length of the tetrahedral cluster consequently, tetrahedral vibration is greater than octahedral vibration [19, 20]. The presence of ν1 and ν2 bands ratify the successful preparation of hexaferrite [21, 22]. While weak bands at 1434 and, 3430 cm−1 are due to the carboxylate group (C–O) [23] and for stretching vibration of (O–H) respectively [24]

Fig. 4
Fig. 4
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FTIR of BaFe12O19 and BaCo0.5Fe11.5O19 hexaferrite samples

The range of absorption bands obtained is in agreement with that reported by Mahadevan et al. [19]. The values of transmittance bands v1 and v2 shift to a lower wave number due to numerous reasons. The first one is the average atomic weight of Co2+ ions which is smaller compared to Fe3+ ions [25]. Secondly, for all the samples, the transmission band at about 1635 cm−1 exhibits the stretch and bending of the surface hydroxyl group (–OH) of M-type hexaferrite [23].

3.1.3 EDAX and Mapping Analyses

EDAX analyses for BHF and BCHF are shown in Fig. 5a–b. The typical peaks in the spectrum include Ba, Fe, Co, and O. The atomic percentage (at%) and weight percentage (wt%) of component elements are obtained theoretically using the formulas BHF and BCHF. The data acquired from EDAX elemental analysis is depicted in the figure as an inset.

Fig. 5
Fig. 5
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EDAX for a BHF and b BCHF. The inset table provides quantitative estimates of elements determined directly from the spectrum via weight and atomic percentages

The distribution of the elements is approved by EDAX mapping images for synthetizing samples. The results indicate that Ba, Fe, Co, and O are uniformly distributed in the hexaferrite structure. Thus, the samples have highly homogeneous structures, as shown in Fig. 6.

Fig. 6
Fig. 6
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EDAX elemental mapping images of a BHF, and b BCHF

3.1.4 Microstructural Features

Figure 7 shows the HRTEM of hexaferrite samples. HRTEM images of the testified samples display hexagonal structures.

Fig. 7
Fig. 7
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a–g HRTEM micrographs of BaCo0.5Fe11.5O19 hexaferrite samples

The particles of various sizes and shapes are observed. The particles have mostly hexagonal with rectangular shapes which are compatible with the hexaferrite structure. The doped samples have spherical together with hexagonal shaped particles. Similar morphology was observed for Barium hexaferrite by Jun Young et al. [26]. The doping with a small amount of Co2+ has small effect on the morphology of particle size as detected from the figure. The magnetic nature of the cobalt causes small agglomerations of nano particles as shown in the images. The selected area electron diffraction (SAED) indicates the well-defined crystalline nature and single phase nano BHF and BCHF hexaferrites (Fig. 7c, g). The data are fitted to the Gaussian distribution and the average particle size of samples are in the range of 95 nm for BHF and BCHF respectively. The obtained data from HRTEM result is in agreement with XRD results.

3.1.5 Surface Topography

Figure 8a–b show the AFM height images of BHF and BCHF nanoparticles. The height of the images reveals that the physical dimensions and thickness of the nano samples can be easily obtained from the height profiles.

Fig. 8
Fig. 8
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a–b AFM images for grain boundaries with comparable grain sizes for two samples of a BaFe12O19 and b BaCo0.5Fe11.5O19 hexaferrites

The roughness on the surface is indicative of the high degree of crystallographic defects, which tend to possess higher free energy and a higher surface activity [27]. Figure 8 illustrates powder's grain boundaries with comparable grain sizes for BHF and BCHF. From the images, one can detect that the grain roughness for the BHF sample is smoother than the grain roughness for the BCHF sample. The maximum peaks height of the roughness is obtained as 4.64, 6.17 nm For BHF and BCHF respectively. The difference in roughness between substituted BHF and BCHF ferrites is due to the larger particle size of the former. The roughness profile section height difference (Rdc) values for BHF and BCHF are 4.38 nm and 4.92 nm, respectively. The skewness of the roughness profile (Rsk) of BHF and BCHF are − 2.26 and − 1.41 respectively.

Better values of magnetic properties as will be discussed later exhibit by samples with cobalt. This is related to a more regular grain shape and a narrow grain size distribution. One can observe from AFM images that BCHF shows a regular grain shape and a larger grain roughness compared with the parent sample.

3.1.6 XPS Analysis

The composition and elements state, as well as the electronic structure and states' density in the material, can all be studied using the XPS technique. In the present work, XPS is used to study the densities of states of each element in BHF. The complete range XPS survey spectrum in the range of energies 0 to 1200 eV for BCHF with Co = 0 and 0.5 is shown in Fig. 9a–b. The peaks of Fe 2p, Ba 3d, and O 1s are detected in both samples from the survey scan spectra Fig. 9a. The Co 2p peaks, on the other hand, are exclusively detected in BCHF (Fig. 9b). The contaminating carbon is responsible for the C 1s peak at 286.24 eV [28]. The binding energy of elements demonstrates that the BCHF is single-phase with no additional impurity phases. The peak with binding energy ≅ 531.08 eV corresponds to O confirm the presence of O2−.

Fig. 9
Fig. 9
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a–b XPS for BHF and BCHF samples

3.2 Magnetic Susceptibility

The thermo-magnetic plots (χ) of the BHF and BCHF samples at the different applied fields are shown in Fig. 10, together with their derivative with respect to temperature. It is clear that the χm decreases until reaching zero point (Tc) with increasing temperature due to thermal energy which causes disordering of the dipoles in field direction and the material begins to act as paramagnetic material. From the figure, the increase in the field intensity reduces the values of χm. This is due to the alignment of spin magnetic moments in field direction until all of them reach saturation (the magnetization nearly become constant) resulting in the decrease of χm. A prominent increase of nearly 2% in the Curie temperature (Tc) has been detected in BCHF samples. The derivative shows a single Tc at 698 °C, 710 for BHF and BCHF respectively. This indicates that the studied samples consist of a single Ba M phase. Higher Tc will enable this material to be used in high-temperature magnetic devices.

Fig. 10
Fig. 10
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Magnetic susceptibility as a function of temperature for BHF and BCHF at different field intensities

The obtained data obeys the well-known Curie–Weiss law. The values of the Curie constant (C), the Curie temperature (Tc) Curie–Weiss constant (θ), and the effective magnetic moment (μeff) are calculated from the reciprocal of χm with temperature (not illustrated here) as previously discussed by Ateia et al. [29, 30]. The calculated data are tabulated in Table 2.

Table 2 The Curie temperature (Tc), Curie constant (C), Curie–Weiss constant (θ) and effective magnetic moment (μeff)
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It is clear from Table 2 that the change in magnetic parameters is associated with partial substitution of Co2+ ions at the octahedral site of the BHF lattice. The divalent cobalt ion Co2+ substituted for the trivalent iron ion Fe3+ will change the valence state, and consequently, a reasonable valence changes from Fe3+ Fe2+ at 2a or 4f2 sites is detected. Additionally, the valence transformation will also affect the strength of double-exchange interactions between spin-up and spin-down sublattices interaction, and Tc should be changed accordingly. The positive θ values in the Table give a strong indication of ferromagnetic behavior for the studied samples.

On the other side, the effective magnetic moment μeff slightly increases with the addition of Co. This result may be due to the occupation of Co to the tetrahedral positions which reduces the negative contribution since in BHF; the tetrahedral sites oppose the spin of most of the octahedral sites resulting in producing the net moment [31].

3.3 Heavy Metal Removal (HMR)

3.3.1 Effect of Cr6+ Ion Concentration

Chromium is a highly toxic element that has a harmful effect on living organisms. Additionally, to the best of my knowledge, no such study has been reported.

The BHF and BCHF nano particles are examined at numerous experimental parameters as heavy metal concentration HMC, as well as pH to determine the optimum conditions for HMR.

The adsorption of Cr6+ is increased on sample surfaces by increasing the HMC till it achieved the maximum adsorption of 9%, 21% for BHF and BCHF respectively as exposed in Fig. 11a–b.

Fig. 11
Fig. 11
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a Effect of heavy metal concentration (Cr6+), and b Effect of pH on the removal efficiency of BHF and BCHF

There is no further increase in the adsorption of HM after the optimum concentration (40 ppm). This behavior can be clarified due to the available active sites on the sample surface which leads to adsorption of HM at a lower concentration. By increasing the HMC, these active sites are occupied by Cr6+ ions more and more till the optimum concentration is achieved. At this concentration, all active sites are fully occupied and consequently, increasing the HMC above its optimum value, causing no change in adsorption of Cr6+ ion.

3.3.2 Effect of pH on Cr6+ Removal

The effect of the pH solution on adsorption performance is illustrated in Fig. 11b. The BHF, as well as BCHF, have the same trend in which the increase of pH is accompanied by increasing the adsorption of heavy ions. The small value of the adsorption of Cr (VI) at low pH can be attributed to a large amount of H+ which competes with Cr6+ on the available active sites [32]. The increase of pH value to 7, causes a decrease in the number of H+. Consequently, more active sites become available for Cr (VI) adsorption [33]. However, the ability of HM ions to form precipitates has occurred at the basic medium (pH 8). Therefore, the removal of Cr (VI) ions is due to precipitation as well as adsorption. Finally, it can be detected that the adsorption efficiency of BCHF is higher compared to BHF. This result attributes to the fact that finer particles size of BCHF leads to higher removal efficiency of Cr (VI). It can be determined that the small crystallite size, the high surface area, as well as relatively high magnetic properties are the main causes for the observed behavior. Moreover, the rough BCHF media is able to collect a high concentration of the HMC on its surface rather than the smooth BHF.

The Langmuir and Freundlich isotherms models are used to clarify the adsorption process. Langmuir and Freundlich isotherms are expressed by Eqs. (4) and (5) respectively [34].

$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{k}_{l}{q}_{m}}+\frac{{C}_{e}}{{q}_{m}}$$
(4)
$$\mathrm{ln}\;{q}_{e}=\mathrm{ln}\;{k}_{f}+ \frac{1}{n}\mathrm{ln}\;{C}_{e}$$
(5)

where qe and qm (mg g−1) are the adsorption capacity at equilibrium and maximum adsorption respectively, and Kl (L mg−1) is the affinity binding constant. while Kf and n are physical constants signifying the adsorption capacity and intensity of adsorption, respectively.

The monolayer and heterogeneous surfaces are related to the Langmuir and Freundlich isotherms respectively [35, 36]. The investigated samples are fitting well with the Langmuir neither Freundlich isotherm models. Consequently, Cr6+ adsorption on the surface of prepared samples occurs through monolayer adsorption as detected in Fig. 12a–d.

Fig. 12
Fig. 12
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a–d The Langmuir and Freundlich isotherms for BaFe12O19 and BaFe11.5Co0.5O19 respectively

Kinetics of HM adsorption is principally studied through pseudo-first-order (P.F.O.) or pseudo-second-order (P.S.O) kinetics. P.F.O.M. kinetic explains the physisorption. Physisorption is weak and occurs using Van der Waals forces without any chemical bonding. As a result, this process is reversible. Though, P.S.O.M. kinetic is related to chemisorption adsorption which is happened by two reactions. The first reaction extends equilibrium quickly. While the second one leaks slowly and reaches equilibrium after a long time [37, 38]. In chemisorption, bond formation exists between adsorbents and adsorbates via electron sharing. Subsequently, it is tougher than physisorption. Another kinetic model is testified by Weber and Morris ID.M. They found that the intra particle diffusion model (IDM) is the single rate –determining the stage and the removal of adsorbate is done thought-out a rapid procedure.

The three adsorption kinetics models (P.F.O, P.S.O, & I.D.) can be clarified from the following equations

$${\text{P}}.{\text{F}}.{\text{O}}.{\text{M}}.\,\,\ln \left( {q_{e} - q_{t} } \right) = \ln q_{e} - \frac{{k_{1} }}{2.303}t$$
(6)
$${\text{P}}.{\text{S}}.{\text{O}}.{\text{M}}.\,\,\,\frac{{\text{t}}}{{q_{t} }} = \frac{1}{{k_{2} q_{e}^{2} }} + \frac{{\text{t}}}{{q_{e} }}$$
(7)
$${\text{I}}.{\text{D}}.{\text{M}}.\,\,\,q_{t} = k_{3} t^{1/2} + C$$
(8)

where k1, k2, and k3 are the P.F.O, P.S.O, and ID rate constants in (min−1) and (g mg−1 min−1), respectively.

The kinetics of adsorption is studied for a contact time ranging 0–60 min. The experimental data is fitted to the P.F.O and P.S.O kinetic model (Fig. 13a–d). The reported R2 value indicates that the experimental results show better fit to P.S.O kinetic model. Consequently, the adsorption of Cr (VI) by using BHF and BCHF is done through Langmuir isotherm and the adsorption mechanism kinetics is P.S.O.M

Fig. 13
Fig. 13
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a–b Pseudo-first-order model, c–d pseudo-second-order model and e–f intra particle models for the adsorption of Cr6+ at BHF and BCHFsurfaces

The use of only 0.02% of low-cost cobalt doped barium hexaferrite has been utilized for 35% removal of heavy metal Cr (IV) from wastewater. The specific advantages of the prepared samples are ease of separation, high adsorption per unit area, low cost as well as recycling with remarkable efficiency. This is the main novelty of the present work. Table 3 compares the heavy metals ion adsorption capacity between the present sample with other adsorbents from the literature.

Table 3 Comparison of maximal heavy metal ion adsorption capacity by various adsorbents
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The Table shows that the removal efficiency of the BHCF is much higher compared to the other adsorbents, like silica grafted, ZnOFe2O3, [39, 40] but lower than rick husk [41]. Overall, Cr (VI) has shown lower adsorption capacities when compared to other metals as Pb II [38], and Cd II [40] This may be due to the adsorption mechanism of Cr (VI) ions [35].

It is very essential to take into account the different experimental conditions such as initial concentration, pH, temperature and contact time to understand and compare the adsorption behavior of each metal.

4 Conclusion

The diffraction patterns showed characteristic diffraction peaks related to the M-type Ba-ferrite structure.

The Co2+ ions completely diffused into magneto plumbite structure for samples BaFe12O19 causes a decrease in the crystallite size of the samples.

HRTEM data reveal that the samples have mostly hexagonal and the particle sizes are in the nano range.

The particle surface roughness can affect the magnetization of ferromagnetic particles. It facilitates their magnetization and enhanced the transition temperature.

The investigated samples are fitting well with the Langmuir neither Freundlich isotherm models. Additionally, the furthermost of heavy metal ions adsorption on the surface of prepared samples has well-fitting with the pseudo-second-order kinetic model.