Ceramics International 51 (2025) 4593–4612

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Ceramics International
journal homepage: www.elsevier.com/locate/ceramint

Removal of radioactive Co(II) and Sr(II) using (Co0.5Zn0.5)Fe2O4

nanoparticles co-doped with barium and antimony
Sajida Rmeid a , Amani Aridi b,* , Khulud Habanjar a , Ehab M. Abdel Rahman c , Waleed F. Khalil c ,
Gehan M. El-Subruiti d , Ramadan Awad e,f
a
Department of Physics, Faculty of Science, Beirut Arab University, Beirut, Lebanon
b
Public Health Department, Faculty of Health Sciences, Modern University for Business and Science, Beirut, Lebanon
c
Nuclear and Radiological Safety Research Centre (NRSRC), Egyptian Atomic Energy Authority (EAEA), Cairo, Egypt
d
Department of Chemistry, Faculty of Science, Alexandria University, Alexandria, Egypt
e
Department of Physics, Faculty of Science, Alexandria University, Alexandria, Egypt
f
Department of Basic Sciences, Faculty of Computer Science and Artificial Intelligence, Pharos University in Alexandria, Alexandria, Egypt

A R T I C L E I N F O A B S T R A C T

Handling Editor: Dr P. Vincenzini Radioactive pollution poses significant environmental and health risks, including increased cancer rates, genetic
mutations, and ecosystem damage, making the removal of radioactive ions from water crucial. Spinel ferrite
Keywords: nanoparticles are promising adsorbents for this purpose owing to their magnetic and adsorption properties.
Ferrites Herein, the novel synthesis of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (0 ≤ x ≤ 0.1) is reported as an effective
Co-precipitation method
solution for adsorbing radioactive Co(II) and Sr(II) ions. Co-doping with Ba and Sb enhances the structural
Soft magnetic materials
stability, magnetic behavior, and adsorption efficiency of these nanoparticles. X-ray diffraction analysis
Adsorption of radioactive ions
confirmed sample purity with minimal hematite phase, while Fourier transform infrared spectroscopy verified
the spinel structure. Transmission electron microscopy analysis revealed spherical morphology and indicated
that increasing x from 0.00 to 0.10 reduced crystallite and grain sizes of nanoparticles from 11.28 to 6.49 nm and
24.47 to 10.88 nm, respectively. Energy-dispersive X-ray spectroscopy confirmed pure elemental compositions
and the successful substitution of host ions with dopant ions in the nanoparticles, while X-ray photoelectron
spectroscopy analysis provided insights into the chemical oxidation states. Vibrating sample magnetometer re­
sults indicated soft ferromagnetic behavior. In adsorption experiments, we examined contact time (0–180 min),
pH (2–10), adsorbent dosage (0.04–0.10 g), initial ion concentration (10–250 mg L− 1), and temperature
(293–313 K) to determine their effects on adsorption efficiency and optimize conditions for maximum
contaminant removal. Nanoparticles with x = 0.06 exhibited the highest ion-removal efficiencies, achieving
88.3 % for Co(II) and 96.3 % for Sr(II) after contact time of 180 min, with maximum efficiencies observed at pH 8
for Co(II) and pH 6 for Sr(II). Co(II) adsorption followed the Freundlich isotherm, while Sr(II) adsorption adhered
to the Temkin model. Kinetics for both ions conformed to the pseudo-second-order model, demonstrating the
potential of Ba and Sb co-doped ferrite nanoparticles for efficient radioactive-water purification.

1. Introduction addition, these isotopes are characterized by long half-lives, i.e., 5.3
years for cobalt and 30 years for strontium [3]. These radioactive species
Water is a vital natural resource for sustaining life. However, human are toxic owing to their emission of gamma rays and beta particles [3,4].
activities pose a direct threat to water systems. Radioactive pollution Moreover, their high solubility in water facilitates easy absorption and
associated with the increasing demand for nuclear technology presents accumulation in human bodies and aquatic organisms [5]. This accu­
serious environmental and health risks [1]. The discharge of untreated mulation can lead to adverse health effects such as cancer, mutations,
radioactive waste into water bodies is a major cause of water pollution. and reproductive issues [6,7]. Therefore, removal of radioactive ions
Co(II) and Sr(II) ions are two primary radioactive isotopes found in from water is of paramount importance.
wastewater from nuclear power plants and research reactors [2]. In Several treatment methods such as ion exchange, membrane

* Corresponding author. P.O. Box: 113-7501, Beirut, Lebanon.

E-mail address: aridiamani@gmail.com (A. Aridi).

https://doi.org/10.1016/j.ceramint.2024.11.433
Received 31 August 2024; Received in revised form 13 November 2024; Accepted 26 November 2024
Available online 28 November 2024
0272-8842/© 2024 Elsevier Ltd and Techna Group S.r.l. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
S. Rmeid et al. Ceramics International 51 (2025) 4593–4612

filtration, and precipitation can be used to remove radioactive ions from 2. Methods and materials
aqueous solutions [8]. However, the high maintenance and operational
costs, along with the generation of toxic byproducts, limit the practical 2.1. Synthesis
application of these methods for the removal of radioactive species from
real water samples [3]. Recently, the adsorption technique has been The co-precipitation method was employed to synthesize
considered as one of the most practical options owing to its high effi­ (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles where x = 0.00, 0.02, 0.04,
ciency, low cost, and suitability for large-scale applications [9]. 0.06, and 0.10. High-purity raw materials were used to prepare
Among various adsorbents used in the adsorption process, ferrite (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles. Precise quantities of cobalt
nanoparticles stand out as the most promising candidates owing to their (II) chloride hexahydrate (CoCl2.6H2O), zinc chloride (ZnCl2), barium
large surface area, high surface reactivity, and tunable surface chemistry (II) chloride dihydrate (BaCl2.2H2O), antimony(III) chloride (SbCl3),
[10–12]. Spinel ferrites are particularly important among various types and iron(III) chloride hexahydrate (FeCl3.6H2O) were weighed and
of ferrite nanoparticles owing to their magnetic properties and stability. dissolved in deionized water. The prepared solutions were mixed and
Specifically, CoFe2O4 and ZnFe2O4 nanoparticles exhibit improved stirred using a magnetic stirrer for 30 min. Subsequently, a solution of
properties that enable their application in wastewater treatment [10, citric acid monohydrate (C6H8O7.H2O), serving as a capping agent, was
13]. Notably, CoFe2O4 nanoparticles possess an inverse spinel structure added to the mixture. A 3-M sodium hydroxide (NaOH) solution was
and exhibit semi-hard magnetic characteristics [14]. In this structure, then added dropwise to adjust the pH to 12. The resulting solution was
Co2+ ions occupy the octahedral sites and Fe3+ ions are distributed be­ heated with continuous stirring at 353 K for 2 h. The co-precipitated
tween octahedral and tetrahedral sites [15]. Conversely, ZnFe2O4 powder was filtered and washed several times with deionized water
nanoparticles possess a normal structure and are classified as soft ma­ until the pH was neutralized to 7. The drying process was conducted at
terials [16]. This classification arises from the occupation of Zn2+ and 100 ◦ C for 24 h. Finally, the dried powders were calcined in air at 550 ◦ C
Fe3+ ions in the tetrahedral and octahedral sites, respectively. Both for 4 h.
CoFe2O4 and ZnFe2O4 nanoparticles have been employed as adsorbents
in previous studies to remove radioactive ions from water. For instance, 2.2. Characterization techniques
Huang et al. [17] evaluated the adsorption performance of clinoptilo­
lite/CoFe2O4 nanoparticles for the removal of radioactive Sr2+ ions from The crystal structure and purity of (Co0.5− xZn0.5− xBaxSbx)Fe2O4
water. Results showed that the adsorption process adhered to the nanoparticles were studied using a Bruker D8 X-ray diffractometer with
Langmuir isotherm model, achieving an adsorption capacity of 20.58 Cu-Kα radiation with a wavelength (λ) of 1.5406 Å and diffraction angle
mg g− 1. Furthermore, 98.3 % of U(VI) was removed using ZnFe2O4 (2θ) ranging from 20◦ to 80◦ . FTIR analysis was performed using a
doped with TiO2, demonstrating a high adsorption capacity of 716.2 mg Thermo Scientific Nicolet iS5 FTIR spectrometer. Furthermore, XPS
g− 1 [18]. spectra were obtained with a state-of-the-art Thermo Scientific K-Alpha
Several studies have examined the structural, magnetic, and elec­ X-ray photoelectron spectrometer. The spectra were recorded in a con­
trical properties of cobalt zinc ferrite nanoparticles prepared by stant pass energy mode at 200 eV, while high-resolution XPS spectra
combining different ratios of Co and Zn [19–22]. However, the use of were acquired at a pass energy of 50 eV. The morphology and elemental
equal ratios of these divalent metals to prepare Co0.5Zn0.5Fe2O4 has composition were assessed using SEM coupled with EDX spectroscopy
garnered considerable attention from scientists owing to its mixed via a JEOL JSM-IT200 instrument. To investigate the magnetic proper­
magnetic phase [23]. Co-doping enhances the properties of ferrite ties of the prepared samples, the VSM Lakeshore 7410 was used.
nanoparticles, resulting in superior performance compared to
single-doped or pure nanoparticles [24]. For instance, improved pho­ 2.3. Batch adsorption experiments
tocatalytic and antibacterial performance was observed in CoFe2O4
nanoparticles co-doped with Cu and Bi compared to Bi-doped and pure The adsorption performance of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nano­
CoFe2O4 nanoparticles [25]. Furthermore, substituting Co with Ba in particles (0.00 ≤ x ≤ 0.10) was assessed for the removal of Co(II) and Sr
CoFe2O4 nanoparticles led to improved charge-storage and -transport (II) ions. To evaluate the influence of contact time on removal efficiency,
properties [26]. 0.1 g of the synthesized adsorbents was mixed with 200 mL of 100 ppm
This study aims to evaluate the influence of co-doping with Ba and Sb Co(II) and Sr(II) solutions for a duration of 180 min. The effect of pH was
on the structural, optical, and adsorption properties of Co0.5Zn0.5Fe2O4 subsequently analyzed by adjusting the pH using HCl and NaOH.
nanoparticles. A review of the literature revealed no documented studies Various adsorbent dosages (0.04, 0.08, and 0.10 g) were tested with 200
on CoFe2O4, ZnFe2O4, or Co0.5Zn0.5Fe2O4 nanoparticles co-doped with mL of 100 ppm Co(II) and Sr(II) solutions. Additionally, the impact of
Ba and Sb. This lack of prior research highlights the novelty of the initial concentration was investigated using solutions of Co(II) and Sr(II)
synthesized nanoparticles as a new adsorbent for the removal of radio­ at concentrations of 10, 25, 50, 100, and 250 mg L− 1. The influence of
active Co(II) and Sr(II) ions. The (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nano­ temperature on the removal efficiency of Co(II) and Sr(II) ions was
particles, where x = 0.00, 0.02, 0.04, 0.06, and 0.10, were prepared evaluated through adsorption reactions conducted at 293, 303, and 313
using the co-precipitation method. Their structural and morphological K. For all experiments, the adsorption suspension was agitated at 200
properties were investigated using X-ray diffraction (XRD), Fourier rpm for 4 h and subsequently centrifuged at 5000 rpm for 30 min.
transform infrared (FTIR) spectroscopy, X-ray photoelectron spectros­ Samples were extracted from the reaction mixture at specified time in­
copy (XPS), transmission electron microscopy (TEM), scanning electron tervals and analyzed using atomic absorption spectroscopy (AA900H) to
microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy. quantify the remaining concentrations of Co(II) and Sr(II) in the solu­
Additionally, a vibrating sample magnetometer (VSM) was used to tion. To ensure accuracy and reliability, all adsorption experiments were
evaluate the magnetic properties of the synthesized samples. Finally, the conducted in triplicate. The amounts of Co(II) and Sr(II) adsorbed at
adsorption efficiency for the removal of radioactive Co(II) and Sr(II) was equilibrium and at time t, denoted as qe and qt, were determined by
assessed considering the effect of various parameters such as contact applying the following equations [27]:
time, pH, adsorbent dosage, initial ion concentration, and temperature.
(C0 − Ce ) × V
qe = (1)
m

(C0 − Ct ) × V
qt = (2)
m

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Fig. 2. Variation of a and D with x in (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nano­

particles here 0.00 ≤ x ≤ 0.10.

observed at 30.1◦ , 35.4◦ , 37.1◦ , 43.2◦ , 53.45◦ , 57.0◦ , 62.7◦ , 70.9◦ , 73.8◦ ,
and 79.0◦ correspond to the (220), (311), (222), (400), (422), (511),
(440), (620), (533), and (444) crystal planes, respectively. The XRD
patterns indicate the formation of a cubic spinel ferrite phase, as detailed
in JCPDS card No. 22–1086 [29]. Additionally, the presence of a sec­
ondary phase, mainly hematite (α-Fe2O3), is confirmed by the appear­
ance of a minor peak at 33.7◦ , as identified in JCPDS card No. 33–0664
[30]. The assessment of the refinement quality is based on the reliability
factors in Rietveld refinement [31]. Therefore, the refinement parame­
ters, mainly weighted profile factor (Rwp), expected profile factor (Rexp),
the goodness of fit (χ2), and phase percentages for
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 and α-Fe2O3, were determined via the
Material Analysis Using Diffraction software. The obtained values are
presented in Table 1. The χ2 values, ranging from 1.11 to 1.29, indicate a
Fig. 1. XRD pattern of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with x = good fit for the experimental data. As the value of x increases from 0.00
0.00, 0.02, 0.04, 0.06, and 0.10. to 0.10, the phase percentage of α-Fe2O3 increases from 0.46 % to 1.31
%. Although co-doping (Co0.5Zn0.5)Fe2O4 with Ba and Sb promotes the
formation of the hematite phase, only a small percentage of hematite is
Table 1 formed, confirming the purity of the prepared samples.
Refinement parameters of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles where x The following equation was applied to determine the lattice
= 0.00, 0.02, 0.04, 0.06, and 0.10 parameter a [32]:
x Rwp (%) Rexp (%) χ2 Phase percentage (%) √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
a = d h2 + k 2 + l2 (4)
(Co0.5-xZn0.5-xBaxSbx)Fe2O4 α-Fe2O3
0 9.79 8.91 1.11 99.52 0.46 where h, k, and l signify the Miller indices and d denotes the interplanar
0.02 9.49 8.26 1.17 99.5 0.49
spacing. As shown in Fig. 2, a increases from 8.399 to 8.415 Å as x in­
0.04 9.6 8.41 1.15 99.24 0.75
0.06 10.47 8.65 1.19 98.96 1.03 creases from 0 to 0.10. This increase is attributed to the difference in
0.1 12.12 8.52 1.29 98.68 1.31 ionic radii between the dopant and the host ions. Both dopant ions, Ba2+
(1.34 Å) and Sb3+ (0.76 Å) have larger ionic radii than the host ions,
Co2+ (0.72 Å) and Zn2+ (0.75 Å) [33–35]. Therefore co-doping
where the initial and equilibrium concentrations of Co(II) and Sr(II) are Co0.5Zn0.5Fe2O4 with Ba2+ and Sb3+ expands the lattice parameter.
denoted as C0 (mg.L− 1) and Ce (mg.L− 1), respectively. In addition, the Similar results were reported in a previous study, where the lattice
volume of the Co(II) and Sr(II) solutions is represented by V (L), and the parameter increased upon doping CoFe2O4 nanoparticles with Sb [36].
mass of the adsorbent is denoted as m (g). The removal efficiency (%) The average crystallite size (D) of the prepared samples was deter­
was calculated using the following equation [28]: mined using the following formula [37]:
C0 − Ct Kλ
Removal efficiency (%) = × 100 (3) D= (5)
C0 β cos θ

3. Results and discussion where λ represents the wavelength of the X-ray, K denotes a shape factor
set at 0.9, and β indicates the full width at half maximum. The D values
3.1. Structural and morphological properties shown in Fig. 2 indicate that as the concentrations of Ba2+ and Sb3+
increase from 0.00 to 0.10, the crystallite size decreases from 11.28 to
Fig. 1 presents the XRD patterns for (Co0.5− xZn0.5− xBaxSbx)Fe2O4 6.49 nm. The increased energy required for the formation of Ba2+–O2−
nanoparticles with 0.00 ≤ x ≤ 0.10. The prominent diffraction peaks and Sb3+–O2− bonds, along with the strain induced by the larger Ba2+

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Fig. 4. Full scan XPS spectra of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles

with x = 0.00, 0.02, and 0.10.

Fig. 3. FTIR spectra of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles.

designated as ν1 is attributed to the stretching vibrations of metal­
–oxygen bonds at tetrahedral sites. Conversely, the low-frequency en­
Table 2 ergy band located at 436–414 cm− 1 and designated as ν2 is associated
Absorption bands (ν1 and ν2), force constants (FT and FO), and Debye tempera­ with the stretching vibrations of metal–oxygen bonds at octahedral sites.
ture (θD) of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles. As the content of Ba and Sb dopants in the ferrite nanoparticles in­
Sample ν1 (cm− 1) ν2 (cm− 1) FT (N/m) FO (N/m) θD (k) creases, ν1 shifts to a lower wavenumber while ν2 shifts to a higher
wavenumber, as detailed in Table 2. This shift in the position of the
0.00 581.19 414.98 246.18 125.51 716.25
0.02 577.33 425.71 242.92 132.08 721.19 absorption bands is attributed to cation redistribution between tetra­
0.04 577.15 426.97 242.77 132.87 721.96 hedral and octahedral sites [41]. The introduction of Ba and Sb dopants
0.06 575.65 427.99 241.51 133.50 721.62 into the ferrite lattice leads to the substitution of Co and Zn. This sub­
0.1 569.29 435.86 236.21 138.46 722.70 stitution can alter the bond length owing to the differences in ionic radii
and masses of Ba and Sb compared to Co and Zn, ultimately influencing
and Sb3+ ions, impedes the crystal growth process. Consequently, the the bond strength.
nanoparticles formed during the co-doping process are smaller than the The force constant (F) can be calculated using the following equation
undoped (Co0.5Zn0.5)Fe2O4 nanoparticles. A similar phenomenon was [42]:
observed with an increase in Sb dopant concentration in Ni0.5Zn0.5Fe2O4 F = 4π2 c2 ν2 μ (6)
and CoFe2O4 [38,39].
FTIR analysis was conducted at room temperature (298 K) to where c, ν, and μ denote the speed of light in vacuum, the sublattice
examine the existing functional groups. The FTIR spectra of frequency, and the reduced mass of Fe3+ and O2− ions (μ = 2.061 ×
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles, recorded in the range of 10− 26 kg), respectively. In addition, the values of FT and FO, representing
3800–380 cm− 1, are presented in Fig. 3. The presence of water trapped the force constants for the tetrahedral and octahedral sites, are pre­
within the synthesized nanoparticles is confirmed by bands observed at sented in Table 2. The observed decrease in FT and increase in FO with
approximately 3429.3 and 1637.4 cm− 1. These bands are attributed to increasing Ba and Sb dopant contents can be attributed to the structural
the stretching vibrations of O–H and the bending vibrations of H–O–H, and electronic changes induced by the larger ionic radii and differing
respectively [40]. Furthermore, the band observed at 2365.1 cm− 1 is bonding characteristics of Ba and Sb in comparison to those of Co and
associated with the adsorption of CO2 on the surface of the nanoparticles Zn. These changes affect the bond lengths, bond strengths, and cation
[32]. distribution between the tetrahedral and octahedral sites within the
Spinel ferrites are characterized by the presence of two metal­ ferrite lattice [43].
–oxygen bands in FTIR spectra, which range from 350 to 600 cm− 1. The To study the various solid-state phenomena related to lattice vibra­
characteristic high-frequency energy band located at 581–569 cm− 1 and tions, a factor known as the Debye temperature (θD) was calculated

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Fig. 5. HR-XPS spectra of (a) Co-2p, (b) Zn-2p, (c) Fe-2p, and (d) O-1s of (Co0.5− xZn0.5− xBaxSbx) Fe2O4 nanoparticles with x = 0.00, 0.02, and 0.10.

using the following equation [44]: evident that the Debye temperature increases with increasing dopant
content. Consequently, the nanoparticles with x = 0.10, which achieve
հсνav
θD = (7) the highest θD value, demonstrate the greatest stiffness. By contrast, the
KB
pure nanoparticles, which have the lowest θD value, exhibit the most
where h, KB, and νav represent Planck’s constant (h = 6.626 × 10− 34 J s), brittle nature among the prepared samples.
Boltzmann’s constant (KB = 1.3806 × 10− 23 J K− 1), and the average XPS measurements were conducted to investigate the elemental
wavenumber, respectively. The estimated values of the Debye temper­ composition and chemical oxidation states of the samples. The XPS
ature for the synthesized nanoparticles are presented in Table 2. It is spectra of the (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with x = 0.00,

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Fig. 6. HR-XPS spectra of (a) Ba-3d and (b) Sb-3d of (Co0.5− xZn0.5− xBaxSbx) Fe2O4 nanoparticles with x = 0.00, 0.02, and 0.10.

Fig. 7. Particle-size distribution and TEM images of (Co0.5− xZn0.5− xBaxSbx) Fe2O4 nanoparticles where (a) x = 0.00, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, and (e) x
= 0.10.

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absence of any other elements confirms the purity of the samples. The
observed C-1s peak in the XPS spectra arises from the exposure of the
sample to air before analysis, as noted in a previous study [40].
To further elucidate the oxidation state and cationic distribution,
high-resolution XPS (HR-XPS) spectra were obtained for Co-2p, Zn-2p,
Ba-3d, Sb-3d, Fe-2p, and O-1s. As illustrated in Fig. 5(a), the HR-XPS Co-
2p spectra show primary regions corresponding to Co-2p3/2 and Co-2p1/
2 accompanied by three satellite peaks at 786.5, 790.5, and 802.4 eV. In
the Co-2p3/2 region, two peaks are observed, centered at 779.78 and
782.98 eV, corresponding to octahedral and tetrahedral coordination,
respectively [40]. Furthermore, the Co-2p1/2 region displays similar
results, with two main peaks corresponding to tetrahedral and octahe­
dral sites at 795.3 and 796.5 eV, respectively. The changes in peak area
indicate a redistribution of Co2+ ions between octahedral and tetrahe­
dral sites as a result of Ba2+ and Sb3+ substitutions [40].
The HR-XPS spectra of Zn-2p, shown in Fig. 5(b), reveal the presence
of two primary regions of Zn-2p3/2 and Zn-2p1/2 centered at 1021.1 and
1044.1 eV, respectively. This is attributed to the presence of Zn2+ at the
tetrahedral site of the ferrite crystal structure [42].
The HR-XPS spectra of Fe-2p, as illustrated in Fig. 5(c), reveal two
main components, Fe-2p3/2 and Fe-2p1/2, resulting from spin–orbit
Fig. 8. Variation of crystallite size (D) and grain size (DTEM) with Ba and Sb
splitting. Satellite peaks are observed at 716.3, 719.5, 730.9, and 733.8
content in (Co0.5− xZn0.5− xBaxSbx) Fe2O4 nanoparticles here 0.00 ≤ x ≤ 0.10. eV. The Fe-2p3/2 region exhibits two peaks with binding energies of
approximately 710.4 and 712.7 eV, corresponding to Fe3+ in octahedral
and tetrahedral sites, respectively. Similarly, the Fe-2p1/2 region shows
0.02, and 0.1 are shown in Fig. 4. These spectra indicate the presence of
peaks around 723.8 and 726.3 eV, indicating Fe3+ in octahedral and
Co, Zn, Fe, and O in all prepared samples, with additional peaks corre­
tetrahedral positions. This analysis confirms that all Fe ions exist in the
sponding to Ba and Sb in the samples where x = 0.02 and 0.1. The
Fe3+ state and are distributed between tetrahedral and octahedral sites

Fig. 9. SEM images and EDX spectra for (Co0.5− xZn0.5− xBaxSbx) Fe2O4 nanoparticles with x = 0.00, 0.02, and 0.10.

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Table 3 related to oxygen vacancies. Furthermore, the third peak, labeled as OH,
Experimental atomic percentage (%) of the elements composing is located around 532.1 eV and is associated with the presence of hy­
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with x = 0.00, 0.02, and 0.10 droxyl groups on the surface [45].
x 0.00 0.02 0.10 The HR-XPS spectra of the dopant ions are presented in Fig. 6. As
Co 6.06 ± 0.13 5.74 ± 0.12 4.96 ± 0.14
illustrated in Fig. 6(a), the HR-XPS spectra of Ba-3d reveal two primary
Zn 5.46 ± 0.15 4.52 ± 0.13 4.16 ± 0.17 peaks centered at binding energies of 779.9 and 795.1 eV in the Ba-3d3/2
Ba 0 0.28 ± 0.02 1.23 ± 0.05 and Ba-3d1/2 regions, respectively. These peaks confirm the +2 oxida­
Sb 0 0.27 ± 0.03 1.13 ± 0.05 tion state of the Ba2+ ions [46,47]. Similarly, the HR-XPS spectra of
Fe 25.16 ± 0.21 27.43 ± 0.20 23.72 ± 0.27
Sb-3d display two main regions, Sb3d5/2 and Sb3d3/2, located at binding
O 63.32 ± 0.48 61.76 ± 0.47 64.80 ± 0.56
energies of 530.5 and 539.5 eV, respectively, as shown in Fig. 6(b).
These peaks confirm the +3 oxidation state of the Sb3+ ions.
TEM analysis was conducted to assess the morphology and grain size
of the synthesized (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles. The
spherical morphology of the samples, characterized by agglomerated
nanoparticles, is clearly illustrated in Fig. 7. The average grain size
(DTEM), determined using ImageJ software, decreases from 24.47 nm to
10.88 nm as the concentrations of Ba and Sb increase from 0 to 0.1. This
trend of decreasing grain size, observed through TEM analysis, is
consistent with the reduction in crystallite size identified in the XRD
analysis, as presented in Fig. 8. However, the grain sizes estimated from
TEM analysis are larger than the crystallite sizes derived from XRD data,
primarily due to particle agglomeration and the presence of multiple
crystallites within each particle. The observed agglomeration is attrib­
uted to magnetic interactions among the nanoparticles [48]. Nonethe­
less, the degree of agglomeration and the discrepancy between the
crystallite and grain sizes diminishes upon the co-doping of nano­
particles with Ba and Sb. This phenomenon is attributed to the substi­
tution of Co2+ and Zn2+ ions by the non-magnetic dopants Ba2+ and
Sb3+ ions.
To further investigate the morphology and elemental composition of
the synthesized samples, SEM-EDX analysis was conducted on
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles, with x values of 0.00, 0.02,
and 0.10. As illustrated in Fig. 9, the synthesized particles display a
Fig. 10. Atomic percentages of the Co, Zn, Ba, and Sb elements in (Co0.5-xZn0.5- spherical morphology accompanied by a considerable degree of aggre­
xBaxSbx)Fe2O4 nanoparticles where x = 0.00, 0.02, and 0.10.
gation. This aggregation may be attributed to the magnetic interactions
occurring between the nanoparticles [48]. The average grain sizes of the
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles were determined to be
25.32 nm, 24.62 nm, and 22.88 nm for x values of 0.00, 0.02, and 0.10,
respectively. The decrease in grain size observed in the SEM analysis
aligns with the variations in crystalline and particle sizes obtained from
XRD and TEM measurements.
The elemental compositions of the synthesized
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (x = 0.00, 0.02, and 0.10)
were assessed using EDX analysis. The absence of any additional ele­
ments confirms the purity of the synthesized nanoparticles, as illustrated
in Fig. 9. The experimental atomic percentages of the elements, detailed
in Table 3, indicate the substitution of the host Co2+ and Zn2+ ions by
the dopants Ba2+ and Sb2+ ions. This is evident from the reduction in the
atomic percentages of Co and Zn, accompanied by an increase in the
atomic percentages of Ba and Sb as x increases from 0.00 to 0.10. Thus,
the EDX analysis validates the successful synthesis of (Co0.5Zn0.5)Fe2O4,
(Co0.48Zn0.48Ba0.02Sb0.02)Fe2O4, and (Co0.4Zn0.4Ba0.1Sb0.1)Fe2O4 nano­
particles. Furthermore, Fig. 10 corroborates the substitution of the host
Co2+ and Zn2+ ions by the dopants Ba2+ and Sb2+ ions, as indicated by
the decrease in the atomic percentages of Co and Zn, paired with an
increase in the atomic percentages of Ba and Sb as x increases from 0.00
to 0.10.
Fig. 11. M − H loop of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with an
inset (upper left) displaying the M − H loop in the low magnetic field region. 3.2. Magnetic properties

[40]. The magnetic properties of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nano­

Three peaks are identified from the deconvolution of HR-XPS spectra particles were examined at room temperature using a VSM. As illus­
for O-1s, as illustrated in Fig. 5(d). The first peak, labeled as OL, is sit­ trated in Fig. 11, the M − H hysteresis loops indicate the weak
uated around 529.7 eV and corresponds to the lattice metal–oxygen ferromagnetic behavior of the synthesized samples. Additionally, a
bonds. The second peak, observed at 530.9 eV, is attributed to defects zoomed-in view of the low applied magnetic field in the inset of Fig. 11
reveals narrow hysteresis loops, suggesting the soft magnetic

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Table 4 octahedral and tetrahedral sites, respectively, the ionic magnetic

Magnetic parameters of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles. moment of Co2+ is 3 μB, while Zn2+, Ba2+, and Sb3+ are classified as non-
x Ms (emu/g) Mr (emu/g) SQR Hc (G) Keff (erg/g) ηB (μB) magnetic ions [39,50]. Initially, as the content of Ba and Sb increases to
0.02, the Ms increases. This rise may be attributed to the substitution of
0.00 47.68 1.51 0.032 26.35 1281.76 2.03
0.02 51.14 1.64 0.032 20.52 1070.47 2.20 Co2+ and Zn2+ ions at tetrahedral sites by the non-magnetic dopants
0.04 47.46 1.11 0.023 16.92 819.61 2.07 Ba2+ and Sb3+ ions. This substitution could facilitate the migration of
∑
0.06 47.45 1.15 0.024 18.20 881.48 2.09 Co2+ from tetrahedral to octahedral sites, resulting in an increase in
0.10 39.29 0.62 0.016 11.86 475.57 1.77 MO. Consequently, this leads to an increase in the net magnetization due
to a higher concentration of Co2+ in the octahedral sites. However, when
characteristics of the prepared samples. the substitution of Ba2+ and Sb3+ is further increased, particularly for x
The extracted magnetic parameters, including saturation magneti­ ≥ 0.02, Ba2+ and Sb3+ occupy both the tetrahedral and octahedral sites.
zation (Ms), remanent magnetization (Mr), coercivity (Hc), squareness As a result, there is reduced migration of Co2+ from the tetrahedral to
ratio (SQR), effective anisotropy constant (Keff), and magnetic moment the octahedral site. Consequently, the magnetic moment at the octahe­
(μ), are presented in Table 4. As the value of x increases from 0.00 to dral site decreases, leading to a reduction in Ms. Similar findings were
0.02, Ms rises from 47.68 to 51.14 emu/g. However, with a further in­ reported in a previous study on the doping of CoFe2O4 with
crease in Ba and Sb content to 0.20, Ms decreases to 39.29 emu/g. In non-magnetic Bi2+ ions [51].
spinel ferrites, the total magnetization of the nanoparticles is deter­ The impact of Ba and Sb co-doping on the magnetic properties of
mined by the difference in net magnetization between the octahedral (Co0.5Zn0.5)Fe2O4 is evidenced by the reduction in coercivity values. As
and tetrahedral sites, as expressed by the following equation [49]: x increases from 0.00 to 0.10, Hc decreases from 26.35 to 11.86 G. This
∑ ∑ reduction in crystallite size of the synthesized nanoparticles, attributed
MTotal = MO − MT , (8) to the increasing Ba2+ and Sb3+ content, contributes to the decline in Ms
and Hc values [32]. The squareness ratio (SQR = Mr/Ms), presented in
∑ ∑
knowing that MO and MT represent the net magnetizations of Table 4, exhibits a decrease from 0.032 to 0.016 with the increasing

Fig. 12. Influence of contact time on the removal efficiency of (a) Co(II) ions and (b) Sr(II) ions evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nano­
particles (0 ≤ x ≤ 0.1).

Fig. 13. Influence of pH on the removal efficiency of (a) Co(II) ions and (b) Sr(II) ions evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (0 ≤ x
≤ 0.1).

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Fig. 14. Effect of the adsorbent dosage on the removal efficiency of (a) Co(II) ions and (b) Sr(II) ions evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4
nanoparticles (0 ≤ x ≤ 0.1).

Fig. 15. Effect of the initial concentration of (a) Co(II) ions and (b) Sr(II) ions on the removal efficiency evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4
nanoparticles (0 ≤ x ≤ 0.1).

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Fig. 16. Effect of the initial concentration of (a) Co(II) ions and (b) Sr(II) ions on the adsorption capacity evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4
nanoparticles (0 ≤ x ≤ 0.1).

dopant concentration from 0.00 to 0.10, mirroring the trend observed in synthesized (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (0 ≤ x ≤ 0.1)
Hc. It is worth noting that all SQR values are below 0.5, indicating the were utilized as adsorbents for the removal of Co(II) and Sr(II) ions. To
formation of a single magnetic domain [52]. assess the impact of contact time on removal efficiency, 0.1 g of the
The magnetic moment (ηB) was calculated using the following for­ prepared adsorbents was mixed with 200 mL of 100 ppm Co(II) and Sr
mula [42]: (II) solutions. Fig. 12(a and b) illustrate that the removal efficiency of Co
(II) ions increases sharply with contact time, reaching a maximum of 90
Mw Ms
ηB = (9) min, whereas for Sr(II) ions, the increase continues up to 60 min.
5585
However, a further extension of contact time only results in a marginal
where Mw is the molecular weight. As shown in Table 4, ηB increases improvement in removal efficiency. Consequently, adsorption equilib­
from 2.03 to 2.20 μB as x increases from 0.00 to 0.02. However, with a rium is attained after 90 min for Co(II) ions and 60 min for Sr(II) ions,
further increase in x to 0.10, ηB decreases to 1.77. This variation is ex­ indicating that Sr(II) ions achieve optimal removal efficiency in a shorter
pected since ηB is directly proportional to Ms. contact time compared to Co(II) ions. This difference may be attributed
The values of the effective anisotropy constant (Keff) are estimated by to the distinct mechanisms involved in the adsorption of Co(II) and Sr(II)
applying the following equation [42]: ions by the prepared samples. These mechanisms will be elaborated
upon in the subsequent sections.
Keff Among the prepared adsorbents, the (Co0.5− xZn0.5− xBaxSbx)Fe2O4
Hc = 0.98 (10)
Ms nanoparticles with x = 0.06 exhibit the highest removal efficiency.
The effective anisotropy constant of the prepared nanoparticles ex­ Specifically, 88.3 % of Co(II) ions and 96.3 % of Sr(II) ions are adsorbed
hibits a significant decrease with increasing dopant concentration. This after a contact time of 180 min. By contrast, the undoped (Co0.5Zn0.5)
constant is directly proportional to coercivity; thus, the reduction in the Fe2O4 nanoparticles display the lowest removal efficiency, with 86.9 %
effective anisotropy can primarily be attributed to the decrease in of Co(II) ions and 94 % of Sr(II) ions adsorbed after the same contact
coercivity. Co-doping (Co0.5Zn0.5)Fe2O4 nanoparticles with Ba and Sb time. Consequently, the incorporation of Ba and Sb dopants into
results in a reduction of crystallite size and modifies the distribution of (Co0.5Zn0.5)Fe2O4 nanoparticles results in a slight enhancement of
cations in both octahedral and tetrahedral sites, thereby influencing the adsorption performance. This improvement is particularly notable for Sr
magnetic properties of the synthesized samples. (II) ions, indicating a greater affinity of the adsorbent for Sr(II) ions
compared to Co(II) ions.

3.3. Adsorption performance 3.3.2. Influence of pH

The pH of the solution affects the adsorption process by altering the
3.3.1. Effect of contact time ionization state of the adsorbent surface and the speciation of the ions
After evaluating the structural and optical properties, the

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Fig. 17. Non-linear adsorption isotherm for Co(II) adsorption onto (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles where (a) x = 0.00, (b) x = 0.02, (c) x = 0.04, (d) x =
0.06, and (e) x = 0.10.

being removed. To study the influence of pH on the adsorption of Co(II) 3.3.3. Effect of adsorbent dose
and Sr(II) ions, experiments were performed in different media with pH To study the influence of adsorbent dosage on the removal efficiency
levels ranging from 2 to 10. Fig. 13(a) shows that the maximum removal of Co(II) and Sr(II) ions, adsorption experiments were conducted using
efficiency for Co(II) ions onto the prepared adsorbents was achieved at 0.04, 0.08, and 0.10 g of each prepared adsorbent. As shown in Fig. 14(a
pH 8. Similarly, the maximum removal efficiency for Sr(II) ions was and b), increasing the adsorbent dosage from 0.04 to 0.10 g boosted the
realized at pH 6, as shown in Fig. 13(b). The lowest removal efficiencies removal efficiency for Co(II) and Sr(II) ions. As the mass of the adsorbent
for Co(II) and Sr(II) ions were observed under acidic conditions. This increases, the number of active sites available for adsorption also in­
phenomenon can be attributed to the presence of hydrogen ions in acidic creases. Therefore, the greater availability of active sites facilitates
media, which compete with Co(II) and Sr(II) ions for adsorption sites on increased ion–adsorbent interactions, thereby improving the overall
the prepared adsorbents. However, a slight decrease in the removal ef­ removal efficiency. These findings align with the results of previous
ficiency is noted at high pH values, which may be ascribed to the pre­ studies indicating that an increase in the α-Fe2O3 adsorbent dosage en­
cipitation of Co(II) and Sr(II) ions as hydroxides. Comparable results hances the removal efficiency of Co(II) ions [54]. Furthermore, the
were reported by Abbar et al. [53], who assessed the removal of Zn(II), highest removal efficiency was exhibited by (Co0.5− xZn0.5− xBaxSbx)
Pb(II), and Cu(II) from aqueous solutions. Fe2O4 nanoparticles with x = 0.06. At an adsorbent mass of 0.1 g, 88.3 %

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Fig. 18. Non-linear adsorption isotherm for Sr(II) adsorption onto (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles where (a) x = 0.00, (b) x = 0.02, (c) x = 0.04, (d) x =
0.06, and (e) x = 0.10.

of Co(II) ions and 96.3 % of Sr(II) ions were adsorbed after a contact time concentrations, specifically at 10 and 25 mg L− 1. By contrast, at higher
of 180 min. initial concentrations, such as 250 mg L− 1, approximately 56 % of Co(II)
and 54 % of Sr(II) ions are adsorbed. This trend can be attributed to the
3.3.4. Influence of initial Co(II) and Sr(II) concentrations greater availability of active sites on the adsorbent at lower concentra­
The initial concentration considerably affects the adsorption process, tions, which enhances the adsorption of Co(II) and Sr(II) ions. As the
prompting experiments that were conducted by varying the initial initial concentration increases, these active sites become occupied,
concentrations of Co(II) and Sr(II) between 10 and 250 mg L− 1. The resulting in a decrease in the removal efficiency.
relationship between removal efficiency and initial concentration is An opposite trend was observed regarding the variation of adsorp­
shown in Fig. 15, while the corresponding variation in adsorption ca­ tion capacity with initial concentration. As shown in Fig. 16, the
pacity is presented in Fig. 16. Furthermore, co-doping (Co0.5Zn0.5)Fe2O4 adsorption capacity increases with increasing initial concentrations of
with barium and antimony has a slight impact on the removal efficiency Co(II) and Sr(II). For example, when the initial Co(II) concentration is
and adsorption capacity. As shown in Fig. 10, increasing the initial increased from 10 to 250 mg L− 1, the adsorption capacity of (Co0.5Zn0.5)
concentrations of Co(II) and Sr(II) reduces the removal efficiency. Fe2O4 nanoparticles increases from 50 to 716 mg g− 1. Similarly, the
Complete removal of Co(II) and Sr(II) is achieved at lower initial adsorption capacity of (Co0.5Zn0.5)Fe2O4 nanoparticles increases from

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Table 5 liquid phase, thereby enhancing the overall adsorption capacity.

Parameters extracted from fitting non-linear adsorption isotherm models.
x 0.00 0.02 0.04 0.06 0.10 3.3.5. Adsorption isotherms
Non-linear adsorption isotherms play an important role in studying
QL bL Ce
Non-linear Langmuir isotherm: qe =
1 + b L Ce the type of adsorption processes of Co(II) and Sr(II) ions onto the syn­
Co(II) QL (mg.g )
¡1
379.37 379.22 310.62 378.46 306.97 thesized adsorbents. Consequently, the data obtained from assessing the
Removal effect of initial Co(II) and Sr(II) concentrations were used to apply the
bL 5.47 − 2.88 2.63 2.06 6.73
​
non-linear forms of adsorption isotherms, specifically the Langmuir,
​ R2 0.293 0.296 0.273 0.299 0.333
Sr(II) QL (mg.g¡1) 464.35 377.06 311.94 314.03 312.25
Freundlich, and Temkin models. The plots illustrating the fitting of these
Removal isotherms for the removal of Co(II) and Sr(II) ions are presented in
​ bL − 1.19 2.62 1.39 − 4.03 − 3.29 Figs. 17 and 18, respectively. Furthermore, the extracted parameters are
​ R2 0.541 0.316 0.272 0.333 0.281 listed in Table 5 and the equations of the isotherms are expressed as
Non-linear Freundlich isotherm: qe = KF Ce 1/n follows:
Co(II) KF ((mg.g¡1) 236.59 239.25 214.79 240.17 241.39
Removal (mg.L¡1)1/n) QL bL Ce
n 4.25 4.31 3.90 4.35 4.55 Non − linear Langmuir isotherm : qe = (11)
​
1 + bL Ce
​ R2 0.988 0.987 0.987 0.987 0.987
Sr(II) KF ((mg.g )
¡1
276.19 254.43 258.66 273.39 258.21
Removal (mg.L¡1)1/n) Non − linear Freundlich isotherm : qe = KF Ce 1/n (12)
​ n 4.73 4.77 4.82 5.10 4.88
​ R2 0.931 0.935 0.919 0.889 0.920 RT
RT Non − linear Temkin isotherm : qe = ln(KT Ce ) (13)
Non-linear Temkin isotherm: qe = ln(KT Ce ) bT
bT
Co(II) KT (L.mg¡1) 99.19 100.89 11.57 104.37 116.34 The Langmuir isotherm model is an invaluable tool for character­
Removal
izing the properties of a homogeneous surface and the monolayer in­
​ bT (J.mol )
¡1
36.57 36.66 26.30 36.97 38.48
​ R2 0.908 0.911 0.975 0.914 0.922
teractions between adsorbent and adsorbate molecules [55]. It assumes
Sr(II) KT (L.mg¡1) 12.04 12.22 14.48 18.28 14.90 that the interactions between the adsorbed molecules are negligible. The
Removal parameters derived from the non-linear Langmuir isotherm correspond
​ bT (J.mol¡1) 24.37 26.05 26.38 26.91 26.73 to the Langmuir constant and the maximum monolayer adsorption ca­
R2 0.937 0.981 0.969 0.946 0.971
pacity, denoted as bL and QL, respectively.
​

However, the Freundlich isotherm model is not limited to monolayer

50 to 775 mg g− 1 as the initial Sr(II) concentration is increased from 10 adsorption; it applies to multilayer adsorption and characterizes the
to 250 mg L− 1. Notably, at high Co(II) and Sr(II) concentrations, a strong distribution of active sites, thereby revealing surface heterogeneity [27].
driving force for mass transfer is present. This highlights the effective­ The Freundlich constant (KF) signifies the adsorption capacity and in­
ness of the nanoparticles in attracting Co(II) and Sr(II) ions from the dicates the strength of the adsorption bond. Moreover, the parameter n
represents the heterogeneity factor, reflecting the distribution of bonds,

Fig. 19. Effect of temperature on the removal efficiency of (a) Co(II) ions and (b) Sr(II) ions evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles
(0 ≤ x ≤ 0.1).

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The coefficient of determination (R2), which determines how well

the data fit the applied models, is listed in Table 5. Among the applied
models, extremely low R2 values are obtained from fitting the non-linear
Langmuir isotherm, which is accompanied by some negative bL values.
Therefore, the non-linear Langmuir isotherm model fails to accurately
describe the adsorption process of Co(II) and Sr(II) ions onto the pre­
pared adsorbents.
For the adsorption of Co(II) ions, the highest R2 values are observed
in the fitting of the non-linear Freundlich isotherm model. This indicates
that the adsorption process of Co(II) adheres to the non-linear Freund­
lich isotherm, suggesting the formation of multilayers. Furthermore, the
n values, which range from 3.90 to 4.55, indicate that Co(II) ions are
favorably adsorbed onto (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles. It
is noteworthy that the values of KF and n exhibit slight variations upon
co-doping the ferrite nanoparticles with barium and antimony.
The highest R2 values were achieved by fitting the non-linear Temkin
isotherm model to the removal of Sr(II) ions. Consequently, the Temkin
model provides the most comprehensive explanation for the adsorption
mechanism of Sr(II) ions onto the synthesized nanoparticles. Addition­
ally, (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with x = 0.06 demon­
strated the highest values of KT and bT, which are 18.28 L mg− 1 and
26.91 J mol− 1, respectively. Furthermore, the co-doping of the ferrite
nanoparticles with barium and antimony enhances the interaction be­
tween Sr(II) ions and the prepared adsorbents.

3.3.6. Effect of temperature

Temperature plays a crucial role in the adsorption process. To assess
the impact of temperature on the removal efficiency of Co(II) and Sr(II)
ions, adsorption experiments were conducted at varying temperatures,
specifically 293, 303, and 313 K. The results, as shown in Fig. 19,
demonstrate that elevated reaction temperatures enhance the removal
efficiency of Co(II) and Sr(II) ions. Therefore, the adsorption of Co(II)
and Sr(II) ions is more favorable at higher temperatures. Similarly, Yu
et al. [7] reported that increasing the temperature improves the
Fig. 20. Removal efficiency of (a) Co(II) ions and (b) Sr(II) ions evaluated at adsorption of Co(II) and Sr(II) onto covalent triazine frameworks.
different temperatures after a contact time of 180 min in the presence of To identify the most efficient adsorbent, the removal efficiency was
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (0 ≤ x ≤ 0.1). evaluated at different temperatures after a contact time of 180 min in the
presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (0 ≤ x ≤ 0.1), as
illustrated in Fig. 20. Among the prepared samples, enhanced removal
Table 6 efficiency for Co(II) and Sr(II) ions was observed with
Thermodynamic parameters for the adsorption reaction of Co(II) and Sr(II) (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles at x = 0.06. Specifically, 94
evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (0 ≤ x % of Co(II) ions and 99 % of Sr(II) ions were adsorbed at 313 K after 180
≤ 0.1). min. The higher removal efficiency for Sr(II) ions compared to Co(II)
x 0.00 0.02 0.04 0.06 0.10 ions suggests a greater affinity of the adsorbent for Sr(II) ions. This
Co(II) 0
ΔH (kJ. 14.69 29.58 25.83 28.01 20.51
finding is consistent with previously reported results regarding the in­
Removal mol¡1) fluence of contact time.
​ ΔS0 (J. 65.87 116.6 104.42 112.88 86.52
mol¡1.K¡1) 3.3.7. Thermodynamic parameters
ΔG0 (kJ. − 5.27 − 5.75 − 5.81 − 6.20 − 5.71
Performing adsorption experiments at different temperatures allows
​
mol¡1)
Sr(II) ΔH0 (kJ. 43.53 48.47 63.99 51.07 42.06 the determination of thermodynamic parameters, including the change
Removal mol¡1) in enthalpy (ΔH0), the change in entropy (ΔS0), and the change in Gibbs
​ ΔS0 (J. 171.63 188.77 242.42 201.87 166.63 free energy (ΔG0). The following equations are used to calculate these
mol¡1.K¡1) thermodynamic parameters, as shown in Table 6 [57]:
​ ΔG0 (kJ. − 8.47 − 8.73 − 9.46 − 10.09 − 8.42
mol¡1) Cac
Kd = , (14)
Ce
with values ranging from 1 to 10 indicating favorable adsorption.
ΔH0 ΔS0
The Temkin isotherm model proposes that the heat of adsorption for ln(Kd ) = − + , (15)
RT R
each layer decreases linearly with increasing coverage, influenced by
the interactions between the adsorbent and adsorbate [56]. It suggests a ΔG0 = ΔH0 − TΔS0 (16)
uniform distribution of binding energies, constrained by a maximum
binding energy. In the non-linear Temkin model equation, T denotes the where Kd signifies the equilibrium constant, Cac represents the adsorbed
temperature and R represents the universal gas constant (R = 8.314 J concentration of Co(II) or Sr(II) at equilibrium, and Ce is the concen­
mol− 1 K− 1). The parameters derived from this model, referred to as bT tration of Co(II) or Sr(II) remaining in the solution. ΔH0 and ΔS0 can be
and KT, correspond to the coefficient associated with the sorption heat determined from the slope and intercept of the plot of ln(Kd) versus 1/T,
and the equilibrium binding constant, respectively.

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Fig. 21. Plot of ln(Kd) versus 1/T for the adsorption of (a) Co(II) ions and (b) Sr(II) ions evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles (0 ≤
x ≤ 0.1).

Fig. 22. Plot of the pseudo-first-order kinetic model for the adsorption of (a) Co(II) ions and (b) Sr(II) ions evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)Fe2O4
nanoparticles (0 ≤ x ≤ 0.1).

Fig. 23. Plot of the pseudo-second-order kinetic model for the adsorption of (a) Co(II) ions and (b) Sr(II) ions evaluated in the presence of (Co0.5− xZn0.5− xBaxSbx)
Fe2O4 nanoparticles (0 ≤ x ≤ 0.1).

as shown in Fig. 21. The positive value of ΔH0 indicates the endothermic enthalpy value for the adsorption of Sr(II) exceeds 40 kJ mol− 1, sug­
nature of the adsorption process, which is consistent with the increase in gesting that the adsorption process involves chemisorption [58]. This
the removal efficiency with increasing the reaction temperature. The implies a chemical interaction between the prepared adsorbents and Sr
enthalpy value for the adsorption of Co(II) is lower than 40 kJ mol− 1, (II) ions. Specifically, Co(II) ions are adsorbed through physisorption,
indicating the dominance of physisorption [57]. By contrast, the which involves weak van der Waals forces, leading to multilayer

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Table 7 3.3.8. Adsorption kinetics

Parameters obtained from the fitting of pseudo-first-order and pseudo-second- Pseudo-first-order and pseudo-second-order models were utilized to
order kinetic models. identify the kinetic model that best fits the experimental data. The
x 0.00 0.02 0.04 0.06 0.10 corresponding plots for these models are presented in Figs. 22 and 23.
Pseudo-first-order
The equations that characterize these models are presented as follows:

Co(II) k1 (min¡1) 0.032 0.033 0.027 0.029 0.042 ln(qe − qt ) = − k1 t + ln(qe1 ), (17)
Removal
​ qe1 (mg.g¡1) 313.24 357.59 231.59 251.32 650.26 t 1 t
R2 0.963 0.988 0.979 0.986 0.981 = + , (18)
​
qt k2 q2e2 qe2
Sr(II) k1 (min¡1) 0.024 0.022 0.021 0.025 0.025
Removal
​ qe1 (mg.g¡1) 203.64 210.71 213.29 202.46 188.74 where k1 and k2 denote the pseudo-first-order and pseudo-second-order
​ R2 0.959 0.906 0.651 0.738 0.849 rate constants, respectively. Furthermore, qe1 and qe2 indicate the
​ ​ Pseudo-second-order amount of Co(II) and Sr(II) ions adsorbed at equilibrium, as determined
Co(II) k2 £ 10¡4 (g 1.88 1.55 2 2.02 1.21 from the plots of the pseudo-first-order and pseudo-second-order kinetic
Removal mg¡1.min¡1)
​ qe2 (mg.g¡1) 462.96 473.93 462.96 469.48 483.09
models, respectively. The extracted parameters are listed in Table 7. The
​ R2 0.999 0.999 0.999 0.999 0.998 linear fit of the pseudo-second-order model yielded higher R2 values (R2
Sr(II) k2 £ 10¡4 (g 1.68 1.75 1.69 2.68 2.52 ≥ 0.992) compared to the pseudo-first-order model, indicating that the
Removal mg¡1.min¡1) adsorption kinetics of Co(II) and Sr(II) ions are more accurately repre­
qe2 (mg.g¡1) 502.51 497.51 495.05 500.01 490.19
​
sented by the pseudo-second-order model. Similar outcomes were
​ R2 0.999 0.999 0.992 0.998 0.999
documented in a prior study, where the removal of Sr(II) using MgFe2O4
nanoparticles followed the pseudo-second-order model, with an
adsorption capacity of 109.28 mg g− 1 [59]. However, the present study
indicates a greater adsorption capacity, ranging from 490.19 to 502.51
mg g− 1, for the removal of Sr(II) ions. Among the synthesized adsor­
bents, the highest rate constants for the adsorption of Co(II) and Sr(II)
ions are identified for (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with x
= 0.06. The rate constants are 2.02 × 10− 4 g mg− 1.min− 1 for Co(II) and
2.68 × 10− 4 g mg− 1.min− 1 for Sr(II).

3.3.9. Reusability of the prepared adsorbents

Desorption is one of the most crucial features of an adsorbent,
emphasizing the assessment of the quantity, efficiency, and recovery
mechanisms of metals adsorbed onto the adsorbent surface. This process
not only facilitates the separation of valuable metals but also evaluates
the potential for adsorbent reuse. Analyzing desorption offers valuable
insights that can improve the environmental sustainability and eco­
nomic efficiency of the adsorption process, ultimately reducing the
overall costs associated with adsorbent production, usage, and pro­
cessing [60]. Fig. 24 illustrates the successive regeneration cycles for the
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with x = 0.06 in a 0.1-M
HNO3 elution solution. As a monoprotic acid, HNO₃ fully dissociates,
Fig. 24. Reusability of (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles with x = mildly reacting with the adsorbent in the presence of nitrate ions and
0.06 for the adsorption of Co(II) and Sr(II) ions. achieving efficient desorption with minimal damage [61]. Over three
adsorption–desorption cycles, HNO₃ successfully recovers substantial
amounts of Co(II) and Sr(II), with recovery rates ranging from 67 % to
formation and requiring a longer time to reach equilibrium (90 min). By
95 %. Furthermore, the use of HNO3 leads to minimal damage to the
contrast, the adsorption of Sr(II) ions involves chemisorption, involving
nanoadsorbents, reducing production and operational costs. A gradual
stronger chemical bonds that may result in a shorter time to reach
decline in adsorption capacity was observed for Co(II) and Sr(II) on
equilibrium (60 min). The stronger interactions associated with chem­
nanoparticles as the number of cycles increased. After four cycles, the
isorption facilitate faster adsorption, thereby explaining the shorter
adsorption capacity for Co(II) and Sr(II) remained at 90 % and 80 % of
contact time required for Sr(II)-ion removal. Furthermore, the positive
their initial levels, respectively. This decline may be attributed to
value of ΔS0 indicates an increase in disorder during the adsorption
adsorbent loss during repeated washing steps and incomplete desorp­
process. Finally, the negative values of ΔG0 reveal that the adsorption of
tion, which left some active sites occupied by radioactive ions.
Co(II) and Sr(II) ions onto (Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles
These results indicate that the prepared nanoparticles serve as
occurs spontaneously.

Table 8
Comparison of results of this and previous studies for the adsorption of Co(II) and Sr(II) onto various adsorbents.
Adsorbent Adsorbate Initial Concentration (ppm) Adsorption Capacity (mg/g) Removal Efficiency (%) Reference

(Co0.44Zn0.44Ba0.06Sb0.06)Fe2O4 Sr(II) 100 481.5 96.3 This Study

Co(II) 441.5 88.3
carbon-coated ZrO2/Mn-Mg-Zn ferrite Sr(II) 100 – 80 [62]
Mg0.4Zn0.6Fe2O4 Sr(II) – 74 – [63]
Alg-CoFe2O4 Sr(II) – 47.5 – [64]
C@ ZrO2/Mn0.5Mg0.25Zn0.25Fe2O4 Co(II) 100 82.51 – [65]
CuFe2O4 Sr(II) 10 23.04 – [66]

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S. Rmeid et al. Ceramics International 51 (2025) 4593–4612

Fig. 25. Schematic representation of the adsorption of (a) Co(II) and (b) Sr(II) ions onto the prepared nanoparticles.

effective and regenerable adsorbents for wastewater treatment, main­ structure of the nanoparticles, while an increase in Ba and Sb content
taining high adsorption capacities for radioactive metals with minimal resulted in larger lattice parameters and smaller crystallite sizes. XPS
capacity loss over multiple cycles. In summary, Co–Zn ferrite nano­ analysis validated the elemental composition and oxidation states of the
particles doped with Ba and Sb demonstrate exceptional performance synthesized nanoparticles, confirming the presence of Co, Zn, Fe, and O
for the removal of radioactive contaminants, particularly Co(II) and Sr in all samples, as well as Ba and Sb in those with x = 0.02 and 0.10. HR-
(II). Their maximum adsorption capacities exceed those of comparable XPS spectra provided insights into the cationic distribution and oxida­
nanoparticles, as shown in Table 8. tion states, indicating a redistribution of Co2+ between octahedral and
tetrahedral sites due to the incorporation of Ba2+ and Sb3+ ions. TEM
3.3.10. Adsorption mechanism analysis revealed a spherical morphology, and EDX results confirmed
The adsorption mechanisms of Co(II) and Sr(II) ions onto the syn­ the successful substitution of Co2+ and Zn2+ ions by Ba2+ and Sb3+ ions.
thesized nanoparticles exhibit distinct interactions, as shown in the The VSM results indicated weak ferromagnetic behavior and soft mag­
schematic diagram in Fig. 25. Initially, Co(II) and Sr(II) ions interact netic properties, with the saturation magnetization (Ms) increasing from
with the adsorbent surface, leading to their adsorption. For Co(II) ions, 47.68 to 51.14 emu/g as x increased from 0 to 0.02, before decreasing to
this process primarily occurs through physisorption, as shown in Fig. 25 39.29 emu/g at x = 0.20. This variation in Ms values was attributed to
(a). This type of adsorption involves weak van der Waals forces, changes in the distribution of Co2+ between octahedral and tetrahedral
allowing the formation of multilayers on the nanoparticle surface, as sites.
indicated by the Freundlich isotherm model. Conversely, Sr(II) ions are The adsorption experiments demonstrated an enhanced performance
adsorbed via chemisorption, indicating stronger chemical bonding with of Ba and Sb co-doped nanoparticles, with (Co0.44Zn0.44Ba0.06Sb0.06)
the adsorbent. As shown in Fig. 25(b), the Temkin isotherm model best Fe2O4 exhibiting the highest removal efficiencies. As the initial con­
describes this mechanism, indicating uniform binding energies across centration of contaminants increased, the available active sites on the
the nanoparticle surface. At equilibrium, the rates of adsorption and adsorbents became occupied, resulting in a decrease in removal effi­
desorption become equal, resulting in saturated layers of Co(II) and Sr ciency. However, at elevated concentrations of Co(II) and Sr(II), a strong
(II) ions on the adsorbent surface. driving force for mass transfer was observed, highlighting the nano­
particles’ capacity to effectively attract these ions from the liquid phase,
4. Conclusion thereby augmenting the overall adsorption capacity. The adsorption of
Co(II) was primarily governed by physisorption, as indicated by an
This study effectively synthesized and employed enthalpy value of less than 40 kJ/mol, whereas the adsorption of Sr(II)
(Co0.5− xZn0.5− xBaxSbx)Fe2O4 nanoparticles as efficient adsorbents for was characterized by chemisorption.
the removal of radioactive Co(II) and Sr(II) ions from aqueous solutions. Among the synthesized adsorbents, the (Co0.44Zn0.44Ba0.06Sb0.06)
The nanoparticles, with varying compositions of x = 0.00, 0.02, 0.04, Fe2O4 nanoparticles demonstrated the highest rate constants for the
0.06, and 0.10, were synthesized utilizing the co-precipitation method. adsorption of Co(II) and Sr(II) ions, exhibiting pseudo-second-order rate
Characterization through XRD and FTIR confirmed the purity and spinel constants of 2.02 × 10− 4 g mg− 1.min− 1 for Co(II) and 2.68 × 10− 4 g

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