CoFe2O4@HaP as Magnetic Heterostructures for Sustainable Wastewater Treatment

The aim of this study was to synthesize a CoFe2O4@HaP nanocomposite (HaP-Hydroxyapatite) through the coprecipitation method in aqueous solution, with the purpose of using it in adsorption processes for the removal of Congo Red dye from aqueous solutions. Fourier Transform Infrared Spectroscopy (FT-IR) was used to characterize the synthesized material, identifying absorption bands specific to the functional groups of cobalt ferrite (Fe-O and Co-O at 603 and 472 cm−1) and hydroxyapatite PO43− at 1035, 962, 603 and 565 cm−1. Powder X-ray diffraction confirmed the cubic spinel structure of cobalt ferrite (S.G Fd-3m) and the hexagonal structure of hydroxyapatite (S.G P63/m). The nanocomposite’s crystallite size was calculated to be 57.88 nm. Nitrogen adsorption/desorption isotherms and BET specific surface area measurements were used to monitor textural parameters, revealing an increase in specific BET surface area when cobalt ferrite nanoparticles (15 m2/g) were introduced into the hydroxyapatite heterostructure (34 m2/g). Magnetic properties were investigated by interpreting hysteresis curves in the ±10 kOe range, with the nanocomposite showing a saturation magnetization of 34.83 emu/g and a coercivity value of 0.03 kOe. The adsorption capacity of the CoFe2O4@HaP nanocomposite is up to 15.25 mg/g and the pseudo-second-order kinetic model (Type 1) fits the data with a high correlation coefficient of 0.9984, indicating that the chemical adsorption determines the rate-determining step of the process. The obtained nanocomposite is confirmed by the analyses, and the absorption measurements demonstrate that it can be utilized to degrade Congo Red dye.


Introduction
The adsorption technique is becoming one of the most effective alternatives for industrial water treatment, due to the fact that it presents a technology that can be used for the removal of wide classes of contaminants. This technique also occupies an important place in wastewater treatment processes to remove pollutants that are not easily biodegradable, and at the same time, through adsorption processes, the materials used can be recovered and regenerated. A general definition of adsorption could be expressed as a surface phenomenon where the substance to be retained from a solution (adsorbate) is adsorbed on the surface of a material called an adsorbent. The adsorption phenomenon can be of two types: physisorption, in which the adsorbate binds to the adsorbent due to van der Waals interactions, and chemisorption [1].
Due to the increase in the world population, the demand and need for clean and potable water have greatly increased. As a consequence of this demographic evolution, the amount of lethal chemicals such as heavy metals and pharmaceuticals, discharged into the sewers, has also greatly increased and at the same time can produce harmful effects on the environment [2,3]. The pollution of water with various dyes, used in the textile and chemical industries, has led to an increase in the interest of scientists to discover new and effective methods of eliminating this category of contaminants. For example, Birniwa and co-workers synthesized a polypyrrole-polyethyleneimine (PPy-PEI) adsorbent characterized a HaP/Iron oxide type composite to be tested in medical and environmental protection applications [16].
The use of nanomaterials as adsorbents has become an increasingly fertile research area. Due to their reduced volume, nanomaterials thanks to their large surface area, containing a large number of free adsorbate sites, and high porosity, make these materials one of the strongest competitors on the market for contaminant extraction from various aqueous solutions [17]. Nanomaterials such as magnetic nanoparticles, carbon nanotubes, and porous materials are widely used as new adsorbents. The efficiency of adsorption processes can be improved by functionalizing nanomaterials, a procedure that leads to an increase in adsorption and the attraction between the adsorbent molecules and the molecules to be removed becoming greater [18][19][20][21].
Previous studies have been carried out on the synthesis of ferrites by various techniques. The experience gained regarding different ferrite synthesis methods led to the realization of the present study. Starting from ferrites, the synthesis of a nanocomposite composed of cobalt ferrite and hydroxyapatite was desired with the aim of being used in various applications.
The aim of the present study is to investigate the potential application of CoFe 2 O 4 @HaP nanocomposite for the removal of Congo Red dye from aqueous solutions. To achieve this goal, the study carries out the synthesis of the nanocomposite using the coprecipitation method, followed by the characterization of the material using complementary techniques such as Fourier Transform Infrared Spectroscopy (FT-IR), Powder X-ray diffraction, and magnetic measurements. Valuable insights into the suitability of the CoFe 2 O 4 @HaP nanocomposite for practical applications in water treatment were determined through adsorption studies by assessing the adsorption capacity of the CoFe 2 O 4 @HaP nanocomposite and by analyzing the kinetic models that govern the process.

Chemicals
The reagents used in this study for CoFe 2

Synthesis
The nanocomposite of cobalt ferrite and hydroxyapatite (CoFe 2 O 4 @HaP 1:1 mass ratio between the magnetic component and the biomaterial) was obtained in two steps. The starting point was cobalt ferrite nanoparticles, which were synthesized by the coprecipitation method in aqueous solution, adapted from [22,23]. A solution of CoCl 2 •6H 2 O 0.2 M was mixed with a solution of FeCl 2 •6H 2 O 0.4 M under continuous magnetic stirring. To prevent particle agglomeration, a 1% carboxymethyl cellulose (CMC) solution was added as a surfactant. The precipitation of the oxides was done by adding a 3 M NaOH solution. The pH was checked and adjusted to reach a value between 11-12. The black precipitate obtained was washed with distilled water to remove NaCl resulting from the synthesis process. After the synthesis of cobalt ferrite nanoparticles, we proceeded to introduce them into the hydroxyapatite structure. Cobalt ferrite nanoparticles were dispersed in 25 mL ethanol, together with 0.1 g CTAB (surfactant agent) and 25 mL 0.3 M H 3 PO 4 under continuous mechanical stirring at 60 • C. The pH of the mixture was corrected with NH 4 OH followed by dropwise addition of 0.5 M Ca(NO 3 ) 2 solution and allowed to age overnight. The particles were washed with distilled water until the pH reached a neutral value. Then they were separated from the solution by decantation, dried in an oven for 24 h at 70 • C, and calcined for 4 h at 500 • C, in a high-temperature furnace. The working protocol was modified according to [24].

Characterization Methods and Equipment
The structural properties of the cobalt ferrite and hydroxyapatite nanoparticles were investigated by complementary analytical techniques. Fourier Transform Infrared Spectroscopy (FT-IR) was performed using a Jasco 660 Plus FT-IR spectrophotometer (JASCO INTERNATIONAL Co., Ltd., Tokyo, Japan) via the pilling method. X-ray diffraction (XRD) was conducted using a Shimadzu LabX XRD-6000 difractometter (Shimadzu, Kyoto, Japan) with a copper radiation source (λ = 1.54 Å) in the 2θ range of 20 • to 80 • at a speed of 2 • /min, and the data were processed with X'Pert HighScore Plus software, version 2.2.3, which includes ICSD databases of analyzed compounds.
The textural properties of the nanocomposite were analyzed by nitrogen adsorption/desorption isotherms and BET specific surface area measurements using a Nova 2000e Quantachrome instrument (Quantachrome Instruments, Boynton Beach, FL, USA). The magnetic properties were measured using a vibration magnetometer in the ±10 kOe range at room temperature.
The general diagram of the stages covered in this study is presented in Figure 1.
resulting from the synthesis process. After the synthesis of cobalt ferrite nanoparticles, we proceeded to introduce them into the hydroxyapatite structure. Cobalt ferrite nanoparticles were dispersed in 25 mL ethanol, together with 0.1 g CTAB (surfactant agent) and 25 mL 0.3 M H3PO4 under continuous mechanical stirring at 60 °C. The pH of the mixture was corrected with NH4OH followed by dropwise addition of 0.5 M Ca(NO3)2 solution and allowed to age overnight. The particles were washed with distilled water until the pH reached a neutral value. Then they were separated from the solution by decantation, dried in an oven for 24 h at 70 °C, and calcined for 4 h at 500 °C, in a high-temperature furnace. The working protocol was modified according to [24].

Characterization Methods and Equipment
The structural properties of the cobalt ferrite and hydroxyapatite nanoparticles were investigated by complementary analytical techniques. Fourier Transform Infrared Spectroscopy (FT-IR) was performed using a Jasco 660 Plus FT-IR spectrophotometer (JASCO INTERNATIONAL Co., Ltd., Tokyo, Japan) via the pilling method. X-ray diffraction (XRD) was conducted using a Shimadzu LabX XRD-6000 difractometter (Shimadzu, Kyoto, Japan) with a copper radiation source (λ = 1.54 Å) in the 2θ range of 20° to 80° at a speed of 2°/min, and the data were processed with X՚Pert HighScore Plus software, version 2.2.3, which includes ICSD databases of analyzed compounds.
The textural properties of the nanocomposite were analyzed by nitrogen adsorption/desorption isotherms and BET specific surface area measurements using a Nova 2000e Quantachrome instrument (Quantachrome Instruments, Boynton Beach, FL, USA). The magnetic properties were measured using a vibration magnetometer in the ±10 kOe range at room temperature.
The general diagram of the stages covered in this study is presented in Figure 1. The study has three main stages: the synthesis of the CoFe2O4@HaP nanocomposite, the characterization of the obtained material, and adsorption studies of Congo Red dye.

Dye Adsorption Experiments
Adsorption experiments with a focus on CoFe2O4-HaP dosage, contact time, and temperature were carried out in order to establish the ability of the material synthesized as a potential adsorbent for the treatment of wastewater contaminated with Congo Red dye. The general diagram of the Congo Red dye removal process is presented in Figure 2.

Characterization of the nanocomposite synthesized
Adsorbtion studies The study has three main stages: the synthesis of the CoFe 2 O 4 @HaP nanocomposite, the characterization of the obtained material, and adsorption studies of Congo Red dye.

Dye Adsorption Experiments
Adsorption experiments with a focus on CoFe 2 O 4 -HaP dosage, contact time, and temperature were carried out in order to establish the ability of the material synthesized as a potential adsorbent for the treatment of wastewater contaminated with Congo Red dye. The general diagram of the Congo Red dye removal process is presented in Figure 2. The adsorption studies were made using a UV-3100PC UV-Vis spectrophotometer (VWR International Europe bvba, Leuven, Belgium). The absorbance of the supernatant of each sample was measured at 497 nm. The following equation was used to calculate the adsorption capacity: where is the adsorption capacity (mg/g), and are the initial dye concentration and The adsorption studies were made using a UV-3100PC UV-Vis spectrophotometer (VWR International Europe bvba, Leuven, Belgium). The absorbance of the supernatant of where q e is the adsorption capacity (mg/g), C i and C f are the initial dye concentration and equilibrium dye concentration (mg/L), V is the volume of dye solution (L), and m is the quantity of CoFe 2 O 4 -HaP used (g).

Powder X-ray Diffraction Analysis (XRD)
In order to determine the structure and phases of the synthesized nanocomposites, their analysis was carried out by X-ray powder diffraction. Diffractograms of the CoFe 2 O 4 , CoFe 2 O 4 @HaP, and Hydroxyapatite (HaP) are presented in Figure 3. The cobalt ferrite sample (CoFe 2 O 4 ) corresponds to a single phase with a cubic structure belonging to the space group Fd-3m (ICSD 98-007-9524). For the sample of cobalt ferrite and hydroxyapatite, in addition to the characteristic structure of ferrite, a hexagonal structure specific to hydroxyapatite with space group P63/m was identified. Analysis of the hydroxyapatite nanoparticles again confirmed the corresponding hexagonal structure as well as their membership in the space group P63/m (ICSD 98-009-4268).  The crystallite sizes of the three samples were calculated using the Debye-Scherrer equation [25]: The values of the crystallite size for the three samples are presented in Table 1.  The crystallite sizes of the three samples were calculated using the Debye-Scherrer equation [25]: The values of the crystallite size for the three samples are presented in Table 1. Lattice parameter-a (Å), unit cell volume-V (Å 3 ), and density-ρ XRD (g/cm 3 ) obtained from the interpretation of the diffractograms of the three samples are given in Table 2. A comparison of the values in Table 2 with those from the ICSD database shows very good agreement. The CoFe 2 O 4 @Hap nanocomposite was successfully obtained.  Table 2 with those from the ICSD database shows very good agreement. The CoFe2O4@Hap nanocomposite was successfully obtained.   In the spectrum of the cobalt ferrite sample, an absorption band was identified at 3432 cm −1 corresponding to the valence vibration of the O-H bond in the water molecules.

FT-IR Spectroscopy
At 1637 cm −1 , the characteristic absorption band of the COO − group from the unreacted surfactant molecules (CMC) was observed. The metal-oxygen, Fe-O and Co-O bonds, in the ferrite structure were found following the identification of the absorption bands at 603 and 472 cm −1 [26]. The infrared spectrum of the hydroxyapatite sample showed absorption bands at 3570 cm −1 and 3432 cm −1 . These correspond to the deformation and valence vibrations of the OH group from hydroxyapatite, respectively. At 1646 cm −1 and 1416 cm −1 the absorption bands of the CO 3 2− group were identified. The PO 4 3− groups from the hydroxyapatite structure contributed the following absorption bands: 1035, 962, 603, and 565 cm −1 [27]. In the FT-IR spectrum of the CoFe 2 O 4 @HaP nanocomposite, the absorption bands corresponding to the Fe-O and Co-O bonds in the ferrite structure were found. Also, the presence of PO 4 3− and OH groups from hydroxyapatite were identified in the spectrum of the nanocomposite. The results illustrate the successful synthesis of the CoFe 2 O 4 @HaP nanocomposite and provide insight into its structural properties. As mentioned above, the infrared spectra of the individual components, i.e., cobalt ferrite and hydroxyapatite, are indicated by the presence of characteristic absorption bands corresponding to their respective functional groups. These results suggest the potential use of the CoFe 2 O 4 @HaP nanocomposite in applications such as wastewater treatment through adsorption processes.

Magnetic Properties
To identify the magnetic properties, the synthesized samples were analyzed using the VSM (Vibrating Sample Magnetometer) technique, with a PMC MicroMag 3900 VSM equipment (PMC-Princeton Measurement Corporation, Princeton, Montgomery, NJ, USA) within ±10 kOe, at room temperature. The hysteresis loops as well as the magnetizations obtained for the two samples are shown in the graphs in Figure 5  the CoFe2O4@HaP nanocomposite in applications such as wastewater treatment through adsorption processes.

Magnetic Properties
To identify the magnetic properties, the synthesized samples were analyzed using the VSM (Vibrating Sample Magnetometer) technique, with a PMC MicroMag 3900 VSM equipment (PMC-Princeton Measurement Corporation, Princeton, USA) within ±10 kOe, at room temperature. The hysteresis loops as well as the magnetizations obtained for the two samples are shown in the graphs in Figure 5 (CoFe2O4) and Figure 6 (CoFe2O4@HaP).
Through the hysteresis loops shown in the graphs in Figures 5 and 6, specific magnetic parameters such as saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), and Mr/Ms ratio could be identified and are presented in Table 3.  Through the hysteresis loops shown in the graphs in Figures 5 and 6, specific magnetic parameters such as saturation magnetization (M s ), remanent magnetization (M r ), coercivity (H c ), and M r /M s ratio could be identified and are presented in Table 3.
The low values of the M r /M s ratios obtained for the two samples analyzed indicate the presence of fractions of superparamagnetic particles. Superparamagnetism usually occurs for ferrimagnetic or ferromagnetic nanoparticles made up of very small particles. After analyzing the hysteresis loops from Figures 5 and 6, a decrease in the saturation magnetization of the nanocomposite was observed (Table 3). This may be caused by the mass ratio used in the synthesis process. As this was 1:1, we have in the composition of the material 50% cobalt ferrite and 50% hydroxyapatite. It is known that hydroxyapatite has no magnetic properties, so its presence at a percentage of 50% in the structure of the nanocomposite led to a decrease of approximately 50% in the saturation magnetization. There was also a decrease in coercivity for the nanocomposite. The reason for this decrease may be the fact that after the introduction of ferrite in the hydroxyapatite structure there was an increase in the crystallite size of the nanocomposite (Table 1), and from the literature, it was observed that an increase in coercivity with an increase in the size of the magnetic material until reaching the value of the critical diameter can be followed by a decrease in coercivity values [28][29][30].

BET Analysis (Brunauer-Emmett-Teller)
The textural properties of cobalt ferrite and hydroxyapatite were identified following the analysis of the nitrogen adsorption-desorption processes and the interpretation of the isotherms corresponding to the two processes (Figure 7(a(1),b(1),c(1))).
The graphs in Figure 7(a(1),b(1),c(1)) show the adsorption-desorption isotherms for CoFe 2 O 4 -a(1), Hydroxyapatite (HaP)-b(1) and CoFe 2 O 4 @HaP-c(1) samples. They show the appearance of a type IV isotherm, with a hysteresis of type H3 according to the IUPAC classification, with a capillary condensation at high relative pressures, characteristic of mesoporous materials with slit-type pores [31].
The textural parameters resulting from processing the adsorption-desorption isotherms for the synthesized samples are shown in Table 4.
As can be seen from the data in Table 4, the introduction of the cobalt ferrite nanoparticles into the hydroxyapatite matrix led to an increase in the BET specific surface area and a decrease in the total pore volume, while the pore diameter remained constant. Magnetic cobalt ferrite nanoparticles are prone to agglomerate, which is the reason for adding carboxymethylcellulose (CMC) in the synthesis process (Section 2. Materials and methods, Section 2.2. Synthesis). Thus, upon the addition of the surfactant solution (CMC 1%), the nanoparticles remained dispersed in the stabilized suspension. However, when they were separated and dried, the nanoparticles agglomerated and showed a low relative BET specific surface area. Therefore, in order to keep an increased specific surface area in the solid state, in the present study, the prepared cobalt ferrite nanoparticles were dispersed in a solid matrix of hydroxyapatite to avoid their agglomeration in the solid phase and to increase their contact surface. Therefore, the use of the surfactant cetrimonium bromide (CTAB) had a double role in the process of introducing cobalt ferrite nanoparticles into hydroxyapatite: firstly to disperse the nanoparticles in solution around which the hydroxyapatite will precipitate and a second role to creates porosity around magnetic cobalt ferrite nanoparticles. Thus, the addition of cobalt ferrite in the hydroxyapatite matrix led to the dispersion and not the agglomeration of the particles, and the value of the BET specific area for the nanocomposite, as expected, was found to be twice that of the pure nanoparticles. magnetization of the nanocomposite was observed (Table 3). This may be caused by the mass ratio used in the synthesis process. As this was 1:1, we have in the composition of the material 50% cobalt ferrite and 50% hydroxyapatite. It is known that hydroxyapatite has no magnetic properties, so its presence at a percentage of 50% in the structure of the nanocomposite led to a decrease of approximately 50% in the saturation magnetization. There was also a decrease in coercivity for the nanocomposite. The reason for this decrease may be the fact that after the introduction of ferrite in the hydroxyapatite structure there was an increase in the crystallite size of the nanocomposite (Table 1), and from the literature, it was observed that an increase in coercivity with an increase in the size of the magnetic material until reaching the value of the critical diameter can be followed by a decrease in coercivity values [28][29][30].

Adsorption Studies
Adsorption studies were made using UV-Vis spectrophotometry.
In this study, three experimental factors were investigated: CoFe 2 O 4 -HaP dosage, contact time, and temperature. The adsorption results are depicted in Figure 8. One of the experimental factors of the adsorption process that has an impact on the adsorption capacity of the adsorbent is the quantity of the material used. Thereby, in this study, five CoFe 2 O 4 -HaP dosages were investigated (Figure 8a). A CoFe 2 O 4 -HaP dosage of 0.47 g/L leads to an adsorption capacity of 14.9 mg/g. By increasing the dosage to 0.49 g/L, a slight increase in the adsorption capacity is observed to 15.25 mg/g. At the same time, the results show that higher dosages of CoFe 2 O 4 -HaP adsorbent cause lower adsorption capacities-0.96 g/L, 1.9 g/L, and 3.04 g/L CoFe 2 O 4 -HaP dosages led to adsorption capacities of 13.07 mg/g, 12.58 mg/g, and 9.15 mg/g, respectively. Another adsorbent reported previously in the literature presented the same behavior. Zhao and co-workers explained this trend by that higher adsorbent dosages cause the adsorption sites to overlap and not be effectively exploited [32]. Based on the results obtained, the subsequent CoFe 2 O 4 -HaP dosage used for the effect of the other two adsorption factors, contact time and temperature, was 0.49 g/L. Figure 8b compares the adsorption capacities of CoFe 2 O 4 -HaP material at various time intervals, from 5 min to 120 min, using an initial dye concentration of 30 mg/L. According to the data obtained, the Congo Red dye is adsorbed rapidly in the first 5 min of contact time: 10.83 mg/g, which represents~71% of the total adsorption capacity of the CoFe 2 O 4 -HaP adsorbent. The presence of numerous active sites on the surface of the adsorbent could explain the fast adsorption [33][34][35]. After 8 min of contact time, the CoFe 2 O 4 -HaP nanocomposite has the ability to adsorb 12.07 mg/g of dye. The adsorption capacity slightly increases from 15.14 mg/g to 15.24 mg/g between 10 min and 50 min of contact time. After 60 min and 120 min of contact time, the adsorption capacity is unchanged. So,~50 min of contact time can be considered sufficient in order to achieve the maximum adsorption capacity. The literature shows an adsorbent synthesized based on hydroxyapatite nanoparticles that shows the same trend: the adsorption process occurs quickly (5-10 min) [36]. In addition, Sirajudheen and co-workers prepared an adsorbent based on chitosan-supported graphene oxide-hydroxyapatite composite and used it to clean water sources polluted with different dye contaminants. Their results show that the adsorption process is very fast, with a contact time of 40 min being enough to attain the equilibrium [37].
A brief comparison of the data obtained in this study versus other materials used for Congo Red dye reported in the literature is further discussed. It is recommended to take into consideration that the value of adsorption capacity strongly depends on the type of material used and the experimental adsorption conditions. AgNPs-functionalized-HaP material was synthesized by the research group of Azeez and co-workers. The investigation showed that using an adsorbent dosage of 0.5 g/50 mL and an initial concentration of 50 mg/L, the material has an ability to adsorb 24.41 mg/g of dye molecules [38].
Simonescu and co-workers synthesized CoFe 2 O 4 nanoparticles and CoFe 2 O 4 -Chitosan composite material and tested them for Congo Red dye adsorption. In order to establish how the contact time affects the adsorption capacity, the samples were collected at different time intervals until 360 min. Using an adsorbent dosage of 0.01 g/25 mL Congo Red dye solution of 102.81 mg/L initial concentration, the results showed that the adsorption capacities are 16.18 mg/g for CoFe 2 O 4 -Chitosan adsorbent and 146.5 mg/g for CoFe 2 O 4 adsorbent [40].
The data obtained from the experiment regarding the effect of contact time were fitted using different kinetic models: pseudo-first-order, pseudo-second-order (Type 1, Type 2, Type 3, and Type 4), and intraparticle diffusion. The plots are shown in Figure 8c-h and the kinetic parameters for the investigated models calculated from the slope and intercept using the equations presented in the literature [41,42] are listed in Table 5. Table 5. Kinetic parameters of the investigated models.

Kinetic Model Equation Results
Pseudo-first-order kinetic model log(q e − q t ) = log q e − (k 1 t)  Table 5 it can be seen that according to the pseudo-first-order kinetic model, the correlation coefficient, R 2 , has a lower value. The pseudo-first-order rate, k 1 , calculated from the slope is 0.0672 min −1 .
The R 2 determined using the pseudo-second-order kinetic model, Type 1 is 0.9996 while pseudo-second-order kinetic model Type 2, pseudo-second-order kinetic model, Type 3, and pseudo-second-order kinetic model, Type 4 show values of R 2 below 0.85. Moreover, the experimental adsorption capacity (q e ) and the calculated adsorption capacity value (q e cal) for pseudo-second-order, Type 1 have close values, 15.25 mg/g vs. 15.45 mg/g. The pseudo-second-order rate, k 2 , calculated for the other three types of pseudo-second-order model are 0.0280 g/mg min, 0.0434 g/mg min, and 0.0218 g/mg min. The q e cal for pseudo-second-order kinetic models, Type 2-Type 4, are 16.13 mg/g, 15.92 mg/g, and 16.59 mg/g, respectively.
By plotting q t vs. t 1/2 (Figure 8h) it was found that the adsorption process of Congo Red dye using CoFe 2 O 4 -HaP adsorbent takes place in two stages. In addition, the intraparticle diffusion model does not fit the adsorption data, with the R 2 values corresponding for both stages being lower.
Therefore, it can be highlighted that the rate-determining step of the adsorption process for Congo Red using CoFe 2 O 4 -HaP adsorbent is determined by chemical adsorption [43,44]. Figure 8i contains the results of the impact of the temperature of the dye solution on the CoFe 2 O 4 -HaP nanocomposite's ability to adsorb the dye. The working temperatures were 296 K, 307 K, and 319 K using an initial dye concentration of 30 mg/L and 0.49 g/L CoFe 2 O 4 -HaP dosage. In addition, the effect of temperature was studied with intermittent stirring at natural pH. The results show that an increase in temperature of 11 K leads to an increase in adsorption capacity from 15.25 mg/g to 20.17 mg/g, while at a temperature of 319 K, the CoFe 2 O 4 -HaP adsorbent has an adsorption capacity of 27.97 mg/g.
The type of the adsorption process was established based on Gibbs free energy (∆G • ), entropy change (∆S • ), and enthalpy change (∆H • ) thermodynamic parameters, according to Equations (3) and (4) [38], and the results are list in Table 6. The ∆H • and ∆S • parameters were determined from the slope and intercept plot of lnK D vs. 1/T. The ∆H • parameter showed that the adsorption process is endothermic. The negative values of ∆G • obtained for all three temperatures indicate the spontaneity and feasibility of the adsorption process. The affinity of the CoFe 2 O 4 -HaP adsorbent for Congo Red dye adsorption is highlighted by the positive values of ∆S • . The data are in agreement with the study performed by Babakir and co-workers [45].
The adsorption isotherm study was predicted using Langmuir and Freundlich isotherms ( Figure 9). Materials 2023, 16, x FOR PEER REVIEW 14 of 18 particle diffusion model does not fit the adsorption data, with the R 2 values corresponding for both stages being lower. Therefore, it can be highlighted that the rate-determining step of the adsorption process for Congo Red using CoFe2O4-HaP adsorbent is determined by chemical adsorption [43,44]. Figure 8i contains the results of the impact of the temperature of the dye solution on the CoFe2O4-HaP nanocomposite's ability to adsorb the dye. The working temperatures were 296 K, 307 K, and 319 K using an initial dye concentration of 30 mg/L and 0.49 g/L CoFe2O4-HaP dosage. In addition, the effect of temperature was studied with intermittent stirring at natural pH. The results show that an increase in temperature of 11 K leads to an increase in adsorption capacity from 15.25 mg/g to 20.17 mg/g, while at a temperature of 319 K, the CoFe2O4-HaP adsorbent has an adsorption capacity of 27.97 mg/g.
The type of the adsorption process was established based on Gibbs free energy (ΔG°), entropy change (ΔS°), and enthalpy change (ΔH°) thermodynamic parameters, according to Equations (3) and (4) [38], and the results are list in Table 6. The ΔH° and ΔS° parameters were determined from the slope and intercept plot of lnKD vs. 1/T. The ΔH° parameter showed that the adsorption process is endothermic. The negative values of ΔG° obtained for all three temperatures indicate the spontaneity and feasibility of the adsorption process. The affinity of the CoFe2O4-HaP adsorbent for Congo Red dye adsorption is highlighted by the positive values of ∆S°. The data are in agreement with the study performed by Babakir and co-workers [45].
The adsorption isotherm study was predicted using Langmuir and Freundlich isotherms ( Figure 9). By evaluating R 2 , the correlation coefficient, of both models, it was observed that the Freundlich model fits the adsorption data with an R 2 value of 0.9844 compared with the By evaluating R 2 , the correlation coefficient, of both models, it was observed that the Freundlich model fits the adsorption data with an R 2 value of 0.9844 compared with the Langmuir model (R 2 = 0.8916). The Freundlich constant, K F , is 2.7 L/g and n has a value of 0.28.
The results obtained suggest that the nanocomposite synthesized in this study can be employed as a potential adsorbent to adsorb the Congo Red dye.

Evaluation of Reuse of CoFe 2 O 4 @HaP Composite
In order to establish the capacity of CoFe 2 O 4 @HaP composite to be re-used, cycles of adsorption/desorption were performed. For the desorption study, a solution of 0.1 M HNO 3 was used. The results obtained ( Figure 10) show that the CoFe 2 O 4 @HaP composite can be re-used as an adsorbent for the dye proposed, the adsorption capacity being 10.22 mg/g after three cycles.
Langmuir model (R 2 = 0.8916). The Freundlich constant, KF, is 2.7 L/g and n has a value of 0.28.
The results obtained suggest that the nanocomposite synthesized in this study can be employed as a potential adsorbent to adsorb the Congo Red dye.

Evaluation of Reuse of CoFe2O4@HaP Composite
In order to establish the capacity of CoFe2O4@HaP composite to be re-used, cycles of adsorption/desorption were performed. For the desorption study, a solution of 0.1M HNO3 was used. The results obtained ( Figure 10) show that the CoFe2O4@HaP composite can be re-used as an adsorbent for the dye proposed, the adsorption capacity being 10.22 mg/g after three cycles.

Conclusions
The synthesized material exhibits great potential as an adsorbent for the removal of Congo Red dye, as demonstrated by the adsorption experimental results. The adsorption capacity is affected by the dosage of CoFe2O4-HaP, with higher dosages resulting in a decreased capacity. Moreover, increasing the temperature from 296 K to 319 K enhances the adsorption capacity from 15.25 mg/g to 27.97 mg/g. Equilibrium is achieved after approximately 50 min of contact time, with a rapid adsorption rate in the first few minutes. The pseudo-second-order kinetic model (Type 1) fits the data with a high correlation coefficient of 0.9984, indicating that chemical adsorption determines the rate-determining step of the process. Furthermore, the thermodynamic analysis suggests that the dye adsorption process is feasible, spontaneous, and endothermic in nature. Further research could include exploring the effectiveness of the CoFe2O4@HaP nanocomposite in the removal of other types of dyes or contaminants from wastewater, optimizing the synthesis process to improve the properties of the nanocomposite, and investigating the feasibility of scaling up the production of the nanocomposite for practical applications in water treatment. Furthermore, investigations on the photodegradation performance of the CoFe2O4@HaP nanocomposite for dye removal under visible light irradiation could be considered, in the expectation that such contributions will demonstrate the potential application of CoFe2O4@HaP nanocomposite as a highly effective adsorbent and photocatalyst for the removal of harmful dyes from wastewater.

Conclusions
The synthesized material exhibits great potential as an adsorbent for the removal of Congo Red dye, as demonstrated by the adsorption experimental results. The adsorption capacity is affected by the dosage of CoFe 2 O 4 -HaP, with higher dosages resulting in a decreased capacity. Moreover, increasing the temperature from 296 K to 319 K enhances the adsorption capacity from 15.25 mg/g to 27.97 mg/g. Equilibrium is achieved after approximately 50 min of contact time, with a rapid adsorption rate in the first few minutes. The pseudo-second-order kinetic model (Type 1) fits the data with a high correlation coefficient of 0.9984, indicating that chemical adsorption determines the rate-determining step of the process. Furthermore, the thermodynamic analysis suggests that the dye adsorption process is feasible, spontaneous, and endothermic in nature. Further research could include exploring the effectiveness of the CoFe 2 O 4 @HaP nanocomposite in the removal of other types of dyes or contaminants from wastewater, optimizing the synthesis process to improve the properties of the nanocomposite, and investigating the feasibility of scaling up the production of the nanocomposite for practical applications in water treatment. Furthermore, investigations on the photodegradation performance of the CoFe 2 O 4 @HaP nanocomposite for dye removal under visible light irradiation could be considered, in the expectation that such contributions will demonstrate the potential application of CoFe 2 O 4 @HaP nanocomposite as a highly effective adsorbent and photocatalyst for the removal of harmful dyes from wastewater.