Heavy Metal Ions Removal from Aqueous Solution using NiZnFe2O4/TiO2 Nanocomposites Adsorbent

This research explores the adsorption effectiveness of NiZnFe2O4/TiO2 nanocomposites regarding Cr(VI). The nanocomposites were effectively synthesized utilizing coprecipitation and Stöber methods, incorporating diverse molar ratios of TiO2. The samples were subjected to characterization using methods such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), Fourier-transform infrared (FTIR), vibrating sample magnetometer (VSM), and ultraviolet-visible spectroscopy (UV-Vis). These analyses were conducted to evaluate the crystal structure, morphology, chemical bond formation, optical properties, magnetic properties, and removal efficiency of the specimens. XRD results showed that NiZnFe2O4 and TiO2 have a cubic and tetragonal structure. The crystallite size decreased as the TiO2 concentration increased. TEM image of NiZnFe2O4 reveals the formation of clusters, indicating uneven dispersion under agglomerated conditions. The average particle size is measured at (10.6 ± 0.8) nm. The Fourier-transform infrared (FTIR) analysis demonstrated the presence of functional groups O-H, C-H, and H-O-H, indicating the successful synthesis of the material. Moreover, the identification of MO-octahedral, MO-tetrahedral, and Ti-O functional groups suggested the formation of NiZnFe2O4/TiO2 nanocomposites. The incorporation of TiO2 had an impact on both the saturation magnetization and coercivity values, which fell within the ranges of 12.4 to 22.9 emu/g and 47 to 55 Oe, respectively. This finding indicates the presence of advantageous magnetic properties. The absorbance spectrum of these nanocomposites displayed a shift to the right (redshift), allowing them to absorb ultraviolet rays. The band gap of these nanocomposites ranges from (2.85 ± 0.02) to (3.29 ± 0.02) eV. Notably, NiZnFe2O4/TiO2 nanocomposites with a concentration ratio of 1:5 exhibit effective Cr(VI) removal efficiency, achieving a degradation value of 65.6%. The pseudo-kinetic model was first investigated to describe kinetic data and Cr(VI) removal determination. The SEM-EDX adsorbent results after adsorption showed the presence of Cr(VI) in the nanocomposites. Therefore, these results can promote NiZnFe2O4/TiO2 nanocomposites as a promising candidate in the removal of heavy metal waste.


Introduction
The impact of pollution caused by textile effluents can certainly affect water quality and lead to various environmental losses.In addition to dye waste, textile also contains toxic heavy metals such as Chromium (VI) [1].Heavy metals have a negative impact on aquatic biota and on humans who consume these biota.Increasing concentrations of heavy metal content accumulate in the cells of organisms and can lead to death [2].Treatment of industrial color waste has been implemented to minimize its negative impact.There are several methods to address environmental pollution caused by industrial dye effluents, including approaches such as adsorption, biosorption, and coagulation.Nevertheless, two of these methods exhibit specific limitations that need to be addressed for more effective and sustainable solutions [3].The biosorption method is an environmentally friendly waste treatment technology capable of economically and feasibly removing toxic compounds from industrial wastewater [4].However, the binder is easily saturated, making this method less effective to use.On the other hand, the coagulation method is a water treatment process in which very small solids and colloids are combined to form flocs by adding chemicals [6].The drawback of this method is that it produces a lot of sludge after use, so it is considered less environmentally friendly [7].
Adsorption is the process of binding a fluid to a liquid or solid, forming a thin layer on its surface due to Van der Waals forces [5].This approach provides advantages such as cost-effectiveness, ease of implementation, and high efficiency [6].One of the semiconductor materials that are widely used in photocatalyst applications is TiO2.This material has various advantages that attract many researchers [7].Besides being widely used as a photocatalyst material, TiO2 can also be applied in adsorption.With stable characteristics, a diminutive particle size, and an expansive surface area, it exhibits enduring properties [8].Compared to large-sized adsorbents of the same volume, nanoparticle adsorbents have a larger particle surface area and greater capacity to adsorb adsorbates [9].However, TiO2 materials also have drawbacks, including challenges in separating them from pollutants due to their extremely small size and the requirement for relatively high costs in subsequent processing [10].
Recent developments involve integrating semiconducting and magnetic materials to enhance adsorption activity [11].One magnetic material with potential as an adsorbent is NiZnFe2O4, offering several advantages, including a substantial surface area and ease of separation from the medium [12].Sahebi et al. synthesized Fe3O4/TiO2 for Cr(VI) removal and conducted research on adsorption activity using Fe3O4/TiO2 nanocomposites, achieving a removal efficiency of >90%.Sahoo and Hota examined adsorption with ZnO/ZnFe2O4 nanocomposites, reaching a removal efficiency of >70%.Yang et al. investigated Fe3O4/TiO2 nanocomposites as an adsorbent for Cr(VI) removal, achieving a removal efficiency of 66.5%.Etemadinia et al. (2019) successfully carried out the modification of ZnFe2O4 with SiO2, achieving degradation results of up to 94%.However, on the other hand, SiO2 is a material with good affinity properties in water, making it highly stable when dissolved.This stability poses a challenge for separation [13].Nanoparticles that readily dissolve in water may experience reduced performance in degradation, waste removal, and repeated use (reusability).
Incorporating magnetic nanoparticles into TiO 2 nanoparticle-based adsorbents has effectively addressed the aforementioned problems [14].Combining magnetic nanoparticles with semiconductors can prevent the nanoparticles from agglomerating and oxidizing easily.Another advantage is that the concentration of hydroxyl compounds is sufficiently high to facilitate electrostatic interactions with other materials during the adsorption process [15].The insoluble nature of TiO2, when combined with NiZnFe2O4 magnetic material, results in adsorbents that can be easily separated from the solution.This compatibility allows for the reuse or reusability of the adsorbent.This research focuses on the synthesis of nanocomposites, specifically NiZnFe 2 O 4 /TiO 2 nanocomposites.The synthesis will employ a twostage method, utilizing the coprecipitation method for NiZnFe2O4 synthesis and the Stöber method for composite formation.The concentration of TiO2 will be varied during the process.Additionally, the research will investigate the microstructure, morphology, magnetism, optical properties, and adsorption ability of the materials.

Synthesis using Stöber method
Mixing NiCl2•6H2O and ZnSO4•7H2O with 20 mL aquadest as solvent named solution 1. Dissolved of FeCl3.6H2O in aquadest named solution 2. Then mix solutions 1 and 2 with added HCl.Next step is prepare the coprecipitant solution, NaOH dissolved in aquadest and then stirred for 10 minutes.After that mix solution 1 and 2 dripped into the NaOH solution constantly using pipette for 1 hour.The resulting precipitate was dried by funace at 90 ℃ for 4 hours.

Synthesis using coprecipitation method
Various of TiO2 concentration used in NiZnFe2O4/TiO2 nanocomposites as in table 1.After the compounding process, the mixture of the two solutions was then permanent magnet washing for 15 minutes for 7 times with a neutral pH.The resulting precipitate was dried by funace at 100℃ for 2 hours.

Characterization of NiZnFe2O4/TiO2
The structure and crystal phase analysis of NiZnFe2O4/TiO2 samples were tested using X-ray diffraction spectroscopy (XRD).The particle size and morphology of the NiZnFe2O4/TiO2 samples were tested using Transmission Electron Microscopy (TEM) with a voltage source of 120kV.The functional group analysis of nanocomposite samples was tested using an Fourier Transform Infra Red (FTIR).Analysis of the magnetic properties of the nanocomposites was carried out using Vibrating Sample Magnetometer (VSM).And analysis of the optical properties was tested using a UV-Vis spectrometer with a wavelength of 200-800 nm.

Result and Discussion
The XRD spectra of NiZnFe2O4/TiO2 nanocomposites are shown in Figure which identifies crystals with a tetragonal structure [9].The formation of NiZnFe2O4 and TiO2 phases at the XRD peaks of all samples confirmed that NiZnFe2O4/TiO2 nanocomposites have been successfully synthesized.The XRD pattern and crystal size of NiZnFe2O4/TiO2 composite were analyzed using the Rietvield Refinement method through Maud software Eq. ( 1) [9].
The decrease in crystallinity of NiZnFe2O4 due to the increase in TiO2 concentration is supported by the phase composition shown in table 2. This cause the higher the intensity of the XRD peak, the more diffraction reflections are detected.Overall, the level of crystallinity can be determined by the average crystallite size of each sample.TEM was employed to analyze the morphology and particle size of NiZnFe2O4 nanoparticles.However, Figure 2 indicates that the TEM morphology was indiscernible, primarily attributed to particle agglomeration resulting from attractive forces between crystals in the sample [17].By utilizing equation 4.6, a particle size distribution graph for NiZnFe2O4 nanoparticles is generated, as depicted in Figure 2 Figure 2 displays the EDX spectra representing the elements at specific energies that constitute the NiZnFe2O4/TiO2 nanocomposite, with a concentration variation of 1:5.The elemental composition of NiZnFe2O4/TiO2 is presented in terms of mass percent, revealing O 37.2%, Ti 29.5%, Fe 6.9%, Zn 4.8%, and Ni 3.9%.EDX analysis provides confirmation of the presence and relative proportions of these elements in the nanocomposite.In Figure 3  Infrared Fourier transform spectra of NiZnFe2O4, TiO2, and NiZnFe2O4/TiO2 were identified within the range of 400-4000 cm -1 , as shown in Figure 4.The IR spectra samples exhibit absorption peaks at wave numbers ranging from 3410.1 to 3425.5 cm -1 , indicative of stretching mode vibrations related to O-H hydroxyl groups.This suggests the physical absorption of water molecules onto the nanoparticle surface [18].The wavelengths of 401.    5 as shown nanoparticles are soft magnetic which have ferromagnetic properties close to superparamagnetic based on the narrow curve shape and by the value of remanent magnetization and small coercivity [21].The values of saturation magnetization (Ms) of NiZnFe2O4 and NiZnFe2O4/TiO2 are detailed in table 3. Hybridization of TiO2 non-magnetic nanoparticles can inhibit the growth of NiZnFe2O4 and the motion of magnetic moments in the domain is hindered [22].Coercivity value NiZnFe 2 O 4 nanoparticles before and after being modified with TiO 2 experienced fluctuating changes.The addition of TiO2 can affect the distribution of the magnetostatic field in the mixed material.Variations in the field can affect the orientation of magnetic domains and lead to fluctuations in coercivity values [12].Although there is a weakening of the saturation magnetization value, it does not affect the magnetic properties of the nanocomposite which is characterized by the NiZnFe2O4/TiO2 nanocomposite being easily and quickly attracted by an external magnet.It can be confirmed that the NiZnFe2O4/TiO2 nanocomposites are reusable.The UV-vis spectra of NiZnFe2O4, TiO2, and NiZnFe2O4/TiO2 nanocomposite are shown in figure 6.The absorption peaks of this nanocomposite concentration variation are respectively at wavelengths of 233.2, 330.1, 324.6, 322.2, 321.7, and 321.1 nm for the variations of 1:0, 1:1, 1:2, 1:3, 1:4, and 1:5 concentrations.While TiO2 has a wavelength of 320.4 as presented in table 4 [23].
The absorbance spectrum was subsequently analyzed using the Tauch Plot technique derived from the Kubelka-Munk equation.This method was employed to ascertain the band gap energy (  ), as illustrated in the provided equation.NiZnFe2O4/TiO2 nanocomposites show their respective wavelengths as presented in table 4. Figure 6 exhibits the absorbance characteristics of NiZnFe2O4/TiO2 nanocomposites, showcasing alterations in the absorption edge peak and an expansion of the absorption region with varying concentrations of TiO2.
(ℎ) = (ℎ −   )  (3) where  represents the absorption constant, ℎ stands for the Planck constant (6.626 × 10 -34 Js),  denotes the light frequency,  is a constant,   signifies the band gap energy, and n is a constant value, which varies based on the specific transition type be it direct, indirect, or forbidden indirect transition [24].As indicated in table 4, the rise in the  value for NiZnFe2O4/TiO2 is ascribed to the quantum confinement The acidic Cr(VI) solution (< 6.5) directly affects the adsorption performance of these nanoparticles.pH at acidic conditions the most dominant molecules are CrO4 2-and Cr2O7²ˉ which allows increased absorption results from electrostatic interactions between the positively charged section of the adsorbent surface and the negatively charged CrO 4 2-anion.Conversely, at high pH, the hydroxy groups on the TiO2 surface will dissociate into TiO -which results in no electrostatic recreation between the TiO2 surface and Cr(VI) [25].In alkaline conditions, a high pH will convert Cr2O7²ˉ into Cr 3+ , where Cr 3+ ions are ions that easily precipitate making it difficult to be adsorbed by adsorbents.At elevated pH levels, chromium ions undergo precipitation to form Cr(OH)3, diminishing the solubility of Cr ions in the solution.This, in turn leads to a decreased quantity of Cr ions available for absorption by the cell surface [26].
The percentage of removal efficiency shown in Figure 7 (a) shows a slow adsorption process during the first 20 minutes for concentration variations of 1:0, 1:1, and 1:2.While the concentration variations of 1:4 and 1:5 have increased in the first 20 minutes.This is because with the addition of TiO2, the active sites and pores are wider and large enough empty surface to facilitate the interaction of molecular charges that trigger the adsorbent to quickly absorb the adsorbate [27].Based on Figure 7(a), the adsorption process occurs quite quickly in the first 20 minutes for concentration variations (1:4) and (1:5) due to the binding of nanoparticles with waste Cr(VI).This can be elucidated by noting that, initially, a majority of the vacant surface sites are accessible for adsorption, while the remaining sites become challenging to utilize because of the repulsive force between Cr(VI) molecules.In Figure 7(c), the optimal correlation coefficient (R 2 ) value is achieved, closely aligning with one of the parameters in the linear equation.This suggests that, compared to first-order and secondorder pseudo-adsorption kinetics, modeling based on second-order pseudo-adsorption kinetics is the most fitting.This model suggests that the adsorption capacity varies in proportion to the number of surface active sites and is contingent upon the capability of NiZnFe2O4/TiO2 nanoparticles to adsorb metal waste [28].In general, the adsorption mechanism is dominated by electrostatic interactions between adsorbent and adsorbate.The inclusion of TiO2 concentration has the potential to enhance the removal efficiency.As previously explained, TiO2 has an abundant number of hydroxyl group compounds on its surface.In acidic conditions, many positive charges are generated on the surface of TiO2 [29].As shown in Figure 8, elevated concentrations of TiO2 and hydroxyl groups are generated, leading to an intensified electrostatic interaction with Cr(VI).[30].Increased concentrations of TiO2 and hydroxyl groups are produced and the electrostatic interaction with Cr (VI) will also increase.
Conversely, the Cr(VI) anion will also engage with the positive charge resulting from the protonization process on the surface of NiZnFe2O4.This interaction aids in maximizing the adsorption process, rendering it more efficient.The increase in removal efficiency values and increase in TiO2 concentration confirms that NiZnFe2O4/TiO2 nanocomposites do not overshadow each others ability, this causes the combination of the two can provide maximum results.Functional group characterization results on NiZnFe 2 O 4 /TiO 2 (1:5) samples that have not been used for adsorption and those that have been used for adsorption.not yet used for adsorption and those that have been used for adsorption can be seen in Figure 9.After adsorption, the stretching vibration peak of the bound O-H group shifts from 33.5 to 33.5.O-H bound group shifted from 3394.72 cm -1 to 3382.43 cm -1 .While for C-H stretching vibrations which is 2283.72 cm -1 .The Ti-O group experienced shifted from 1427.32 cm -1 to 1401.12 cm -1 .

Conclusion
The synthesis of NiZnFe2O4/TiO2 nanocomposites has been effectively accomplished through the combination of coprecipitation and the Stöber method.Crystal size before and after modified respectively in the range of values of 3.3 to 5.9 nm.Analysis through FTIR indicates wavelengths of 3410.2 and 2924.1 cm -1 for O-H and C-H, indicating successful synthesis.In addition, the presence of MO-octahedral, MO-tetrahedral functional groups at 563.2 cm -1 and Ti-O at 1473.6 cm -1 indicate that nanocomposites have been formed.The magnetic characteristics of the nanocomposite closely resemble superparamagnetism, with a reduction in the saturation magnetization value observed as the concentration of TiO2 increases.NiZnFe2O4/TiO2 nanocomposites with superparamagnetic ability were successfully separated using an external magnet [31].
Figure2displays the EDX spectra representing the elements at specific energies that constitute the NiZnFe2O4/TiO2 nanocomposite, with a concentration variation of 1:5.The elemental composition of NiZnFe2O4/TiO2 is presented in terms of mass percent, revealing O 37.2%, Ti 29.5%, Fe 6.9%, Zn 4.8%, and Ni 3.9%.EDX analysis provides confirmation of the presence and relative proportions of these elements in the nanocomposite.In Figure3(b), a mapping image of NiZnFe2O4/TiO2 elemental distribution is presented, revealing the absence of any additional impurities in the structure.The distribution of Fe exhibits a slightly higher concentration when compared to Zn and Ni, both of which have nearly identical levels.This observation is evident in the mapping image, where the dominant Zn and Ni regions appear slightly darker in color.The presence of Zn and Ni elements suggests the continued existence of bonds between Ni, Zn, and Fe in the material.Moreover, the SEM characterization results indicate that the nanoparticles exhibit bulkiness and lack homogeneity, primarily attributed to the imperfect grinding process.Furthermore, the detection of Ni, Zn, Fe, O, and Ti elements confirms the successful formation of NiZnFe2O4/TiO2.
1 on the NiZnFe2O4 and NiZnFe2O4/TiO2 nanoparticles are M-O functional group with octahedral position and 563.2 M-O absorption functional group with octahedral position.The absorption peak observed at 374.6 cm-1 corresponds to the stretching vibration of Ti-O [19].Multiple functional groups, including M-O octahedral and Ti-O peaks, indicate the successful formation of these nanocomposites [20].

Figure 10 (
Figure 10 (a) displays the EDX spectra representing the elements at certain energies that make up the NiZnFe 2 O 4 /TiO 2 1:5 nanocomposites after the adsorption process.In Figure 10 (a), the elemental composition of NiZnFe2O4/TiO2 is presented in terms of mass percent Ti 86.9%, Fe 4.9%, Zn 3.7%, Cr 3.1%, and Ni 1.4%.The element Cr with a blue color in the mapping results as shown in Figure 10 (b) is uniformly spread across the surface of nanocomposites, especially NiZnFe2O4.The SEM results before and after adsorption are consistent with the percentage of large Ti element content which is due to the difference in concentration between NiZnFe2O4 and TiO2.In addition, the SEM characterization results show nanoparticles are bulky and are not homogeneous.The presence of Ti, Fe, Zn, and Ni elements shows that there are still bonds between the three elements: Ni, Zn, Fe, and Ti.

Table 2 .
The phase composition and average crystallite size NiZnFe2O4/TiO2 nanocomposites various concentration of TiO2.

Table 5 .
effect induced by the reduction in crystallite size, wherein the size of the crystallites serves as a representative measure of particle size.Band 9