Titan Yellow and Congo Red Removal with Superparamagnetic Iron-Oxide-Based Nanoparticles Doped with Zinc

Abstract

In this work, we present magnetic nanoparticles based on iron oxide doped with zinc synthesized using the wet co-precipitation method for environmental application. The morphology of the samples was revealed by SEM and TEM, which showed particles of granular shape and size of about 15 nm. The specific surface areas of the materials using the BET method were within the range of 85.7 to 101.5 m² g⁻¹ depending on the zinc content in the superparamagnetic iron oxide nanoparticles (SPIONs). Magnetometry was performed to determine the magnetic properties of the particles, indicating superparamagnetism. Synthesized magnetic nanoparticles with different amounts of zinc dopant were used as an adsorbent to remove model pollutant Titan yellow (TY) from the aqueous solutions. Adsorption was determined by investigating the effects of sorbent amount, dye concentration, and contact time. The synthesized material removed Titan yellow quickly and efficiently within the physical adsorption. The adsorption isotherms were consistent with the models proposed by Langmuir and Redlich-Peterson. The monolayer adsorption capacities were 30 and 43 mg g⁻¹ for Fe₃O₄ and Fe₃O₄@10%Zn, respectively, for the removal of TY. However, that of Congo red is 59 mg g⁻¹ by Fe₃O₄@10%Zn. The proposed nanoparticles offer fast and cost-effective water purification, and they can be separated from solution using magnets.

1. Introduction

Water pollution is one of the most demanding environmental challenges. Among many compounds generated by anthropogenic activity are dyes from the textile, paper, tanneries, cosmetic, plastic, and food industries, with the production of 1,000,000 tons globally [1]. Dyes used for colorizing textiles, leathers, and paper are mainly based on synthetic chemicals of aromatic structure, which have adverse effects on the environment. One of the most commonly used groups of dyes is azo-based compounds. Their removal from aquatic reservoirs and wastewater is of great concern, so water purification methods have been developed that propose many materials for use as adsorbents. Activated carbon [2], biochar [3], zeolites [4,5], biosorbents [6], and metal organic frameworks (MOFs) [7,8] have all been investigated and used. Recently, nanomaterials have been proposed as novel adsorbents offering a high volume-to-surface ratio, high adsorption capacity, cost-effective processing, and feasibility. Different nanomaterials such as functionalized multi-walled carbon nanotubes [9], hydroxyapatite/chitosan composites [10], supported chitosan [11], Au-decorated Bi₂S₃ nanorods [12], etc. have been used for the removal of dyes from wastewater.

Nevertheless, the great effectiveness of magnetic nanomaterials offers the feasibility of pollutant adsorption as well as sorbent removal from water using magnets. In this regard, much attention has been paid to magnetic metal oxides such as CoFe₂O₄ [13], NiFe₂O₄ [14], magnetic Fe₃O₄/chitosan nanoparticles (MFe₃O₄/CS NPs) and arginine-modified MFe₃O₄/CS nanoparticles [15], magnetic lignosulfonate [16], polyaniline/Fe₃O₄ [17], Fe₃O₄ nanowalls [18], γ-Fe₂O₃ [19], etc. These materials are widely used to treat dyes such as Titan yellow (TY) and Congo red that are released into the environment from industrial and textile effluents. Despite wide application, some studies show a toxicity assay [20,21,22].

Iron oxide, Fe₃O₄, is a promising candidate for aqueous pollution removal, offering non-cytotoxicity, a high surface-area-to-volume ratio, strong adsorption capacities, and photostability. They can be easily doped with several metals, including zinc [23], for biomedical [24,25,26] or environmental use such as the removal of phenol and ketoprofen [27], methylene blue dye [28], methyl orange, Congo red [29] and rhodamine B dyes [30], malachite green [31], H₂S [32], Hg(II) ions [33], and Th(IV) ions [34]. Additionally, their magnetic properties can facilitate the removal of nanoparticles with adsorbed pollutants from an aqueous suspension with an external magnet.

In this paper, we present an application of iron-oxide-based nanoparticles doped with zinc for aqueous pollution treatment. The proposed material is environmentally friendly and has a high surface area. Its magnetic properties make it possible to magnetically separate the sorbent after adsorption in a facile way. Nanoparticles prepared using co-precipitation techniques have a high affinity to the adsorption of dye-based pollutants on their surface, and post-treatment, they can be collected on magnets and removed from the solution, leading to high water remediation efficiency. The obtained material has superparamagnetic properties, spherical shape, a size below 20 nm and a developed surface that makes it suitable for aqueous pollution treatment. Titan yellow and Congo red removal via magnetic separation was investigated using UV-vis spectrometry, revealing the potential of this material for environmental studies. Zinc dopant improves the magnetization saturation of nanoparticles and adsorption efficiency, while optimal content must be maintained. The proposed nanoparticles have the advantage of magnetic filtration in the separation of dye from water, making them a promising material for environmental remediation.

2. Experimental Section

2.1. Chemicals

Iron (III) chloride hexahydrate FeCl₃·6H₂O was supplied from Aldrich ACS reagent 97%, and iron (II) chloride tetrahydrate FeCl₂·4H₂O p.a. ≥99% (RT) were supplied from Sigma Aldrich (St. Louis, MO, USA). ZnCl₂·4H₂O was purchased from Fluka Analytical, pure p.a. >98%. Deionized water with resistivity of 18.2 MΩ·cm at 25 °C was obtained using the Milli-Q ultra-pure water-filtering system from Merck, Germany. Titan yellow and Congo red of analytical grade were purchased from Warchem Sp. z o.o., Poland.

2.2. Synthesis of Nanoparticles

Nanoparticles were synthesized using the wet co-precipitation method, where 1.35 g of FeCl₃·6H₂O was placed in a beaker and diluted in 25 mL of Milli-Q water and continuously stirred at 600 rpm at 85 °C. Then, different amounts of the salt of FeCl₂·4H₂O and ZnCl₂·4H₂O were added depending on the dopant content. The ratio between the salts is presented in

Table 1. Mass of particular salts used for synthesis of iron oxide-based nanoparticles.

Source of Ions	FeCl3·6H2O	FeCl2·4H2O	ZnCl2·4H2O
Fe3O4	1.35 g	497.0 mg	-
Fe3O4@1%Zn		493.6 mg	3.40 mg
Fe3O4@2%Zn		490.2 mg	6.80 mg
Fe3O4@5%Zn		472.1 mg	24.85 mg
Fe3O4@10%Zn		447.3 mg	49.70 mg

.

2.3. Characterization

The morphology of the nanoparticles was investigated using scanning electron microscopy (SEM), specifically a Zeiss Crossbeam 350 microscope equipped with energy-dispersive X-ray spectroscopy (EDS) and electron-backscattered diffraction (EBSD) detectors. EDS and EBSD data were analyzed using EDAX TEAM software. The powder sample was placed on PELCO^® double-sided carbon-based conducing tape, and the tape was glued to an aluminum sample holder. As a second technique, the transmission electron microscopy (TEM) Zeiss Libra 120 Plus (Stuttgart, Germany), operating at 120 kV, was used, where the aqueous suspension was placed on a copper mesh covered with a formvar polymer layer. Then, the sample was left on air (under the fume-hood) for drying. Brunauer–Emmett–Teller (BET) analysis was performed using Asap 2060 micromeritics. The saturation magnetization of SPIONs was measured using a homemade vibrating sample magnetometer (homemade VSM) under the maximum applied field of 11 kOe at room temperature. The semiconductor properties were measured using potentiostat chi660c. X-ray diffraction spectra (XRD) were registered on a Rigaku Ultima IV diffractometer using Cu-Kα (λ = 1.54056 Å) with a step angle of 0.030°, a scanning rate of 0.04285° s⁻¹, and 2θ degree in the range of 10–70°. Adsorption studies were performed using a UV-vis/NIR spectrometer (Perkin Elmer Lambda 1050+). TGA-DTG analysis from room temperature to 500 °C at 5 °C min⁻¹ in air was performed on a macro-thermogravimetric system (Montpellier, France) to determine the thermal behaviors of the Fe₃O₄@10%Zn before and after adsorption TY.

2.4. Adsorption Study

The effectiveness of Titan yellow dye removal from aqueous solutions was investigated using UV-vis spectrometry in a wavelength range from 250 nm to 800 nm. Initially, 2 mL of 25 ppm solution (0.36 mM) were exposed to the adsorbent containing different amounts of zinc dopant, and each measurement was performed at least three times. The amount adsorbed onto the nanoparticles was calculated using Equation (1).

$$Amount adsorbent=\frac{(c_b-c_a)}{adsorbent{ }_{mass} \left(g\right)}\times V$$

(1)

C_a and C_b are concentration before and after adsorption (mg L⁻¹), while V is the volume of the solution (L). Additionally, we also analyzed the effect of the contact time, and the adsorbate concentration.

Maximum adsorption efficiency for all nanostructures was calculated using Equation (2).

$$\%H=\frac{c_0-c_x}{c_0}\times 100\%$$

(2)

where C₀—initial concentration of dye (ppm), C_x—amount of dye adsorbed on the surface of nanoparticles (ppm), and %H—adsorption efficiency (%).

3. Results

3.1. Morphology Studies

As can be seen in Figure 1, the morphology of the particles is similar, where grain-like structures of a size below 20 nm can be distinguished [35]. Nanostructures agglomerate, while cavities and pores in between the bulk of agglomerates are visible. Since samples are dried in air from aqueous suspension, they tend to agglomerate during the drying process. Despite the differences in the content of dopants, there are minor changes in morphology. EDS mapping revealed the differences in zinc distribution in the sample for particular samples with different doping levels. The morphology is similar to the SEM images described in the literature [36,37].

Additionally, the features of the particles were examined using TEM. As shown in Figure 2, the size of the average particles is about 15 nm. Nanoparticles form spherical structures, with darker spots the effect of overlapping [38].

3.2. BET Analysis

The BET-specific surface area and micropore area of the nanoparticles were investigated using BET (Brunauer–Emmett–Teller) isotherms. Based on the adsorption–desorption isotherms, the porosity of the five samples was analyzed from the N₂ adsorption method using an ASAP 2060 micromeritics analyzer. The specific surface area was determined using the BET method, and the microporous area and volume were estimated using a t-plot. The mesoporous area and volume, estimated using BJH, was also conducted. Results are given in

Table 2. Surface areas and pore volumes of five samples.

Sample	STotal (m2 g−1)	SMicro (cm2 g−1)	SMeso (cm2 g−1)	VMeso (cm3 g−1)	dMean pore (nm)
Fe3O4	97.9	0	100.4	0.17	6.78
Fe3O4@1%Zn	85.7	2.3	54.5	0.03	2.41
Fe3O4@2%Zn	93.1	0	70.0	0.04	2.29
Fe3O4@5%Zn	91.4	1.6	66.9	0.04	2.30
Fe3O4@10%Zn	101.5	5.2	63.3	0.04	2.40

. It can be seen that all five samples had quite similar values of S_Total, ranging from 85.7 to 101.5 m² g⁻¹, where the highest values were obtained for pristine Fe₃O₄ and Fe₃O₄@10%Zn. Moreover, their S_Micro and V_Micro values were close to zero, suggesting that the porosity of these samples is not given by the micropores, but mostly by the mesopores formed by the aggregates inside the samples. The materials with different percentage of Zn (from 1% to 10%) had mostly the same mesoporous structures, expressed by the similarities in S_Meso, V_Meso and d_Mean pores, as seen in the literature [39,40]. However, the results of Fe₃O₄ indicated that the pristine material had a higher mesopore volume, which was composed of larger amounts of mesopores compared to the materials doped with Zn. It is suggested that the Zn content added to the Fe₃O₄ was attached to the macropores reducing the pore size and volume of these structures. However, the highest surface area was recorded for Fe₃O₄ and Fe₃O₄@10%Zn, which agrees with the TY adsorption studies (these samples removed dye at the highest efficiency).

3.3. Crystallinity

Due to the different compositions of synthesized particles, the crystal structure was determined using the X-ray diffraction (XRD) technique. As shown in Figure 3, the XRD diffractograms of the samples doped with zinc are similar, while for undoped Fe₃O₄, minor shifts in the position of particular patterns are visible. All the samples exhibited the typical crystal structure of Fe₃O₄ (JCPDS card 02-088-0315), including peaks at 30.1°, 35.4°, 43.1°, 53.5°, 57.1°, and 63.2°. These can be ascribed to the (220), (311), (400), (422), (511), and (440) planes, respectively [38,41,42,43]. At the same time, in all doped samples, the peak at 30.1° is increased, corresponding to the (220) plane of the Zn_xFe_(1−x)Fe₂O₄ [23,44]. Aghazadeh also presented the growth of peaks after the Zn doping of iron oxide [45]. Similar results were presented by Paz-Díaz et al. [46] and Shanmugavaniand and Selvan [47]. As there are no additional peaks in the diffractograms for a multiphase system as in the literature [48,49,50], it can be assumed that the particles contain one phase. The particles obtained within this work were estimated using the Debye–Scherrer Equation (3).

D = (Kλ/β cos θ),

(3)

where D is the size of the crystallite (nm), K is known as the Scherer’s constant (K = 0.94), λ is the X-ray wavelength (λ = 1.54056 Å), β is full width at half maximum (FWHM) of the diffraction peak, and θ is the angle of diffraction.

The following size of crystallites were determined: Fe₃O₄—11.01 nm, Fe₃O₄@1%Zn—12.74 nm, Fe₃O₄@2%Zn—12.77 nm, Fe₃O₄@5%Zn, 11.98 nm, and Fe₃O₄@10%Zn—10.48 nm.

3.4. Magnetic Properties

The magnetic properties of the nanoparticles were evaluated using a homemade vibrating sample magnetometer, where the measurements were recorded at 300 K. The hysteresis loops of the magnetic samples are shown in Figure 4. The magnetization increases with the increase of zinc content in the crystal structure. The saturation magnetization (M_s) values are in the order 60.89 emu g⁻¹ for Fe₃O₄, 63.32 emu g⁻¹ for Fe₃O₄@1%Zn, 64.28 emu g⁻¹ for Fe₃O₄@2%Zn, 68.69 emu g⁻¹ for Fe₃O₄@5%Zn, and 68.72 emu g⁻¹ for Fe₃O₄@10%Zn. Similar values of saturation magnetization (M_s) and low coercivity (H_c) of all samples were recorded, and be ascribed to superparamagnetic properties. The obtained values are similar to the data described in the literature [50,51]. It reveals the highest effect of zinc dopant with 10% wt.%. on the saturation magnetization.

3.5. Optical Absorption Properties

The band gap of the prepared nanoparticles was determined using a UV-vis spectrometer. The optical absorption spectra relating to the transition of the electrons from the valence band to the conduction band were recorded to estimate the band-gap energy E_g for SPIONs. Based on the UV-vis spectra and Tauc equation (see Equation (4)) the optical band-gap energy was calculated.

αhν = K(hν − E_g)_n

(4)

where: α is the absorption coefficient of semiconductor, hν is a photon energy, K is the frequency-independent material constant, and exponent n is the electronic transition, where it depends on the nature of transition in the semiconductor that is equal to ½ allowing for direct transition.

The band gap for undoped Fe₃O₄ is equal to 2.35 eV, and similar values were also recorded for the 1–5% dopants. However, for SPION@10%Zn, the band gap had a lower value. The corresponding Tauc plots yielding the energy band gap are in good agreement with the literature [52]. However, as the optical properties depend on experimental conditions, the values can be easily disturbed. Figure 5 illustrates the Tauc plot for the SPIONs with different dopant content, and it is clearly seen that the band-gap energy (an average value from three measurements) for all samples is almost the same. The values for samples are also presented in

Table 3. Band-gap value for samples with different concentration of zinc in the crystal structure of nanoparticles measured three times for each sample type.

Nanoparticles	Band-Gap Energy [eV]
Fe3O4	2.35 ± 0.08
Fe3O4@1%Zn	2.35 ± 0.10
Fe3O4@2%Zn	2.34 ± 0.29
Fe3O4@5%Zn	2.33 ± 0.30
Fe3O4@10%Zn	2.24 ± 0.08

.

3.6. Adsorption Studies of Titan Yellow

Titan yellow dye has two characteristic absorption peaks: the first is located at 405 nm, and the second is at about 333 nm. The effect of the Fe₃O₄ doped with Zn nanoparticles on the removal of Titan yellow was evaluated by adding various amounts of adsorbent—10, 20, 40, 100, and 200 mg—into beakers containing 20 mL with an initial dye concentration of 25 mg L⁻¹ at pH 7.0. The dye was exposed to the adsorbent (adsorbent was added to the beaker and the solution was not mixed) for 30 min. Prior to UV-vis measurement, the nanoadsorbents were placed in a beaker filled with dye solution. Then, the suspension was mixed mechanically and, after a particular contact time, the nanoadsorbent was separated magnetically, and collected at the bottom of the beaker. Next, after nanoadsorbent treatment, the solution was removed from the beaker using a pipette and placed in a quartz cuvette.

Measurements were performed for all SPIONs containing 0%, 1%, 2%, 5%, and 10% zinc dopant. By increasing the amount of the adsorbent, the absorbance of dye remaining in the solution decreased as more active sites on the SPIONs’ surface became available for absorbing the molecules of dye, as shown in Figure 6. The presented spectra were recorded for 30 min contact time as an overview to present the absorbance drop with an increase of the adsorbent mass for a particular type of sample. The survey spectra for doses affected TY adsorption. Based on the recorded spectra, 20 mg was proposed to be used in Titan yellow adsorption studies.

Within this part of adsorption studies, the optimal dose of particles was investigated. Based on the recorded spectra, a more intensive peak was used to optimize the experimental conditions for optimal dye removal from solutions. Figure 7 reveals the effect of mass and dopant on the effectiveness of dye adsorption, where each measurement was repeated a minimum of three times, and the average values are presented on the graphs. The curves present maximum adsorption efficiency, %H, in the function of time, where each dopant was marked with different colors. As can be seen in Figure 7a, the effectiveness of the TY from the highest, in order, is Fe₃O₄@10%Zn > Fe₃O₄ > Fe₃O₄@1%Zn > Fe₃O₄@2%Zn > Fe₃O₄@5%Zn for the 10 mg, 20 mg, and 40 mg of the adsorbent dose. With the following increase of the adsorbent dose, still, Fe₃O₄ and Fe₃O₄@10%Zn absorb the dye similarly with the highest efficiency.

After the mass-effect studies, UV-vis measurements were followed with the contact time of the nanostructures on the adsorbate, where %H was measured as a function of time. Based on the mass-effect studies, it can be seen that 100 and 200 mg doses are large enough to remove the pollutant. However, from an economical point of view, such an amount is not cost-effective, so the 20 mg dose was proposed. Moreover, for these data, the kinetics of the process was examined because, at higher masses, the kinetic adsorption process is too fast, and it would be hard to notice the kinetic profile. The measurements were performed at the same adsorbate concentration. Figure 8 shows the highest effect for pristine iron oxide and SPIONs doped with 10% zinc, where within 60 min the maximal adsorption efficiency was observed.

3.7. Adsorption Kinetics of Titan Yellow

Based on the experimental results obtained within the UV-vis spectrometry, the kinetics of the adsorption mechanism was investigated; see Figure 9. The kinetics of the adsorption processes were determined using Lagergren’s pseudo-first-order law and McKay–Ho’s pseudo-second-order law mode given in Equations (5) and (6), respectively:

$$ln\left(q_e-q_t\right)=\ln \left(q_e\right)-k_{1}t$$

(5)

where q_t (mg g⁻¹) is the adsorption capacity at time t, q_e (mg g⁻¹) is the adsorption capacity at equilibrium, and k₁ (min⁻¹) is the pseudo-first-order adsorption rate constant.

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$$

(6)

where k₂ (g min⁻¹ mg⁻¹) is the pseudo-second-order rate constant for adsorption.

As shown in Figure 9, based on the R² values for pure iron oxide, the adsorption process follows the kinetics of the pseudo-second order. The adsorption kinetics for undoped iron oxide is consistent with the literature [35,53]. The value R² for 10% of zinc is similar to the pseudo-first order; however, the pseudo-second-order sorption can be approximated more appropriately by the pseudo-second-order kinetic model; see Figure 10. The samples containing dopants lower than 10% undergo the pseudo-first-order kinetics.

Pseudo-first order of adsorption process:

Pseudo-second order of adsorption kinetics:

Additionally, thermogravimetric analysis (TGA) was performed for the samples removing TY most efficiently, i.e., Fe₃O₄ and Fe₃O₄@10%Zn. Measurements were taken on samples before adsorption and after dye removal in the optimized conditions from the 25 ppm of TY. As can be seen in Figure 11, in all samples, mass loss is observed. The initial mass loss above 100 °C corresponds to the desorption of water, while for the samples after dye treatment the weight loss associated with the thermal degradation of Titan yellow. Complementary to the UV-vis studies, TGA analyses confirms the adsorption of TY on the nanoparticles. As the TY degrades under the thermal treatment, nanoparticles can be re-used for adsorption studies.

3.8. Adsorption Isotherm of Titan Yellow

The experimental data obtained from the adsorption of Titan yellow (TY) using Fe₃O₄ and Fe₃O₄@10%Zn were fitted into the non-linear equation of Langmuir, Freundlich, and Redlich-Peterson adsorption isotherms. Therefore, the plot of the amount adsorbed at equilibrium (q_e) versus the equilibrium adsorbate concentrations (C_e) is shown in Figure 12. The non-linear equations of Langmuir, Freundlich, and Redlich-Peterson are presented in Equations (7)–(9), respectively.

$$q_e=\frac{q_{max}{C}_e{K}_{L }}{\left(1+K_L{C}_e\right)}$$

(7)

$$q_e=K_F{C}_e^{1/n_F}$$

(8)

$$q_e=\frac{K_{RP}{C}_e}{\left(1+a_{RP}{C}_e^g\right)}$$

(9)

where $C_e$ is the equilibrium concentration (mg L⁻¹), $q_e$ is the equilibrium adsorbed (mg g⁻¹), $q_{max}$ is the maximum adsorption capacity (mg g⁻¹), $K_L$ is Langmuir equilibrium constant (L mg⁻¹), $K_F$ is the Freundlich constant (mg g⁻¹ (mg L⁻¹)^−1/nF), $n_F$ is the dimensionless exponent of Freundlich, where K_RP (L g⁻¹) and a_RP (mg L⁻¹)^-g are the Redlich-Peterson constants and g is dimensionless.

Langmuir and Freundlich assume homogeneous and heterogeneous adsorption processes, while Redlich-Peterson made up the limitations of both Langmuir and Freundlich isotherms by incorporating the features of the two isotherms [54]. The parameters of the equations, adjusted coefficient of determination $({\mathrm{R}}_{\mathrm{adjusted}}^2$), and standard deviation (SD) obtained from the curve fit of the isotherms are shown in

Table 4. Adsorption parameters.

Parameters	Fe3O4	Fe3O4@10%Zn
Langmuir
qmax (mg g−1)	30.0	43.0
KL (L mg−1)	0.041	0.094
${\mathrm{R}}_{\mathrm{adjusted}}^2$	0.966	0.985
SD (mg g−1)	1.72	1.77
Redlich-Peterson
KRP (L g−1)	0.843	4.28
aRP (mg L−1)-g	0.004	0.107
g	1.39	0.981
${\mathrm{R}}_{\mathrm{adjusted}}^2$	0.985	0.983
SD (mg g−1)	1.13	1.86
Freundlich
KF (mg g−1) (mg L−1)−1/nF	4.66	10.36
nF	2.79	3.40
${\mathrm{R}}_{\mathrm{adjusted}}^2$	0.889	0.937
SD (mg g−1)	3.14	3.61

. The respective values of the R²_adjusted and SD of Langmuir and Redlich-Peterson for Fe₃O₄@10%Zn are approximately near each other; this implies that the two isotherms can accurately describe the adsorption of TY on Fe₃O₄@10%Zn. In the case of Fe₃O₄, R²_adjusted of Redlich-Peterson is higher than that of Langmuir, while the SD is lower. However, to ascertain the conformation of the experimental data with Redlich-Peterson, the value of g must be less than 1 [55]. As shown in Table 1, the value of g obtained for Fe₃O₄@10%Zn is less than 1, which indicates that the Redlich-Peterson (RP) isotherm model is suitable for explaining the adsorption process of TY on the Fe₃O₄@10%Zn. When g in RP equations is equal to 1, then it reduces to Langmuir. Therefore, the overlapping of Langmuir and RP in Figure 12b could be because g (0.9817) of RP is close to 1. Despite the higher value of R²_adjusted and lower SD of Redlich-Peterson compared to Langmuir for Fe₃O₄, the value of g that is higher than 1 makes it inapplicable for the adsorption process of TY on Fe₃O₄. The experimental data are also not well fitted into the Freundlich isotherm for both Fe₃O₄ and Fe₃O₄@10%Zn.

The monolayer adsorption capacity Fe₃O₄@10%Zn for the removal of TY is 43 mg g⁻¹, while that of Fe₃O₄ is 30 mg g⁻¹, which are higher than some of the adsorbents reported in the literature (

Table 5. Comparison of the maximum adsorption ($q_{max}$) of different adsorbents for the removal of Titan yellow.

Adsorbents	qmax (mg g−1)	References
NiFe2O4	19.193	[56]
Chitosan	58.76	[57]
Saccharum spontaneum	3.984	[58]
Walnut Husks	7.6982	[59]
Bio-Waste Aloe vera Leaves	55.25	[60]
Fe3O4	30.0	This work
Fe3O4@10%Zn	43.0	This work

). The higher adsorption capacity of Fe₃O₄@10%Zn than pristine Fe₃O₄ is related to the difference in their specific surface area.

3.9. Effect of pH and Ionic Strength on the Adsorption of TY

The role that effect of pH plays in understanding the adsorption process of any contaminant, especially for the wastewater treatment [61]. Given this, the study of the effect of pH on the adsorption of TY by Fe₃O₄@0% Zn and Fe₃O₄@10% Zn was investigated. The two materials were chosen based on their better performance compared to other synthesized materials used in this research work. The results are shown in Figure 13a. Increasing the solution pH decreases the amount of TY that was absorbed by Fe₃O₄@0% Zn and Fe₃O₄@10% Zn. The highest amounts, 23 and 24.4 mg g⁻¹ of TY, were absorbed by Fe₃O₄@0% Zn and Fe₃O₄@0% Zn, respectively, at pH 2. At pH 10, the amount reduced to 5.7 and 7.9 mg g⁻¹ by Fe₃O₄@0% Zn and Fe₃O₄@10% Zn, respectively. The reason for this is the anionic nature of TY dye [62], which is expected to be favorably adsorbed on a positively charged surface of an adsorbent. Our previous study [35] showed that iron oxide nanoparticles are positively charged at pH below 7. Therefore, the highest amount adsorbed at pH below 7 is due to an electrostatic interaction between the anionic TY dye and the positively charged Fe₃O₄@0% Zn and Fe₃O₄@10% Zn. One would expect an electrostatic repulsion at pH 8 and 10, but TY was adsorbed at these pH values, by Fe₃O₄@0% Zn and Fe₃O₄@10% Zn, though lower than the amount adsorbed at acidic pH values. The explanation for this is provided in the further discussion on the effect of ionic strength.

In textile industries, salts such as NaCl are used during the dyeing process, which causes effluents from this industry to contain salts [63]. The effects of salts in terms of ionic strength on the adsorption of TY has been studied by varying the concentration of NaCl within the range of 0.04 to 0.2 mol L⁻¹. Since the trend of adsorption of TY by Fe₃O₄@0% Zn and Fe₃O₄@10% Zn with respect to pH followed the same pattern, we investigated the effect of ionic strength on Fe₃O₄@10% Zn as a model for other materials, since it performed better that the rest of the materials. As shown in Figure 13b, the amount of TY adsorbed by Fe₃O₄@10% Zn increased from 20.9 to 24.1 mg g⁻¹ in the presence of 0.04 mol L⁻¹ of NaCl. Further increase in ionic strength did not lead to a significant increase in the adsorption of TY. This shows that the presence of salts in dye effluent that contains TY will promote favorable adsorption. It is worth noting that the presence of ionic strength could increase, decrease or not affect the adsorption of organic molecules [64,65]. Whenever the presence of salts leads to an increase in the adsorption of organic molecules, as with TY, hydrophobic interaction has been credited with the adsorption process [65,66]. Based on the results from the effect of pH and ionic strength, this implies that both electrostatic attraction and hydrophobic interaction are the two mechanisms that are involved in the adsorption of TY. Therefore, the adsorption of TY at alkaline pH is due to the involvement of these two adsorption mechanisms.

4. Photocatalytic Studies

Since the nanoparticles used in this work are semiconducting, they can be used not only for TY removal via adsorption but also for photocatalytic degradation. Based on adsorption studies, the optimal experimental conditions were proposed. So far, the photocatalytic effect of nanostructures on dye degradation of the 20 mg of nanoparticles of all dopants was based on the 20 mL of the 25-ppm Titan yellow solution. Then, the solution was exposed to a mercury lamp (UV-vis source) for 60 min with the fan cooling set up to protect the solution from overheating. The photodegradation of the TY was investigated as complementary to the adsorption studies, where dark adsorption was performed prior to photodegradation to present only the photodegradation effect.

In contrast to the literature where, for example, Vidya et al. proposed the use of zinc oxide nanoparticles to degrade 20-ppm Titan yellow solution with 60 mg adsorbent, in this work, only 20 mg were used to reach similar effectiveness [62,67]. As the highest effectiveness of the adsorption was proved for undoped Fe₃O₄ and Fe₃O₄@10%Zn nanoparticles, the same conditions were proposed to investigate the effectiveness of the TY degradation. Figure 14a shows the photocatalytic efficiency for all samples from 0% to 10% zinc dopants, where it is clearly seen that the highest effectiveness of the particles in the TY treatment is observed for Fe₃O₄@10%Zn and Fe₃O₄, respectively. Undoped iron oxide and 1–2% doped nanoparticles have similar results. The Fe₃O₄@10%Zn reduces the amount of dye in the water solution from 25 ppm to 6.6 ppm, reaching 74% removal efficiency, while Fe₃O₄@5%Zn reaches about 62% efficiency. At this point, analogically to the %H determination without the additional exposition of the solution on the UV irradiation, measurements were also performed under a UV-vis lamp. The reference sample, containing only TY solution, was also illuminated at the same time, without SPION addition to check the degradation of the dye, and no changes in the spectra were recorded. Figure 14b shows the efficiency of the photodegradation process after 60 min of exposition on the samples under a UV-vis light compared with the results obtained for adsorption studies without external light exposition. Interestingly, the highest efficiency is recorded only for Fe₃O₄@5%Zn; however, the effect is only slightly higher than without light exposure. The low effectiveness of the samples on the photocatalytic effect are caused by a too-low band-gap energy compared to the location of the particular TY bands corresponding to the absorption of a certain wavelength of light. Therefore, the proposed samples work better as adsorbents than as photocatalysts for TY treatment, so for photocatalytic studies, material with broader band-gap energy or compounds absorbing blue–green light should be used.

4.1. Adsorption Studies of Congo Red

As the removal of Titan yellow, which is a sulphonic dye, was effective, the following studies were also performed for Congo red—another sulphonic dye—which can also be commonly found in wastewater [68]. Figure 15a presents the absorption spectra for the 20-ppm Congo red solution treated within 20 mg of adsorbent for contact time from 5 to 120 min. Based on spectra similar to TY, efficiency H% was estimated. The highest efficiency was observed for the sample doped with 10% of zinc, reaching about 80% of H%, see Figure 15b. As follows, Figure 15c shows the Congo red solution before treatment and post-treatment with adsorbent.

4.2. Adsorption Kinetics of Congo Red

Next, the kinetics of CR adsorption was investigated. As can be seen in Figure 16a,b, the Fe₃O₄ and Fe₃O₄@1%Zn nanoparticles undergo first-order kinetics during adsorption of CR. The following graphs presented in Figure 16c–e corresponding to the Fe₃O₄@2%Zn, Fe₃O₄@5%Zn, and Fe₃O₄@10%Zn, respectively, where the results were compared with the pseudo-second-order kinetics presented in Figure 17, based on the R² fitting.

Adsorption kinetics models for Fe₃O₄@2%Zn, Fe₃O₄@5%Zn, and Fe₃O₄@10%Zn undergo pseudo-second adsorption, which agrees with the literature [69].

4.3. Effect of pH and Ionic Strength on the Adsorption of CR

The adsorption study of Congo red dye on the proposed adsorbent requires a separate approach due to the different structure of the dye. The influence of pH and ionic strength on the adsorption of the dye were analyzed. The pH study is presented in Figure 18a. With an increase in the solution pH, the amount of CR was less adsorbed for all samples. The highest value of adsorbent was obtained in a pH equal 4, where the attained amounts are about 14.4 mg g⁻¹ for Fe₃O₄@1%Zn and 17.1 mg g⁻¹ for Fe₃O₄@2%Zn. However, such a low pH is not recommended for the proposed nanoparticles due to their partial dissolution in acidic media below pH 4. At the pH 10, the amount was reduced to 4.2 mg g⁻¹ for Fe₃O₄@10%Zn and 3.6 mg g⁻¹ for Fe₃O₄@2%Zn, respectively. CR has anionic character [70], which is also similar to the TY case favorably adsorbed on a positively charged surface of an adsorbent, so acidic or neutral pH is better than alkalic for the effective removal of CR. Since the absorption efficiency was similar for all samples having different dopant content at pH 6, the following studies (ionic strength effect and isotherms) were performed at pH 6.

Next, complementary to TY studies, the effect of ionic strength on CR removal was also investigated at different concentrations of NaCl solution in a range from 0.04 mol dm⁻³ to 0.20 mol dm⁻³ concentration. In this case, we investigate this effect for Fe₃O₄@2%Zn and Fe₃O₄@10%Zn because we obtain the best results for these samples with CR. As shown in Figure 18b, for 0.04 concentration of salt, the amount of absorbency achieved is 11.8 mg g⁻¹ for Fe₃O₄@2%Zn and 11.4 mg g⁻¹ for Fe₃O₄@10%Zn. Increasing salt concentration to 0.20 mol dm⁻³, the amount of absorbency achieved is 14.9 mg g⁻¹ for Fe₃O₄@2%Zn but for Fe₃O₄@10%Zn it is only 12.1 mg g⁻¹.

4.4. Adsorption Isotherm for Congo Red

Complementary to the TY adsorption isotherms studies, measurements for CR were also performed, where analysis was made for the Fe₃O₄@10%Zn sample revealing the highest adsorption capacity. Figure 19 presents the experimental results and fitting to particular models.

Based on the isotherms’ parameters as listed in Table 5, the R² adjusted and SD of Langmuir and Redlich-Peterson are relatively the same, which could imply that both isotherms are applicable for describing the adsorption of Congo red on Fe₃O₄@10%Zn. Meanwhile, the value of g in Redlich-Peterson as shown in

Table 6. Adsorption parameters.

Fe3O4@10%Zn
Langmuir
qmax (mg g−1)	59
KL (L mg−1)	0.033
${\mathrm{R}}_{\mathrm{adjusted}}^2$	0.984
SD (mg g−1)	2.07
Redlich-Peterson
KRP (L g−1)	1.64
aRP (mg L−1)-g	0.009
g	1.2
${\mathrm{R}}_{\mathrm{adjusted}}^2$	0.985
SD (mg g−1)	2.04
Freundlich
KF (mg g−1) (mg L−1)−1/nF	5.183
nF	2.06
${\mathrm{R}}_{\mathrm{adjusted}}^2$	0.945
SD (mg g−1)	3.75

is 1.2. However, since the value of g must be less than 1, before it can be accepted to be suitable to describe the adsorption process, the best isotherm for the adsorption of Congo red using Fe₃O₄@10%Zn is Langmuir. This implies the monolayer adsorption of CR on the surface of Fe₃O₄@10%Zn.

Table 7. Comparison of the maximum adsorption ($q_{max}$) of different adsorbents for the removal of Congo red.

Adsorbents	qmax (mg g−1)	References
ZnO-polypyrrole magnetic nanocomposite	38.0	[71]
ZnO	25.0	[72]
magnetic HKUST-1	49.5	[73]
mesoporous-TiO2/Fe3O4	50.0	[74]
Fe3O4	28.46	[75]
α-Fe3O4	57.2	[76]
SnO2	24.27	[77]
Fe3O4@10%Zn	59.0	This work

shows the comparison of the experimental results obtained in this work for CR adsorption capacity with the literature.

4.5. Photocatalytic Studies for Congo Red

For Congo red, the highest photocatalytic efficiency is recorded for Fe₃O₄@10%Zn, see Figure 20a,b. Based on TY research into photo-assisted degradation, Congo red dye used 20 mg of nanoparticles to 20 mL concentration 25-ppm model pollution. In the case of photocatalysis, Congo red achieved better results than Titan yellow. The highest % of removal was accomplished for 10% zinc dopped from 25 ppm to 7.1 ppm. After 60 min for Fe₃O₄ and Fe₃O₄@1%Zn and Fe₃O₄@5%Zn, the results are similar. Comparing photodegradation results, both dyes using Fe₃O₄@10%Zn removal are similar, at more than 70%. Fe₃O₄@2%Zn under UV-vis lamp obtains a higher percentage removal of CR than TY.

5. Conclusions

In this work, superparamagnetic iron oxide-based nanoparticles containing different levels of zinc dopant up to the 10% of the wt.% were synthesized and applied as an effective adsorbent of Titan yellow and Congo red sulphonic model dyes. The morphology of the prepared nanoparticles was estimated with SEM and TEM, revealing uniform round shape and size up to 15 nm. The increase of zinc content in the SPION structure improved their magnetization saturation, from 60.89 emu g⁻¹ for pristine Fe₃O₄ to 68.72 emu g⁻¹ for Fe₃O₄@10%Zn. A minor shift was observed in the diffractogram of the all the samples that contained Zn, which could be due to substitution of Fe with Zn. The BET studies show the total surface area is in the range of 85.7 to 101.5 m² g⁻¹, where the highest values were obtained for pristine Fe₃O₄ and Fe₃O₄@10%Zn. The amount of Titan yellow that was removed increased in line with an increase in the mass of the adsorbent, with the highest adsorption efficiency at 60 min of contact time. UV-vis studies also proved the highest adsorption efficiency for pristine Fe₃O₄ and Fe₃O₄@10%Zn for TY removal. The experimental data obtained using Fe₃O₄ fits well with the isotherm models as proposed by Langmuir, while that of Fe₃O₄@10%Zn was in accordance with the isotherm models proposed by Langmuir and Redlich-Peterson for TY. According to the Congo red adsorption process, Fe₃O₄@10%Zn could be fitted with the same models. The monolayer adsorption capacities for TY were 30 and 43 mg g⁻¹ for Fe₃O₄ and Fe₃O₄@10%Zn towards TY treatment, respectively, and 59 mg g⁻¹ Fe₃O₄@10%Zn for CR adsorption. These values are higher than some of the reported adsorbents in the literature for materials offering similar adsorption efficiency. It can be seen that even minor changes to the physicochemical properties of the proposed material have different effects on the adsorption efficiency and the photocatalytic degradation of dyes. The main advantage of the proposed material is magnetic separation that enables the fast removal of sorbent from the solution without filtration and/or centrifuging. We show that direct dye treatment with a functional magnetic sorbent can be an easier solution and less energy-consuming than the photo-assisted method. The proposed material is effective in water pollutant treatment. However, there is a lack of commercial solutions for their management. The next step will include different dyes, including non-sulfonic molecules, their mixtures, and real wastewater treatment.