Fe₃O₄@Granite: A Novel Magnetic Adsorbent for Dye Adsorption

Abstract

Magnetic granite (MG), a new and low-cost adsorbent, was prepared by the chemical co-precipitation of Fe²⁺ and Fe³⁺ using granite (G), which is a magmatic rock type. The adsorption of the Reactive Black 5 (RB5) dye from aqueous solutions on Fe₃O₄-modified granite was examined in a batch system. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray fluorescence spectrometry (XRF), X-ray diffractometry (XRD), N₂ adsorption–desorption, vibrating-sample magnetometry (VSM), and point-of-zero charge (pH_pzc) analysis were used to characterize the prepared MG. Magnetic granite displayed significant magnetization and could be easily separated using external magnets. The maximum adsorption capacity was 29.85 mg/g at 298 K. According to kinetic and isothermal examinations, the pseudo-second-order model and Langmuir isothermal adsorption were the best fit for adsorption. It was found that the enthalpy change ΔH (kJ/mol) was −31.76, and the entropy change ΔS (kJ/mol) was 0.096 for a temperature change of 298–330 K. The ΔG° (kJ/mol) value was negative at all temperatures (298 K, −2.86 kJ/mol; 303 K, −2.85 kJ/mol and 313 K, −1.50 kJ/mol), indicating that the adsorption of RB5 on MG was spontaneous.

1. Introduction

The dye content of wastewater from the paper, pharmaceutical, plastic, leather, and, especially, textile industries is very considerable. More than 1000 tons of dyes—45% of which are reactive dyes—are discharged into natural waters by the effluents of various enterprises each year [1]. Dyes take an important place in these sectors. Dyes belong to the class of aromatic and heterocyclic compounds with azo bonds (-N=N-) and exhibit toxic, mutagenic, and even carcinogenic properties. They cause considerable damage to the central nervous and digestive systems and the liver [2]. The discharge of untreated wastewater with high dye contents into the environment where living organisms are present is among the main sources of water pollution [3]. The above-mentioned negative properties impact the life system in nature, including plants, animals, and humans. Therefore, decolorizing wastewater from industries in which dyes are used is also important for the whole ecosystem [4].

Reactive Black 5 is a diazo acidic reactive dye known as Remazol Black B in the industry (

Table 1. The physical and chemical properties of the RB5 dye.

Chemical Name	Reactive Black 5 (Remazol Black B)
Chemical formula	C26H21N5Na4O19S6
Molecular weight	991.82 g/mol
Maximum absorbance	597 nm
Ionic structure	Anionic dye
Water solubility	Highly soluble
Molecular structure

). Reactive dyes are widely used due to their low cost, bright color, and the property of providing effective coloring by binding to materials through covalent bonds. Studies reported that prolonged exposure to RB5 may cause skin rashes, bladder cancer, chromosomal abnormalities, respiratory and kidney failure, and asthma [5,6,7]. Furthermore, reactive dyes are highly soluble in water. Therefore, it is challenging to remove them from wastewater using conventional methods [8].

Numerous methods have been reported in the literature for dye removal, such as electrocoagulation [9], the Fenton [10,11] and photocatalytic reaction [12,13,14,15], and adsorption [16,17]. Adsorption is preferred among the said methods because of its simple application, low cost, and reproducibility [16,18].

However, using the adsorption method, the efficient separation of adsorbents from water is difficult and may take a long time [19]. Fe₃O₄ magnetic particles are characterized by a large surface area and their easy separation from reaction media. The large surface area, biocompatibility, non-toxicity, and easy synthesis of Fe₃O₄ enable its use in various fields [20]. Magnetic particles and composite materials made with magnetic particles are considered potential materials for the removal of pollutants from aqueous media [21,22,23,24,25]. Magnetic materials are preferred to centrifugation or filtration separation methods as they are more effective, with respect to the time and the energy required, in adsorption processes [26]. After adsorption, magnetic materials containing the adsorbed compounds are separated by a magnet. This separation method is reported to be faster, safer, and more efficient than centrifugation and filtration [27,28,29].

Some studies were conducted on the removal of the RB5 dye from aqueous media using various adsorbents. Low-cost agricultural products [30,31,32,33], pumice [34], activated carbon [35], zeolite [8,36], clay [37], and various composite materials [38,39,40] were used for RB5 removal.

The present study used granite, a magmatic rock type, as the basic material constituting the adsorbent’s structure [41]. Granite is a rock type comprising quartz and alkali feldspar from 20% to 60% [42]. To the best of our knowledge, no study in the literature has examined the use of granite or magnetic granite as an adsorbent. Granite was magnetized and used for RB5 removal. On the basis of its magnetic properties, it was separated from the environment quickly and without energy consumption after the process. The results showed that MG could be utilized as a novel alternative adsorbent in wastewater treatment.

2. Materials and Methods

2.1. Materials

The granite used in the study was obtained from the Davulalan region, Sivas. RB5 dye, FeSO₄·7H₂O, FeCl₃, and NH₄OH were acquired from Sigma-Aldrich (St. Louis, MO, USA).

2.2. RB5 Stock Solution Preparation

The RB5 dye was utilized without any prior preparation. The stock solution used in the study was prepared at 1000 mg/L and diluted to the desired concentrations. The pH of the RB5 solutions was adjusted using 0.1 M NaOH and 0.1 M HCl.

2.3. Synthesis of Magnetic Granite (MG)

Magnetic granite was prepared by the co-precipitation method. FeCl₃ and FeSO₄·7H₂O were used, and the Fe²⁺/Fe³⁺ mole ratio was adjusted to 1/2. The solution prepared with Fe²⁺ and Fe³⁺ (1:1 volume ratio) was stirred at 80 °C until homogenization. Then, 2.5 g of granite was added to the Fe²⁺ and Fe³⁺ solution, stirring at 80 °C for 2 h. In the end, 25% NH₄OH solution was slowly added to the mixture. The solution’s color turned black, indicating that magnetic particles had formed. The residual ammonium ions were eliminated from the mixture by repeatedly washing it with distilled water. The washed magnetic granite particles were dried at 70 °C [43]. Figure 1 shows the synthesized MG.

2.4. Granite and Magnetic Granite Characterization

Granite and magnetic granite were characterized by FTIR, SEM, EDX, XRF, XRD, N₂ adsorption–desorption, VSM, and pHpzc.

The morphology of granite and magnetic granite was investigated using SEM and EDX for qualitative chemical analysis. For the purpose of identifying potential functional groups in the structures of granite and magnetic granite, FT-IR analysis in the 400–4000 cm⁻¹ range was carried out. The elemental analyses of granite and magnetic granite were conducted by XRF. The crystal structure of granite and magnetic granite was determined by XRD. To determine the surface and pore characteristics, N₂ adsorption–desorption analysis was performed. The VSM analysis was conducted to identify the magnetic properties of magnetic granite.

2.5. RB5 Removal

A 100 ppm RB5 stock solution was prepared by dissolving 0.1 g of RB5 dye in 1 L of distilled water. Various parameters, such as pH (3.0–7.0), initial dye concentration (20–200 mg/L), adsorbent dosage (0.25–5 g/L), contact time (0–300 min), and temperature (25–40 °C), were examined. The “one-factor-at-a-time method” was employed to design the experimental system. The best parameter conditions were determined sequentially. Batch system experiments were conducted in a 50 mL volume. RB5 concentration was measured by determining the absorbance at 597 nm with a UV–visible spectrophotometer (Merck Spectroquant Pharo 300, KGaA, Darmstadt, Germany). The % removal efficiency and the adsorption capacity were determined according to the following equations (Equations (1) and (2)):

Removal efficiency (%) = [(C_o − C_e)/C_o] × 100

(1)

$$q_e=\frac{(C_o-C_e)V}{m}$$

(2)

C_o refers to the initial concentration (mg/L), C_e is the concentration of the substance remaining unadsorbed at equilibrium (mg/L), V denotes the solution volume (L), and m represents the adsorbent dosage (g).

2.6. Reusability

We added 0.05 g of MG to 200 mg/L of RB5 solution and submitted it to the dye adsorption process for 120 min at ambient temperature (pH: 4.0). After this process, the desorption process with 0.1 M NaOH was applied to RB5-adsorbed MG to determine the desorption efficiency. After rinsing with distilled water to remove any remaining NaOH MG was dried in an oven. These processes were repeated five times, and the regeneration efficiency was calculated by Equation (3).

Regeneration efficiency = (Amount of the dye desorbed/Amount of the dye adsorbed) × 100

(3)

2.7. Mathematical Modeling of the Adsorption System

2.7.1. Adsorption Isotherms

The monolayer or multilayer adsorption of RB5 on MG was analyzed through the use of adsorption isotherms. The Langmuir, Freundlich, and Temkin isotherm models were used for the description of the adsorption system (Table 2).

Table 2. Adsorption isotherms and equations.

Isotherm Model	Equation	Reference
Langmuir Isotherm	$\frac{1}{q_e}=\frac{1}{q_m}+\frac{1}{b.q_m}\frac{1}{C_e}$	[44]
Freundlich Isotherm	$\ln q_e=\ln K_F+\frac{1}{n}\ln C_e$	[45]
Temkin Isotherm	$q_e=\frac{RT}{b_T}\ln (A{C}_e)$	[46]

Table 2. Adsorption isotherms and equations.

Isotherm Model	Equation	Reference
Langmuir Isotherm	$\frac{1}{q_e}=\frac{1}{q_m}+\frac{1}{b.q_m}\frac{1}{C_e}$	[44]
Freundlich Isotherm	$\ln q_e=\ln K_F+\frac{1}{n}\ln C_e$	[45]
Temkin Isotherm	$q_e=\frac{RT}{b_T}\ln (A{C}_e)$	[46]

2.7.2. Adsorption Kinetics

The adsorption kinetic model explained the adsorption mechanism, diffusion phenomena, and the connection between equilibrium time and adsorption capacity (Table 3).

Table 3. Kinetic models and equations.

Kinetic Model	Equation	Reference
Pseudo-first-order kinetic model	$\ln (qe-qt)=\ln qe-k_{1}t$	[47]
Pseudo-second-order kinetic model	$\frac{t}{q_t}=\frac{1}{k_2{q_e}^2}+\frac{1}{q_e}t$	[48,49]
Intraparticle diffusion	$q_t=k_d{t}^{1/2}+C$	[50]

Table 3. Kinetic models and equations.

Kinetic Model	Equation	Reference
Pseudo-first-order kinetic model	$\ln (qe-qt)=\ln qe-k_{1}t$	[47]
Pseudo-second-order kinetic model	$\frac{t}{q_t}=\frac{1}{k_2{q_e}^2}+\frac{1}{q_e}t$	[48,49]
Intraparticle diffusion	$q_t=k_d{t}^{1/2}+C$	[50]

2.7.3. Adsorption Thermodynamics

The Gibbs free energy change (∆G°), enthalpy change (∆H°), and entropy change (∆S°) of RB5 adsorption were evaluated. The ∆G° value is essential since it indicates the spontaneity of a chemical reaction, which can be computed in the following way (Equation (4)):

$$∆\mathrm{G}°=-RT\mathrm{l}\mathrm{n}{K}_c$$

(4)

where K_c ((amount of RB5 in the adsorbent)/(amount of RB5 in the solution)) represents the distribution coefficient.

The enthalpy change (∆H) and entropy change (∆S) were estimated from the equation below (Equation (5)):

$$\ln K_c=\frac{\Delta S}{R}-\frac{\Delta H}{RT}$$

(5)

3. Results

3.1. Characterization of Granite and Magnetic Granite

3.1.1. FTIR Results

A FTIR analysis was conducted with the objective of determining the functional groups in the material structure and explaining the interaction between the materials.

In the FTIR spectrum of granite (Figure 2), the bands at 3691 and 3615 cm⁻¹ originate from the OH stretching of water in the layers of granite [51]. The peaks at 1797 cm⁻¹ and 1442 cm⁻¹ correspond to H-O-H bending vibrations and organic–inorganic hybrid bond peaks, respectively [52]. The stretching vibration of the Si-O-Si and Al-O-Si groups of aluminosilicate is related to the important peak at 1024 cm⁻¹ in the IR spectra of granite [52,53]. This absorption band also indicates the presence of albite [26]. The medium band at 774 cm⁻¹ is attributed to the Si-O-Si bond’s tetrahedral bending vibration; this peak is representative of crystalline quartz [54]. The ring vibrations of framework silicates are responsible for the four extremely weak bands between 538 cm⁻¹ and 700 cm⁻¹ [53].

Upon comparing the FTIR spectra of granite and magnetic granite, it was seen that the wave number did not change in many peaks. However, the transmittance values of the peaks in the magnetic granite spectrum were found to increase (Figure 2). These results showed that the internal structure of granite did not change following modification with iron [55,56,57].

The absorption peak of magnetic granite at ~550 cm⁻¹ is the characteristic band of Fe-O stretching vibrations in Fe₃O₄ particles. It is possible to find the specific vibrations of Fe-O bonds around 460 cm⁻¹ and 570 cm⁻¹. When the granite and magnetic granite peaks were compared, we observed that some peaks were shifted to new values while other peaks disappeared [56,57,58]. In fact, the peaks at 643 cm⁻¹ and 783 cm⁻¹ disappeared. These changes confirmed the modification with Fe₃O₄.

3.1.2. SEM-EDX Results

Figure 3 presents the SEM images of granite, magnetic granite, and magnetic granite after RB5 adsorption. The SEM image of granite shows that it had an agglomerated structure which did not display a homogeneous distribution. It was observed that spherical structures were formed when Fe₃O₄ was added to granite. A similar spherical structure was also observed in a kaolin-based magnetic zeolite [59]. After adsorption, the spherical structure decreased, and the gaps on the surface disappeared.

Figure 4 shows the EDX analysis results of granite, MG, and RB5-adsorbed MG. In the EDX spectrum of granite, it was observed that the structure of granite mainly contained 55.43% O, 9.77% Al, and 23.72% Si as well as low amounts of Na, Mg, K, Ca, and Fe. The EDX spectrum analysis of MG, which was imparted magnetic properties by loading iron to granite, determined that 60.94% Fe was present. C, N, and S in the EDX spectrum of MG after adsorption derived from the RB5 dye, indicating that the RB5 dye was adsorbed by MG.

3.1.3. XRF Results

An XRF analysis was carried out for the purpose of revealing the elemental composition of granite and magnetic granite (

Table 4. XRF results of granite and magnetic granite.

	Element (%)
	Mg	Al	Si	Ti	V	Mn	Fe	Zn	Zr	Nb	Mo	Pb	Bi
G	0.53	21.97	74.775	0.477	0.151	0.285	1.65	0.015	0.047	0.012	0.003	0.032	0.053
MG	0.15	5.14	16.97	0.111	0.03	0.16	77.41	0.01	0.02	0.005	0.002	0.032	0.02

).

The XRF analysis of granite showed that its elemental composition mainly consisted of Si (74.77%) and Al (21.97%). The granite structure contained a very low amount of Fe (1.65%). The percentage contents of the other elements in the table were determined as <1%.

In the XRF analysis of magnetic granite, the Fe (77.41%) ratio increased due to modification, whereas the Si (16.97%) and Al (5.14%) percentages decreased.

The elemental composition of granite and magnetic granite was determined by EDX and XRF analyses. In XRF analysis, analyzing elements with atomic numbers less than 11 is impossible. Therefore, the element “O” is not included in the XRF analysis results, which causes the percentages of elements in EDX and XRF analyses to differ.

Nevertheless, the EDX and XRF results showed a high proportion of Al and Si in the structure of granite and an increased proportion of Fe in the structure of magnetic granite, which was similar in the two analyses.

3.1.4. XRD Results

An XRD analysis was performed at 2θ = 0–65° to determine the crystal phase and structural features of granite and magnetic granite, and the results are presented in Figure 5. The sharp peaks at 2θ = 20.9° and 26.54° are the characteristic peaks belonging to quartz in the granite structure [60]. The peak positions in the XRD pattern of granite were in line with the literature [60,61,62,63]. The peaks at 2θ = 30.26°, 35.6°, 43.32°, 57.36°, and 62.94° in the XRD pattern of iron-modified granite confirmed the addition of Fe₃O₄ to the granite structure [64].

3.1.5. Adsorption–Desorption Analysis

Table 5. N₂ adsorption–desorption results of granite and magnetic granite.

		Granite	Magnetic Granite
Surface area (m2/g)	Single-point surface area at P/Po	0.4532 m2/g	19.0604 m2/g
	BET Surface Area	0.4638 m2/g	19.1211 m2/g
	BJH adsorption cumulative surface area of pores with width between 17.000 Å and 3000.000 Å	0.1485 m2/g	4.006 m2/g
Pore volume (cm3/g)	Single-point adsorption total pore volume of pores	0.000287 cm3/g	0.013088 cm3/g
	BJH Adsorption cumulative volume of pores with width between 17.000 Å and 3000.000 Å	0.000099 cm3/g	0.004697 cm3/g
Pore size (Å)	BJH Adsorption average pore width (4V/A)	39.137 Å	46.901 Å

reports the surface area and pore sizes of granite and magnetic granite. According to the N₂ adsorption–desorption characterization results, the surface area of granite modified with Fe₃O₄ loading was determined by the BET and BJH methods. The surface area of magnetic granite was larger compared to that of granite. While the surface area of granite was 0.46 m²/g, the surface area after iron loading was 19.12 m²/g. The pore size measurement results showed that both materials were mesoporous (d = 2–50 nm).

The increase in the specific surface area with the addition of Fe₃O₄ could be attributed to the partial penetration of iron oxide into the granite layers, which led to the formation of new layers, increasing both the surface area and the interlayer porosity. These results agree with the studies reporting that the modification of a support with Fe₃O₄ increased surface area and pore size [56,65,66].

3.1.6. VSM Results

The magnetic characteristics of granite loaded with Fe₃O₄ were examined with a vibrating sample magnetometer operating between 0 and 15 K. The internal hysteresis loop of magnetic granite demonstrated an “S”-shaped curve at magnetic ambient temperature (Figure 6). The saturation magnetization (Ms) value of iron-modified granite was calculated as 46.04 emu/g, with a magnetization value of 3.5314 emu for 0.0767 g.

A study in the literature reported the Ms value of Fe₃O₄ nanoparticles and Fe₃O₄ in bulk form as 53.81 emu/g and 92 emu/g, respectively [67]. Another study found the MS value of Fe₃O₄ nanoparticles to be 59 emu/g. It was reported that when Fe₃O₄ nanoparticles were coated with chitosan, the MS value was 51.4 emu/g, and the polymer coating process reduced magnetization [68].

The VSM analysis determined the maximum field for iron-modified granite as 25,000 Oe, coercivity (Hci) as 24,515 Oe, and permanence (Mr) as 0.13901 emu. These values confirmed that iron-modified zeolite has adequate magnetic features to be attracted by a permanent magnet [69]. When the external magnetic field was removed, the particles were redispersed and reinstated [70].

3.2. Adsorption Studies

3.2.1. Effect of the Initial pH and of the pH_PZC of MG

A solution’s pH is among the most significant parameters that impact adsorption, since it influences the degree of ionization, the surface charge of the adsorbent, and the structure of the dye to be removed. The effect of pH on the removal of RB5 dye from MG was studied in the pH range from 3 to 7, keeping constant the dye concentration (50 mg/L), the adsorbent dosage (1.5 g/L), the temperature (23 °C), and the contact time (240 min). The results obtained are presented in Figure 7. The adsorption capacity of the MG adsorbent was 25.99 mg/g and 25.53 mg/g at pH 3.0 and 4.0, respectively. The adsorption capacity decreased with an increasing pH of the solution. The lowest adsorption efficiency was 0.2 mg/g at pH 6.98, which was the pH value of the solution. Dye removal was favored at pH values of 3 and 4, and the subsequent experiments were performed at pH 4.0. Electrostatic attraction is stronger in acidic solutions due to the anionic ${\mathrm{SO}}_4^{-}$ centers in the structure of RB5, which leads to higher adsorption efficiency at low pH values [71].

The pH_PZC was determined to better define the adsorption mechanism. The surface charge of an adsorbent is positive at pH < pH_PZC and negative at pH > pH_PZC [72,73,74,75,76]. The pH_PZC value of the MG adsorbent was found to be 6.42, and the MG surface charge was positive at pH < 6.42 and provided the maximum adsorption of RB5 dye, an anionic dye, at pH 4.0.

3.2.2. Effect of the Adsorbent Dosage

The effect of the adsorbent dosage on dye removal was studied in the range of 0.25–5 g/L, keeping constant the dye concentration (50 mg/L), the pH (4.0), the temperature (23 °C), and the contact time (240 min). Figure 8 shows the results of the removal of RB5 with MG. The removal percentage of RB5 dye increased with an increasing adsorbent dosage. The removal efficiency was determined to be 22.42% and 88.09% at 0.25 g/L and 5 g/L, respectively. Increasing the adsorbent dosage from 2.5 g/L to 5 g/L did not change the removal efficiency. The increased removal efficiency is also related to an increased number of active centers involved in the removal as the adsorbent dosage increases [77]. Figure 8 shows a decrease in the adsorption capacity of MG with the increase in the adsorbent amount. The adsorption capacity was found to be 45.93 mg/g and 9.02 mg/g at 0.25 g/L and 5 g/L, respectively. This explained why the unit adsorption capacity decreased with the increasing adsorbent dosage, since some areas on the adsorbent surface remained empty [78,79]. For the RB5 removal with MG, 2.5 g/L was determined as the adsorbent dosage, and further experiments were conducted with this amount.

3.2.3. Effect of the Dye Concentration and Isotherm Modeling

The initial dye concentration in the removal of RB5 dye with MG was investigated at pH 4.0, with contact time of 240 min and 2.5 g/L of adsorbent. The RB5 concentration was studied in the 60–200 mg/L range. The calculated adsorption capacity was 17, 22, 23, 28, and 29 mg/g at initial dye concentrations of 60, 80, 100, 150, and 200 mg/L, respectively. The adsorption capacity increased with the increasing RB5 concentration, but no significant difference was observed in the presence of 150 and 200 mg/L contaminant concentrations (Figure 9). The saturation of the adsorption centers explains why the adsorption capacity does not change significantly at high concentrations [80]. The increasing dye concentration increased the required interaction forces by overcoming the resistance related to mass transfer between MG and RB5. Accordingly, a higher adsorption capacity was obtained [81].

The equilibrium data for RB5 removal with MG were characterized by the Langmuir, Freundlich, and Temkin isotherms (Figure 10).

Table 6. The Langmuir, Freundlich, and Temkin isotherm constants for RB5 adsorption on MG.

	Isotherm Constants	298 K	303 K	313 K	323 K
Langmuir	Qmax	29.85	28.01	26.45	21.41
	b	0.121	0.113	0.057	0.143
	R2	0.89	0.95	0.95	0.74
Freundlich	kF	12.95	5.87	7.73	12.29
	1/n	0.16	0.39	0.22	0.10
	R2	0.83	0.71	0.82	0.49
Temkin	At	12.46	0.29	1.84	653.76
	bt	0.64	0.23	0.61	1.51
	R2	0.86	0.60	0.84	0.47

reports Q_max, b, and the correlation coefficient computed from the Langmuir isotherm model, k_F and 1/n, and the A_t and b_t constants computed from the Temkin isotherm.

Table 7 reports the RB5 removal results with various adsorbents. The maximum adsorption capacity obtained at 298 K for RB5 removal with MG was 29.85 mg/g. As seen in the table, very different results were obtained because the parameters selected in each study were different. Table 7 allowed us to acquire an approximate idea of the results of RB5 removal with MG.

Table 7. RB5 removal with various adsorbents.

Adsorbent	qe (mg/g)	Ref.
Magnetic iron oxide nanoparticles	18	[5]
ZnO–chitosan composite beads	189.44	[38]
Activated carbon (prepared from orange peel)	11.85	[82]
Brazilian pine-fruit shells (Araucaria angustifolia)	74.6	[32]
Activated carbon (prepared from Brazilian pine-fruit shells)	446.2	[32]
TiO2-G5/PET films	155.04	[83]
Acidic modification of natural stone (pumice)	10	[84]
Polyacrylamide cryogels modified with polyethyleneimine	201	[85]
Cu/natural zeolite	0.4299	[86]
Surfactant-modified zeolite	12	[8]
Fly ash	7.93	[87]
Powdered activated carbon	58.52	[87]
Pre-treated Aspergillus flavus	26.95	[88]
Magnetic granite	29.85	This study

Table 7. RB5 removal with various adsorbents.

Adsorbent	qe (mg/g)	Ref.
Magnetic iron oxide nanoparticles	18	[5]
ZnO–chitosan composite beads	189.44	[38]
Activated carbon (prepared from orange peel)	11.85	[82]
Brazilian pine-fruit shells (Araucaria angustifolia)	74.6	[32]
Activated carbon (prepared from Brazilian pine-fruit shells)	446.2	[32]
TiO2-G5/PET films	155.04	[83]
Acidic modification of natural stone (pumice)	10	[84]
Polyacrylamide cryogels modified with polyethyleneimine	201	[85]
Cu/natural zeolite	0.4299	[86]
Surfactant-modified zeolite	12	[8]
Fly ash	7.93	[87]
Powdered activated carbon	58.52	[87]
Pre-treated Aspergillus flavus	26.95	[88]
Magnetic granite	29.85	This study

3.2.4. Contact Time’s Effects and Kinetic Model

To study the impact of contact time on the adsorption of RB5 dye on MG, studies were performed at various contact times (5–300 min). Figure 11 displays the impact of the contact time on the adsorption capacity. The q_e-t graph shows an increase in the amount adsorbed as the contact time increased. The removal efficiencies for 30, 60, and 300 min were 60%, 70%, and 74%, respectively, for a 60 mg/L RB5 concentration, whereas the removal efficiencies for 30, 60, and 300 min were 28%, 28%, and 36%, respectively, for a 200 mg/L RB5 concentration. Very high removal rates were reached in the first minutes due to the high number of active sites. However, because of the decrease in and the occupancy of the said zones and the rapid equilibrium, the dye removal gradually decreased. The reason for this is the strong attraction force between dye molecules and adsorbent materials; intra-particle diffusion takes place after rapid surface diffusion. Similar results were obtained for RB5 removal using magnetic chitosan [89]. The equilibrium time for RB5 removal with MG was determined to be 60 min.

Kinetic studies are essential for determining the best adsorption time of a pollutant on an adsorbent. The most commonly used kinetic models were analyzed to determine the kinetic parameters for RB5 dye adsorption on MG.

Table 8. Kinetic data for the adsorption of RB5 on MG.

T (K)	Co (mg/L)	qe (exp.) (mg/g)	Pseudo-First-Order			Pseudo-Second-Order			Inrtaparticle Diffusion
			qe (cal.) (mg/g)	k1 (min−1)	R2	qe (cal.) (mg/g)	k2 (g·mg−1 min−1)	R2	ki (mg·g−1 min−1/2)	C	R2
298	60	18.35	4.60	0.0092	0.66	18.51	0.0044	0.99	1.416	−13.1	0.72
	80	22.16	7.10	0.0105	0.76	22.62	0.0046	0.99	0.644	12.69	0.81
	100	23.77	7.91	0.0103	0.77	24.27	0.0040	0.99	0.679	13.56	0.85
	150	25.39	8.27	0.0850	0.87	29.94	0.0017	0.98	0.969	13.00	0.93
	200	28.99	9.48	0.0059	0.78	25.44	0.0077	0.99	0.571	17.94	0.85

presents the data obtained from the PFO, PSO, and IPD kinetic models. Upon comparing the experimental q_e values obtained from the adsorption system with the calculated q_e values, the system was determined to be in accordance with the PSO kinetic model.

The intraparticle diffusion model was used to analyze the adsorption mechanism (Figure 12). The relationship between diffusion, equilibrium time, and adsorption capacity is described by investigating the adsorption mechanism. The adsorption of RB5 on MG was realized in two steps. In the first step, when adsorption took place, RB5 was adsorbed on the adsorbent surface. Most adsorption occurred in this first step, according to boundary layer diffusion. In the second step, the process was slower, as shown by the lower slope of the line. This step involved intraparticle diffusion. In the diffusion of RB5 on MG, boundary layer diffusion occurred in the first 60 min, and intraparticle diffusion occurred afterward. The equilibrium time of the adsorption system was determined to be 60 min.

3.2.5. Temperature’s Effects and Thermodynamic Parameters

The impact of temperature on the adsorption of RB5 on MG was studied in the range of 23–40 °C with 60–200 mg/L of RB5, at pH 4.0, and 0.25 g of adsorbent. The adsorption capacity was observed to decrease as the temperature increased (Figure 13). At the concentration of 200 mg/L RB5, the adsorption capacity was 28.99 mg/g at 23 °C, while the it decreased to 19.66 mg/g when the temperature increased to 50 °C. Likewise, at the concentration of 60 mg/L RB5, the adsorption capacity was 18.35 mg/g at 23 °C and decreased to 12.26 mg/g as the temperature increased to 50 °C. The decreased adsorption capacity with the increasing temperature indicated that the system was exothermic.

Thermodynamic parameters, such as ∆G, ∆H, and ∆S, were calculated for RB5 adsorption with magnetic granite. The experiments were conducted at 25–40 °C with 60 mg/L of RB5.

Table 9. Thermodynamic parameters for RB5 adsorption with magnetic granite.

Temperature (K)	∆G° (kJ/mol)	∆H° (kJ/mol)	∆S° (kJ/mol K)	R2
298	−2.86	−31.76	0.096	0.90
303	−2.85
313	−1.50

shows the calculated ∆G, ∆H, and ∆S values. The negative ∆H values at all temperatures and concentrations showed that the adsorption was exothermic. The negative ∆G values demonstrated that the adsorption was spontaneous.

3.3. Regeneration

The adsorbent’s regeneration is an important factor in the efficiency and economy of an adsorptive process. The regeneration of the used adsorbent and the reuse of the regenerated adsorbent in adsorption systems are important from an environmental and economic point of view. To determine the reusability of MG used in the study, 0.1 M NaOH was used as an eluent for the regeneration process.

MG used for RB5 adsorption was oven-dried at 50 °C for 3 h in the regeneration experiments. In the first cycle after regeneration, it was determined that magnetic granite removed 85.06% of RB5 (Figure 14). In the 5th cycle, the RB5 removal rate decreased to 49.15%. The results obtained proved that MG is reusable.

4. Conclusions

In this study, magnetic granite was synthesized using natural rock-type granite by the chemical co-precipitation method for RB5 adsorption. The characterization results showed the success of magnetization. Magnetic granite was found to have a larger surface area (G: 0.4638 m²/g, MG: 19.1211 m²/g) than granite. According to the VSM results for MG, the saturation magnetization (Ms) value was 46.04 emu/g. The presence of Fe₃O₄ peaks in the XRD pattern of magnetic granite proved that Fe₃O₄ was added to the granite structure. The spectra obtained from FTIR analysis also confirmed the modification with Fe₃O₄. In the EDX analysis of MG, the peaks appearing after adsorption confirmed RB5 adsorption. The adsorption of RB5 on magnetic granite showed a good fit with the Langmuir isotherm model and the pseudo-second-order kinetic model. The adsorption of magnetic granite was spontaneous and exothermic. This research demonstrated that magnetic granite could be efficiently utilized for RB5 adsorption from textile wastewater.