Adsorption Studies of Ternary Metal Ions (Cs⁺, Sr²⁺, and Co²⁺) from Water Using Zeolite@Magnetic Nanoparticles (Z@Fe₃O₄ NPs)

Abstract

The mixture of three metal ions (Cs⁺, Sr²⁺, and Co²⁺) is commonly found in radioactive waste, which induces several negative health effects. The removal of multiple metal ions is a true challenge for researchers due to the competitive adsorption of ions onto adsorbents. In this study, three metal ions, namely Cs⁺, Sr²⁺, and Co²⁺, have been successfully removed simultaneously from water using zeolite@magnetic nanoparticles (Z@Fe₃O₄ NPs). The optimized condition for the adsorption of ternary metal ions was obtained at an adsorbent weight of 0.2, pH of 6.0~7.0, and contact time of 60 min. The adsorption mechanism of ternary metal ions onto the surface of Z@Fe₃O₄ NPs was studied using the Pseudo-first-order, Pseudo-second-order, Elovich, and Intra-particle diffusion models. The Dubinin–Radushkevich Temkin, Freundlich, and Langmuir isotherm models were used to study the isotherm adsorption. The ternary metal ion adsorption (Cs⁺, Sr²⁺, and Co²⁺) on Z@Fe₃O₄ NPs was followed by the Pseudo-second-order model (PSO) with correlation coefficient (R²) range of 0.9826–0.9997. Meanwhile, the adsorption isotherms of ternary metal ions on Z@Fe₃O₄ NPs were in line with the Langmuir model with R² values higher than 0.9206, suggesting monolayer chemisorption with maximum adsorption capacities of 48.31, 15.02, and 10.41 mg/g for Cs⁺, Sr²⁺, and Co²⁺, respectively. Thus, the selectivity trend in the ternary metal ions system towards the Z@Fe₃O₄ NPs is observed to be Cs⁺ > Sr²⁺ > Co²⁺, which indicates that the competitive effect of Cs⁺ is the strongest compared to Sr²⁺ and Co²⁺ions.

1. Introduction

A huge volume of radioactive liquid waste is frequently created during the performance of nuclear power plants, fuel recycling plants, hospitals, research institutes, and defense research institutes, which can cause potential environment. Also, radioactive waste can generate various negative health effects and is also the main cause of many dangerous diseases in humans, such as carcinoma of the liver, neurological disorders, liver cancer, kidney failure, and leukemia [1]. Thus, radioactive waste disposal is an urgent issue for the sustainable development of the nuclear, defense, and medical industries. Several methods, including ion exchange [2], membrane filtration [3], and adsorption [4,5], have been used to treat radioactive liquid waste to meet environmental discharge standards. Compared to these methods, inorganic adsorption by zeolite is attractive and promising for being more effective and economical due to its simplicity, selectivity, radiation stability, and good compatibility with the waste material [6]. In recent decades, many adsorption experiments used natural zeolites, synthetic zeolites, and a blend of the two for the decontamination of radioactive Cs and Sr [7,8]. However, the difficulty in separating the zeolite absorbent from the aqueous solution remains a drawback in this material. In the adsorption process, magnetic separation is an optimal solution due to its advances in operation compared to filtration, centrifugation, or gravitational separation. Furthermore, magnetic iron oxide (Fe₃O₄) materials have attracted much attention in many environmental applications because Fe₃O₄ has good stability, high surface area, high co-effectivity, low toxicity, excellent magnetic responsivity, high dispersibility, and ease of surface modification [9,10,11,12]. Thus, zeolite could be engineered to become a magnetic material by loading Fe₃O₄ NPs onto the zeolite surface. Moreover, this material can also improve the surface area, resulting in increased adsorption capacity.

As we know, Cs, Sr, and Co are the most harmful radionuclides in radioactive waste. There are many studies on the reparation of a single radionuclide from waste solutions by inorganic sorbents [4,5,6,7,13,14], whereas few have considered the competitive adsorption of binary, ternary, or multiple radionuclides [15,16,17]. In addition, the metal ions in the aquatic environment interact with each other and compete to absorb into the adsorbent, which can influence the adsorption process. This is a challenge for removing multiple metal ions. Herein, zeolite@magnetic nanoparticles (Z@Fe₃O₄ NPs) were synthesized by loading magnetic nanoparticles (Fe₃O₄ NPs) onto the zeolite molecular sieve 4A surface using a precipitation method. Z@Fe₃O₄ NPs were applied as adsorbents for the removal of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺) from water.

2. Results and Discussion

2.1. Characterizations of Z@Fe₃O₄ NPs

The field-emission scanning electron microscopy (FE-SEM) results in Figure 1a,b were analyzed to investigate the surface morphology of zeolite and Z@Fe₃O₄ NPs. The FE-SEM result of Z@Fe₃O₄ NPs exhibits that the spherical particle in the range of 25–50 nm has formed on the surface of the zeolite. Furthermore, the electron dispersive X-ray spectra (EDX) of Z@Fe₃O₄ NPs show the presence of Fe (Figure 1c), indicating that the loading of iron oxide nanoparticles onto the zeolite surface is successful. Meanwhile, X-ray diffraction (XRD) analysis exhibited a different crystal structure of Z@Fe₃O₄ NPs in comparison with the zeolite (Figure 1d). Before loading magnetic nanoparticles (Fe₃O₄ NPs), all diffraction peaks were assigned to zeolites [18].After loading magnetic nanoparticles (Fe₃O₄ NPs), the diffraction peaks were divided into two groups. The first group includes peaks at 2θ of 10.3, 12.7, 16.3, 21.8, 24.2, 27.3, 30.2, and 34.4°, which are assigned from zeolites, while the second group includes two diffraction peaks at 2θ of 35.5 and 62.6°, corresponding to the reflection of the (311) and (440) lattice planes, which can be indexed to the face-centered cubic structure of Fe₃O₄ according to JCPDS card No. 19-0629 [14,19].

The surface chemistry of Z@Fe₃O₄ NPs was characterized by X-ray photoelectron spectroscopy (XPS) (Figure 2a–c). A survey spectrum showed peaks corresponding to C 1s, O 1s, Si 2p, and Fe 2p, which demonstrated the existence of Fe on the zeolite surface (Figure 2a). As shown in Figure 2b, the Fe 2p exhibited two peaks at 712 and 724 eV, which corresponded to Fe_2p3/2 and Fe_2p1/2, respectively. Moreover, the high-resolution Fe 2p_3/2 spectrum of Z@Fe₃O₄ NPs (Figure 2c) exhibited multi-component features that can be fitted by two Gaussian peaks at 710.0 and 711.8 eV, which can be indexed to Fe²⁺ and Fe³⁺, respectively [20,21]. These indicate the successful formation of the Fe₃O₄ phase on the zeolite surface.

The formation of the Fe₃O₄ nanoparticle on the zeolite surface resulted in the enhancement of the nitrogen gas adsorption–desorption of Z@Fe₃O₄ NPs in comparison with zeolite 4A (Figure 2d). In addition, as shown in the surface area plot (Figure S1a,b), the surface area of the Z@Fe₃O₄ NPs (183.46 m²/g) was higher than the zeolite (18.52 m²/g). Therefore, the loading of Fe₃O₄ NPs onto the zeolite molecular sieve 4A surface not only makes it easy to separate Z@Fe₃O₄ NPs adsorbent from the solution using an external magnetic field, but also enhances the surface area. This will increase the Z@Fe₃O₄ NPs’ ability to absorb metal ions.

2.2. Adsorption Studies of Ternary Metal Ions Using Z@Fe₃O₄ NPs

2.2.1. Influence of the Adsorbent Weight and pH

The adsorbent weight is described as a significant parameter that influences the adsorption process. The adsorption process of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺) from an aqueous solution is performed at diverse amounts of Z@Fe₃O₄ NPs in the range from 0.1 to 0.5 g at room temperature in the glove box. The pH, contact time, and initial concentration of ions are maintained at 6.9, 60 min, and 105 mg/L of each metal ion, respectively.

When the adsorbent weight rose from 0.1 to 0.5 g, the removal efficiency (R_e %) of each metal ion in a mixed solution (Cs⁺, Sr²⁺, and Co²⁺) increased from 44.65 to 91.20%, 5.37 to 71.14%, and 0 to 47.14%, respectively. An enhancement in the removal efficiency along with an increase in the weight of Z@Fe₃O₄ NPs is due to the increase in the number of active adsorption sites and empty sites on the adsorbent surface (Figure 3). Although the enhancement amount of adsorbent provides additional active sites, the Z@Fe₃O₄ NPs adsorbent underwent agglomeration at high adsorbent concentrations, thereby decreasing the unoccupied adsorption active sites and effective surface area [22]. In addition, the initial concentration and volume of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺) are constant, which results in the number of ions contacted and adsorbed by the adsorbent per unit mass decreasing with the increase in the adsorbent dose, and the active sites of the adsorbents are not saturated [23,24]. However, only the adsorption capacity of Cs⁺ in mixed metal ions decreases from 46.89 to 19.15 mg/g as the adsorbent weight increases from 0.1 to 0.5 g. Contrarily, the adsorption capacity of Sr²⁺ and Co²⁺ in mixed metal ions initially increases in the adsorbent mass range (0.1–0.2 g) and reaches a maximum of 18.33 and 11.90 mg/g at 0.2 g, respectively, due to strong affinity of Z@Fe₃O₄ NPs with Cs⁺. After that, the solid adsorption capacity of both ions then becomes almost constant as the adsorbent mass increases from 0.2 to 0.5 g. Because the adsorption capacity values of ternary metal ions (Cs⁺, Sr²⁺, Co²⁺) were optimized at an adsorbent weight of 0.2 g, we chose the adsorbent weight of 0.2 g to survey the influencing factors.

The influence of the pH on the removal efficiency (Re%) and adsorption capacity (q_e) of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺) from the aqueous solution are performed at a pH from 3.0 to 10.0 and at room temperature (Figure 4a,b). HCl 0.1 M and NaOH 0.1 M were added to the adsorption system to adjust the pH values of the solution ranging from 3 to 10. The initial concentration of ions, adsorbent mass, and contact time are maintained at 105 mg/L of each metal ion, 0.2 g, and 60 min, respectively. Figure 4 depicts that the removal efficiency and adsorption capacity of three ions slightly increased as the pH solution increased from 3.0 to 8.0. Subsequently, it showed that the removal efficiency and adsorption capacity of Cs⁺ ions decreased, while Sr²⁺and Co²⁺ ions exhibited a rapid increase and their removal efficiency and adsorption capacity were higher than Cs⁺. As we know, the pH of the aqueous solution was a critical parameter for the absorption of metal ions experiments because it affected the surface charge of the adsorbent as well as the metal ion chemistry structure in the solution [25,26,27,28]. The surface charge of the adsorbent depends on their zero-point charge (Zpc) [27,28]. When the pH of the solution is above the Zpc value, the adsorbent surface becomes more negatively charged. On the contrary, the surface charge of the adsorbent is more positive as the pH is lower than Zpc. Furthermore, the Zpc value depends on the nature of the nanoparticle [25,26]. According to previous works, the Zpc values of the magnetic materials vary between 4.0 and 6.5 [28,29,30,31,32,33]. Meanwhile, Belachew et al. reported that the Zpc of the zeolite 4A was determined to be 6.8 [34]. Since the Z@Fe₃O₄ NPs adsorbent was synthesized by loading Fe₃O₄ NPs onto the zeolite 4A surface using a precipitation method, we suggested that the zero-point charge (Zpc) of the Z@Fe₃O₄ NPs adsorbent is between 4.0 and 7.0. In addition to affecting the surface charge of the adsorbent material, the pH value also affects the metal ion chemistry structure in the solution, resulting in changes in the charge and size of the metal ions [26,27,28,29]. Ali et al. observed the distributions of Cs, Sr, and Co species (C = 100 mg/L) as a function of pH [35]. The authors indicated that the metal ion chemistry structure of Cs in the solution was the dominant Cs+ ion at a pH ranging from 1.0 to 12.0. Meanwhile, the metal ion chemistry structure of Sr was the dominant Sr²⁺ ion at a pH ranging from 1.0 to 8.0 and was the Sr(OH)⁺ ion at a pH above 8.0 [35]. For Co, the metal ion chemistry structure was the dominant Co²⁺ ion at a pH ranging from 1.0 to 8.0 and was Co(OH)₂ at a pH above 8.0 [12,35]. Thus, we propose that when the pH solution increases from 3.0 to 8.0, the surface charge of Z@Fe₃O₄ NPs becomes gradually negative, while the metal ions’ chemistry structure in the solution is the dominant species of cation. These lead to the gradual enhancement of the electrostatic attraction between the positive charge on the adsorbent surface and cations, the removal efficiency, and the adsorption capacity of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺). However, there is no significant increase in the removal efficiency and adsorption capacity. Table S2 shows the pH of the solution after adsorption increases as compared to before adsorption at a pH ranging from 3.0 to 7.0. This indicates that competition between H⁺ ions and three metal ions for the adsorption on Z@Fe₃O₄ NPs occurs, which makes the removal efficiency and adsorption capacity of the three ions see no significant increase.

In addition, it was found that the removal efficiency and adsorption capacity of Cs⁺ in mixed metal ions decreased at a pH above 8.0, while Sr²⁺and Co²⁺showed rapid increases, with their removal efficiency and adsorption capacity being higher than Cs⁺. Because the Cs and Sr exist mainly in the form of Cs⁺ and Sr(OH)⁺ at pH values above 8.0 [27,35], the radius of the Sr(OH)⁺ ion (260 pm) is higher than the Cs⁺ion (186 pm) [36]. This result leads to Sr(OH)⁺ ions having higher binding power and higher adsorption than Cs⁺ ions [14,35]. For Co²⁺ ions, the strong increase in the removal efficiency and adsorption capacity at pH values above 8.0 is due to the formation of hydroxide precipitation [12,35]. Therefore, a part of Co²⁺ ion removal is due to the formation of Co(OH)₂ precipitation at a pH above 8. For this reason, the next absorption experiment should be carried out at a pH range of 6.0–7.0.

Furthermore, Figure 4a,b exhibit q_e values of ternary metal ions following the order Cs+ > Sr²⁺ > Co²⁺. This indicates that there is competitive absorption for mixtures of metal ions onto Z@Fe₃O₄ NPs. The competitive adsorption of Cs⁺, Sr²⁺, and Co²⁺ ions onto Z@Fe₃O₄ NPs depends on how these metal ions interact with the adsorbent. In addition, the interaction between Cs⁺, Sr²⁺, and Co²⁺ions and Z@Fe₃O₄ NPs depends on the metal’s molecular mass, ion charges, ionic radius, hydration energy, and electrostatic charge [37,38]. The difference in the radius of metal ions has a significant influence on the absorption capacity; ions with a smaller radius have high mobility in aqueous solutions, and thus, they have a lower tendency to absorb onto magnetic nanoparticle adsorbents [35]. Moreover, the order of the ionic radii of three ions is as follows: Cs⁺ (186 pm) > Sr²⁺ (132 pm) > Co²⁺ (79 pm) [27]. Hence, the Cs⁺ ion with the highest ionic radius has the highest binding power and highest adsorption onto Z@Fe₃O₄ NPs as compared to Sr²⁺ and Co²⁺ ions at pH values below 8. This result reveals that Z@Fe₃O₄ NPs behaved more selectively toward Cs⁺ ions at pH values below 8. However, it has been found that the order adsorption capacity of ternary metal ions changed (such as Sr²⁺ > Cs⁺) as pH values reached above 8. As we discussed above, the interaction between metal ions and Z@Fe₃O₄ NPs was affected by the ionic radius. Furthermore, Cs and Sr exist mainly in the form of Cs⁺ and Sr(OH)⁺ at pH values above 8.0, which results in the radius of the Sr(OH)⁺ ion (260 pm) being higher than Cs⁺ (186 pm) [36]. Thus, the Sr(OH)⁺ ions with higher ionic radii resulted in higher binding powers and higher adsorptions as compared to Cs⁺ with pH values above 8.0.

2.2.2. A Study on Adsorption Kinetic

One of the important factors that affect the adsorption process of ternary metal ions is the contact time. The effect of the contact time on the adsorption capacity (q_e) of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺) from aqueous solutions is performed at a contact time range from 5 to 180 min and at room temperature. The pH, initial concentration of each ion, and the adsorbent mass are maintained at 6.3, 105 mg/L (each metal ion), and 0.2 g, respectively. Table S3 and Figure S2a clearly show that with an increase in the contact time, the adsorption capacity of all studied metal ions gradually increased until reaching equilibrium at 50 min. Therefore, the contact time of 60 min is an ideal time to investigate the effect of other factors on the adsorption of ternary metal ions (Co²⁺, Sr²⁺, and Cs⁺).

Kinetic adsorption models illustrate the mechanism of adsorption of metal ions and particularly determine the rate of removal of metals ion from the aqueous medium, which will provide the optimized design parameters such as the adsorbate residence time and kinetic rate constant. In this study, Pseudo-first-order, Pseudo-second-order, Intra-particle diffusion kinetic, and Elovich models were used to simulate the absorption kinetics of ternary ion adsorption (Cs⁺, Sr²⁺, and Co²⁺) onto Z@Fe₃O₄ NPs. Four kinetic models have been investigated as follows [39]:

Pseudo-first-order equation:

$$\ln ({\mathrm{q}}_{\mathrm{e}}-\mathrm{q})=\ln ({\mathrm{q}}_{\mathrm{e}})-{\mathrm{k}}_1\mathrm{t}$$

(1)

Pseudo-second-order equation:

$${\displaystyle \frac{1}{\mathrm{q}}}=\left({\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}\right)\mathrm{t}+\left({\displaystyle \frac{1}{{\mathrm{k}}_2\times {{\mathrm{q}}_{\mathrm{e}}}^2}}\right)$$

(2)

Elovich equation:

$$\mathrm{q}={\displaystyle \frac{\ln {\mathrm{a}}_{\mathrm{e}}{\mathrm{b}}_{\mathrm{e}}}{{\mathrm{b}}_{\mathrm{e}}}}+{\displaystyle \frac{1}{{\mathrm{b}}_{\mathrm{e}}}}\ln \mathrm{t}$$

(3)

Intra-particle diffusion equation:

$$\mathrm{q}={\mathrm{k}}_{\mathrm{i}\mathrm{n}\mathrm{t}}{\mathrm{t}}^{0.5}$$

(4)

where q_e (mg/g) is the amount of metal ions at equilibrium; k₁ (min⁻¹) is the kinetic rate constant for the Pseudo-first-order model calculated by a linear plot of ln (q_e − q) against time; k₂ (g/mg∙min) is the equilibrium rate constant for the Pseudo-second-order model obtained from the intercept of the linear plot of t/q against t; a_e (mg/g∙min) is the initial sorption rate; b_e (g/mg) is the constant related to the surface coverage and activation energy of chemisorption in the Elovich mode; and k_int (mg/g∙min^0.5) is the Intra-particle diffusion rate constant calculated from the slope of the plot q versus t^0.5.

From Figure 5a and

Table 1. The parameters of kinetic absorption for ternary metal ions on Z@Fe₃O₄ NPs.

Kinetic Adsorption Parameters		Ternary Ions Absorption
		Cs+	Sr2+	Co2+
Pseudo-first-order	qe.exp(mg/g)	43.29	20.46	14.70
	qe.cal (mg/g)	4.43	10.00	8.58
	k1 (min−1)	0.048	0.023	0.021
	R2	0.4622	0.9415	0.9573
Pseudo-second-order	qe.cal(mg/g)	43.29	20.33	14.56
	k2 (g/mg·min)	0.026	0.007	0.007
	R2	0.9997	0.9917	0.9826
Elovich	ae (mg/g·min)	5.61 × 105	4.22 × 101	1.54 × 101
	be (g/mg)	3.87 × 10−1	3.96 × 10−1	5.18 × 10−1
	R2	0.8214	0.9563	0.9483
Intra-particle diffusion	kint1(mg/g∙min0.5)	3.59	1.80	1.34
	R21	0.9770	0.9579	0.9927
	kint2(mg/g·min0.5)	0.042	0.542	0.568
	R22	0.0212	0.8707	0.9132

q_e._exp: experimental equilibrium capacity; q_e._cal: calculated equilibrium capacity.

, the experimental adsorption data showed a good fit to the Pseudo-first-order model with correlation coefficient (R²) values of 0.9415 for Sr²⁺ and 0.9573 for Co²⁺. In Cs⁺ adsorption, the model did not fit well with the experimental result with a low R² value of 0.4622. Moreover, the calculated equilibrium capacities (q_e) obtained from the analysis of Cs⁺ (4.43 mg/g), Sr²⁺ (10.00 mg/g), and Co²⁺ (8.58 mg/g) were lower than the experimental values for Cs⁺ (43.29 mg/g), Sr²⁺ (20.46 mg/g), and Co²⁺ (14.70 mg/g). These results suggested that the pseudo-first-order model might not be applicable for the adsorption of ternary ion adsorption (Cs⁺, Sr²⁺, and Co²⁺) onto Z@Fe₃O₄ NPs.

The Pseudo-second-order kinetic model was also applied for the adsorption kinetics of ternary metal ion adsorption. The calculated amount of three metal ions at equilibrium (q_e) was in the following order: Co²⁺ < Sr²⁺ < Cs⁺. This result confirms that the Z@Fe₃O₄ NPs adsorbent has a stronger affinity for Cs⁺ than Sr²⁺ and Co²⁺ions.Furthermore; the correlation coefficient (R²) of three metal ions has the highest values and is closest to unity as compared to the other three models, which indicates that the pseudo-second-order kinetic model provides almost a perfect fit to the experimental data. In addition, the q_e of Cs⁺ (43.29 mg/g), Sr²⁺ (20.33 mg/g), and Co²⁺ (14.56 mg/g) values obtained from the slope of the plot of t/q versus t were the closest to the experimental data (

Table 2. The relative parameters of the adsorption isotherms for ternary ions on Z@Fe₃O₄ NPs.

Kinetic Adsorption Parameters		Ternary Ions Absorption
		Cs+	Sr2+	Co2+
Dubinin–Radushkevich isotherm	qmax (mg/g)	26.95	13.17	9.77
	β(mol2/J2)	4.0 × 10−7	7.0 × 10−8	1.0 × 10−7
	E (kJ/mol)	1.12	2.67	2.24
	R2	0.7959	0.8168	0.6676
Temkin isotherm	δT (kJ/mol)	0.24	1.46	2.23
	KT (L/g)	2.15	149.51	251.56
	R2	0.9546	0.6829	0.5583
Freundlich isotherm	nF	1.70	5.43	6.89
	KF (mg/g)	8.23	7.70	5.94
	R2	0.8908	0.7408	0.6477
Langmuir isotherm	qmax(mg/g)	48.31	15.02	10.41
	KL (L/mg)	0.19	2.07	0.69
	R2	0.9758	0.9206	0.9441

q_max: calculated maximum capacity.

). Hence, we assumed that the adsorption of ternary ions onto Z@Fe₃O₄ NPs was a chemisorption process.

The Elovich model was applied to further characterize the assumption of chemisorption for the heterogeneous system. As seen in Table 1 and Figure 5c, the initial adsorption rate (a_e) of three metal ions was in the following order: Cs⁺ > Sr²⁺ > Co²⁺. Meanwhile, the constant related to the surface coverage and activation energy of chemisorption (b_e) was in the following reverse order: Co²⁺ > Sr²⁺ > Cs⁺. However, in Cs⁺ adsorption, the model also did not fit well with the experimental result with a low R² value of 0.8555. Whereas, a good fit of Sr²⁺ and Co²⁺ adsorption data was obtained with a higher R² value (0.9563 and 0.9483). Thus, the Elovich model suggests that chemisorption might be significantly involved in Sr²⁺ and Co²⁺, but not in Cs⁺ adsorption onto Z@Fe₃O₄ NPs.

The Intra-particle diffusion adsorption kinetic model is one of the most widely applied for the adsorption kinetics of metal ions. The plot of q_t versus t^1/2 (Figure 5d) for ternary ion adsorption (Cs⁺, Sr²⁺, and Co²⁺) onto Z@Fe₃O₄ NPs demonstrated multi-linear plots. In the first plots, the high slopes indicated the high diffusion rate constant values (k_int1) of three metal ions, which was due to the availability of a massive number of active sites on the Z@Fe₃O₄ NPs surface [40]. It means that this first step is instantaneous adsorption or external surface adsorption [40]. In the second plot, the low slopes indicated the low diffusion rate constants (k_int2), which are due to a decrease in the concentration gradient [40]. In addition, the diffusion rate constants (k_int2) of Cs⁺, Sr²⁺, and Co²⁺ions into the mesopores of the Z@Fe₃O₄ NPs were lower than the first step. These suggested that the second step was the progressive adsorption or intra-particle diffusion stage [40]. On the other hand, a good fit of three metal ion adsorption data at the first step was obtained with higher R² values of 0.9770, 0.9579, and 0.9927, respectively. Nevertheless, this model did not fit well at the second step of adsorption with a low R² value of 0.0212 for Cs⁺ and 0.8707 for Sr²⁺, respectively, while a good fit of Co²⁺adsorption data at the second step was obtained with a high R² value of 0.9132. These results indicate that the intra-particle diffusion model might not be applicable for Cs⁺ and Sr²⁺, but is significantly involved in Co²⁺ adsorption onto Z@Fe₃O₄ NPs.

2.2.3. A Study on Absorption Isotherms

The initial concentration was also one of the important factors that affected the adsorption process. The effect of the initial concentration on the adsorption capacity (q_e) of ternary metal ions from an aqueous solution has been characterized by using 0.2 g of Z@Fe₃O₄ NPs with 100 mL of a mixed metal ion solution with different initial concentrations (from 10 to 150 mg/L). The pH and the contact time are maintained at 6.2 for 60 min under room temperature with stirring. Figure S2b and Table S4 showed that with a rise in the initial concentration of ternary metal ions, the adsorption capacity of three ions enhanced rapidly until reaching a maximum at 105 mg/L for Cs⁺ and 80 mg/L for Sr²⁺ and Co²⁺. After that, the adsorption capacity of all three ions decreased as the initial concentration increased from 80 to 150 mg/L. This observation could be attributed to the ratio between the active sites of the Z@Fe₃O₄ NPs adsorbent and the fact that the number of moles of ions was huge at a low initial metal ion concentration range (10 to 105 mg/L). Nevertheless, the ratio between the number of moles of ions and the active sites of the Z@Fe₃O₄ NPs adsorbent decreased as the initial concentration of ions enhanced over 105 mg/L; this was because the available number of moles of metal ions is still enhanced, while the active sites of the Z@Fe₃O₄ NPs are constant. This leads to a decrease in the adsorption capacity of the Z@Fe₃O₄ NPs adsorbent.

Dubinin–Radushkevich (D–R), Temkin, Langmuir, and Freundlich isotherm models were applied to simulate the adsorption mechanism of ternary metal ions onto the Z@Fe₃O₄ NPs surface. Four isotherm models have been investigated as follows:

Dubinin–Radushkevich model [41]:

$${\mathrm{lnq}}_{\mathrm{e}}={\mathrm{lnq}}_{\max }-\mathsf{\beta }{\mathsf{\epsilon }}^2$$

(5)

Temkin model [42]:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{\mathrm{R}\mathrm{T}}{{\mathsf{\delta }}_{\mathrm{T}}}}\ln {\mathrm{K}}_{\mathrm{T}}+{\displaystyle \frac{\mathrm{R}\mathrm{T}}{{\mathsf{\delta }}_{\mathrm{T}}}}\ln {\mathrm{C}}_{\mathrm{e}}$$

(6)

Freundlich model [43]:

$${\mathrm{lnq}}_{\mathrm{e}}={\mathrm{lnK}}_{\mathrm{F}}+{\displaystyle \frac{1}{{\mathrm{n}}_{\mathrm{F}}}}\mathrm{l}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$$

(7)

Langmuir model [43]:

$${\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}={\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}}$$

(8)

where β (mol²/J²) is a constant related to the adsorption energy, q_max (mg/g) is the maximum capacity of the adsorbent, and ε (kJ·mol⁻¹) is the adsorption potential. The adsorption potential ε (kJ/mol) is calculated using Equation (15):

$$\mathsf{\epsilon }=\mathrm{R}\mathrm{T}\ln \left(1+{\displaystyle \frac{1}{{\mathrm{C}}_{\mathrm{e}}}}\right)$$

(9)

The parameter β could be used to estimate the mean free energy E (kJ/mol) using the following equation:

$$\mathrm{E}={\displaystyle \frac{{10}^{-3}}{\sqrt{2\mathsf{\beta }}}}$$

(10)

K_T (L/g) is the Temkin isotherm equilibrium binding constant; δ_T (kJ/mol) is the Temkin isotherm constant; R is the universal gas constant (8.314 J/mol·K); and T is the temperature at 298K. K_F (mg/g) is the Freundlich isotherm constant, n_F is the adsorption intensity, and C_e is the equilibrium concentration of Cs⁺, Sr²⁺, and Co²⁺ ions (mg/L). K_L (L/mg) is the Langmuir isotherm constant and q_max(mg/g)is the maximum monolayer coverage capacity. The vital features of the Langmuir isotherm can be described by a separation factor R_L. The separation factor or equilibrium parameter R_L is a dimensionless constant, which is defined by the following equation [41]:

R_L = 1/(1 + K_LC₀)

(11)

The R_L value assumes the nature and the feasibility of the adsorption process, such as linear (R_L = 1), irreversible (R_L = 0), unfavorable (R_L > 1), or favorable (0 < R_L < 1).

Figure 6 and Table 2 illustrate the resulting plots of the Dubinin–Radushkevich, Temkin, Freundlich, and Langmuir isotherm models of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺ions) onto the surface of Z@Fe₃O₄ NPs. The Langmuir isotherm model was found to be the best model (with R² values higher than 0.9206) to describe the ternary metal ion absorption by Z@Fe₃O₄ NPs. This indicates that the ternary metal ion adsorption process is characterized by monolayer adsorption. The ternary metal ions absorption by Z@Fe₃O₄ NPs was based on the electrostatic attraction. Furthermore, the important characteristics of the Langmuir isotherm can be described by a separation factor R_L, which was obtained in the range from 0 to 1 (Figure S3). This indicates that the adsorption of ternary metal ions by Z@Fe₃O₄ NPs is favorable. Furthermore, the maximum monolayer coverage capacities (q_max) for three ions follow the order of Cs⁺ (48.31 mg/g) > Co²⁺ (15.02 mg/g) > Sr²⁺ (10.41 mg/g), which is the same as their ionic radius sequence. The Cs⁺ (186 pm) ion has the highest ionic radius as compared to Co²⁺ and Sr²⁺, resulting in higher binding power and higher adsorption. Thus, Z@Fe₃O₄ NPs behaved more selectively toward Cs⁺ ions in the competitive adsorption of ternary metal ion systems (Cs⁺, Sr²⁺, and Co²⁺ ions).

3. Materials and Methods

3.1. Chemical and Materials

In this work, iron (II) chloride tetrahydrate (FeCl₂·4H₂O, 99%), calcium hydroxide (Ca(OH)₂, 99%), cobalt (II) sulfate heptahydrate (CoSO₄·7H₂O, >99%), hydrochloric acid (HCl, 36%), strontium chloride (SrCl₂, 99%), sodium hydroxide (NaOH, 99%), ethanol (C₂H₅OH, 99.5%), and cesium chloride (CsCl, 99%) were provided from Merck, Darmstadt, Germany. The zeolite molecular sieve 4A was purchased from Guangdong Xintao Technology Co., Ltd. Zhuhai City, Guangdong province, China. Before the synthesis of zeolite@magnetic nanoparticles (Z@Fe₃O₄ NPs), the zeolite molecular sieve 4A was carefully crushed and sieved to an 80 mesh particle size.

3.2. Synthesis of Zeolite@Magnetic Nanoparticles (Z@Fe₃O₄ NPs)

The zeolite@magnetic nanoparticles (Z@Fe₃O₄ NPs) were synthesized by loading magnetic nanoparticles (Fe₃O₄ NPs) onto the zeolite molecular sieve 4A surface using a precipitation method, as shown in Scheme 1 [9,44]. Firstly, 1 g zeolite was mixed with 100 mL saturated Ca(OH)₂ solution by stirring for 30 min. FeCl₂ 2.4 M was then added dropwise into the as-prepared suspension with a stirring speed of 700 rpm until the pH of the mixture about 7~8. The reaction mixture was stirred for 30 min at room temperature. After the reaction time was completed, the brown precipitate was separated from the reaction mixture by centrifugation at 10,000 rpm for 5 min. The precipitate was washed with distilled water (three times) and ethanol (three times). To remove water and ethanol, the solid sample was dried at 50 °C in a vacuum oven for 24 h. Finally, we obtained brown powder (Z@Fe₃O₄ NPs). The reaction mechanism that occurs during the synthesis of Z@Fe₃O₄ NPs is shown through the equations below [9,14]:

Ca(OH)₂ + FeCl₂→ CaCl₂ + Fe(OH)₂

(12)

1/2O₂ + Fe(OH)₂ → Fe₂O₃ + 2H₂O

(13)

Fe(OH)₂→ FeO + H₂O

(14)

FeO + Fe₂O₃ → Fe₃O₄

(15)

Field-emission scanning electron microscope (FE-SEM) using a Regulus 8230 (Hitachi, Tokyo, Japan) and energy dispersive X-ray (EDX) analyses were performed to investigate the microstructure of the Z@Fe₃O₄ NPs and zeolite. For analyzing a Z@Fe₃O₄ NPs’ surface chemistry, X-ray photoelectron spectroscopy (XPS) was applied using a Thermo Scientific K-Alpha (Manchester, UK) with an Al Kα(1486.6 eV) source at a pass energy of 1–400 eV. The crystal structure of Z@Fe₃O₄ NPs and zeolite was determined by X-ray diffraction (XRD) spectra using D8 Discover, Bruker, Karlsruhe, Germany. Nitrogen (N₂) adsorption–desorption isotherms of Z@Fe₃O₄ NPs and zeolite at 77K were analyzed with Quantachrome mod. NOVA 2200e, Boynton Beach, FL, USA. The specific surface area of Z@Fe₃O₄ NPs and zeolite were typically determined by using the Brunauer–Emmett–Teller (BET) analysis. The concentration of Cs⁺, Sr²⁺, and Co²⁺ ions in the aqueous solution was analyzed by inductively coupled plasma–mass spectrometry (plasma Agilent 8900 Triple Quadrupole ICP-MS, Santa Clara, CA, USA).

3.3. Adsorption Experiments

The adsorption of ternary metal ions (Cs⁺, Sr²⁺, and Co²⁺) onto Z@Fe₃O₄ NPs has been studied by using batch-adsorption techniques. The absorption properties of Z@Fe₃O₄ NPs for ternary metal ions were investigated by changing the effective factors, such as the adsorbent weight (0.1 to 0.5 g), pH (3 to 10), contact time (5 to 180 min), and initial concentration of ions (10 to 150 mg/L). The detailed experiment of the adsorption of ternary metal ions from water using Z@Fe₃O₄ NPs was explained and summarized in the supporting information. The Cs⁺, Sr²⁺, and Co²⁺ concentrations in the aqueous solution after the adsorption process were analyzed by ICP-MS spectrometry. The removal efficiency (R_e%) and adsorption capacity (q_e) was calculated by Equations (5) and (6) as follows [11]:

$$\mathrm{Re\%}={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_0}}\times 100\left(\mathrm{\%}\right)$$

(16)

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{V}}_0({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})}{\mathrm{m}}}$$

(17)

where C₀ and C_e are the initial and equilibrium concentrations of the metal ions, respectively (mg/L); V₀ is the volume of the aqueous phase (L); and m is the adsorbent mass (g).

4. Conclusions

In this work, the zeolite@magnetic nanoparticles (Z@Fe₃O₄ NPs) were successfully synthesized to remove ternary metal ions (ternary metal ion systems), including Cs⁺, Sr²⁺, and Co²⁺ ions, from water. In particular, loading Fe₃O₄ NPs onto the zeolite molecular sieve 4A surface not only easily separates the Z@Fe₃O₄ NPs adsorbent from the solution using an external magnetic field, but also enhances the surface area, leading to an increase in the Z@Fe₃O₄ NPs’ ability to absorb metal ions. These adsorption results revealed that the ternary metal ion adsorption is dependent on the adsorbent mass, solution pH, contact time, and initial metal ion concentration. The optimized condition for the removal of ternary metal ions is obtained at the adsorbent mass of 0.2, pH of 6.0~7.0, and contact time of 60 min. In kinetics studies, the kinetic adsorption of ternary metal ion systems fitted well to the pseudo-second-order model. Thus, the adsorption of ternary ions onto Z@Fe₃O₄ NPs was a chemisorption process. Furthermore, based on the isotherm adsorption investigations of the ternary metal ion system, it shows that the Langmuir isotherm model was found to be the best model (with R² values higher than 0.9206) to describe the ternary metal ion absorption by Z@Fe₃O₄ NPs. This indicates that the ternary metal ion adsorption process is characterized by monolayer adsorption. Furthermore, the maximum adsorption capacities of Cs⁺, Sr²⁺, and Co²⁺obtained from the Langmuir model are 48.31, 15.02, and 10.41 mg/g, respectively. Thus, the selectivity trend in the ternary metal ion system towards the Z@Fe₃O₄ NPs is observed to be Cs⁺ > Sr²⁺ > Co²⁺, which indicated that the competitive effect of Cs⁺ was the strongest.