2.1 Materials

All the chemical reagents used in this study were of analytical reagent grade [AR Grade]. FeCl₃·6H₂O (Aldrich 98%), FeCl₂·4H₂O (Aldrich 99%) and ammonium hydroxide (Aldrich 28–30% of ammonium) were used for magnetic particle preparation. Caesium and strontium were supplied as caesium chloride and strontium chloride, from Merck Company.

2.2 Synthesis of magnetite zeolite composite

Zeolite A was synthesized according to the following procedure [9]. Sodium hydroxide and sodium aluminate were dissolved in water, then stirred and boiled. The prepared solution was added to the hot solution of sodium metasilicate under vigorous stirring. After hydrothermal crystallization, the product was washed with distilled water and dried overnight in an oven at 100 °C.

MZC was prepared through the co-precipitation of Fe³⁺ and Fe²⁺ in the presence of zeolite powder. First, 100 ml of aqueous ammonia solution (1 M), were placed in a round bottom flask and a known amount of zeolite powder was added to the ammonia solution. The mixture was homogenized by using vigorously mechanical stirring and deoxygenated by bubbling of N₂ gas for 30 min. Solutions of ferric chloride (1 M in water) and ferrous chloride (2 M in HCl 2 M) with volume ratio of 4:1 were prepared and mixed together. The Fe³⁺/Fe²⁺ solution was added dropwise to the zeolite/ammonia mixture while the mixture was vigorously stirred under N₂ atmosphere. The prepared composite was separated with the aid of a permanent magnet. Finally, the product was washed four times with deionized–deoxygenated water and then dried at 50 °C for 5 h. Five different composites were prepared by adding different amounts of zeolite A.

2.3 Characterization of the samples

The structure of the synthesized zeolite and the magnetic composite was examined using a Bruker, D8ADVANCE X-ray diffractometer. The chemical composition of the samples was determined by a PANalytical Magix X-ray fluorescence spectrometer. FT-IR spectra were obtained by a Shimadzu, IR Prestige-21 Model spectrophotometer. Thermogravimetric analysis of the samples was conducted on Mettler, TG-50 thermal analyzer with heating rate of 10 °C/min. A vibration–sample magnetometer (Meghnatis Daghigh Kavir Co.) was used to characterize the magnetic properties of the composite. The size distribution, shape and morphology of the samples were examined by SEM. The images were taken using Seron technology AIS2100. The cation exchange capacity (CEC) of the synthesized zeolite and the magnetic composites was determined by shaking 1.0 g of the samples with 100 mL of 1 M NH₄NO₃ solution at 25 °C for 72 h. The ammonium concentration was determined by the Kjeldahl method [10]. The theoretical cation exchange capacity (TCEC) was calculated from chemical composition of the samples as a sum of exchangeable cations present in 1.0 g of zeolite.

2.4 Metal ion uptake experiments

Adsorption behavior of zeolite A and the magnetic composites was studied by batch technique. First, 0.2 g of zeolite or composite was equilibrated with 20 mL of each ion solution in a polypropylene bottle on a thermostat rotary shaker. After equilibration, the zeolite samples were separated by centrifugation while the composite separation was performed by a permanent magnet. Ion concentration was measured by ICP-AES method. The effect of pH on the cations uptake was studied by adjusting the initial pH at 3–8, using 0.1 N HNO₃ and 0.1 N NaOH solutions. For kinetic experiments, the adsorbents were contacted with ions solution for different period of time (5–120 min) at 298 K. To study the effect of temperature on the cations removal, the experiments were conducted at four different temperatures of 298, 313, 328 and 343 K, respectively. In order to investigate the effect of iron oxide content on sorption capacity, five composites with different iron oxide contents were prepared. The adsorption isotherms were studied with 0.001–0.1 N cation solutions. Cation uptake (q), expressed as the amount of ions removed per unit mass of the adsorbent was calculated according to:

3.1 Adsorbents characterization

Fig. 1 shows the XRD patterns of the synthesized zeolite and MZC samples. Comparing the diffraction lines at 2θ ranges of 5–45° with those of the reference indicated that the microporous structure of zeolite A was well obtained (Fig. 1a) [11]. In the XRD pattern of the MZC sample (Fig. 1b), the relative line intensity and line position related to zeolite A remained unchanged, indicating that during composite preparation, the crystal structure of zeolite A was stable. The diffraction line of iron oxide was observed at 2θ of 35.55° [12]. By applying the Scherrer's equation [13], the particle size of magnetite in the composite was found to be 21 nm.

The chemical composition of the synthesized zeolite and the composite obtained by XRF method is shown in Table 1. The Si/Al ratio of zeolite A was 1.189 and similar to that of reference [14]. The experimental and theoretical adsorption capacity of zeolite A and the composite are listed in Table 2. The CEC value of MZC sample was only 5% lower than that of zeolite A while the zeolite content of the sample was 75%. This showed that iron oxide also contributed to the adsorption process according to the following reaction:

The FT-IR spectra of magnetite, synthesized zeolite and MZC are recorded between 400 to 4000 cm⁻¹ (Fig. 2). In the spectra of magnetite, the band observed at 579 cm⁻¹ is assigned to Fe−O−Fe [12]. The bands at 465, 560 and 1005 cm⁻¹ are, respectively, related to the vibration of T–O bending, double ring and TO₄ asymmetric stretch (T = Si or Al) [15]. The band at 3000–3600 cm⁻¹ represent inter and intermolecular hydrogen bonding. The adsorption band at 1640 cm⁻¹ is related to interstitial bonded water. In the spectrum of the MZC sample, all the adsorption bands related to zeolite A were observed and overlapped with the band related to Fe−O−Fe.

In the thermal curves of zeolite A and MZC sample, the weight loss peak around 100 °C is attributed to the loss of adsorbed water (Fig. 3). Since the water molecules are bonded to different cations so, dehydration has taken place at two different temperatures. After 250 °C, the adsorbents exhibited no dehydration peak up to 800 °C, indicating that the adsorbents are thermally stable.

In the SEM image of zeolite A, the presence of zeolite particles with homogenous morphology and particle size of about 2 μm was observed (Fig. 4a) In the composite image, the presence of magnetite nanoparticles on the surface of zeolite was observed (Fig. 4b).

3.2 Separation of the composite from the solution

One of the most important features of this composite is its magnetic properties that were characterized with the VSM method. Fig. 5 shows the magnetization curve of the composite (zeolite to iron oxide ratio of 3:1) at 298 K. When the external magnetic field intensity increased, the magnetization is increased and then saturation is occurred. By applying the reverse external magnetic field, the magnetization achieved the reverse saturation. The hysteresis loop of the composite showed a “S”-shaped curve with no remanence. Therefore, the MZC has good superparamagnetic properties and also when external magnetic field is removed, the composite particles can be redispersed quickly with shaking. The saturation magnetization value was 19.03 emu/g confirming that in practice, the composite can be easily separated with a permanent magnet. On the other hand, the adsorption capacity of the composite with different zeolite to iron oxide ratio is shown in Fig. 6. The adsorption capacity of MZC is decreased as the magnetite to zeolite ratio increased. Due to the desirable magnetic properties and adsorption capacity, the composite with zeolite to iron oxide ratio of 3:1 was used for adsorption experiments.

3.3 Effect of pH

The effect of pH on Cs⁺ and Sr²⁺ removal by zeolite A and the composite is shown in Fig. 7. Adsorption capacity of both the adsorbents increased with increasing initial pH. The lower uptake in the acidic medium is attributed to the competition of hydronium ions with Cs⁺ and Sr²⁺ for the exchange sites. Since the magnetite particles act as weak acid or base, the pH of the solution has an extra effect on the sorption behavior of the MZC sample. By increasing the pH, the magnetite surface was deprotonated and the exchange capacity of the magnetite increased. The maximal adsorption was occurred at pH 8.

3.4 Effect of contact time

The effect of contact time on the removal efficiency of Cs⁺ and Sr²⁺ is shown in Fig. 8. The adsorption was initially fast and then reached equilibration after 2 h. Between 80−90% of the maximal capacity was obtained within 30 min, indicating that the reaction is kinetically fast.

3.5 Kinetic sorption modeling

Four kinetic models, including the pseudo-first-order, pseudo-second-order, Elovich and intra-particle diffusion model, were examined for the evaluation of the experimental data. The linear form of Lagergren pseudo-first-order model is expressed as:

The results are listed in Table 3. Although the R² have not extremely low values, calculated q_e are not consistent with the experimental data. Therefore, the adsorption process does not follow a pseudo-first-order model. The linear form of pseudo-second-order kinetic model is written as:

and initial sorption rate can be obtained from:

Comparing of k₂ values indicated that the adsorption of both ions onto MZC is faster than on zeolite A.

The relationship between the amount of adsorbed ions and the square root of time indicates the effect of the intra-particle diffusion on the overall rate of ion exchange. The intra-particle diffusion equation is expressed as:

Comparison of calculated q_e and the correlation coefficient for kinetic models evidence that pseudo-second-order has the best match with the experimental data. These results indicated that the overall sorption rate is controlled by the chemical sorption process [16]. The k₂ values were selected for the evaluation of E_a using Arrhenius equation (Fig. 9).

The calculated data are represented in Table 5. The values of E_a was less than 42.0 J/mol, suggesting that the adsorption of Cs⁺ and Sr²⁺ proceeded with a low potential energy [17].

3.6 Sorption thermodynamics

The effect of sorption temperature on the removal of Cs⁺ and Sr²⁺ was studied in the temperature range from 298 to 343 K (Fig. 10). Adsorption capacity increased with increasing temperature. Thermodynamic parameters, including enthalpy change (ΔH°) and entropy change (ΔS°) can be obtained using Vant Hoff equation.

A plot of ln k_d versus 1/T is shown in Fig. 11. The free energy change of the adsorption (ΔG°) is calculated from:

The calculated thermodynamic parameters are summarized in Table 5. Since ΔH° values are positive the adsorption process is endothermic in nature. The positive values of ΔS° suggested that the randomness at the solid−solution interface increases during the sorption process. The negative values of ΔG° indicated that the Cs⁺ and Sr²⁺ adsorption reaction is spontaneous. A decrease in ΔG° values with an increasing temperature showed that the spontaneous nature of sorption is directly proportional to temperature. Therefore, the adsorption process is favored when temperature is increased. The absolute value of ΔG° for Sr²⁺ adsorption is higher than the values for Cs⁺ adsorption, indicating that the sorption of Sr²⁺ is more favorable than that of Cs⁺.

3.7 Sorption isotherm

In order to investigate the effects of initial concentration, Cs⁺ and Sr²⁺ solution with various concentrations were equilibrated with a constant amount of adsorbents (Figs. 12 and 13). The initial fast adsorption was followed by a slow approach to equilibrium in higher concentration. Three isotherm models, including Langmuir, Freundlich and Dubinin–Radushkviech (D–R) models were used to examine the experimental data.

3.7.1 Langmuir isotherm model

Langmuir isotherm assumes that the sorption occurs at specific homogeneous sites within the adsorbent. This model is expressed by:

Ce/qe=1/Q0b+Ce/Q0

(12)

where Q_o the saturation adsorption capacity (mg/g) and b the Langmuir constant related to the free energy of adsorption that were calculated from the slope and the intercept (Tables 6 and 7). The plot of C_e/q_e versus C_e is shown in Fig. 14. The important parameter of Langmuir isotherm is the dimensionless constant, R_L, which could be calculated from:

RL=1/1+bC0

(13)

where C₀ is the highest initial metal ion concentration (mg/L). The value of this parameter indicates the unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1) and irreversible (R_L = 0) types of the adsorption isotherm [18]. The obtained data confirmed that Cs⁺ and Sr²⁺ adsorption is favorable.

Table 6

Adsorption isotherm parameters for Cs⁺.

Isotherm	Sorbents	Model parameters
		Q0 (mg/g)	b × 103 (L/mg)	RL×103	R2
Langmuir	Zeolite A	222.9	2.569	28.45	0.9965
MZC	207.4	2.161	33.64	0.9988
		Model parameters
		n	Kf (mg/g)	R2
Freundlich	Zeolite A	2.611	7.907	0.9664
MZC	2.304	4.843	0.9376
		Model parameters
		β × 10−9 (mol2/kJ2)	qm (mmol/g)	E (kJ/mol)	R2
Zeolite A		5.127	2.094	9.875	0.9990
MZC		6.375	2.230	8.856	0.9901

Table 7

Adsorption isotherm parameters for Sr²⁺.

Isotherm	Sorbents	Model parameters
		Q0 (mg/g)	b × 103 (L/mg)	RL × 103	R2
Langmuir	Zeolite A	99.11	18.32	12.31	0.9985
MZC	83.68	13.86	16.20	0.9993
		Model parameters
		n	Kf (mg/g)	R2
Freundelich	Zeolite A	2.985	8.629	0.9665
MZC	2.702	5.17	0.9637
		Model parameters
		β × 10−9 (mol2/kJ2)	qm (mmol/g)	E (kJ/mol)	R2
D–R	Zeolite A	3.814	1.367	11.45	0.9964
MZC	4.615	1.226	10.41	0.9951

The maximum adsorption capacities of MZC for the removal of Cs⁺ and Sr²⁺ have been compared with some adsorbents reported in the literature (Table 8). It can be seen that the MZC exhibit high adsorption capacity for these ions. On the other hand, the magnetic composite was easily separated from the solution by a permanent magnet.

Table 8

Adsorption capacity of Cs⁺ and Sr²⁺ by some adsorbents.

Adsorbent	Qm (mg/g)	Reference
	Cs+	Sr2+
Clinoptilolite	49.26	6.69	[19]
Montmorillonite	57.04	13.26	[20]
Phosphate-modified Montmorillonite	93.87	12.56	[20]
Taiwan laterite	39.87	−	[21]
Activated carbon	−	44.40	[22]
Magnetic zeolite composite	207.4	83.68	This work

3.7.2 Freundlich isotherm model

Freundlich isotherm model makes the assumption that multi-layer sorption occurs at a heterogeneous surface. This model was used to estimate the intensity of the adsorption process and relative sorption capacity. The linear form of the Freundlich equation is shown as:

logqe=logKf+1/nlogCe

(14)

where K_f is the Freundlich constant related to the adsorbent capacity and n is the constant indicative to the intensity of the adsorption process. The Freundlich isotherm is shown in Fig. 15 and the isotherm constants are represented in Tables 6 and 7. The Freundlich constant (n) was greater than unity, indicating that an increasing tendency of adsorption process occurred with ion concentration increasing. The K_f value for Sr²⁺ was greater than for Cs⁺, confirming a greater adsorption tendency of the adsorbents towards Sr²⁺.

3.7.3 Dubinin–Radushkviech isotherm model

D–R isotherm was used to distinguish between the physical and chemical adsorptions of ions and to estimate the apparent free energy of adsorption. This equation is given as follows:

lnqe=lnqm−βε2

(15)

where q_m is the maximum adsorption capacity (mequiv/g), β is the D–R constant related to the sorption energy (mol²/K/J²) and ɛ is the Polanyi potential:

ε=RTln1+1/Ce

(16)

where R is the gas constant (kJ/mol/K) and T is the absolute temperature (K). The D–R isotherm is plotted in Fig. 16 and the obtained parameters are represented in Tables 6 and 7. The free energy when 1 mol of ion is transferred to the surface of the adsorbent from infinity in the solution is called the mean energy of adsorption (E) and is calculated as:

E=2β−1/2

(17)

The magnitude of E in the range of 8–16 KJ/mol indicates that the sorption process is done through an ion exchange interaction. The reaction mechanism can be related to the magnitude of E. When the E value is in the 8–16 kJ/mol range, the sorption process is followed by ion exchange, but when E < 8.0 kJ/mol, the physical forces are responsible for the process [23].