NanoTafla Nanocomposite as a Novel Low-Cost and Eco-Friendly Sorbent for Strontium and Europium Ions.

Now the wide use of nanooxides is attributed to their remarkable collection of properties. Nanocomposites have an impressive variety of important applications. A thermal decomposition approach provides a more optimistic method for nanocrystal synthesis due to the low cost, high efficiency, and expectations for large-scale production. Therefore, in this study a new eco-friendly nanooxide composite with sorption characteristics for europium (Eu(III)) and strontium (Sr(II)) was synthesized by a one-step thermal treatment process using earth-abundant tafla clay as a starting material to prepare a modified tafla (M-Taf) nanocomposite. The synthesized nancomposite was characterized by different techniques before and after sorption processes. Different factors that affected the sorption behavior of Eu(III) and Sr(II) in aqueous media by the M-Taf nanocomposite were studied. The results obtained illustrated that the kinetics of sorption of Eu(III) and Sr(II) by the M-Taf nanocomposite are obeyed according to the pseudo-second order and controlled by a Langmuir isotherm model with maximum sorption capacities (Q max) of 25.5 and 23.36 mg/g for Eu(III) and Sr(II), respectively. Also, this novel low-cost and eco-friendly sorbent has promising properties and can be used to separate and retain some radionuclides in different applications.

Introduction

During the last decades, as we’ve entered a new era of globalization, the purification and treatment of radioactive wastes became urgent demands to all countries before these wastes are discharged into the environment.[1,2] In the different nuclear industries, various types of massive radionuclides such as europium Eu(III) and strontium Sr(II) are generated, which can pollute the natural environment and seriously threaten living things.[2−6] Therefore, current efforts have been directed to reduce the risk of radiotoxicity of such radionuclides either before or after they are released into the environment, and we regard this as an issue of mounting concern due to the radioactivity and harmful impact on human health and aquatic environments.[7−11] The effective removal and efficient remediation of these radionuclides has become an urgent challenge in nuclear-waste management.[12−14] According to the International Energy Agency (IEA), by 2030 the capacities of the international nuclear industries will be developed by more than 40%, and this will promote the requirement of U6+ consumption and lead to the production of huge amounts of radioisotopes that might be released to the environment.[4] Strontium (90Sr) and europium (152,154Eu) are regarded as the main radionuclides in nuclear effluents and have severe biological toxicity including bone sarcoma, soft tissue cancer, arteriosclerosis diseases, rib bone and liver damages, leukemia, and endocrine system disruption.[15−17] Therefore, the removal of these radionuclides has acquired great attention and wide concern. So, a number of techniques, such as coagulation precipitation, extraction, filtration, precipitation, and adsorption, were applied to treat radioactive wastes and remove these radionuclides from aqueous solutions.[4,18−22] On the one hand, compared with various separation techniques and due to its advantages such as high selectivity, effectiveness, eco-friendly quality, and cost-effectiveness, sorption methods have been considered a quite promising technique,[15,16,23−25] where the sorbents reduce the radionuclide amount of large volumes of wastewater to a small volume of solid by different adsorption modes.[12] On the other hand, nanosorbents have various promising characteristics, like low cost, considerable surface area, porous structures, and low sorption equilibrium time, and therefore they are considered promising sorbents used to remove different types of radionuclides.[26,27] In recent decades, a variety of different sorbents and ion exchangers has been utilized to remove some radionuclides from wastewater. Clay minerals,[16,28−31] zeolites,[32−36] montmorillonite,[37,38] and carbonaceous materials (carbon nanotubes, graphene, and activated carbon)[39,40] are examples of these sorbents that have attracted great attention during recent decades. The preparation of graphene oxide (GO)-based nanocomposites has been reported using the self-assembly of chitosan and graphene oxide and an in situ reduction approach.[41] Molybdenum compound nanoclusters supported on N-doped reduced graphene oxide have been prepared by carbonization and hydrothermal processes.[42,43] Also, poly(vinyl alcohol)/poly(acrylic acid)/carboxylate graphene oxide nanosheet@polydopamine (PVA/PAA/GO–COOH@PDA) samples have been prepared by an electrospinning technique, thermal treatment, and polydopamine modification.[44]

Therefore, this work has been directed to prepare a novel nanoTaflanancomposite (M-Taf) as a low-cost and eco-friendly sorbent for strontium and europium species. The prepared nanocomposite was characterized by various techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy. Various parameters, such as contact time, pH, volume-to-mass (V/m) ratio, initial metal ions concentration, and temperature, influenced the sorption processes, which were investigated.

Experimental Section

Chemicals and Materials

Herein, all the reagents and materials employed are of analytical grade and used as received without any further refining. Europium(III) nitratepentahydrate (Eu(NO3)3·5H2O), strontium chloridehexahydrate (SrCl2·6H2O), nitric acid, and hydrochloric acid were supplied by Sigma-Aldrich, while NaOH was supplied by Adwic company for chemicals.

Synthesis of Nanocomposite

Tafla clay, which was used as a precursor for the nanooxides composite preparation, was collected from the Katameyya region, Cairo, Egypt. First, natural Tafla clay was crushed and washed several times with distilled water and stirred for several hours to remove impurities and other adhering materials. Then, the mixture was filtrated, and the filtrate was dried in the sunlight for 2.0 d. After it dried in the sunlight, the Tafla clay particles were sieved to obtain the particle size of 53 μm. The resulting Tafla powder was thermally treated in the muffle furnace (in an air atmosphere) by ramping the temperature (50 °C/10 min) until it reached 1000 °C, when it was held for 4 h. No other solvents or chemicals were used in the process of preparing this M-Taf nanoparticle.

The characterizations of the prepared modified Tafla (M-Taf nanocomposite) were achieved using a scanning electron microscope (JSM-6510A model), Fourier transmission infrared spectroscopy (Nicolet spectrometer). XRD measurements were made on a Shimadzu X-ray diffractometer obtained from Shimadzu Kyoto (model XD-Dl). The nanostructure of the M-Taf nanocomposite was inspected through a transmission electron microscope (JEM-2100). The chemical composition of the nanocomposite sample was analyzed by a PW-2400 Philips XRF spectrometer.

Batch Sorption Experiments

Batch adsorption experiments were done by shaking 0.05 g of new nanocomposite with 25.0 mL of 100 mg/L of Eu(III) and Sr(II) in a thermostated shaker bath (G.F.L. 1083) adjusted at 25 °C.

The concentration of Sr(II) was measured using an atomic absorption spectrophotometer, model S4 Series, Thermo-electron Corporation. The concentration of europium ions was determined spectrophotometrically by the Arsenazo-III method using a Shimadzu UV–visible double beam spectrophotometer model 160A. The pH measurements were performed by a Hanna pH meter with a resolution of 0.01 pH and accuracy of ±0.01.

Batch experiments were conducted to adsorb Eu(II) and Sr(II) from aqueous media onto the prepared nanocomposite at ambient temperature (25 °C ± 1). A 0.025 L solution of 100.0 mg/L for each investigated metal ion was mixed with 0.05 g of the new nanocomposite at the stirring rate of 400 rpm for 60.0 min at pH ≈ 5.0 for Eu(II) and 8.0 for Sr(II), unless otherwise cited.

In the sorption kinetics and isotherms, 0.05 g of the nanocomposite was transferred into a bottle containing 25.0 mL of a definite concentration of Eu(III) and Sr(II) ions. The mixture of nanocomposite and aqueous phases was mixed in a thermostated shaker bath for different time periods. The parameters affecting the sorption processes of Eu(III) and Sr(II) by a prepared nanocomposite were investigated and studied. The samples were withdrawn at certain times for analysis.

The percentage removal (%R) of Eu(III) and Sr(II) as well as the amount of metals ions sorbed at time t, qt (mg/g), was calculated bywhere both Ao and At are the initial concentration and concentration at time t of Eu3+and Sr2+ per unit volume, V (L) is the volume of the aqueous solution, while m (g) is the weight of the nanocomposite.

The thermodynamic parameters of the sorption processes, namely, ΔH°, ΔS°, and ΔG°, were calculated by eqs –5).

From Equation in Equation where ΔH0, ΔS0, ΔG0, and Kd are the standard enthalpy, entropy, Gibbs free energy change, and the distribution coefficient (mL·g–1), respectively, while R and T are the general gas constant (R = 8.314 J mol–1 K–1) and absolute temperature (K), respectively. The relations between ln Kd against T–1 are linear. The slope of this relation gives the value of ΔH0, and the intercept gives the value of ΔS0.

Reusability Experiments

In this work, 100 mL of hot 5% HCl was utilized to desorb Eu(III) and Sr(II) from the prepared nanocomposite. The concentration of Eu(III) and Sr(II) in the desorption solution was measured spectrophotometrically, as shown above. %R of Eu(III) and Sr(II) was calculated using eq .

Results and Discussion

Characterization of the Original Tafla and Prepared M-Tafnanocomposite

The chemical composition of the fabricated nanocomposite was analyzed by X-ray fluorescence (XRF) spectrometry. According to XRF analyses, the main chemical compositions are SiO2 (62.911%), Al2O3 (13.9%), Fe2O3 (13.88), K2O (2.48%), TiO2 (2.72%), and other minor compositions. Most of these oxides have chemical and thermal stabilities under the investigated conditions.

The SEM images of original Tafla clay are represented in Figure with different magnifications. Microstructures of the original particles demonstrated that the particles are composed of a highly crystalline form of sheets like polygonal flakes and are structured to form the stacks.[45,46]Figure also shows the surface morophology of the prepared nanocomposite at different magnifications. The micrograph of the prepared nanocomposite illustrates a vital change in the morophology structures compared with the original Tafla. The effect of the modification process on the Tafla samples shows that the surface area of the prepared nanocomposite has been increased due to the opening of the platelets, which also leads to a more porous characteristic in the nanocomposite structure.[47] The surface of the prepared nanocomposite has many advantages that enhance sorption processes such as irregularity, roughness, porosity, and heterogeneity.[48] These images indicate that the particles of the prepared nanocomposite have a polygonal shape. The successful formation of the new nanocomposite under the investigated conditions is confirmed by TEM and XRD analyses, as illustrated in Figures and 3.

Figure 1

Images of SEM of the original tafla (a, b) and synthetic nanocomposite (c, d) with different magnification values.

Figure 2

TEM images of prepared nanocomposite with magnification values of (a) X = 60 000 and (b) X= 40 000.

Figure 3

Crystal structure of prepared nanocomposite.

The data obtained by TEM measurements revealed the recognizable platy and hexagonal crystals of the prepared nanocomposite. These results emphasize the formation of structures obtained by the SEM analysis, Figure . Also, Figure displays a propagation of a homogeneous morphology with conglomerate particles and relatively broad size distributions.[49]

The XRD pattern of the prepared M-Taf-nanocomposite is shown in Figure . The prominent bands at 20.9, 26.8, 33.4, 35.7, 41.9, 54.6, and 68.1 are characteristic bands that confirm the presence of a kaolinite mineral and quartz in the prepared nanocomposite.[45,50,51] The occurrence of more kaolinite approves the increase in the pore sizes of the prepared nanocomposite.[45]

The FTIR spectra of the prepared nanocomposite and the original Tafla were investigated and represented in Figure a,b to identify the function groups and to check for possible changes that could result from the modification processes.[52] For the original Tafla, Figure b, the absorption peaks observed at 3700 and 3400 cm–1 may be due to the stretching modes of Al–OH and the stretching vibrations of −OH in the tetrahedral and octahedral sheets of the original Tafla.[52,53] These peaks are changed and decreased for the prepared nanocomposite (Figure a) due to a dehydroxylation process.[54] Also, strong vibration bands detected at ∼1034 and 16 331 cm–1 may be due to the presence of O–Si–O, and these vibration bands are changed to appear at 1086, 1354, and 1432 cm–1 in the prepared nanocomposite due to surface chemistry changes that occur by a thermal activation.[53−55]

Figure 4

FTIR spectra of prepared nanocomposite and original Tafla.

The vibration band at 913 cm–1 may be assigned to the presence of Al–O–H bending and inner hydroxyl groups. Also, absorption bands of Al–OH and Al–O–Si appear at ∼914, 961, and 531 cm–1. The bands that appear in the range of 791–424 cm–1 are characteristic of Tafla and assigned to Si–O–Al or Si–O–Si vibrations.[53−55] In the prepared nanocomposite, the loss of −OH groups and Al–O–Si peaks approves the formation of amorphous SiO2 and Al2O3 nanoparticles, and these are accompanied by the destruction of Al–O–Si bonds of the original Tafla.[55−57] Also, two main characteristic bands for the formation of silica nanoparticles appeared at ∼1086 and 791 cm–1 and are assigned to Si–O–Si peak vibration modes.[58,59]

Sorption Studies

Influence of Contact Time

The effect of contact time on the removal of Eu(III) and Sr(II) from aqueous solution using 0.05 g of nanocomposite was investigated as a function of mixing time in the range of 0.0–90 min using a batch adsorption experiment, and the results obtained are represented in Figure . The maximum removal percentage and equilibrium were reached for both Eu(III) and Sr(II), respectively. The results obtained reveal that the adsorption capacities of Eu(III) and Sr(II) by 0.05 g of nanocomposite increased rapidly by increasing the contact time within the first 15 min and then became slow to reach the equilibrium. This can be due to the large availability of the active sites on the nanocomposite. With further increase in time, there is almost no further increase in the sorption of both metals ions, and this can be due to fewer active sites being available on the nanocomposite.[56]

Figure 5

Effect of contact time on the removal of Eu(III) and Sr(II) by nanocomposite ([Eu] = 100.0 mg L–1, pHEu = 5.0, [Sr] = 100.0 mg/L, pHSr = 8.0, v = 0.025 L, dose = 0.05 g, T = 25 ± 1 °C).

Adsorption Kinetic

In order to better understand the adsorption rates and pathway of the investigated sorption processes, the dynamics of adsorption processes of Eu(III) and Sr(II) by nanocompositeis were studied until an equilibrium reached.[61] Pseudo-first-order and pseudo-second-order processes were investigated and illustrated in Figure a,b, respectively, to analyze and calculate the rate constants and adsorption data, as in eqs and 8.[15,26,60,61]Here, qe (mg·g–1) and qt (mg·g–1) are the amounts of sorption at the equilibrium time and time t. k1 (min–1) is the rate constant of the pseudo-first-order sorption.

Figure 6

Adsorption kinetics of Eu(III) and Sr(II) ions with 0.05 g of nanocomposite. (a) Pseudo-first order and (b) pseudo-second order.

K2 (g mg–1 min–1) is the rate constant of the pseudo-second-order sorption.

The kinetic parameters of the adsorption processes, such as qeexp, k1, qecal, R2, and k2, for a linear fitting of the experimental results are calculated from the intercept and slope of the plot represented in Figure a,b and illustrated in Table . The data dissected that the Eu(III) and Sr(II) by nanocomposite shows an excellent fitting with the pseudo-second order (R2 ≥ 0.992 for Eu(III) and ≥0.986 for Sr(II)), indicating the sorption processes are a chemisorption process and agree well with the results reported in previous works.[12,15,26,58]

Table 1

Adsorption Kinetic Parametersa for the Adsorption of Eu(III) and Sr(II) Ions with 0.05 g of Nanocomposite at 298 K

	linear pseudo-first order
metal ion	qe calc mg/g	K1, min–1	R2
Eu(III)	39.09	339.0 × 10–3	0.866
Sr(II)	26.21	254.64 × 10–3	0.925
	linear pseudo-second order
	qe calc mg/g	k2, g/mg·min	R2
Eu(III)	22.32	35.45 × 10–3	0.992
Sr(II)	19.16	24.55 × 10–3	0.986

([Eu] = 100.0 mg/L, [Sr] = 100.0 mg/L, v = 0.025 L, dose = 0.05 g, Shaking time = 15.0 min, T = 25 ± 1 °C).

Influence of pH

The effect of pH (1.0–10) on the sorption of Eu(III) and Sr(II) by 0.05 g of nanocomposite was investigated at the optimum conditions, as shown in Figure , to examine how the metals ion sorption is strongly influenced by the initial pH of the solution.

Figure 7

Effect of pH on the removal of Eu(III) and Sr(II) ions with 0.05 g of nanocomposite. ([Eu] = 100.0 mg L–1, [Sr] = 100.0 mg/L, v = 0.025 L, dose = 0.05 g, shaking time = 15.0 min, T = 25 ± 1 °C)

The amounts of Eu(III) and Sr(II) adsorbed by nanocomposite were increased by increasing the pH values due to the electrostatic repulsion between positive charges on the nanocomposite and Eu(III) and Sr(II) ions.[26] The repulsion decreases with increased pH in this region and leads to an increase in the capacities values. At higher pH, sorption capacities of the nanocomposite rapidly reduce, which is due to the change in the charges on the surface of the nanocomposite.

All of these results may be explained depending on the fact that, at a low pH value, two mechanisms may be investigated to interpret the results. Here, the main Eu(III) species was Eu(III).[62,63] Here, the main species, namely, Eu(III), was at pH < 6.0.[12] Subsequently, the lower adsorption rate by the nanocomposite may be due to the competitive adsorption between Eu(III) ions and positively charged surface H+/M+ ions in ion exchange sites of the nanocomposite due to the electronic repulsion.[62] At higher pH, the dominant species of Eu(III) in the aqueous phase are Eu (OH)3, EuOH2+, and Eu(OH)2+, and therefore these species are sorbets in the higher pH range.[62] For Sr(II), as reported by Qi et al.,[64] Sr(II) is the prominent species when pH < 11.0. Therefore, the sorption rate is low at lower pH values, and it increased with further increase in the pH of media. With increases in the pH value of the solution, the surfaces of a nanocomposite become more negatively charged, and strong electrostatic interactions between Sr(II) with nanocomposite will form; therefore, the sorption capacities are enhanced, and more Sr(II) ions are removed.[65,66] This behavior may be due to weak acid cation exchange properties of the nanocomposite.[63]

Influence of V/m Ratio

The dependence of Eu(III) and Sr(II) removal on the V/m ratio (L/g) was investigated in the range as it was varied from 0.15 to 0.5 L/g by the nanocomposite. The data obtained, Figure , show that the rate of sorption first decreased rapidly with increases in the V/m ratio (L/g) until 0.35 g/L, and then it decreased slowly with further increases in the V/m ratio (L/g). This decrease in sorption efficiency is attributed to the fact that, with increases in the number of active sites on the nanocomposite surface, the ratio of these active sites to the number of sorbate ions is increased.[63]

Figure 8

Effect of aqueous volume (V) on the adsorption of Eu(III) and Sr(II). [Eu] = 100.0 mg L–1, pHEu = 5.0, [Sr] = 100.0 mg/L, pHSr = 8.0, shaking time = 15.0 min, T = 25 ± 1 °C.

Influence of Initial Metal Ion Concentrations

The effect of initial concentrations of Eu(III) and Sr(II) ions on the sorption processes by nanocomposite varying from 50 to 150 mg L–1 was studied, and the data obtained are represented in Figure . It is observed that, as the initial concentrations of Eu(III) and Sr(II) increase from 50 to 150 mg L–1, the sorption capacity of Eu(III) and Sr(II) decreased from ∼80 to 40% for Eu(III) and from 58 to 38% for Sr(II), respectively. This may be explained by the fact that, with increases in the initial metal ion concentrations of Eu(III) and Sr(II), the available active sites in the surface of nanocomposite decreased, and saturation of the these sites occurred.

Figure 9

Effect of initial Eu(III) and Sr(II) concentrations on the adsorption rate by nanocomposite. [Eu] pH = 5.0, [Sr] pH = 8.0, v = 0.025 L, m = 0.05 g, shaking time = 15.0 min, T = 25 ± 1 °C.

Therefore, the rates of adsorption of the studied cations were dropped.[65,67] The experimental data indicate that the affinity of the nanocomposite toward Eu(III) are higher than that for Sr(II).

Sorption Isotherm and Modeling

Different isotherm models are commonly used to evaluate the solid–liquid sorption systems.[26] Therefore, Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin isotherm models are investigated and applied, as in eqs –15 to study the best fitting models, as presented in Figure a–d.

Figure 10

(a) Langmuir, (b) Freundlich, (C) Dubinin–Radushkevich, and (d) Temkin isotherm models for the adsorption of Eu(III) and Sr(II) by (M-Taf)-nanocomposite.

The Langmuir isotherm model has been applied extensively for dilute media and to describe the monolayer sorption processes, as in eq where, Ce is the equilibrium concentration (mg L–1) of the Eu(III) and Sr(II) ions, qe is the Eu(III) and Sr(II) concentration (mg·g–1) in the nanocomposite, and Qmax is the maximum monolayer sorption capacity (mg·g–1) as represented in Figure a.

The Freundlich isotherm model supposes that the surface of a nanocomposite contains active sites that have several binding energies. The linearity equation iswhere Kf (mg·g–1) and n are Freundlich constants that integrate the influence of all parameters, like the intensities of sorption and sorption capacities, on the sorption process. The Freundlich model considers the heterogeneity of a sorbent surface and the exponential distribution of active sites and their energy. For a favorable sorption, the values of n must be much greater than 1.0.

For many adsorption processes, the Dubinin–Radushkevich isotherm is more appropriate compared with the Freundlich mode, where it is not limited by constant sorption potential assumptions and homogeneous surfaces. It is represented by[64]where β is a constant related to the sorption energy and qm the sorption capacity, and ε is the Polanyi potential and can be determined as in eq .

The mean free energy of sorption (EDR) is determined from the β values by

The Temkin isotherm model explains the interaction between sorbent and sorbate, as in eq where BT (kJ·mol–1) is the Temkin constant, which is related to the heat of adsorption, and kT (L·mg–1) is the equilibrium binding constant related to the maximum binding energies. The values of BT and kT may be calculated from the slopes and intercepts of straight lines of the plots of qe versus ln Ce (Figure d).

The experimental results were fitted with the investigated isotherm models, Figure a–d, and their fitted factors and correlation coefficients (R2) were calculated and represented in Table . The experimental results and calculations were well-represented by a Langmuir isotherm with R2 values of 0.999 and 0.996 for Eu(III) and Sr(II), respectively, and with maximum adsorption capacities Qe (mg/g) of 25.5 and 23.36 mg·g–1 for Eu(III) and Sr(II), respectively. Therefore, the adsorption processes of Eu(III) and Sr(II) using 0.05 g of nanocomposite are obedient to the Langmuir isotherm model. These results agree with those reported in previous works.[2,62,64,68]

Table 2

Isotherm Parametersa for Adsorption Processes of Eu(III) and Sr(II) by Nanocomposite

		Freundlich parameters			Langmuir parameters				Dubinin–Radushkevic				Temkin
metal ions	qo, exp mg/g	Kf(mg/g)	n	R2	Qo (mg/g)	b (ml/mg)	RL	R2	qm	β	R2	EDR	KT	bT	B	R2
Eu(III)	24.66	15.5	9.27	0.847	25.5	0.337	0.029	0.999	0.239	0.0011	0.879	21.52	420.4	1040.56	2.38	0.865
Sr (II)	20.95	7.5	4.35	0.814	23.4	0.089	0.101	0.996	0.502	0.0025	0.841	14.09	2.0	612.20	4.05	0.845

[Eu] = 100.0 mg/L, pHEu = 5.0, [Sr] = 100.0 mg/L, pHSr = 8.0, m = 0.15 g, v = 0.025 L, shaking time = 15.0 min.

Acoording to the calculated value of R2 for the investigated isotherm models of the adsorption processes for Eu(III) and Sr(II) by nanocomposite, these adsorption processes obey the order Langmuir > Frendlich > Dubinin–Radushkevic > Temkin models.

Influence of Temperature and Thermodynamics Study

The influence of temperature on sorption processes has a considerable impact on the sorption capacities of the internal pores of sorbents and the diffusion rates of metal ions.[60] To calculate and evaluate the thermodynamic parameters of the investigated sorption processes, the influences of temperature on sorption capacity were studied (see Figure S1). The sorption rates of Eu(III) and Sr(II) by a nanocomposite at the optimum conditions are not affected or have an insignificant effect; increases in the temperature showed the stability of the bond between the metal ions and the surface of the prepared nanocomposite.

To understand the mechanisms involved in the investigated sorption processes of Eu(III) and Sr(II) by a nanocomposite at the optimum conditions under the effect of temperature, different thermodynamic parameters were calculated including ΔH0, ΔS0, and ΔG0 from eqs –7 for the sorption systems under study.[60] As illustrated in Figure , the relationship between ln Kd and 1/T is linear, with the slope of −ΔH0/R and the intercept of ΔS°/R, as illustrated in Figure S2. The values of thermodynamic parameters calculated by the linear relations are represented in Table . The positive values of ΔH0 indicate an endothermic sorption process for both Eu(III) and Sr(II) by nanocomposite at the optimum conditions.

Table 3

Thermodynamic Parameters for Adsorption of Eu(III) and Sr(II) by Nanocomposite

metal ions	temp (K)	enthalpy change (ΔH°), kJ mol–1	entropy change (ΔS°), J mol–1 K–1	free energy change (ΔG°), kJ mol–1
Eu (III)	298	8.28	105.25	–23.08
	308			–24.14
	318			–25.19
	328			–26.24
	338			–27.29
Sr (II)	298	8.83	86.39	–16.91
	308			–17.78
	318			–18.64
	328			–19.51
	338			–20.37

The positive values of ΔS0 suggest the strong affinity of the nanocomposite toward both the metal ions during adsorption and high degree of randomness. It also indicates an increasing randomness in the adsorption systems and the irreversible processes that favor the complexation and stabilities of the adsorption systems.[45,60,64,67,68] The negative values of ΔG0 imply that the sorption reactions are spontaneous and feasible at all the investigated temperatures.[67] It can be concluded that the sorption reactions of Eu(III) and Sr(II) onto a nanocomposite are endothermic and spontaneous processes, and these results agree with those reported before with various adsorbents.[45,60,64,67−71]

Reusability Study

The sorption–desorption reusability procedure was investigated using hot 5% HCl and repeated through five cycles. The data obtained illustrated that the sorption capacities of the prepared nanocomposite remained unchanged after five sorption/desorption runs. This is one of the most promising properties of the prepared Tafla-nanocomposite.

Comparison of the Sorption Behavior of (M-Taf)-Nanocomposite with Different Adsorbents toward Eu(III) and Sr(II)

The sorption capacities of the new nanocomposite toward Eu(III) and Sr(II) are compared with those of other various types of sorbents cited in the literature.[62,64,69−74] As reported in Table , the equilibrium times of adsorption processes of Eu(III) and Sr(II) onto nanocomposite are faster than that for the other adsorbents. In our work the equilibrium is faster and reached at ∼15 min for both Eu(III) and Sr(II), while the least equilibrium times reported by other works were reached at ∼30 min for Eu(III)[62] and Sr(II).[70]

Table 4

Comparison of the Adsorption Behavior of Nanocomposite with with Different Sorbents Materials toward Eu(III) and Sr(II)

	adsorption capacity (mg g–1) (%R)a
adsorbent	Eu3+	Sr2+	equilibrium time, min	ref
MX-80 bentonite	46.06 (91.66%)	NRb	>30	(62)
polyacrylamide modified graphene oxide composites	NRb	2.11 mmol/g (93%)	360	(64)
Na-HEU zeolite	0.66 (mequiv/g) (87%)	NRb	240	(69)
“graphene oxide–magnetite” hybrid	NRb	14.3 (80%)	30	(70)
magnetic graphene oxides	NRb	14.706 (80%)	∼360	(71)
red clay	5.079(90%)	NR	720	(72)
dolomite powder	NR	1.172 (∼40%)	120	(73)
Sr-HAP	30.62(92.2%)	NR	∼1440	(74)
(Mg–Cd)HAP	30.91(93.6%)	NR	∼1440	(74)
NanoTaflaNanocomposite	25.5(89.15%)	23.36 (51.46%)	15	this study

%R: removal percent.

NR: not reported.

Also the nanocomposite has considerable sorption capacities for both Eu(III) and Sr(II) compared with other materials, as in Table . Therefore, nanocomposite can be considered as an eco-friendly and promising nanomaterial utilized to separate and remove such metal ions from different wastes.

Conclusion

A novel nanoTafla nancomposite was prepared as a low-cost and eco-friendly sorbent for strontium and europium ions from aqueous media. Under the optimum conditions, the investigated sorption processes suggest the strong affinity of nanocomposite toward both Eu(III) and Sr(II). The data obtained show an endothermic and spontaneous characteristic that is feasible at all the investigated temperatures at the optimum conditions with increases in the randomness of the processes and is obedient to the pseudo-second order. The sorption processes are pH-controlled and fitted with Langmuir isotherm models. Therefore, this promising nanocomposite can be used to separate and retain radionuclides in different applications.