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Journal of Radioanalytical and Nuclear Chemistry

, Volume 309, Issue 2, pp 761–776 | Cite as

Thorium removal from weakly acidic solutions using titan yellow-impregnated XAD-7 resin beads: kinetics, equilibrium and thermodynamic studies

  • Ahmad Hosseini-Bandegharaei
  • Ahmad Allahabadi
  • Abolfazl Rahmani-Sani
  • Ayoob Rastegar
  • Ramzanali Khamirchi
  • Mohammad Mehrpouyan
  • Reza Hekmat-Shoar
  • Zahra Pajohankia
Article

Abstract

To remove Th(IV) ion from acidic solutions (pH 2.5–2.7), an extractant-impregnated resin (EIR) was fabricated by impregnation of Ambelite XAD-7 resin beads with titan yellow as extractant. Various physicochemical factors such as pH, contact time, temperature, sorbent dose and initial concentration of thorium were investigated. The isotherm data was well interpreted by the Langmuir model. Kinetic experiments data showed that the sorption process could be described by Weber–Morris kinetic model. Thermodynamic studies revealed the feasibility, spontaneity and endothermic nature of sorption process. Desorption experiments showed that the EIR could be reused without significant losses of its initial capacity.

Keywords

Extractant-impregnated resin Titan yellow Amberlite XAD-7 Sorption Th(IV) ion 

List of symbols

ARE

Average relative error (%)

B

Tempkin constant related to the heat of sorption

b

Langmuir constant related to the free energy of sorption (L mg−1)

bM

Langmuir constant related to the free energy of sorption (L mol−1)

C

Intial concentration of thorium ion in solution

Ce

Equilibrium concentration of the metal ion in the bulk solution (mg L−1)

Cr

Total concentration of the exchangeable ion in resin phase

Db

Diffusion coefficient in the solution bulk

Db

Intra-particle diffusion coefficient (m2 s−1)

E

Mean sorption energy estimated from Dubinin–Radushkevich (J mol−1)

k1

Pseudo-first order rate constant (min−1)

k2

Pseudo-second order rate constant (g mg−1 min−1)

Kd

Distribution coefficient (mL g−1)

Kf

Freundlich constant indicative of the relative sorption capacity of the EIR (mg1−(1/n) L1/n g−1)

Kid

Intra-particle diffusion constant (mg g−1 min−1/2)

KT

Equilibrium binding constant, Tempkin constant (L g−1)

I

Intercept in the intraparticle diffusion model (mg g−1)

m

EIR dose, weight of EIR per liter of solution (g L−1)

N

Number of measurements

n

Freundlich constant indicative of the heterogeneity factor

q0

Maximum sorption capacity based on Dubinin–Radushkevich model (mol g−1)

qe

Amount of metal ion sorbed per unit weight of EIR at equilibrium (mg g−1)

qe.cal

Theoretical q e values obtained from the kinetic or isotherm models (mg g−1)

qe.exp

Experimental q e values (mg g−1)

qmax

Maximum sorption capacity; Langmuir constant (mg g−1)

qmax,exp

Maximum experimental sorption capacity (mg g−1)

qt

Amount of metal ion sorbed at any time t (mg g−1)

R

Universal gas constant (J mol−1 K−1)

R2

Correlation coefficient

r0

Mean radius of the EIR particles (m)

R%

Removal efficiency (%)

RL

Dimensionless separation factor

RMSE

Root mean square error (%)

T

Temperature (K)

t

Time (min)

V

Solution volume (L or mL)

W

Weight of EIR (mg)

Xt

Degree of fractional attainment to equilibrium at time t

Greek letters

ΔG°

Gibb’s free energy change (J mol−1)

H°

Enthalpy change (J mol−1)

S°

Entropy change (J mol−1 K−1)

Δq%

Normalized standard deviation (%)

α

Elovich constant indicative of the initial sorption rate (mg g−1 min−1)

β

Elovich constant indicative of the desorption constant (g mg−1)

δ

Dubinin–Radushkevich constant related to the sorption energy (mol2 J−2)

ε

Polanyi potential

ω

Thickness of the liquid film surrounding the sorbent beads

Introduction

Radioactive metals, including thorium, endanger environment and all forms of life through radiation release and chemo toxic effects. Unfortunately, in addition to natural sources, spread of radioactive metals in the environment goes along with all stages in some industries and, therefore, radioactive pollution originated from these sources has been rapidly increased in the last decades. This problem has triggered extensive investigations aiming at development of more suitable technologies for removal of radioactive metals from polluted waters and wastewaters. Among the various removal technologies reported in the literature, sorption is one of the suitable methods for water and wastewater treatment. Its advantages include simplicity, flexibility, high efficiency, cost-effectiveness, ease of operation and low consumption of reagents [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].

Macroporous polymers, including XAD series resins, are desirable supports for sorption systems owing to the properties of their matrix such as chemical and mechani-cal stability, uniformly accessible pores and high surface area [11]. In addition to their applications as desirable sorbents for the sorption of organic pollutions, XAD resins have been widely applied as promising supports for synthesizing selective chelating ion exchange sorbents for interested metal ions [12, 13, 14, 15, 16, 17]. To enhance the capability and selectivity of the macroporous polymeric resins, few modification approaches, including impregnation of the polymeric beads with desired extractants [12, 14, 15, 18, 19, 20], have been attempted in the last decades. Since the impregnation process does not require surface activation of polymeric supports, which is very difficult and time consuming, impregnating of special chelating extractants into/onto the macroporous polymeric supports is the most convenient and simplest approach to preparing selective chelating ion exchange resin, and extractant-impregnated resins can be easily prepared by contacting the solution of appropriate extractants with desired polymeric supports [21]. Therefore, in the last decade, many extractant molecules containing functional groups with an affinity for specific metal ions have been used for preparing advantageous EIRs and removing toxic metal ions from aqueous solutions [5, 13, 16, 21, 22, 23, 24, 25, 26, 27, 28, 29].

Titan yellow (Fig. 1) is a triazene dye and, as an acid–base indicator, its color changes from yellow to red between pH 12 and 13. The dye is employed as a stain and fluorescent indicator in microscopy and can be used for detection of magnesium [30]. Titan yellow contains several different functional groups which, depending on the conditions, can show a tendency towards specific metal ions in the solution media. On the other hand, the large p-electron delocalization over the entire structure of titan yellow molecule makes a high affinity for impregnation of this extractant onto/into a solid support having desirable properties for impregnation. Based on this assumption, in our earlier contribution, a desirable macroporous polymeric resin, Amberlite XAD-7, was impregnated with titan yellow for preparing a novel extractant-impregnated resin which was successfully used for pre-concentrative separation and determination of trace amounts of uranium and thorium ions in various environmental samples. Beside the simultaneous sorption of uranium and thorium from the low acidic solutions (pH 5), the previous results showed that TY/XAD-7 can selectively sorb Th(IV) ion from acidic solution (pH 2.6) [15]. Therefore, aiming at the need for a sorbent suitable for selective removal of thorium from acidic waters and wastewaters, the present study was conducted to investigate the properties of TY/XAD-7 in the sorption of Th(IV) ion from acidic solutions. The influence of agitation speed, sorbent dose, initial concentration, contact time and temperature on the removal efficiency was investigated in detail to gain insight on the potential applicability of TY/XAD-7 beads for Th(IV) removal from acidic media, using batch sorption experiments. Also, the sorption properties of this novel EIR for thorium were evaluated by applying adequate kinetic and equilibrium models to experimental data, obtaining sorption mechanism and calculating kinetic and thermodynamic parameters.
Fig. 1

Chemical structure of titan yellow

Experimental

Material and apparatus

All reagents used in this research were analytical grade and purchased from Merck (Dramstate, Germany), except Amberlite XAD-7 resin (surface area 500 m2 g−1, average bead diameter 560 µm, pore diameter 45.0 nm and bead size 20–60 mesh) which was supplied by Rohm & Haas (USA). The stock solution of 500 mg L−1 Th(IV) was prepared by dissolving the appropriate amount of its nitrate salt in 50 mL of 1 M HNO3 solution and diluting to the mark (1 L) with deionized water (Milli-Q Millipore, 18.2 MΩ cm−1 resistivity) and all the working solutions of given concentrations were obtained by diluting this stock solution. NaCl solution (2.0 M) was used for adjusting the ionic strength of working solutions at 0.01 M, and the pH of solutions was adjusted using 1.0 M HCl or 1.0 M NaOH solution. The reagent solution of 1.0 % Arsenazo III was prepared daily by dissolving 0.2500 g of this reagent in 25 mL deionized water.

A PHS-3BW Model pH-meter (Bel, Italy) with a combined glass–calomel electrode was employed for measuring pH values in the aqueous solutions. A Gallenkamp automatic shaker model BKS 305-010, UK, was used for the batch experiments. To compare the FT-IR spectra of synthesized EIR with commercial polymer, the spectra were recorded using AVATAR 370-FTIR Thermo Nicolet instrument within the range of 400–4000 cm−1 wave number, using KBr discs. The morphological change of XAD-7 after the impregnation was observed by a field emission scanning electron microscope (FE-SEM, Hitachi S4160) under an acceleration voltage of 30.0 kV. A Shimadzu model UV-1601PC spectrophotometer was used for all absorbance measurements with one pair of 10 mm quartz cells.

Preparation of the EIR

To prepare the best impregnated resin beads based on the reported results, 1 g of dry Amberlite XAD-7 resin was transferred into a glass stoppered bottle containing 200 mL of methanolic solution with TY concentration of 0.55 % (W/V). The mixture was slowly shaken at room temperature for 6 h to complete the impregnation process and then placed into a drying oven at 50 °C to remove the solvent. The resulting yellow EIR beads were then transferred to a porous filter and washed successively with HCl and large amounts of distilled water until the washings were colourless and, finally, the impregnated resins were dried at 50 °C [15]. The morphological features and the differences of surface characteristics between XAD-7 and TY/XAD-7 can be seen from the field emission scanning electron microscopy (FE-SEM) micrographs (Fig. 2). Also, as can be observed in Fig. 2, in comparison to the FT-IR spectrum of pristine XAD-7, there are some additional bands in that of TY/XAD-7, including the bands appeared at 1045, 1528, 1606 and 3215 cm−1, which are the characteristic vibrations bands of titan yellow molecule and denote on proper impregnation of titan yellow into/onto the polymeric beads. Furthermore, it can be observed that some characteristic bands of XAD-7 are present in the impregnated resin spectrum with a slightly red shift (5–12 cm−1), which confirms that the impregnation process has been performed via a physical sorption pathway.
Fig. 2

FE-SEM micrographs (×10,000) and FT-IR spectra of polymeric supports: FE-SEM micrograph of XAD-7 (1), FE-SEM micrograph of TY-impregnated XAD-7 (2) FT-IR spectrum of XAD-7 (3) and FT-IR spectrum of TY-impregnated XAD-7 (4)

Prior to conduct the sorption experiments, 0.05-g portions of dry EIR were suspended in 10 mL 3 M HCl for 24 h and, then, the beads were thoroughly rinsed with deionized water [21].

Sorption and desorption experiments

Adsorption experiments were carried out by batch technique at a known temperature. 0.050-g portions of EIR were placed in 150-mL conical flasks containing 100 mL solution of known thorium concentration with 0.01 M NaCl ionic strength and were shaken at a fixed temperature using a temperature controlled shaker set at a known rpm for a given time period. The solutions were then filtered and the concentration of thorium was measured by UV–Visible spectrophotometry, using arsenazo procedure described in our earlier work [15]. Each sorption experiment was replicated three times and the results were averaged. The sorption capacity of the EIR beads at any time (q t , mg g−1) and at equilibrium (q e, mg g−1), the removal percentage (R%), fractional attainment to equilibrium (X t ) and the distribution coefficient (K d, mL g−1) were computed from the following mass balance equations:
$$q_{t} \; = \;\frac{{\left( {C_{0} - C_{t} } \right)V}}{W}$$
(1)
$$q_{\text{e}} \; = \;\frac{{\left( {C_{0} - C_{\text{e}} } \right)V}}{W}$$
(2)
$$R\% = \frac{{(C_{0} - C_{\text{e}} )}}{{C_{\text{e}} }}\, \times \,100$$
(3)
$$X_{t} = \frac{{q_{t} }}{{q_{\text{e}} }}$$
(4)
$$K_{d} = \frac{{(C_{0} - C_{\text{e}} )}}{{C_{\text{e}} }} \times \frac{V}{W}$$
(5)

In the above equations, q t (mg g−1) is the amount of Th(IV) sorbed onto the sorbent at time ‘t’, q e, (mg g−1) is the amount of Th(IV) sorbed onto the sorbent beads at equilibrium, q max,exp (mg g−1) is the amount of Th(IV) sorbed onto the sorbent beads after several sorption equilibrium cycles, C 0 (mg L−1) is the initial concentration of Th(IV) in the aqueous phase, C t (mg L−1) is the Th(IV) concentration remaining in the solutions at time ‘t’, C e (mg L−1) is the equilibrium concentration of Th(IV) in the aqueous phase, V (L; mL) is the volume of the solution and W (g) is the weight of the sorbent beads used in the sorption experiments.

For the study of pH dependency of Th(IV) sorption onto the EIR beads, the radionuclide solutions with different initial concentrations of 25, 50 and 75 mg L−1 were shaken at 180 rpm for 60 min at ambient temperature (298 K). The pH value of working solutions was adjusted by 1 M NaOH or 1 M HCl solution.

The effect of agitation speed was studied in the range of 0–300 rpm using 100-mL aliquots of aqueous solution (pH 2.6) containing 50.0 mg L−1 Th(IV) and shaking them for 60 min at ambient temperature (298 K).

To evaluate the effect of EIR dosage on the removal extent, 10.0–100.0 mg of fresh EIR was added to 100 mL of aqueous solution (pH 2.6) containing 100.0 mg L−1 Th(IV) and the mixture was shaken at 180 rpm for 60 min at ambient temperature (298 K).

Equilibrium studies were conducted by contacting the EIR beads with thorium solutions (pH 2.6) with various initial concentrations (0–100 mg L−1) and shaking the mixtures at 180 rpm for 60 min. The equilibrium experiments were performed at different temperatures.

To investigate the kinetic behavior of the sorption process, EIR beads were added to the Th(IV) solutions (pH 2.6) with three different concentrations (25, 50 and 75 mg L−1) and the samples were agitated for constant time periods varying from 1 to 90 min at 180 rpm and 298 K.

For gaining enough insight on the best sorption kinetic and isotherm models, the experimental data were fitted with different models and, then, correlation coefficient (R 2), normalized standard deviation ∆q(%), average relative error ARE(%) and root mean square error (RMSE) were exploited for model comparison and for goodness-of-fit evaluation for a given concentration and temperature. The statistical indices of ∆q(%), ARE(%) and RMSE were calculated using the following equations and their values should be as close to ‘zero’ as possible [31, 32, 33, 34].
$$\Delta q(\% ) = 100\sqrt {\frac{1}{N - 1}\sum\limits_{i = 1}^{N} {\left( {\frac{{q_{\exp } - q_{\text{cal}} }}{{q_{\exp } }}} \right)_{i}^{2} } }$$
(6)
$$\text{ARE}(\% ) = \frac{100}{N - 1}\sum\limits_{i = 1}^{N} {\left( {\frac{{q_{\exp } - q_{\text{cal}} }}{{q_{\exp } }}} \right)_{i}^{2} }$$
(7)
$$\text{RMSE }= \sqrt {\frac{1}{N}\sum\limits_{i = 1}^{N} {\left( {q_{\exp } - q_{\text{cal}} } \right)_{i}^{2} } }$$
(8)
where q exp and q cal respectively are the experimental value and the calculated value of the sorption capacity of TY/XAD-7 for thorium at time ‘t’ or equilibrium concentration ‘C e’ and N is the number of measurements made.

The effect of temperature on the sorption of Th(IV) ion was investigated by conducting equilibrium experiments at 288, 298, 308, and 318. In addition, the distribution constant, K d, obtained at the mentioned temperatures was utilized to compute the thermodynamic parameters.

To conduct regeneration studies, the thorium-loaded EIR beads were washed thoroughly with deionized water. Then, desorption of Th(IV) was performed by shaking the thorium-loaded EIR beads with 10 mL of 2 M HCl solution, after equilibrium was reached, the EIR beads were removed from the eluent solution and the concentration of Th(IV) ion was measured to determinate the amount of desorbed radionuclide.

Results and discussion

Effect of pH on Th(IV) sorption by EIR

The study of pH dependency of sorption process may provide insight into the mechanism and forces involved in the binding of metal ions to sorbent surface. From a basic science point of view, functional groups and other characteristics of the sorbent surface can affect the pH dependency of the interaction with metal ions, but the pH-induced changes in the metal ion speciation can also involved in the pH-dependent interaction with the sorbent. In the previous work, the influence of pH on the preconcentration of Th(IV) and U(VI) ions was studied in the presence of different buffers and the results showed that, at the trace concentrations, the degree of sorption for Th(IV) was close to 100 % at the pH ranges of 2.5–2.7 and 4.7–5.2, whereas the U(VI) ion had no detectable sorption at the acidic solutions (pH range of 2.5–2.7) [15]. Therefore, the present study was directed to thorium sorption from the acidic solutions, and the pH dependency of sorption process was studied at different initial thorium concentrations. The effect of initial solution pH on Th(IV) sorption by TY/XAD-7 is shown in Fig. 3. Interestingly, the results indicated that at pH range of 2.5–2.7, the maximum sorption capacity of 49.36, 98.02 and 136.36 mg g−1 respectively occurred at an initial Th(IV) concentration of 25, 50 and 75 mg L−1, whereas the corresponding maximum sorption capacities at pH range of 4.7–5.2 were lower than these amounts. In addition, the maximum sorption capacities of pristine XAD-7 beads were also evaluated at both pH range of 2.5–2.7 and 4.7–5.2, and the results showed that the maximum sorption capacities of pristine XAD-7 at the mentioned initial concentrations were lower than 4.0 mg g−1 which is ignorable. These results can be attributed to both the pH-induced changes in the Th(IV) speciation and the interaction between the active functional groups of the EIR surface and Th(IV) species at the studied pHs. Anyway, due to the higher sorption capacity of TY/XAD-7 at pH range of 2.5–2.7 and the industrial importance of removal of radionuclides from acidic aqueous solutions, pH of 2.6 was selected for the following studies.
Fig. 3

The effect of solution pH on Th(IV) removal by TY/XAD-7 at various initial concentrations (Volume, 100 mL; EIR dose, 0.05 g; initial concentration, 25, 50 and 75 mg L−1; contact time, 60 min; temperature, 298 K; agitation speed, 180 rpm)

Effect of agitation speed on the sorption process

Sorption of a metal ion is a surface process and, therefore, efficient contact between the metal ion and sorption sites is very vital. The effect of agitation speed, which is responsible for effective contact, was studied in the range of 0–300 rpm and the obtained results on thorium removal were presented in Fig. 4. The removal efficiency of thorium increased from 9.93 to 98.03 % when the agitation speed increased from 0 to 180 rpm. Although the change in removal efficiency was appreciable in the range of 0–180 rpm, no detectable increase was observed at the agitation speeds greater than 180 rpm. Therefore, optimal agitation speed was chosen as 180 rpm.
Fig. 4

Removal efficiency (a) and adsorption capacity (b) of Th(IV) ion versus TY/XAD-7 dose (Volume, 100 mL; EIR dose, 0.05 g; initial concentration, 100 mg L−1; contact time, 60 min; temperature, 298 K; agitation speed, 180 rpm)

Effect of sorbent dose

For achieving to higher removal extent, an optimum dose is essentially required to maximize the interactions between radionuclide and chelating sites of EIR in the solution media. Increasing the EIR dose would increase the number of available chelating sites and, thereby, result in the increase in removal percentage (R%) of thorium. The results represented in Fig. 5 indicate that, by increasing the sorbent dose from 10 to 100 mg L−1, the removal efficiency of thorium increased from 14.42 to 98.03 %, while the sorption capacity of EIR, q e, decreased from 144.2 to 98.03 mg g−1. These results show that the removal percentage of Th(IV) ion is dependent largely on the EIR dose.
Fig. 5

Removal efficiency of Th(IV) ion versus agitation speed (Volume, 100 mL; initial concentration, 50 mg L−1; EIR dose, 0.05 g; contact time, 60 min; agitation speed, 0–300 rpm temperature, 298 K)

Effect of contact time and initial concentration

Both the initial concentration of Th(IV) ion in solution and time of contact between Th(IV) ion and EIR beads can play important role in the removal process. The effect of these two parameters on the removal process is dependent on the nature of sorbent and the sorption mechanism and, therefore, their study is very important to predict the efficiency and feasibility of the new EIR for its industrial use in thorium removal. Figure 6 shows that the rate of removal is rapid in initial stages and, during the removal of Th(IV) ions by the sorption onto EIR surface, the uptake increased up to 60 min and then it became constant. In addition, it is clear from this figure that the equilibrium time is almost independent of the initial concentration of thorium and, by increasing the initial concentration of thorium ions from 25 to 75 mg L−1, the removal percentage (R%) decreased from 98.72 to 45.45 at pH 2.6, 0.01 M NaCl ionic strength and 298 K. It should be mentioned that the initial faster rate and the higher sorption capacities at the higher initial concentrations may be due to the higher ratio of the initial number of moles of Th(IV) to the available chelating sites existing on the EIR surface and, also, the higher removal percentages at the low initial concentrations is of great industrial importance.
Fig. 6

Sorption of Th(IV) ion onto the TY/XAD-7 beads versus contact time at different initial concentrations (Volume, 100 mL; EIR dose, 0.05 g; initial concentration, 25, 50 and 75 mg L−1; temperature, 298 K; agitation speed, 180 rpm)

Equilibrium studies and effect of temperature

The equilibrium studies are generally useful for understanding the sorption mechanism and gaining ability to design an industrial sorption system. A solute can be sorbed from aqueous media onto surface of a solid support by several mechanisms. The sorption mechanism is dependent on the nature of sorption sites, surface properties, affinities of the sorbent sites, the type of the sorbate and the bulk properties of the aqueous solution (like pH). For interpretation of the equilibrium sorption data at different temperatures, the Langmuir [35], Freundlich [36], Tempkin–Pyzhev [37] and Dubinin–Radushkevich [38] isotherm models were utilized Eqs. (9)–(12).

The linear form of the Langmuir equation is given by

$$\frac{{C_{\text{e}} }}{{q_{\text{e}} }} = \frac{{C_{\text{e}} }}{{q_{\hbox{max} } }} + \frac{1}{{bq_{\hbox{max} } }}$$
(9)
where q e (mg g−1) and C e (mg L−1) respectively are the amount of metal ion adsorbed on the sorbent surface and the metal ion concentration in solution, both at equilibrium, b (L mg−1) is the Langmuir constant and q max (mg g−1) is the maximum sorption capacity for monolayer formation on the sorbent surface.
The linear form of Freundlich equation is given by the following equation:
$$\log q_{\text{e}} = \log K_{\text{f}} + \frac{1}{n}\log C_{\text{e}}$$
(10)
where K f (mg1−(1/n) L1/ g−1) is the Freundlich constant and n is the heterogeneity factor.
The linear form of Tempkin–Pyzhev (T–P) isotherm is given as Eq. (10):
$$q_{\text{e}} = B\ln K_{T} + B\ln C_{\text{e}}$$
(11)
where K T (L g−1) is the equilibrium binding constant corresponding to the maximum binding energy and constant B is related to the heat of sorption (B = RT/b).
The linear form of Dubinin–Radushkevich (D–R) isotherm is given as the following equations:
$$\ln q_{\text{e}} = \ln q_{0} + \delta \varepsilon^{2}$$
(12)
$$\varepsilon = RT\ln \left( {1 + \frac{1}{{C_{\text{e}} }}} \right)$$
(13)
where, ε is Polanyi potential, R (8.314 J mol−1 K−1) is the gas constant, T (K) is the temperature, C e (mol L−1) is the equilibrium concentration, q 0 (mol g−1) is the maximum sorption capacity based on D–R model and δ (mol2 J−2) is the constant related to the sorption energy.
The mean sorption energy (E, J mol−1) is calculated from the following relation:
$$E = \frac{1}{{\sqrt {2\delta } }}$$
(14)

The E magnitude can give an idea about the type of sorption process whether it is physical or chemical.

The aforementioned isotherm models are basically considering the sorption mechanism for the removal process. The linear plots of different isotherm models at different temperatures are shown in Fig. 7. The equilibrium parameters for the sorption of Th(IV) ion by TY/XAD-7 were evaluated from the slopes and intercepts of the respective linear plots at various temperatures, and the results are reported in Table 1. The maximum experimental q max,exp. values in the various temperatures are in agreement with the calculated values using Langmuir isotherm model. Also, based on both the obtained correlation coefficients (R 2) and the statistical indices, the Langmuir isotherm was the only model that provided the best fit for the experimental equilibrium data, indicating that the sorption process is monolayer and the sorption process occurs at energetically equivalent chelating sites [39, 40, 41, 42, 43, 44].
Fig. 7

Linear plots of different isotherm models for Th(IV) sorption by TY/XAD-7 beads at different temperatures

Table 1

Isotherms parameters statistical indices for the sorption of Th(IV) ion onto TY/XAD-7 surface at different temperatures

Isotherm model

Temperature (K)

 

288

298

308

318

Langmuir

 R 2

0.9998

0.9997

0.9996

0.9999

 q max (mg g−1)

145.0

147.1

149.7

152.1

 b (L mg−1)

1.725

2.073

2.441

2.928

 ∆q e (%)

2.499

2.672

2.817

2.895

 ARE (%)

0.062

0.071

0.079

0.084

 RMSE

1.271

1.201

1.212

1.299

Freundlich

 R 2

0.8117

0.8142

0.8199

0.8205

 K F (mg1−(1/n) L1/n g−1)

74.96

73.91

70.08

64.86

 n

2.931

3.018

3.160

3.347

 ∆q e (%)

32.76

33.26

34.28

34.34

 ARE (%)

10.73

11.04

11.75

11.80

 RMSE

23.22

23.24

23.62

23.77

T–P

 R 2

0.9635

0.9527

0.9516

0.9509

 Kt (L g−1)

25.28

38.24

48.35

62.56

 B

24.32

24.04

23.14

21.71

 ∆q e (%)

26.47

23.81

15.83

15.72

 ARE (%)

7.005

5.671

2.506

2.470

 RMSE

10.41

9.219

9.006

8.838

D–R

 R 2

0.8502

0.8557

0.8621

0.8765

 E (kj mol−1)

14.73

15.81

16.65

17.05

 q 0 (mol g−1)

2.997E−03

2.845E−03

2.721E−03

2.687E−03

 ∆q e (%)

49.43

51.28

52.89

56.30

 ARE (%)

24.44

26.29

27.97

31.70

 RMSE

18.50

18.40

18.85

19.19

 q max,exp (mg g−1)

144.43

146.52

149.35

151.79

The essential characteristics of Langmuir isotherm model can be explained in terms of R L which is a dimensionless constant referred to as separation factor or equilibrium parameter for predicting whether a sorption system is favorable or unfavorable. R L is given by the following equation:
$$R_{L} = \frac{1}{{1 + bC_{0} }}$$
(15)
where C 0 is the initial Th(IV) concentration (mg L−1). The value of R L indicates the nature of the isotherm to be irreversible (R L = 0), linear (R L = 1), favourable (0 < R L < 1) and unfavourable (R L > 1). From the experimental data, the values of RL were found to be lied between 0 and 1 for all of initial Th(IV) concentrations and thereby obeying the Langmuir isotherm model under the temperatures and other conditions used in this study (Fig. 8).
Fig. 8

Langmuir separation factors for Th(IV) sorption by TY/XAD-7 beads at different initial concentrations and temperatures

The results reported in the Table 1 also show that an increase in the temperature results in a relative increase in both theoretical and experimental sorption capacities, indicating the greater equilibrium constants at the higher temperatures and endothermic natural of sorption process. Also, increasing the temperature caused increase in the Langmuire constant, b, indicating the higher sorption rates at the higher temperatures, which can be attributed to both decrease in the solution viscosity and increase in the Th(IV) ion mobility in the solution.

The Langmuir isotherm model also provided the best fit for the sorption of thorium ion onto perlite [45], n-Benzoyl-n-phenylhydroxylamine impregnated Amberlite XAD-4 [46], nanoporous ZnO [6], electrospun polyvinyl alcohol/titanium oxide nanofiber [47], poly(methacrylic acid)-grafted chitosan/bentonite composite [48], carboxylate-functionalised graft copolymer derived from titanium dioxide-densified cellulose [49], PAMAM dendron-functionalized styrene divinyl benzene [11], Tannin-modified poly(glycidylmethacrylate)-grafted zirconium oxide densified cellulose [50], and Cystoseira indica alga [51].

Kinetic modeling

Information on the kinetics of metal uptake is required for gaining insight on the physical chemistry of removal process and selecting the optimum conditions for the design of a full scale industrial sorption system [52]. The sorption of thorium ion onto/into the spherical beads of TY/XAD-16 can be considered as a heterogeneous reaction between solid polymeric phase and thorium solution. The sorption process thorium can be defined by three steps:
  1. (i)

    Mass transfer of thorium ions from bulk solution to TY/XAD-16 surface (called solution mass-transfer, or bulk diffusion).

     
  2. (ii)

    Mass transfer of thorium ions through the liquid film surrounding the EIR beads (called external mass-transfer, or film diffusion).

     
  3. (iii)

    Mass transfer of thorium ions through the bead pores (called internal mass-transfer, or intra-particle diffusion).

     
  4. (iv)

    Chemical reaction of thorium ions with available titan yellow molecules on the interior surface of pores.

     

Therefore, to find out the controlling mechanism of sorption process, such as solution mass-transfer, film diffusion, intra-particle diffusion and chemical reaction, the datasets in Fig. 6 were fitted on several kinetic models and the results are discussed below.

Pseudo-first-order kinetic model

The linear forms of pseudo-first order model can be expressed as follows [53]:
$$\log (q_{\text{e}} - q_{t} ) = \log q_{\text{e}} - \frac{{k_{1} }}{2.303}t$$
(16)
where q e and q t are the amounts of thorium sorbed (mg g−1) at equilibrium and at any instant of time t (min), respectively, and k 1 is the rate constant of pseudo-first order sorption (min−1). The pseudo-first-order parameters determined from the plots of log (q eq t ) versus t (Fig. 9) which their visual examination shows that the pseudo-first-order model has a poor fit to the experimental data, and that the rate of thorium removal with time is not directly proportional to the difference between the equilibrium capacity of EIR and the amount of thorium sorbed at any time (q eq t ). This result is supported by the low correlation coefficients (R 2 ≤ 0.965), large difference between q e,cal and q e,exp values, and high statistical indices obtained for pseudo-first-order model (Table 2).
Fig. 9

Linear plots of different kinetic models for sorption of Th(IV) ion onto TY/XAD-7 beads at different initial concentrations and 298 K

Table 2

Kinetic parameters and statistical indices for the sorption of Th(IV) ion by the TY/XAD-7 beads at different initial concentrations and 298 K

Kinetic model

C 0 (mg g−1)

25

50

75

Weber–Morris

 k ip (mg g−1 min−1/2)

4.147

7.992

11.482

 I

0.386

0.430

0.448

 R 2

0.994

0.995

0.996

 ∆q (%)

2.264

2.334

2.985

 ARE (%)

0.051

0.055

0.089

 RMSE

0.723

1.263

1.811

Elovich

 α (mg g−1 min−1)

12.49

25.07

34.64

 β (g mg)

0.091

0.046

0.033

 R 2

0.929

0.896

0.893

 ∆q (%)

28.59

29.80

29.61

 ARE (%)

8.173

8.880

8.766

 RMSE

3.396

8.317

11.567

Pseudo-first-order

 k 1 (min−1)

0.063

0.072

0.075

 q e,cal (mg g−1)

53.4

123.0

175.5

 R 2

0.965

0.9159

0.880

 ∆q t (%)

34.00

65.76

70.61

 ARE (%)

12.25

43.25

49.86

 RMSE

3.887

12.39

18.85

Pseudo-second order

 k 2 (g mg−1 min−1)

1.825E−3

8.450E−4

6.092E−4

 q e,cal (mg g−1)

54.35

109.89

151.5

 R 2

0.939

0.916

0.914

 ∆q (%)

37.07

37.51

37.57

 ARE (%)

13.74

14.07

14.11

 RMSE

4.270

9.351

13.19

 q e,exp (mg g−1)

49.35

98.04

136.36

Pseudo-second order model

The pseudo-second order kinetic model, based on equilibrium sorption, can be expressed by the following linear equation [54]:
$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{\text{e}}^{2} }} + \frac{t}{{q_{\text{e}} }}$$
(17)
where q e and q t (mg g−1) are the amount of radionuclide sorbed at equilibrium and at time t and k2 (g mg−1 min−1) is the pseudo-second order rate constant. The values of q e and k 2 were, respectively, determined from the slopes and the intercepts of the plots of t/q t versus t (Fig. 9). The pseudo-second order rate constants, k 2, the calculated equilibrium sorption capacities, q e,calc, statistical indices and the linear correlation coefficient values, R 2, obtained by linear regression are listed in Table 2. It was found that the correlation coefficient (R 2) has low value (R 2 ≤ 0.939) for all initial thorium concentrations studied and a very large difference exists between q e,cal and q e,exp values, indicating a poor pseudo-second-order fit to the experimental data. This fact suggests that the overall rate of the thorium sorption on the TY/XAD-7 beads is not controlled by the chemisorption [55].

Elovich kinetic model

The linear form of Elovich model can be given as following equation [56]:
$$q_{t} = \frac{1}{\beta }\ln (\alpha \beta ) + \frac{1}{\beta }\ln (t)$$
(18)
where α (mg g−1 min−1) and β (g mg−1) are the initial sorption rate and the desorption constant, which are, respectively, related to the extent of surface coverage and activation energy for chemisorption. The values of Elovich constants, theoretical calculated capacities, statistical indices and the linear correlation coefficients were determined from the plots of q t versus ln t (Fig. 9) and reported in Table 2. The low correlation coefficients (R 2 ≤ 0.929), high statistical indices and large difference between q e,cal and q e,exp suggest that the Elovich model can not explain the sorption mechanism of thorium. Therefore, the reaction of radionuclide with the chelating group is not the rate-controlling step, and the thorium removal does not take place via a multilayer sorption process [57].

Intra-particle kinetic model

Intra-particle diffusion model is usually given by Weber–Morris equation [58]:
$$q_{t} = k_{\text{ip}} t^{0.5} + I$$
(19)
where q t (mg g−1) is the amount of radionuclide sorbed at time ‘t’, k id (mg g−1 min−0.5) is the pore diffusion rate constant, t (min) is the time and the values of I is proportional to the boundary layer.

The corresponding kinetic parameters for the Weber–Morris model were determined from the plots presented in Fig. 9, and the results are listed in Table 2. The results showed that k ip increased with increasing initial Th4+ concentration and the correlation coefficients were in the range of 0.994–0.996 (Table 2). The higher correlation coefficients (R 2), lower statistical indices and large difference between q e,cal and q e,exp for Weber–Morris model indicate that the sorption process is controlled by pore diffusion which is the rate-controlling step in the sorption of thorium onto TY/XAD-7 beads. This model also indicates that the intra-particle diffusivity is constant and the direction of the diffusion is radial [59, 60]. The applicability of the Weber–Morris model to describe the kinetic characteristics of the sorption of some other radionuclides using chelating ion-exchange polymers have been also reported in some previous studies. For example, similar phenomena have been observed for thorium sorption onto many sorbents, such as polyhydroxyethylmethacrylate-hydroxyapatite [61], 1,4-diaminoantraquinone/1,4-dihydroxyantraquinone impregnated XAD-16 [5], calcined diatomite [62], TBP-impregnated sorbent [63] etc.

Since the TY/XAD-16 beads are spherical, the validity of kinetic results mentioned above can be judged again by correlating kinetic data with homogenous particle diffusion model (HPDM) which describe counter diffusion of two ions in a quasi-homogeneous media [10, 12]. Based on the HPDM model, if the film diffusion is the slowest step, rate-controlling step, in the mass-transfer of ions from a volume of solution into the spherical chelating ion-exchange TY/XAD-7 beads, the relationship between the degree of fractional attainment to equilibrium (X t ) and the sorption time (t, min) can be given as the following equation:
$$- \ln (1 - X_{t} ) = \frac{{3D_{b} C}}{{r_{0} \omega C_{r} }}t$$
(20)
where C and C r are the total concentrations of the exchanged ions in solution and resin phases, respectively, D b is the diffusion coefficient in the solution bulk, X t is the fractional attainment to equilibrium or extent of sorption process, r0 is the average radius of the TY/XAD-7 beads (2.8 × 10−4 m), X t values are equal to q t /q e at various times, and ω is the thickness of the liquid film surrounding the sorbent beads.
Based on the HPDM model, if the intra-particle diffusion is the rate-controlling step in the mass-transfer of thorium ions from the solution into the spherical chelating ion-exchange TY/XAD-7 beads, the relationship between the degree of fractional attainment to equilibrium (X t ) and the sorption time (t, min) can be expressed as the following equation:
$$- \ln (1 - X_{t}^{2} ) = \frac{{2D_{id} \pi^{2} }}{{r_{0}^{2} }}t$$
(21)
where D id is the intra-particle diffusion coefficient, r 0 is the average radius of the TY/XAD-7 beads (2.8 × 10−4 m), and X t values are equal to q t /q e at various times.
Linear plots based on the two previous equations Eqs. (20) and (21) are shown in Fig. 10 for different initial concentrations, and the values of slopes and intercepts are summarized in Table 3 along with the R 2 values. The results suggest that the plots of \(- { \ln }\left( {1 - X_{t}^{2} } \right)\) versus t show higher correlation coefficients and lower intercept values than the plots of \(- { \ln }\left( {1 - X_{t} } \right)\), which again indicate the validity of the intra-particle diffusion as the rate-controlling step for the sorption process at all studied concentrations. Therefore, the slope values of the plots of \(- { \ln }\left( {1 - X_{t}^{2} } \right)\) versus t were used to calculate the effective intra-particle diffusion coefficient (D ip, m2 s−1) for different initial concentrations, and the results are reported in Table 4. The results indicate that the value of D ip increases from 2.73 × 10−9 to 8.09 × 10−9 m2 s−1, as the initial concentration increases from 25 to 75 mg L−1 for the sorption of Th(IV) into the pores of TY/XAD-7 beads.
Fig. 10

Linear plots of the functions of HPDM model for different initial concentrations

Table 3

The slopes, intercepts and R2 values obtained from correlating data with the functions of HPDM model

Concentration

\(- { \ln }\left( {1 - X_{t} } \right)\)

\(- { \ln }\left( {1 - X_{t}^{2} } \right)\)

 

Slope

Intercept

R2

Slope

Intercept

R2

25

0.0441

0.1561

0.9751

0.0282

−0.0005

0.9988

50

0.0467

0.1656

0.9760

0.0306

0.0005

0.9987

75

0.0503

0.1747

0.9774

0.0339

0.0009

0.9992

Table 4

The values of D ip obtained from HPDM model for the sorption of thorium by the EIR beads at different initial concentrations

Initial concentration (mg L−1)

D ip (m2 s−1)

25

6.73 × 10−9

50

7.30 × 10−9

75

8.09 × 10−9

Evaluation of thermodynamic parameters

Evaluation of thermodynamic parameters is of great importance in practical application of a removal system and can be used in order to assess the feasibility and spontaneity of the sorption process. The Gibbs free energy change, ∆G (J mol−1), is related to the heat of sorption, ∆H (J mol−1), and entropy change, ∆S (J mol−1 K−1), at constant temperature (K) by the following relation [64]:
$$\Delta G = \Delta H - T\Delta S$$
(22)
The distribution constant, K d (mL g−1), is related to Gibbs free energy change of the sorption process by following equation [65]:
$$\Delta G = - RT\ln K_{\text{d}}$$
(23)
where R is the universal gas constant (8.314 J mol−1 K−1).
By combining Eqs. (22) and (23), the following relation can be obtained:
$$\ln K_{\text{d}} = \frac{ - \Delta G}{RT} = \frac{\Delta S}{R} - \frac{\Delta H}{RT}$$
(24)
Therefore, the thermodynamic parameters viz.; Gibbs free energy (∆G), the enthalpy change (∆H) and entropy change (∆S) for the sorption of Th(IV) ion onto TY/XAD-7 beads were calculated using the values of distribution constant, K d, at the different temperatures and plotting of ln K d versus 1/T (Fig. 11). The calculated thermodynamic parameters for sorption of Th(IV) ion onto the EIR surface are reported in Table 5. The negative ∆G values indicate the feasibility and spontaneity of the sorption process, which are resulted from the high affinity of both Th(IV) ion to EIR. However, more negative ∆G values at the higher temperatures indicate that the extent of feasibility and spontaneity is proportional to the temperature and the higher temperatures are more favorable for the sorption process. The positive ∆H° value shows the endothermic nature of the overall sorption process, confirming that the intensity of sorption process is enhanced at higher temperatures. The positive ΔS° value confirms the affinity of TY/XAD-7 for thorium ion and corresponds to the increased randomness at the EIR/solution interface and increase in the degree of freedom of sorbed Th(IV) resulted from the liberation of water of hydration during the sorption process [21, 66].
Fig. 11

Thermodynamic plot for sorption of Th(IV) on the EIR beads

Table 5

Thermodynamic parameters for the sorption of thorium by the EIR beads as a function of temperature

Temprature (K)

Parameters

 

∆G° (kJ mol−1)

∆H° (kJ mol−1)

∆S° (J mol−1 K−1)

288

−30.89

  

298

−32.43

13.33

153.5

308

−33.93

  

318

−35.51

  

Desorption and reusability

In the previous study, several eluent solutions were tested and 2 M HCl solution indicated as an efficient eluent for desorption of thorium from TY/XAD-7 resin beads [15]. Taking into account the practical application, the same EIR beads were tested for a series of 25 sorption–desorption cycles, using 10 mL of 2 M HCl as eluent solution. Figure 12 shows the relationship between the sorption capacity of the regenerated EIR and the time for reuse. It can be seen that the efficiency of regenerated EIR retained unchanged and no difference in the sorption capacity was observed during 25 sorption–desorption cycles. These results suggest that TY/XAD-7 resin beads have good reusability.
Fig. 12

Regeneration studies: effect of sorption cycle on the EIR capacity

Conclusion

In this research work, titan yellow-impregnated XAD-7 resin (TY/XAD-7) was synthesized and its sorption behavior was investigated for Th(IV) removal from acidic solutions. The sorption of Th(IV) ion onto TY/XAD-7 was investigated for pH, agitation speed, sorbent dose, initial concentration, time, temperature and reusability. The sorbent showed a high affinity to Th(IV) ion, and the maximum sorption of Th(IV) ion occurred at pH range of 2.6–2.7. The sorption process was relatively fast and the equilibrium could be reached within 55 min. The equilibrium data were analyzed using several isotherm models. Both the correlation coefficients and statistical indices indicated that the Langmuir model fits better than the other isotherm models for the sorption of thorium ion onto the TY-XAD-7 beads. The kinetic data fitted with the Weber–Morris model and, under agitation speed and other experimental conditions applied in this work, the resistance to intra-particle diffusion had the greatest impact on the control of sorption process. In addition, the kinetics was correlated with homogenous particle diffusion model (HPDM) for estimating the intra-particle diffusion coefficients, D ip values, which were of the order of 10−9 m2 s−1. Sorption capacity appeared to increase at elevated temperatures and the thermodynamic studies indicated that the nature of sorption process is spontaneous and endothermic. The radionuclide bound to the sorbent surface was efficiently desorbed by 2 M HCl solution and the recycling was shown to be efficient for at least 25 sorption/desorption cycles.

The studies performed in the present study exhibited that TY/XAD-7 resin can be considered as a good sorbent for thorium in terms of high sorption capacity, selectivity, cost, rapid sorption, etc. Also, since the macroporous XAD series resins are completely stable in all aqueous solutions, including acidic ones, TY/XAD-7 can be utilized as an industrial compatible sorbent for the removal of Th(IV) ion from acidic streams and matrices.

Notes

Acknowledgments

We acknowledge the financial support of the present work from the Central Research Council of Sabzevar University of Medical Sciences (Grant 3930101102). In addition, the authors wish to take this opportunity to express their sincere thanks to Prof. Mohammad Mohammad–Zadeh, the research council president of Sabzevar University of medical Science, for his great helps and supports during the experimental works.

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2016

Authors and Affiliations

  • Ahmad Hosseini-Bandegharaei
    • 1
    • 2
  • Ahmad Allahabadi
    • 1
  • Abolfazl Rahmani-Sani
    • 1
  • Ayoob Rastegar
    • 1
  • Ramzanali Khamirchi
    • 1
  • Mohammad Mehrpouyan
    • 3
  • Reza Hekmat-Shoar
    • 1
  • Zahra Pajohankia
    • 1
  1. 1.Wastewater Division, Faculty of HealthSabzevar University of Medical SciencesSabzevarIran
  2. 2.Department of Engineering, Kashmar BranchIslamic Azad UniversityKashmarIran
  3. 3.Nutrition and Biochemistry Department, Faculty of MedicineSabzevar University of Medical SciencesSabzevarIran

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