J Radioanal Nucl Chem (2011) 289:529–536 DOI 10.1007/s10967-011-1100-4

Removal of thorium from aqueous solutions by sodium clinoptilolite Yones Khazaei • Hossein Faghihian Mahdi Kamali

•

Received: 6 April 2011 / Published online: 22 April 2011 Ó Akade´miai Kiado´, Budapest, Hungary 2011

Abstract Adsorptive behavior of natural clinoptilolite was assessed for removal of thorium from aqueous solutions. Natural zeolite was characterized by X-ray diffraction and X-ray fluorescence. The zeolite sample composed mainly of clinoptilolite. Na-exchanged form of zeolite was prepared and its sorption capacity for removal of thorium from aqueous solutions was examined. The effects of relevant parameters, including initial concentration, contact time, solid to liquid ratio, temperature and initial pH on the removal efficiency were investigated in batch studies. The pH strongly influenced thorium adsorption capacity and maximal capacity was obtained at pH 4.0. Kinetics and isotherm of adsorption were also studied. The pseudo-firstorder, pseudo-second-order, Elovich and intra-particle diffusion models were used to describe the kinetic data. The pseudo-second-order kinetic model provided excellent kinetic data fitting (R2 [ 0.999) with rate constant of 1.25, 1.37 and 1.44 g mmol-1 min-1 respectively for 25, 40 and 55 °C. The Langmuir and Freundlich models were applied to describe the equilibrium isotherms for thorium uptake

Y. Khazaei Department of Nuclear Engineering, Faculty of Advanced Sciences and Technologies, University of Isfahan, 81746-73441 Isfahan, Islamic Republic of Iran H. Faghihian (&) Department of Chemistry, Islamic Azad University, Shahreza Branch, Shahreza, Islamic Republic of Iran e-mail: [email protected]; [email protected] M. Kamali (&) Chemical Processes Research Department, Engineering Research Center, University of Isfahan, 81746-73441 Isfahan, Islamic Republic of Iran e-mail: [email protected]; [email protected]

and the Langmuir model agrees very well with experimental data. Thermodynamic parameters were determined and are discussed. Keywords Thorium  Clinoptilolite  Adsorption  Langmuir and Freundlich models

Introduction The long-lived radionuclides in radioactive waste have been considered to be dangerous pollutants [1]. Thorium is a naturally occurring radioactive element widely distributed over the earth’s crust. Some human activities such as exploitations of ores with associated thorium and nuclear fuel reprocessing can also concentrate this element [2]. Thorium is an important model element for tetravalent actinides in natural waters. It is also useful as a tracer when studying environmentally important process [3]. Since last century, thorium has been extensively used in a variety of applications. These applications produce various gaseous, liquid and solid wastes containing isotopes of uranium, thorium and daughter ions of Rn, Po, Bi, Ra. Liquid wastes are freed into the surface or the ground waters of mines. Solid and liquid wastes are also produced during nuclear fuel production. Direct toxicity of thorium is low due to its stability at ambient temperatures; however thorium fine powder is self-ignitable to thorium oxides [4]. The removal of radiotoxic Th4? from aqueous solutions has been explored using different groups of adsorptive materials [4]. Zeolites are crystalline aluminosilicates with a microporous structure and high chemical and radiation stability. They can withstand high temperatures and chemically harsh conditions, and are used as selective adsorbents and catalysts [5]. Because of their singular structure, zeolites

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exhibit special physicochemical properties and have been widely used as molecular sieves, ion-exchangers, adsorbents, catalysts, detergent builders, etc. [6–8]. Among the naturally occurring species the most abundant are those of the HEU-type [9] (heulandite-clinoptilolite series) usually found in specific types of sedimentary formations and constituting big deposits in certain area of the world [10–12]. One of the major applications of natural zeolitic materials has been the removal of radionuclides from aqueous environments and especially the radio-cesium and -strontium decontamination of polluted waters [13–16]. Their utilization for the removal of other radionuclides, among them thorium, is, at presents, rather limited. Clinoptilolite is the most abundant natural zeolite and its typical unit cell formula is given either as Na6 [(AlO2)6 (SiO2)30]24H2O or (Na2, K2, Ca, Mg)3 [(AlO2)6 (SiO2)30]24H2O [6, 17]. The aim of this work was to study the efficiency of Naexchanged form of clinoptilolite for removal of thorium from aqueous solutions. The effect of initial concentration, pH of the solution, contact time, solid to liquid (S:L) ratio and temperature on adsorption process is also studied. Kinetic and thermodynamic parameters of the process are calculated.

X-ray diffraction pattern of the Na-clinoptilolite was taken by a Bruker, D8ADVANCE X-ray diffractometer ˚ and filter: Ni) using Cu Ka radiation (wavelength: 1.5406 A up to 2h = 70 at ambient temperature. Scanning parameters are step size = 0.04° and time step = 2 s. ICDD cards were used to identify phases in the zeolite sample. Na, K, Ca, Si, and Al contents of the sample were determined by a Bruker, S4 PIONEER X-ray fluorescence spectrometer (XRF).

Experimental

q ¼ ðCi  Cf Þ 

Materials and methods

where q is the amount of metal ion adsorbed by unit mass of zeolite (meq g-1); Ci and Cf are respectively initial and final concentrations (meq L-1); m is the amount of zeolite used (g) and V is the volume of thorium solution (L). To determine optimum conditions, the examinations were designed by QUALITEK-4 software using Taguchi method [20]. This method uses a combination of parameters to determine the best condition that was determined based on three factors in four levels (Table 1). The designed examinations using QUALITEK-4 are presented in Table 2.

All chemical reagent used in this study were of analytical reagent grade (AR Grade). All solutions were prepared in double distilled water. Solutions of thorium were prepared by dissolving 3.5987 g of thorium nitrate (Th(NO3)45H2O) in 500 mL distilled water. pH value of this solution was 2.00. The pH was adjusted by addition of HNO3 and NaOH. Natural clinoptilolite was collected from semnan deposits in Iran. It was crushed and pulverized in mortar and sieved to a particle size of 45–75 lm. The powder was refluxed in distilled water in order to remove soluble salts then washed and dried at 110 °C. The powder was stored in a desiccator over saturated NaCl solution in order to maintain a constant vapor pressure during the whole period of experiments. To prepare the exchanged form of clinoptilolite 5 g of the purified zeolite was shaken with 100 mL solution of 1 mol L-1 NH4Cl at 60 °C for 24 h. The solid was filtered, washed and dried at 110 °C. This solid is NH4?-form of clinoptilolite. The H-form of clinoptilolite was prepared by calcining the NH4?-form at 450 °C for 2 h to remove ammonia molecule. Na-exchanged form was prepared by shaking 5 g of H-form of zeolite with 100 mL of 1 mol L-1 solution of NaCl at 60 °C for 48 h. The solid was separated, washed with distilled water, dried at 110 °C, and stored in the desicator [18].

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Adsorption experiments Adsorption of thorium by Na-clinoptilolite was performed by a batch technique at room temperature. Accurate weight of zeolite was shaken with 50 mL of Th solution with known concentration at different pH (1.0–4.0) and constant temperature for known period of time. After equilibration, filter-separating of solid phase was followed by centrifuging (15,000 rpm for 10 min). The thorium was determined spectrophotometrically using Arsenazo III method as complexing agent at 662 nm against reagent blank, employing Biowave II UV-Spectrophotometer [19]. The amount of adsorbed thorium was estimated from the difference of the thorium concentration in the aqueous before and after the adsorption. V m

ð1Þ

Results and discussions Characterization The XRD patterns of Na-clinoptilolite and the reference are presented in Fig. 1. The position of the lines and their

Table 1 Factors and levels in experimental design Factor pH Initial Th conc. (mg L-1) S:L ratio (g L-1)

Level 1

Level 2

Level 3

Level 4

1

2

3

4

58

290

580

2,900

4

6

8

10

Removal of thorium from aqueous solutions

531

Table 2 Examination design using Taguchi method pH

-1

Initial Th conc. (mg L )

Table 3 Chemical composition of Na-clinoptilolite -1

S:L ratio (g L )

Species

% W/W

1

58

4

SiO2

71.70

1

290

6

Al2O3

11.90

1

580

8

Na2O

2.85

1

2,900

10

Fe2O3

0.756

2

58

6

K2O

0.591

2

290

4

MgO

0.333

2

580

10

CaO

0.147

2

2,900

8

TiO2

0.144

3

58

8

ZrO2

0.016

3

290

10

Nb2O5

0.010

3 3

580 2,900

4 6

CuO SrO

0.008 0.005

4

58

10

LOIa

11.42

4

290

8

Total

99.88

4

580

6

Si/Al

5.31

4

2,900

4

a

relative intensities in the studied sample and the reference are similar indicating that the major phase in zeolite-rich rock examined is clinoptilolite. The chemical composition of the sample obtained by XRF method is shown in Table 3. The Si/Al ratio of the zeolite was 5.31. Different Si/Al ratios have been reported for clinoptilolite but all of them are within the range (4–5.5). Beside sodium as the major cation, traces of K, Ca and Mg are presented in the zeolite channels. Effect of contact time The effect of contact time on thorium adsorption from 58 mg L-1 thorium solution and mass-to-volume ratio of 1:100 at 25, 40 and 55 °C was studied. The initial pH was

Loss on ignition

4.00. The results are shown in Fig. 2. The equilibration was attained after 24 h. From the slope of the curves, it was concluded that adsorption rate was fast at the beginning and became slow with the progress of the reaction. Effect of temperature Adsorption of thorium was measured at constant concentration of thorium solution (58 ppm) and mass-to-volume ratio of 1:100 at three different temperatures (Fig. 2). Up to 55 °C, the adsorption capacity increased as the temperature increased. This could be attributed to the endothermic nature of the process. The calculated thermodynamic parameters reveal the endothermic nature of the reaction (Table 4).

b a

Fig. 1 XRD patterns of (a) the reference pattern of clinoptilolite and (b) Na-clinoptilolite

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Y. Khazaei et al.

Fig. 3 Effect of S:L ratio on thorium adsorption Fig. 2 Effect of contact time and temperature on thorium adsorption

Effect of solid to liquid ratio Data relating to the effect of S:L ratio on thorim adsorption is show in Fig. 3. The figure indicates that the amount of adsorbed thorium increased with high zeolite loading and the optimal sorption occurs in S:L ratio of 10. This may be attributed to the fact that with large adsorbent amount the adsorption sites increases. Effect of initial concentration Fig. 4 Effect of initial concentration on thorium adsorption

The result showed that with increase of initial concentration a decrease in removal efficiency is observed. The highest thorium removal occurs at 58 mg L-1 (Fig. 4). This could be attributed to saturation of sorption sites of the zeolite. At low thorium ion loading, the ratio of the number of thorium ions to the number of available adsorption sites is small and consequently, adsorption is independent of initial concentration, but as the concentration of thorium ions increases, the situation changes and the competition for adsorption sites becomes fierce. Effect of pH The concentration of thorium species gradually change during the adsorption process. And the pH value of the solution depends on the concentration. The effect of pH on the sorption was studied from initial pH 1 to 4 at 25 °C (Fig. 5). pHs higher than 4 were not examined because of precipitation of thorium at pH [ 4. In lower pHs, adsorption capacity decreased because H3O? acts as a competitive ion. The reactions of thorium in aqueous solutions are the subject of much controversy. Because of its large size, Th(IV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides (U, Np, Pu). Thorium(IV) hydrolysis has been studied by a number of researchers, and many of these studies indicated stepwise hydrolysis to yield monomeric products of formula

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Fig. 5 Effect of pH on thorium adsorption

[Th(OH)n]4-n with n = 1-4, in addition to a number of polymeric species and colloid formation [21–25]. In the most recent critical review, a comprehensive set of hydrolysis constants or the stepwise formation of [Th(OH)]3?, [Th(OH)2]2?, [Th(OH)3]1?, and Th(OH)4 have been proposed [25]. The distribution of hydrolyzed species of thorium as a function of pH is shown in Fig. 6. Although it is difficult to define thorium form in aqueous solution because of depending its species to pH and concentration, but the optimum pH find to be 4 for adsorption of thorium because at this pH various monomer forms of thorium including Th4?, [Th(OH)]3?, [Th(OH)2]2?, [Th(OH)3]1? are the major species.

Removal of thorium from aqueous solutions

533

where K2 is the rate constant of pseudo-second-order sorption (g mmol-1 min-1). Integrating this equation and applying boundary conditions for t = 0, q = 0 gives: t 1 t ¼ þ 2 qt K 2 qe qe

ð5Þ

sorption rate can be obtained from Eq. 5: qt ¼ t

1 1 K2 q2e

þ qte



ð6Þ

and the initial sorption rate, h, can be defined as h ¼ K2 q2e

ð7Þ

so Eq. 6 can become Fig. 6 Distribution of thorium species versus pH at 25 °C [26]

qt ¼  Adsorption kinetic and thermodynamics The kinetic of adsorption was evaluated by applying four different models including the pseudo-first-order equation, the pseudo-second-order equation, Elovich equation and intra-particle diffusion model. The models were tested to fit experimental data obtained by batch experiment. The pseudo-first-order equation is generally expresses as follows: dq ¼ K 1 ð qe  qt Þ dt

ð2Þ

where qe and qt are the amount of species adsorbed per unit mass sorbent at equilibrium and any time t, respectively (mmol g-1) and K1 is the rate constant of pseudo-firstorder sorption (min-1). After integration and applying boundary conditions, for t = 0, q = 0, the integrated form of equation becomes: lnðqe  qt Þ ¼ ln qe  K1 t

ð3Þ

The pseudo-second-order equation based on adsorption equilibrium capacity can be expressed as: dq ¼ K 2 ð qe  qt Þ 2 dt

ð4Þ

t 1 h

þ qte



ð8Þ

The initial sorption rate, h (mmol g-1 min-1), the equilibrium sorption capacity, qe, and pseudo-secondorder rate constant, K2, can be determined experimentally from slope and intercept of plotting of t/qt against t. The Elovich equation is given as follows: qt ¼

lnðabÞ lnðtÞ þ b b

ð9Þ

where qt is the sorption capacity at time t, a is the initial sorption rate of Elovich equation (mmol g-1 min-1), and the parameter b is related to the extent of surface coverage and activation energy for chemisorption (g mmol-1). The constants can be obtained from the slope and intercept of a straight line of qt versus ln t. The intra-particle diffusion model is qt ¼ Kdiff t1=2 þ C

ð10Þ

where Kdiff is the intra-particle diffusion rate constant (mmol g-1 min-1/2) and C is the intercept. Kinetic parameters and correlation coefficients for four kinetic models of thorium adsorption on the zeolite sample were calculated from corresponding plots and listed in Tables 4 and 5. The value of R2 of pseudo-second-order kinetic model is higher for Na-form of zeolite.

Table 4 Kinetic parameters obtained for thorium adsorption (pseudo-first-order model and pseudo-second-order model) qe(exp.) (9102) (mmol g-1)

Pseudo-first-order model K1 (910 ) (min-1)

qe(theo.) (910 ) (mmol g-1)

R

25

2.06

3.44

0.45

40

2.42

7.43

1.33

55

2.71

2.46

0.43

T (°C)

3

Pseudo-second-order model -2

2

K2 (g mmol-1 min-1)

qe (9102) (mmol g-1)

R2

0.997

1.25

2.11

0.999

0.953

1.37

2.47

0.999

0.953

1.44

2.76

0.999

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Y. Khazaei et al.

Table 5 Kinetic parameters for thorium adsorption (Elovich and Intra-particle diffusion models) T (°C)

Elovich model a (mmol g

-1

Intra-particle diffusion model -1

-3

min )

-1

b (910 ) (g mmol )

R

2

Kdiff (9104) (mmol g-1 min-1/2)

C (9102) (mmol g-1)

R2

25

7.57

0.769

0.993

1.07

1.68

0.974

40

45.91

0.724

0.944

1.12

2.03

0.935

55

14043.49

0.868

0.939

0.96

2.36

0.958

The pseudo-second-order kinetic model selected as the best model to fit experimental data and was taken for evaluation of Ea. The K2 value obtained in this model is considered as a scale for the rate of the reaction. The activation energy of the adsorption (Ea) is evaluated from the slope of ln K2 (rate constant of pseudo-secondorder sorption) versus 1/T using Arrhenius equation (Fig. 7). ln K2 ¼ ln A 

Ea RT

ð11Þ

where K2 and A are the rate constant and temperature independent factor (g mmol-1 min-1), respectively, Ea is the activation energy of the reaction of adsorption (J mol-1), R is the gas constant (8.314 J mol-1 K-1) and T is the adsorption absolute temperature (K). In order to obtain thermodynamic parameters, the distribution coefficient, kd (mL g-1), value was determined according to equation:   Ci  Cf V kd ¼ ð12Þ  W Cf where Ci and Cf are respectively the initial and final concentrations of the thorium in solution, W is the weight of zeolite (g) and V is the volume of the solution (mL). The values of DH° and DS° were calculated from the slopes and intercepts of the linear variation of ln kd with the reciprocal of the temperature, 1/T (Fig. 8). 



ln kd ¼ ðDH =RT Þ þ ðDS =RÞ

ð13Þ

Fig. 8 Linear least square plots for obtaining thermodynamic parameters

The free energy of the adsorption, DG° is calculated from: DG ¼ DH   TDS

ð14Þ

The activation energy and enthalpy values are positive. It means that adsorption of thorium is an endothermic process. Negative DG° value indicate that the adsorption process is favorable (Table 6). Adsorption isotherms Adsorption isotherms were plotted by data obtained at different concentrations by the following equation (Fig. 9): a ¼ ðCi  Ce Þ 

V m

ð15Þ

a is the amount of metal species adsorbed by unit mass of zeolite (mg g-1) at equilibrium; Ci and Ce are respectively the initial and equilibrium concentration of thorium (mg L-1); m is the amount of zeolite (g) and V is the volume of thorium solution (L). The value of a is valid for equilibrium at particular temperature (here 40 °C). Two adsorption isotherm models were used for fitting of the experimental data. The Freundlich equation: Table 6 Thermodynamic adsorption

Fig. 7 Linear least square plots for obtaining Ea

123

parameters

obtained

for

thorium

Ea (kJ mol-1)

DH (kJ mol-1)

DS (kJ mol-1 K-1)

DG298 (kJ mol-1)

4.01

4.39

0.05144

-10.94

Removal of thorium from aqueous solutions

535

Fig. 9 Adsorption isotherm at 40 °C

a ¼ kF C n

ð16Þ

Fig. 11 Linear least square plots for calculating parameters of Freundlich isotherm

was initially applied in its linear form: ln a ¼ ln kF þ n ln C

ð17Þ

Table 7 Adsorption isotherm parameters for thorium adsorption Langmuir

where kF is the known Freundlich constant related to the adsorbent capacity and n is an exponent related to the adsorption. The applicability of the Longmuir equation was also tested, where Langmuir constant kL the saturation capacity am can be evaluated by regression analysis of the linear form of following equation: am k L C a¼ ð1 þ kL C Þ

ð18Þ

In particular we used the linear expression: 1 1 1 1 ¼ þ  a am am k L C

ð19Þ

Figures 10 and 11 shows respectively the linearized Langmuir and Freundlich adsorption isotherms. The parameters for two isotherms obtained from experimental data and the related correlation coefficients (R2) are presented in Table 7. Langmuir isotherm suggests monolayer coverage of the thorium species at the surface of the zeolite. n \ 1 obtained in Freundlich model depict a favorable adsorption.

3

Freundlich -1

-1

2

kL (cm mg )

am (mg g )

R

1.0

333.3

0.997

kF (cm3 g-1)

n

R2

1.02

0.69

0.967

Conclusions The adsorption behavior of zeolite adsorbent for thorium was investigated. Clinoptilolite is an abundant zeolite and highly stable against radioactive radiations. It was found that Na-form of clinoptilolite is an economical and effective sorbent for Th(IV) removal and exhibited excellent adsorption selectivity for Th(IV). Among two applied isotherm models, Langmuir isotherm gave better correlation with the experimental data. The temperature variation has been used to evaluate the values of DH, DS and DG. The positive values of DH and negative DG indicate endothermic and spontaneous nature of sorption, respectively. Fitting data to kinetic models and values of R2 showed that the pseudo-second-order kinetic model is the best model for kinetics of thorium adsorption. Acknowledgments The authors wish to thank the Office of Graduate Studies of the University of Isfahan for their support. The authors thank Mrs. M. Akbari for his help in the XRD and XRF analysis in Central Laboratory of University of Isfahan, and Mr. R. Sayyari for help in the ICP analysis.

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Fig. 10 Linear least square plots for calculating parameters of Langmuir isotherms

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