Kinetic, equilibrium and thermodynamic studies on sorption of ... - CORE

Journal of Hazardous Materials 286 (2015) 152–163

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Kinetic, equilibrium and thermodynamic studies on sorption of uranium and thorium from aqueous solutions by a selective impregnated resin containing carminic acid Abolfazl Rahmani-Sani a , Ahmad Hosseini-Bandegharaei a,b,∗ , Seyyed-Hossein Hosseini b , Keivan Kharghani c , Hossein Zarei b , Ayoob Rastegar a,b a

Wastewater Division, Faculty of Health, Sabzevar University of Medical Sciences, PO Box 319, Sabzevar, Iran Department of Engineering, Kashmar Branch, Islamic Azad University, PO Box 161, Kashmar, Iran c Water Division, Department of Engineering, Torbat-e-Hydarieh Branch, Islamic Azad University, PO Box 121, Torbat-e-Hydarieh, Iran b

h i g h l i g h t s • • • • •

The objective of the study is to investigate the potential application of a selective EIR for sorption of U(VI) and Th(IV) ions. The effects of several physiochemical parameters were investigated. The sorption kinetics and sorption isotherms were used to explain the sorption mechanism. The thermodynamic studies showed the feasibility of sorption process. The EIR beads showed a great potential for effective removal of U(VI) and Th(IV) ions.

a r t i c l e

i n f o

Article history: Received 18 August 2014 Received in revised form 21 December 2014 Accepted 24 December 2014 Available online 30 December 2014 Keywords: Extractant-impregnated resin Carminic acid Amberlite XAD-16 Sorption U(VI) and Th(IV) ions

a b s t r a c t In this work, the removal of uranium and thorium ions from aqueous solutions was studied by solid–liquid extraction using an advantageous extractant-impregnated resin (EIR) prepared by loading carminic acid (CA) onto Amberlite XAD-16 resin beads. Batch sorption experiments using CA/XAD-16 beads for the removal of U(VI) and Th(IV) ions were carried out as a function of several parameters, like equilibration time, metal ion concentration, etc. The equilibrium data obtained from the sorption experiments were adjusted to the Langmuir isotherm model and the calculated maximum sorption capacities in terms of monolayer sorption were in agreement with those obtained from the experiments. The experimental data on the sorption behavior of both metal ions onto the EIR beads fitted well in both Bangham and intra-particle diffusion kinetic models, indicating that the intra-particle diffusion is the rate-controlling step. The thermodynamic studies at different temperatures revealed the feasibility and the spontaneous nature of the sorption process for both uranium and thorium ions. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Due to the high toxicity of radioactive metals, especially uranium and thorium, exposure to these pollutants has been a quickly growing problem for human health and, hence, removal of these heavy metals from polluted waters and wastewaters is very

∗ Corresponding author at: Wastewater Division, Faculty of Health, Sabzevar University of Medical Sciences, PO Box 319, Sabzevar, Iran. Tel.: +98 532 8250550; fax: +98 532 8250525. E-mail addresses: [email protected], [email protected] (A. Hosseini-Bandegharaei). http://dx.doi.org/10.1016/j.jhazmat.2014.12.047 0304-3894/© 2014 Elsevier B.V. All rights reserved.

necessary [1–3]. Two old methods, namely precipitation and solvent extraction, can be efficiently utilized for removal of uranium and thorium. However, solvent extraction is very expensive on a large scale and results in huge environmental problems, because the toxic organic diluents are used widely [4–6]. In addition, if flammable solvents are concerned, solvent extraction could be very dangerous. On the other hand, in a system as complex as the industrial wastewaters, co-precipitation of other metal ions with thorium and uranium makes precipitation undesirable, difficult, time-consuming, and expensive. Today, there is a growing interest for potential applications of sorption technology in metal separations, hydrometallurgy, water and wastewater treatment, removal and recovery of metal ions,

A. Rahmani-Sani et al. / Journal of Hazardous Materials 286 (2015) 152–163

Nomenclature a ARE B b Ce Dip E K0 k1 k2 Kd Kf kip KT I m N n q0 qe qe,cal qe,exp qmax qmax,exp qt R R2 r0 R% RL RMSE T t t1/2 V W

Bangham constant Average relative error (%) Tempkin constant related to the heat of sorption Langmuir constant related to the free energy of sorption (L mg−1 ) Equilibrium concentration of the metal ion in the bulk solution (mg L−1 ) Intra-particle diffusion coefficient (m2 s−1 ) Mean sorption energy estimated from Dubinin–Radushkevich (J mol−1 ) Bangham constant (mL g−1 L−1 ) Pseudo-first order rate constant (min−1 ) Pseudo-second order rate constant (g mg−1 min−1 ) Distribution coefficient (mL g−1 ) Freundlich constant indicative of the relative sorption capacity of the EIR (mg1−(1/n) L1/n g−1 ) Intra-particle diffusion constant (mg g−1 min−1/2 ) Equilibrium binding constant, Tempkin constant (L g−1 ) Intercept in the intraparticle diffusion model (mg g−1 ) EIR dose, weight of EIR per liter of solution (g L−1 ) Number of measurements Freundlich constant indicative of the heterogeneity factor Maximum sorption capacity based on DubininRadushkevich model (mol g−1 ) Amount of metal ion sorbed per unit weight of EIR at equilibrium (mg g−1 ) Theoretical qe values obtained from the kinetic models(mg g−1 ) Exp experimental qe values (mg g−1 ) Maximum sorption capacity; Langmuir constant (mg g−1 ) Maximum experimental sorption capacity (mg g−1 ) Amount of metal ion sorbed at any time t (mg.g−1 ) Universal gas constant (J mol−1 K−1 ) Correlation coefficient Mean radius of the EIR particles (m) Removal efficiency (%) Dimensionless separation factor Root mean square error Temperature (K) Time (min) Time for half sorption of metal ion onto the EIR particles (min) Solution volume (L or mL) Weight of EIR (mg)

Greek letters G◦ Gibb’s free energy change (J mol−1 ) H◦ Enthalpy change (J mol−1 ) Entropy change (J mol−1 K−1 ) S◦ q% Normalized standard deviation (%) ˛ Elovich constant indicative of the initial sorption rate (mg g−1 min−1 ) ˇ Elovich constant indicative of the desorption constant (g mg−1 ) ı Dubinin–Radushkevich constant related to the sorption energy (mol2 J−2 ) Polanyi potential

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etc [7–15]. When considering these applications along with the simplicity, flexibility, cost effectiveness, ease of operation and low consumption of reagents, sorption method provides a very competitive alternative to both solvent extraction and precipitation. Therefore, there is a great quest for studying new sorbents and, in the recent decades, a number of innovative adsorbents have been prepared and reported for removal of heavy metal cations, including uranium and thorium, from aqueous solutions [10–25]. However, although the most used adsorbents for the sorption of uranium and thorium ions are cation exchange ones, several anion-exchange resins have been utilized for sorption of these radionuclides from both strongly acidic and basic solutions [26–31]. Of the many kinds of solid supports used as sorbent, there are several advantages of using chelating ion exchange supports, such as high sorption capacity, selectivity and reusability. As a result, due to these advantages, there is a great quest for studying new selective chelating ion exchange sorbents for interested metal ions. Macroporous polymeric supports, including XAD series resins, have been utilized extensively due to the properties of their matrix, such as chemical and mechanical stability and high surface area, which are attributed to the existence of large numbers of cross links in their structure [32]. The selective polymeric sorbents can be prepared through a modification step that involves the immobilization of organic molecules containing functional groups as sorption sites which improve the sorption capability and selectivity toward the interested metal ions. This immobilization usually occurs via direct bonding of chelating ligands to the polymeric supports or impregnating the polymeric beads with desired extractants [33–37]. However, due to relative inertness of polymeric surfaces in the ground state, direct bonding of chelating ligands to a polymeric matrix requires surface activation which is very difficult and time consuming. In contrast, extractant-impregnated resins can be easily prepared by contacting the solution of appropriate extractants with desired polymeric supports. Therefore, in the last decade, impregnating methods have been extensively used for preparing selective sorbents, and removing toxic metal ions from waters and wastewaters [38–50]. In our earlier group works, we have reported the application of two antraquinon compounds for preparing high stable extractant-impregnated resins and studding their capability for sorption of some toxic metal ions, including uranium and thorium, from aqueous solutions [50–53]. Among these two EIRs, carminic acid-impregnated XAD-16 resin (CA/XAD-16) showed high selectivity towards thorium(IV) and uranium (VI) ions and, therefore, it was used for devising a method for pre-concentrative separation of trace amounts of uranium (VI) and thorium(IV) ions followed by their spectrophotometric determination using arsenazo III procedure which is very simple and cost-effective. The application of CA/XAD-16 was robust for tackling the complicated matrixs such as environmental and geological samples and, additionally, a sequential elution could be used to separate uranium and thorium ions from each others [53]. Considering the above advantages, the present work was directed towards the application of this selective sorbent for removal of uranium and thorium from waters and wastewaters. The performance and the effectiveness of CA/XAD-16 for the new purpose were evaluated by batch sorption experiments and, after evaluating the effects of some physicochemical parameters, sorption isotherms, kinetics and thermodynamic parameters were also evaluated for gaining insight on the sorption properties of uranium and thorium by this original EIR. 2. Experimental 2.1. Material and apparatus All the chemicals used were of analytical grade from Fluka (Switzerland), except Amberlite XAD-16 resin (surface area

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825 m2 g−1 , pore diameter 14.4 nm and bead size 20–60 mesh) which was supplied by Rohm & Haas (USA). The laboratory glassware was kept overnight in a 5% (v/v) nitric acid solution and then was rinsed thoroughly with deionized double distilled water. Highly pure deionized water (Milli-Q Millipore, 18.2 M cm−1 resistivity) was used for all dilutions throughout this work. The stock solutions of uranium and thorium (500 mg L−1 ) were prepared by dissolving the appropriate amounts of its nitrate salts in 50 mL of 2 M HNO3 solution and diluting to the mark (1 L) with deionized water, and all the working solutions were prepared daily by diluting these stock solutions. The pH and ionic strength of working solutions were, respectively, adjusted to 5.0 and 0.01 M using appropriate ratios of 0.5 M acetic acid and ammonium acetate solutions. 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. A Shimadzu model UV-1601PC spectrophotometer was used for all absorbance measurements with one pair of 10 mm quartz cells. 2.2. Preparation of the EIR

solution, Ct (mg L−1 ) is the U(VI) or Th(IV) concentration remaining in the solutions at time ‘t’, Ce (mg L−1 ) is the equilibrium concentration of U(VI) or Th(IV) in the solutions, V (L; mL) is the volume of the solution and W (g) is the weight of the EIR beads used in the sorption experiments. 2.3.1. Kinetic experiments For studding both the effect of contact time and the kinetic behavior of the sorption process, the sorption capacity of the CA/XAD-16 at different contact times was examined for different initial concentrations varying from 25 to 100 mg L−1 . The sorption experiments of both U(VI) and Th(IV) ions were conducted by application of batch technique at 288, 298, 308 and 318 K and an agitation speed of 180 rpm. For modeling the sorption rate data, the following models were utilized. (i) Pseudo-second order model The pseudo-second order model assumes that the sorption process is a pseudo-chemical reaction process. In this model, the driving force is the difference between the equilibrium capacity of EIR and the amount of radionuclide sorbed at any time (qe –qt ), but the overall sorption rate is proportional to the square of the driving force [54]. The pseudo second-order sorption kinetic equation is expressed by the following equation [55]: dqt = k2 (qe − qt )2 dt

Preparation and characterization of the carminic acidimpregnated XAD-16 has been described in the previous paper [44]. Appropriate amounts of dry EIR were weighed and were suspended in 10 mL 3 M HCl for 24 h prior to use. Then, the EIR beads were thoroughly rinsed with deionized water and used in the sorption experiments.

After integration and applying boundary conditions t = 0–t and qt = 0–qt , the above equation becomes:

2.3. Sorption experiments

which can be rearranged to obtain the following linear form:

Adsorption experiments were carried out in 150 mL conical flasks. To conduct sorption experiments, 0.060 g portions of sorbet, EIR, mixed with radionuclide (uranium or thorium) solutions (100 mL) having known initial concentrations. The pH and ionic strength of the solutions were, respectively, adjusted to 5.0 and 0.01 M using appropriate ratios of 0.5 M acetic acid and ammonium acetate solutions, and then the mixtures were shaken at a fixed temperature using a temperature controlled shaker set at a known rpm for a period of desired time. After that, the suspensions were quickly filtered and the filtrates were analyzed by the arsenazo III procedure described in our earlier work [53]. Each sorption experiment was replicated three times and the results were averaged. The sorption capacity of the EIR beads at any time (qt , mg g−1 ) and at equilibrium (qe , mg g−1 ), the removal efficiency or the removal percentage at equilibrium (R%) and the distribution coefficient (Kd , mL g−1 ) were calculated from the following mass balance equations: qt =

(C0 − Ct )V W

(1)

qe =

(C0 − Ce )V W

(2)

R% =

(C0 − Ce ) × 100 Ce

(3)

(C0 − Ce ) V Kd = × Ce W

1 1 = + k2 t qe qe − qt

In the above equations qt (mg g−1 ) is the amount of the radionuclide sorbed onto the EIR beads at time ‘t’, qe, (mg g−1 ) is the amount of the radionuclide sorbed onto the EIR beads at equilibrium, qmax,exp (mg g−1 ) is the amount of the radionuclide sorbed onto the EIR beads after several sorption equilibrium cycles, C0 (mg L−1 ) is the initial concentration of U(VI) or Th(IV) in the aqueous

(6)

t 1 t = + qt qe k2 q2e

(7)

where qe and qt (mg g−1 ) are the amount of radionuclide sorbed at equilibrium and at time t, k2 (g mg−1 min−1 ) is the pseudo-second order rate constant and t (min) is the time. (ii) Pseudo-first-order Lagergren model The pseudo-first order Lagergren model assumes that the rate of radionuclide removal with time is directly proportional to the difference between the equilibrium capacity of EIR and the amount of radionuclide sorbed at any time (qe –qt ). The linear form of model equation is given as the following equation [56]: log(qe − qt ) = logqe −

k1 t 2.303

(8)

where qe and qt (mg g−1 ) are the amount of radionuclide sorbed at equilibrium and at time t, k1 (min−1 ) is the pseudo-first order rate constant and t (min) is the time. (iii) Elovich model The Elovich model is useful when the reaction of radionuclide with the chelating group is the rate-controlling step. The model implies a multilayer adsorption and assumes that the sorption sites increase exponentially with sorption [57]. The Elovich equation can be written as follows: qt =

(4)

(5)

1 1 ln(˛ˇ) + ln(t) ˇ ˇ

(9)

where ˛ (mg g−1 min−1 ) and ˇ (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. (iv) Intra-particle diffusion model The intra-particle diffusion model assumes that the effect of the diffusion of radionuclide is the rate-controlling step for the sorption process, the intra-particle diffusivity is constant and the direction of


the diffusion is radial [58,59]. This model was used to calculate the intra-particle diffusion rate constant. The model equation is given the following equation: qt = kid t 0.5 + I

(10)

where qt (mg g−1 ) is the amount of radionuclide sorbed at time ‘t’, kid (mg g−1 min−0.5 ) is the intra-particle diffusion rate constant, t (min) is the time and the values of I is proportional to the boundary layer. (v) Bangham model The Bangham model assumes that the diffusion of radionuclide into the pores of EIR beads is the rate-controlling step [60]. The model equation is expressed as the following equation: loglog(

C0 k0 m ) = log( ) + a log t C0 − qt m 2.303V

(11)

where C0 (mg L−1 ) is the initial concentration of radionuclide in the solution, V (mL) is the volume of solution, m (g L−1 ) is the weight of EIR per liter of solution, qt (mg g−1 ) is the amount of radionuclide sorbed at time ‘t’, and a ( 1). The obtained data clearly shows that in all the cases the values of RL were positive and less than unity, indicating thereby a highly


159

Fig. 5. Linear plots of different kinetic models for sorption of U(VI) ion onto EIR beads at different initial concentrations (T: 298 K).

favorable sorption process for both metal ions which obeys the Langmuir isotherm model under the conditions used in this study. The Langmuir isotherm model is based on the several assumptions such as (i) the sorption energy is constant, (ii) the maximum sorption responds to saturated monolayer of metal ions on the EIR surface, (iii) there is no transmigration of radionuclides in the EIR surface, (iv) EIR surface is homogenous, (v) all sorption sites are equivalent and (vi) sorption in an active site is independent of whether the adjacent sites are occupied or not. Therefore, the suitability of Langmuir isotherm model for describing the sorption of both uranium and thorium ions onto EIR surface can be attributed to homogenous distribution of extractant molecules, or chelating ion exchange sites, on the polymeric surface [68].

3.5. Kinetic modeling The kinetic studies are of great importance for both gaining insight on the physical chemistry of the sorption processes and the design of sorption systems. In general, the sorption of uranium

and thorium onto the EIR surface is known to proceed through the following steps: 1. Transfer of radionuclide from bulk solution to EIR surface, which is usually mentioned as bulk diffusion. 2. Diffusion of radionuclide across the external film surrounding the EIR bead (film diffusion). 3. Migration of radionuclide into pores, which is usually mentioned as pore diffusion. 4. Interaction of radionuclide with available chelating sites on the interior surface of pores. The rate limiting step is resulted from one of the above steps or combination of them. For gaining insight on the sorption kinetic, the datasets from Section 3.3 were used in the modeling exercise and results obtained from several models are discussed below. The correlation coefficient (R2 ), normalized standard deviation (q(%)) and average relative error (ARE(%)) were used for model comparison and for goodness-of-fit evaluation for a given concentration and temperature.

160


Fig. 6. Linear plots of different kinetic models for sorption of Th(IV) ion onto EIR beads at different initial concentrations (T: 298 K).

The normalized standard deviation (q(%)) and the average relative error (ARE(%)) are calculated using the following equations and their values should be as close to ‘zero’ as possible [69–71].

N 1 qt,exp − qt,cal 2 q(%) = 100 ( ) N−1

i=1

qt,exp

100 qt,exp − qt,cal 2 ( ) qt,exp N−1 i

(22)

i

N

ARE(%) =

(23)

i=1

where qt,exp and qt,cal respectively are the experimental value and the calculated value of the sorption capacity of EIR for a radionuclide at time ‘t’ and N is the number of measurements made. A visual examination of fitted plots showed that the Bangham and intra-particle models were better than the other studied models (Figs. 5 and 6, Figs. 5–10S), and this was supported by the statistical indices obtained for each model (Tables 4 and 5, Table 1–6S). As can be seen from the Tables, both the Bangham and

intra-particle diffusion models were very successful in explaining the sorption data, and their fits were identical for all concentrations under all temperatures investigated. Only a closer look at the statistical indices in the lower concentrations would indicate subtle differences between Bangham equation and intra-particle diffusion model. For example, in the concentration of 25 mg L−1 at all temperatures, intra-particle diffusion model had lower R2 values and higher statistical indices compared with the Bangham model, which is resulted from stronger effect of the boundary layer conditions in the lower concentrations. Overall, both Bangham and intra-particle diffusion models were the best models for explaining uranium and thorium sorption under all varying conditions. This finding indicates that the diffusion of metal ion into the pores of the EIR beads is the only rate-controlling step for the sorption of both uranium and thorium on to CA/XAD-16 resin. 6.1. Effect of temperature and thermodynamic studies Since the effect of operating temperature is important in the studies of sorption of metal ions, thermodynamic studies were

A. Rahmani-Sani et al. / Journal of Hazardous Materials 286 (2015) 152–163 Table 4 Kinetic parameters of different models and statistical indices for the sorption of U(VI) ion by the EIR beads at different initial concentrations (T: 298).

Table 5 Kinetic parameters of different models and statistical indices for the sorption of Th(IV) ion by the EIR beads at different initial concentrations (T: 298).

C0 (mg g−1 )

C0 (mg g−1 )

Kinetic model

25

50

Bangham k0 (mL g−1 L−1 ) ˛ R2 q(%) ARE(%)

10.672 0.784 0.996 3.276 0.107

10.551 0.761 0.997 2.775 0.077

10.374 0.740 0.998 2.106 0.044

Elovich ˛ (mg g−1 min−1 ) ˇ (g mg) R2 q(%) ARE(%)

4.801 0.107 0.968 24.961 6.230

8.987 0.054 0.964 25.723 6.617

4.147 0.386 0.994 4.815 0.232 0.034 40.142 0.980 19.410 3.767

Intra-particle diffusion kip (mg g−1 min−1/2 ) I R2 q(%) ARE(%) Pseudo-first-order k1 (min−1 ) qe,cal (mg g−1 ) R2 q(%) ARE(%)

Pseudo-second order 1.08E-03 k2 (g mg−1 min−1 ) 44.743 qe (mg g-1 ) R2 0.961 q(%) 8.051 ARE(%) 0.648 39.05 qe,exp (mg g−1 )

161

75

100

Kinetic model

25

50

10.140 0.706 0.999 1.430 0.020

Bangham k0 (mL g−1 L−1 ) ˛ R2 q(%) ARE(%)

10.686 0.784 0.996 2.876 0.083

10.559 0.763 0.997 2.788 0.078

10.384 0.747 0.998 2.090 0.044

10.165 0.709 0.999 1.489 0.022

12.838 0.038 0.963 25.965 6.742

15.937 0.031 0.960 25.982 6.751

Elovich ˛ (mg g−1 min−1 ) ˇ (g.mg) R2 q(%) ARE(%)

4.936 0.105 0.967 25.099 6.299

9.067 0.054 0.964 26.989 7.284

12.914 0.038 0.962 27.809 7.733

15.956 0.030 0.960 28.022 7.852

7.992 0.430 0.995 4.013 0.161

11.482 0.448 0.996 2.806 0.079

14.106 0.458 0.996 2.418 0.058

Intra-particle diffusion kip (mg g−1 min−1/2 ) I R2 q(%) ARE(%)

4.184 0.388 0.993 4.856 0.236

8.072 0.429 0.994 4.034 0.163

11.557 0.454 0.996 2.775 0.077

14.296 0.464 0.998 2.457 0.060

0.032 76.789 0.983 17.955 3.224

0.031 111.045 0.982 16.948 2.872

0.029 134.369 0.985 11.624 1.351

Pseudo-first-order k1 (min−1 ) qe,cal (mg g−1 ) R2 q(%) ARE(%)

0.034 40.683 0.978 26.508 7.026

0.032 77.553 0.983 23.722 5.627

0.031 111.789 0.980 19.288 3.720

0.029 136.176 0.984 16.757 2.808

5.45E-04 86.207 0.958 8.141 0.663 76.25

3.71E-04 124.533 0.956 8.220 0.676 110.08

3.00E-04 153.139 0.954 8.373 0.701 137.50

Pseudo-second order 1.06E-03 k2 (g mg−1 min−1 ) 45.147 qe (mg g-1 ) R2 0.961 q(%) 8.004 ARE(%) 0.641 39.38 qe,exp (mg g−1 )

5.30E-04 87.719 0.958 8.336 0.695 76.99

3.68E-04 125.313 0.956 8.396 0.705 110.79

2.95E-04 155.280 0.954 8.439 0.712 139.33

performed to study how different temperatures affect the sorption of U(VI) and Th(IV) ions onto EIR surface. It is well known that temperature has two major effects on the sorption process. Due to both decrease in the solution viscosity and increase in the metal ion mobility in the solution, increasing the temperature causes increase the rate of diffusion of the metal ions across the external boundary layer and in the internal pores of the EIR beads. In addition, increasing the temperature will change the equilibrium constant

Fig. 7. Plots of ln Kd versus 1/T for the determination of thermodynamic parameters.

75

100

and sorption capacity of the EIR for both radionuclides. Experimental results concerning the effect of temperature on the sorption of metal ions at different concentrations can be observed from the equilibrium and kinetic studies. Depended on the concentration of metal ion, an increase in temperature results in a relative increase in both the sorption percentage and sorption speed, indicating that the sorption of metal ions is an endothermic process. More rapid sorption at higher temperatures can be attributed to acceleration of pore diffusion step which is the rate-controlling step for the sorption of both uranium and thorium on the CA/XAD-16. Also, greater sorption percentages at higher temperatures are due to the bigger equilibrium constants. Evaluation of thermodynamic parameters was used to assess the spontaneity of the sorption process. The thermodynamic parameters viz.; Gibbs free energy (G), the enthalpy change (H) and entropy change (S) for the sorption of U(VI) and Th(IV) ions onto EIR beads were calculated using the values of distribution constant, Kd , at the different temperatures, and plotting of ln Kd versus 1/T (Fig. 7). The calculated thermodynamic parameters for sorption of U(VI) and Th(IV) ions onto the EIR surface are reported in Table 6. The negative values of Gibbs free energy (G) are due to high affinity of both radionuclides to EIR and the spontaneous nature of sorption process for both uranium and thorium ions. However, G value becomes more and more negative with increase in temperature, indicating that the extent of spontaneity is proportional to the temperature and, therefore, higher temperature favors the sorption process for both metal ions. The positive value of H◦ shows the endothermic nature of adsorption process, confirming that the intensity of sorption process is enhanced at higher temperatures. The positive value of S◦ confirms the affinity of the sorbent for both uranium and thorium ions and suggests the increased ran-

162


Table 6 Thermodynamic parameters for the sorption of uranium and thorium by CA/XAD-16. Radionuclide

T (K)

G◦ (kJ mol−1 )

U(VI)

288 298 308 318 288 298 308 318

−24.1 −25.54 −26.91 −28.34 −23.96 −25.38 −26.76 −28.19

Th(IV)

H◦ (kJ mol−1 )

S◦ (J mol−1 K−1 )

16.48

140.94

16.57

140.71

domness at the solid/solution interface during the sorption of metal ions onto CA/XAD-16. The positive value of S◦ may be related to the liberation of water of hydration during the adsorption process causing the increase in the randomness of the system [72]. 7. Conclusion This work demonstrates that Carminic acid-impregnated XAD16 resin beads have high sorption capacity for uranium and thorium, and can be successively used for the fast removal of U(VI) and Th(IV) ions from aqueous solutions. The sorption of U(VI) and Th(IV) ions onto CA/XAD-16 is influenced by several physicochemical parameters, such as initial concentration, temperature, contact time, sorbent dose and pH. The sorption process of both radionuclides occurs rapidly, and tends to complete lesser than 2 h at all tested concentrations and temperatures. Based on the knowledge gained from this work, the following key conclusions can be drawn: (i) The EIR showed very high sorption affinity to both metal ions. The equilibrium studies showed that the Langmuir isotherms were highly fitted to experimental data for both metal ions at all studied temperatures, reflecting that the sorption process for both metal ions occurs via a monolayer sorption mechanism. (ii) The kinetic studies showed that both Bangham and intraparticle diffusion models provided a good description of uranium and thorium sorption datasets when compared with the other models. (iii) The thermodynamic studies were conducted to calculate different thermodynamic parameters, such as G◦ , H◦ and S◦ , from the sorption data, and the obtained results indicated spontaneous and endothermic nature of the sorption process for both thorium and uranium. Acknowledgement 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 Sciences, for his great helps and supports during the experimental works. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2014.12.047. References [1] D. Brugge, J.L. deLemos, B. Oldmixon, Exposure pathways and health effects associated with chemical and radiological toxicity of natural Uranium: a review, Rev. Environ. Health 20 (2005) 177–193. [2] E. Metwally, Kinetic studies for sorption of some metal ions from aqueous acid solutions onto TDA impregnated resin, J. Radional. Nucl. Chem. 270 (2006) 559–566. [3] P. Sharma, R. Tomar, Synthesis and application of an analogue of mesolite for the removal of uranium(VI), thorium(IV), and europium(III) from aqueous waste, Microporous Mesoporous Mater 116 (2008) 641–652.

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