Arab J. Nucl. Sci. & Appl., Vol. 42(1), pp. 91–105, 2009.

Modeling Kinetics and Thermodynamics of Cesium andEuropium Radionuclides Removal from Aqueous Solutions

Using Zirconium Tungstate

E. MetwallyNuclear Chemistry Department, Hot Laboratories Center, Atomic Energy Authority, P. Code

13759, Cairo, Egypt.

ABSTRACT

Zirconium tungstate ion exchanger was used as aneffective sorbent for the adsorption of cesium and europiumradionuclides from their aqueous solutions. Zirconiumtungstate was synthesized with equimolar ratio betweenzirconium and molybdnum ions. The adsorption behavior ofzirconium molybdate was studied towards the uptake of Cs(I)and Eu(III) ions from aqueous solutions. The obtainedisotherm data have been correlated with Langumir,Freundlich, Temkin, Halsey, Handerson and Harkins Juraisotherm models. The effect of temperature on theequilibrium distribution values has been utilized toevaluate the standard thermodynamic parameters such as freeenergy (∆G), enthalpy (∆H) and entropy (∆S). All theexperimented data are discussed revealing the success ofzirconium tungstate as cation exchanger for the adsorptionand separation of the studied elements from their media to alarge extent.

Key Words: Modeling / Kinetics / Thermodynamics / Radionuclides / Zirconium tungstate

INTRODUCTION

Various processes used in the nuclear fuel cycle and in the research application of radionuclides in medicine and in industry, generate a range of low and intermediate level of liquid wastes. Treatment of these radioactive liquid wastes is applied to reduce themto levels which allow safe discharge or to isolate radioactive

contaminants from the environment. Cesium and Europium are of the mostabundant rdionuclides in nuclear fission products. They have a relatively long half-life and are considered as hazardous elements forthe environment(1,2). A large number of insoluble salts have been investigated as inorganic ion exchangers such as zirconium phosphate, other phosphates, arsenates, tungstates, molybdates and hydrous oxides. In addition, antimonates of cerium (IV)(3), have been also investigated. A composite of zirconium molybdate and zirconium tungstate have been used successfully for removal of cesium and strontium ions from acid solutions(4). Inorganic compounds are extremelyproven candidates for the separation and recovery of cesium and strontium from aqueous waste streams(5,6). Sorption study has been applied for the removal of Cd(II) and Pb(II) from aqueous solutions onto coal sorbents(7). A number of adsorbents such as activated carbon(8), sargassum(9), chitosan(10), metal oxide gel(11), saw dust(12), humus–boehmite complex(13), animal bone powder and ceramics(14) and bananastem(15) have been used for some heavy elements removal. Also modeling for heavy metals and some elements adsorption onto sandy soil(16) and tree fern(17) sorbents have been investigated. Modeling, thermodynamics and kinetics studies have been applied recently for removal of Pb(II),As(III) and Cr(III) ions from aqueous solutions using carbon nanotubes, activated alumina and coir pith sorbents respectively(18-20).

The aim of this work is to apply modeling of thermodynamics and kinetics study for the removal of radioactive cesium and europium fromaqueous solutions using zirconium tungstate ion exchanger.

EXPERIMENTAL All chemicals and reagents used were of analytical grade and used without further treatments.

Preparation of zirconium tungstate (ZW) ion exchanger ZW was prepared by mixing equimolar and equivolume of zirconylchloride and sodium tungstate solutions with constant stirring as hasbeen mentioned in reference 21. The stirring was continued for onehour then the precipitate was filtered and washed by distilled waterto constant pH 4.5 and dried at 50oC for 24 hours(21). Then the whitedried precipitate was sieved and fraction of 0.25 mm was taken forfurther use.

Characterization of ZW

All the characterization of the prepared ZW has been mentioned in detail elsewhere(21). The prepared ZW was investigated by DSC thermal, X-ray diffraction and infra red spectrum analysis. The DSC thermogram is shown in Fig.(1a). The endothermic peak at 136.3oC mas loss (11.2%, up to 200 oC) may be due to evaporation of thesurface water. Mass loss continues up to 700 oC wherease the total mass loss between 450 and 700 oC equals 57%. Thus, the compound is stable up to 450 oC [21].The X-ray diffraction of the prepared ZW, in Fig.(1b) shows that ZW is amorphous(21). The infra red spectrum of ZW presented in Fig. (1c) shows a broad adsorption peak at 3242 cm-1 whichis attributed to the stretching vibration of the bonded OH groups of the water molecules having intermolecular H bonds and a relatively sharp peak at about 1628.76 cm-1 which can also be attributed to water molecules. The large compound band at 1000-600 cm-1 is incorporating overlapped peaks at about 955, 868 and 695 cm-1. These peaks are due tothe vibration of metal oxygen bonds, i.e., W-OH, Zr-OH, W-O and Zr-O(21). Sorption isotherms Sorption isotherms for Cs+ and Eu3 were determined over the concentration range of 10-5-5x10-3 M at 0.1M HCl and a constant V/m valueof 200 (cm3.g-1). Experiments were carried out in a shaker thermostat at 30, 40 and 50oC.

Radiometric assay 134Cs and 152+154Eu isotopes were produced from irradiating an appropriate weigh of their corresponding salts in Inshas first reactorER-1. NI scintillation detector connected to Nucleaus gamma counter was used to determine the counting rates of 134Cs and 152+154Eu isotopes. The counting rates were at least 10 times as that of the background. Generally, the net counting rates has a standard deviations less than +3 %. The rate of sorption of cesium and europium was studied by equilibrating 0.1 g of the sorbent with 10 ml of 134Cs or 152+154Eu aqueoussolutions. The distribution ratio and the uptake percent of the element between the aqueous phase and the exchanger were determined using the following equations: Kd = ( Ai - Af ) / Af x V/W (ml/g) (1) Uptake % = ( Ai - Af ) / Ai x 100 (2) Where, Kd , is the adsorption coefficient.

Ai, is the area under the gamma energy peak for the correspondingisotope before contacting the ion exchanger. Af, is the area under the same peak for the corresponding radioisotope after contacting the ion exchanger V, is the volume of aqueous phase in ml. W, is the weight of the dry ion exchanger in gram.

Figure 1. a) IR , b) X-ray diffraction and c) thermal analysis for ZW ion exchanger

Adsorption experiments Batch technique was applied for the adsorption process at differenttemperatures (30, 40 and 50◦C). Adsorption isotherms were recorded over the studied range of solution concentration. A known amount of ZWis then added into the solution and is stirred with thermostatic bath operating at 400 rpm. The amount of ions adsorbed onto ZW, qt (mg/g), was calculated by the following relationship(2) Eq.(3): qt = (C0 − Ct) (V/W). (3)Where, C0 and Ct are the initial and concentration at certain time t of the ions in the liquid-phase (mg/L) respectively. V, is the volume of the solution (L) and W is the weight of ZW used (g). Adsorption of 134Cs and 152+154Eu ions were carried out by a batch technique at different temperature (30, 40 and 50oC). All the radionuclides used in the present study were produced via irradiating an accurate and appropriate weight of their corresponding salts or oxides in the second research reactor at Inshas ERE2, Egypt. A known weight of ZW (W) was mixed with a certain specified volume (V) of ion solutions in a shaker thermostat adjusted to the desired temperature and shaking speed. The solutions were separated after a certain time and the concentration of each studied ion was determined radiometry.

RESULTS AND DISCUSIONSEffect of pH Amount absorbed of Cs(I) and Eu(III) ions onto ZW varies with varying pH values. Uptake % of the studied ions are plotted with pH value, Fig.(2). The uptake % increases with increasing pH values for both Cs and Eu ions. Maximum adsorption percent reaches 94.2% at pH 7.4 for Cs(I) ions and 69.3% at pH 7.4 for Eu(III) ions respectively.

Effect of particle size Particle size of ZW ion exchanger affects the uptake % for cesium and europium ions. As the particle size increases from 0.1 mm to 0.25 mm, the uptake % decreases for both Cs(I) and Eu(III) ions using initial ion concentration of 10-3 M and 10-4 M respectively, Fig.(3).

Figure 2. Effect of pH on the uptake percent of Eu(III) and Cs(I) ions onto ZW ion exchanger from aqueous solution.

Figure 3. Effect of contact time on the adsorption of Eu(III)and Cs(I)ions onto ZW at differenr particle size, using [Eu3+] = 10-4M and [Cs+] = 10-3M ion concentration.

Effect of competing ion Figure 4 shows the effect of Na+ ion concentration on the uptake % of Cs+ and Eu3+ ions respectively. Europium uptake % shows a little and gradual decrease with increasing Na+ ion concentration, while adsorption of Cs+ ions does not affected.

Figure 4. Effect of competing ion concentration on the uptake percent of Eu(III) and Cs(I) ions onto ZW ion exchanger from aqueous solution.

Effect of contact time Effect of contact time on the uptake % of Cs and Eu ions onto ZW isinvestigated using different ion concentration, at pH 7.4 and 3.5respectively, Fig.(5a&b). The plots in these figures indicate that theprocesses were quite rapid and typically 60-70% of the ultimatesorption of each ion occurs within the first 60 min. of contact. Whilethe saturation of the sorption processes was almost reached at 150min. and completed at 260 min. of contact. Effect of contact time onthe amount adsorbed Cs(I) and Eu(III) ions onto ZW at different ionconcentration is represented in fig.(6). The amount of adsorbed ions(qt) increases with time. It increases with increasing ionconcentration from 10-5 to 5x10-3 M for both Cs and Eu ions. Also theabsorbed amount of the studied ions has a relatively high value using5x10-3M ion concentration than in the other ion

Figure 5. Effect of contact time on the adsorption of a) Cs(I) and b) Eu(III) using ZW at pH = 7.4 and 3.5 respectively, using different ion concentration.

concentration. Also, figures (5&6) show that the isotherms for the adsorption of Cs(I) and Eu(III) are regular, positive, and concave to the concentration axis. That indicates the initial rapid sorption which reveals to a slow approach to equilibrium at higher ion concentrations. These results reflect the efficiency of synthetic ZW for the removal of both ions from aqueous solution in a wide range of concentrations.

Figure 6. Effect of contact time on the amount adsorbed of a) Cs(I) and b) Eu(III) ions onto ZW at pH = 7.4 and 3.5 respectively, using different ion concentration.

Equilibrium kinetics studies It is well recognized that the characteristic of sorbent surface isa critical factor that affects the sorption rate parameters and the diffusion resistance plays an importanr role in the overall transport of the solute. To describe the change in the sorption of studied ions with time several kinetic models were tested. The rate constant of each metal ion removal from the solution by ZW was determined using different models. The Lagergren first order rate expression(22,23) is written as: log (qe – qt) = log qe – k1t / 2.303 (4)where, qe and qt are the amount of metal ion sorbed onto ZW at equilibrium and time t (mg/g) respectively. And k1 is the rate constantof first order adsorption (min-1). The slopes and intercept of log (qe– qt) versus t, as shown in Fig.(7), were used to determine the first order rate constant k1

Figure 7. Lagergren plotts for the adsorption of: a) cesium , b) europium ions onto ZW at different ion concentration.

values, which were 0.0102 min-1 and 0.0128 min-1 for Cs(I) and Eu(III) ions respectively. Also their R2 values were 0.99012 and 0.99083 respectively. It was observed that the sorption of both ions followed the Lagergren equation over most of the range of shaking time. Lagergren regression plots for the adsorption of Cs and Eu ions were shown in Fig. (8).

Figure 8. Lagregren regression plotts for the adsorption of: a) Cs(I) and b) Eu(III) onto ZW from 5 x10-3M ion concentration.

Sorption isotherm Relation between Ce and qe for the adsorption of Cs(I) and Eu(III) ions at different temperatures is presented in Fig.(9). The data showed that the amount of Cs+ and Eu3+ sorbed at equilibrium increase with increase in temperature indicating the process is to be endothermic. The sorption studies were carried out at 303, 313 and 333K to determine the sorption isotherms. The isotherm parameters wereevaluated using Freundlich isotherm. Freundlich isotherm equation could be written as: log qe = log kf + 1/n .log Ce (5)

Figure 9. Relation between Ce and qe for the adsorption of a) Cs(I) andb) Eu(III) ions onto ZW at different temperatures.

where, qe is the amount of solute sorbed per unit weight of adsorbent (mg/g), Ce the equilibrium concentration of the solute in the equilibrium solution (mg/l), Kf is a constant indicative of the relative adsorption capacity of the adsorbent (mg/g) and 1/n the constant indicative of the intensity of the sorption process. The Freundlich isotherm for the sorption of the two metal ions on ZW is presented in Fig.(10). The straight lines obtained for the isotherm indicate that the sorption of both ions fit with the investigated isotherm. The corresponding Freundlich parameters along with correlation coefficients are given in Table (1). The slope of Freundlich isotherm for all cases is less than one, indicating a concentration dependent sorption for both studied ions onto ZW sorbent.

Figure 10. Freundlich isotherm for the adsorption of a) Cs(I) and b) Eu(III) ions onto ZW at different temperatures.

Table 1. Constant parameters and correlation coefficients calculated for various adsorption models at different temperatures for Cs(I) and Eu(III) adsorption onto ZW.

Cs(I) Eu(III)

Model Parameters 303 K 313 K 323 K 303 K 313 K 323 K

Freundlich kf (L/g)1/nR2

3.82230.70230.9936

3.45610.67800.9509

4.06400.75590.9866

3.79740.87530.9992

3.34180.68560.9945

3.14910.59400.9922

Temkin B1

KT (L/mg)R2

1.9980314.370.7632

2.7722872.620.8680

3.7717872.310.9720

3.0246 61.2310.9603

3.440023.0270.9556

3.903953.3950.9892

Hasely nkR2

1.73379.75 x100.9839

1.47521.25 x100.9509

1.42352.77 x100.9937

1.89943.99 x100.9709

1.46237.56 x100.9947

1.14292.17 x100.9992

Handerson nkR2

1.70601.09x10-6

0.9839

1.414169.11x10-6

0.9509

1.40283.66x10-6

0.9937

1.844232.88 x100.9709

1.45461.33x10-5

0.9947

1.14214.59x10-5

0.9992Harkins-JuraA

BR2

250.6263.944800.97694

101.722.96130.9317

9.51402.84220.6865

10.1092.92500.7067

9.21402.87670.7695

3.07352.85040.6850

Dubinin–Radushkevich(D-R)

qm x104 (mol/g)k’ x104 (mol2/kJE (KJ/mol)R2

6.6502.78042.400.9936

0.9032.20247.650.9884

0.2582.08049.020.9610

0.6133.45838.060.6133

0.2172.62843.610.9947

0.0981.96150.490.9708

Thermodynammic parameters investigation The adsorption of Cs(I) and Eu(III) onto ZW at differenttemperatures shows an increase in the adsorption capacity when thetemperature is increased. The adsorption capacity varies withtemperature and initial concentration as shown in Fig.(11). Withincrease in temperature from

Figure 11. Effect of temperature on the amount adsorbed of : a) Cs(I) and b) Eu(III) ions

onto ZW at pH = 7.4 and 3.5 respectively using different ion concentration.

303 to 323 K, the adsorption capacity of Cs(I) and Eu(III) ionsincreased from 77.18 mg/g to 95.90 mg/g and from 43.30 mg/g to 51.90mg/g respectively. That was achieved with using initial ionconcentration of 5 x10-3 M at pH 7.4 and 3.5 for Cs(I) and Eu(III) ionsrespectively. Also, Similar trends are observed for all the otherconcentrations. This indicates that the adsorption reaction isendothermic in nature. The enhancement in the adsorption capacitymay be due to the chemical interaction between adsorbates andadsorbent, creation of some new adsorption sites or the increasedrate of intraparticle diffusion of Cs(I) and Eu(III) ions intothe pores of the adsorbent at higher temperatures(24). Thestandard Gibb’s energy was evaluated by the equation: ΔGo = −RT ln Kc

(6) The equilibrium constants Kc was evaluated at each temperatureusing the following relationship: Kc = CAe / Ce

(7) where, CAe is the amount adsorbed on solid at equilibrium and Ce

is the equilibrium concentration. The other thermodynamicparameters such as change in standard enthalpy (∆H◦) and standardentropy (∆S◦) were determined using the following equation: ln Kc = ( S∆ o / R ) -- ( ∆Ho / RT)(8) ∆Ho & S∆ o were obtained from the slope and intercept of the Van’tHoff’s plot of ln Kc versus 1/T as shown in Fig.(12). Positivevalue of ∆H◦ indicates that the adsorption process isendothermic. The negative values of ∆G◦ reflect the feasibilityof the process and the values become more negative with increasein temperature. Standard entropy determines the disorderliness ofthe adsorption at solid–liquid interface. Table (2) summarizesthe results.

Figure 12. Vant Hoff’s plots for Cesium and Europium adsorption onto ZW at different ion concentration.

Equilibrium modeling in a batch system Analysis of equilibrium data is important for developing anequation that can be used to compare different biomaterials underdifferent operational conditions and to design and optimize anoperating procedure(25). Heat of adsorption and the adsorbate-adsorbateinteraction on adsorption isotherms were studied by Temkin andPyzhev(26,27). The Temkin isotherm equation is given as: qe = (RT / b) . ln (KT Ce)(9)Equation (9) can be linearized as: qe = B1 ln KT + B1 ln Ce (10)where, B1 = RT/b, T is the absolute temperature in K, R the universalgas constant, 8.314 J/mol, KT is the equilibrium binding constant(L/mg) and B1 is related to the heat of adsorption. A plot qe versus lnCe at studied temperature is given in Fig.(13). The constant obtainedfor Temkin isotherms are shown in Table (1). The Temkin isothermconstant in Table (1) shows that the heat of adsorption (B1) increaseswith increase in temperature, indicating endothermic adsorption (28).

Figure 13. Temkin isotherm for the adsorption of a) Cs(I) and b) Eu(III) ions onto ZW at different temperatures.

Halsey(29) and Henderson(30) adsorption isotherm can be given, asrespectively, ln qe = [ (1/n) ln k] - (1/n) . ln(1/Ce)(11) ln [-ln(1 - Ce)] = ln K + n . ln qe

(12) These equations are suitable for multilayer adsorption. lnqe versuslnCe Halsey and ln[-ln(1-Ce)] versus lnqe Henerson adsorption isothermsare given in Figs.(14,15). Isotherm constants and correlationcoefficients are summarized in Table (1). The values of constant n forHalsey and Henderson’equation, decrease with increasing temperaturewhich show that the adsorption increased with a decrease in n values,indicating that the process is endothermic(31). Dubinin–Radushkevich (D-R) isotherm equation can be expressed(32) as follows: qe = q’m exp(−K’ ε2)(13)where ε (Polanyi potential) is equal to RT ln(1 + 1/Ce), qe is theamount of the ion adsorbed per unit ZW (mg/g), qm maximum sorptioncapacity (mg/g), Ce the equilibrium concentration of the solution(mg/L), K’ the constant of the adsorption energy (mol2/kJ2), R the gasconstant(kJ/mol ), and T is the temperature (K). The linear form of theD–R isotherm is ln qe = ln qm − K’ ε2(14) K’ is related to mean adsorption energy (E, kJ/mol) as(33)

E = 1 √2K’(15)

The plot of ln qe versus ε2 at 303, 313 and 323 K adsorptiontemperature is presented in Fig. (16). The constants obtained for D–Risotherms are shown in Table 2. The mean adsorption energy (E) givesinformation about chemical and physical adsorption(34). It was found tobe in the range of 38.06–50.49 kJ/mol, which is bigger than the energyrange of adsorption reaction, 8–16 kJ/mol. These results indicate thatthe type of adsorption of Cs(I) and Eu(III) ions onto ZW was definedas chemical adsorption(35). D-R approaches provide best fit over thestudied range of concentration of the system. This suggests that someheterogeneity in the surface or pores of sorbent will play a role inthe metal ion sorption(35).

Figure 14. Halsey isotherm for the adsorption of a) Cs(I) and b) Eu(III) ions onto ZW at different temperatures.

Figure 15. Henderson isotherm for the adsorption of a) Cs(I) and b) Eu(III) ions onto ZW at different temperatures.

ε2 ε2

Figure 16. Dubinin–Radushkevich (D-R) isotherm for the adsorption of a) Cs(I) and b) Eu(III) ions onto ZW at different temperatures.Table 2. Thermodynamic parameters calculated for the adsorption of Cs(I) and Eu(III) onto ZW at different ion concentration.

Cs(I) Eu(III)

Ion concentration

Temperature (K)

Δ Go (KJ/mol)

Δ Ho (KJ/mol)

Δ So (J/mol K)

Δ Go (KJ/mol)

Δ Ho (KJ/mol)

Δ So (J/mol K)

10-4 M 303313323

-2.635-3.125-3.584

120.537

483.4

-2.524-2.761-3.025

51.821

254.227

5x10-4 M 303313323

-2.548-2.632-3.170

70.207 307.618

-2.473-2.659-2.8716

36.332

201.434

10-3 M 303313323

-2.557-2.780-3.003

42.775

225.606

-2.425-2.563-2.728

22.215

153.243

The Harkins–Jura adsorption isotherm can be expressed(36) as: 1 / qe

2 = ( B/A) - (1 / A). log Ce (16)

It accounts to multilayer adsorption which can be explained with theexistence of a heterogeneous pore distribution. Plots of 1 / qe

2 versuslog Ce , Harkins–Jura isotherms are given in Fig. (17). Isothermconstants and correlation coefficients are summarized in Table (1).

Figure 17. Harkins Jura isotherm for the adsorption of a) Cs(I) and b)Eu(III) ions onto ZW at different temperatures.

CONCLUSION

The results, of Cs+ and Eu3+ ions sorption reported, showed that ZW is an efficient sorbent media for the removal of cesium and europium radionuclides from aqueous solutions. The removal of these two metal ions by the sorbent material takes place via a particle diffusion mechanism, and the thermodynamic parameters reflect the feasibility ofthe process. Freundlich, Temkin, Halsey, Henderson, Harkins-Jura and Dubinin-Redushkevich equations were used to describe the adsorption ofCs+ and Eu3+ ions onto ZW sorbent. Freundlich and and Dubinin-Redushkevich (D-R) isotherm models are the best choice to describe theobserved equilibrium data. The sorption of both metal ions is an endothermic process and the results show that ZW can be fruitfully employed for the removal of these metal ions in a wide range of concentrations.

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