Diffusion Mechanism of Cesium, Cobalt and Cadmium Ions in the Particles of Tin Vanadate as a Cation Exchanger

Transcription

1 Arab Journal of Nuclear Sciences and Applications, 45()4-() Diffusion Mechanism of Cesium, Cobalt and Cadmium Ions in the Particles of Tin Vanadate as a Cation Exchanger S.A. Shady, A. B. El-Deeb and I.M. El-Naggar Hot Laboratories Center, Atomic Energy Authority, P. No. 759, Cairo Egypt. ABSTRACT The mechanism of diffusion of Cs +, Co + and Cd + ions in the H-form of tinvanadate as cation exchanger was studied as a function of particle size, concentration of the exchanging ions, reaction temperature and drying temperature. The exchange rate was controlled by a particle diffusion mechanism as a limited batch technique and is confirmed from straight lines of B versus /r plots. The values of diffusion coefficients, activation energy and entropy of activation were calculated and their significance was discussed. The data obtained have been compared with those reported for other inorganic ion exchangers. Key words: Diffusion Mechanism / Cesium / Cobalt / Cadmium / Tin Vanadate / Cation exchanger. -INTRODUCTION A good deal of interest has grown in the last decades in synthetic inorganic ion exchangers [], because of their greater resistance to high radiation doses and highly elevated temperatures which is of a great importance in the nuclear technology []. There has been an increasing interest in studying the kinetics and equilibrium characteristics of inorganic ion exchangers [-5]. Kinetic studies of H-metal ion exchanges on ion exchange materials are important for their analytical applications, since the kinetic studies of a reaction allow us to understand the mechanism, rate determining step and rate laws obeyed by a diffusion process. Energy of activation, entropy of activation and the self-diffusion coefficient of the ions [E a,?s* and D i ) are the fundamental properties of a system. These parameters help us to understand the mechanism of reactions during adsorption /exchange process. Hence it is important to deal with the kinetic behaviour of the material for the exchange of various metal ions [6]. In the course of the successful application of these ion exchange materials, our interest has been directed to study the diffusion mechanism of Cs +, Co + and Cd + on the prepared tin-vanadate under the control of particle diffusion mechanism... Chemical reagents and instruments: -EXPRIMENTAL The main reagents used for the synthesis of the material were obtained from BDH (England) and Loba Chemie (India). All reagents and chemicals were of analytical reagent grade purity and used without further purification. All measurements were performed using an atomic absorption spectrophotometer model AA-67 Shimadzu, Kyoto "Japan"... Preparation of Tin-vanadate:.5M stannic tetrachloride was dissolved in 4M hydrochloric acid then was added to.5m sodium monovanadate in a volume ratio : with constant stirring at room temperature. After the addition was completed, ammonia was added until a precipitation was occurred and -4

2 Arab Journal of Nuclear Sciences and Applications, 45()4-() then the reaction mixture was allowed to settle for 4 hours. The precipitate was filtered, washed several times with deionized water. The precipitate was dried at 5 o C in an electric oven, grained and sieved for different mesh sizes and stored at room temperature... Kinetics Studies: The radius of the particles of the sieved fractions was determined by measuring the diameter of particles with an optical microscope. The particles were assumed to be spherical and a mean equivalent radius was calculated. The reaction takes place between the metal ions and the counter ions in the exchanger and the rates are controlled by a particle diffusion mechanism only. Kinetics experiments were performed by using batch factor of V/m equals mlg - and - M metal chloride solution in a shaker thermostat adjusted at the desired temperatures. After the adjusted interval period, the solid was separated immediately from the solution and the extent of sorption was determined as the following: Ai Af % sorption = x A where A i and A f are the initial and final concentrations of the metal ions in solution. i.. Mathematical model -RESULTS &DISCUSSION The experimental conditions of the present study were set for the particle diffusion mechanism only (as mentioned before in section ). The fractional attainment of equilibrium at time t, (t), was used and is expressed as: Qt Amount of exchange after time, t t () = = Q Amount of exchange after infinite time w for the particle diffusion controlled kinetics, the expression developed by Boyed et al.[7] and improved by Reichenberg [8], Eq.(), which is used in this work: and 6 -n = - e () π B = π r n= n D i where B is a mathematical function, D i is the diffusion coefficient of the ions undergoing exchange within the exchanger, n is an integer number, r refers to the radius of the particles. When (t) is less than, Eq.() can be approximated in a simpler form: 6 (t) = r Dt i π which hold to a fairly good approximation. Therefore, a plot of (t) against the square root of the contact time must be give a straight line passing through the origin inwhich (t) is less than. () () -5

3 Arab Journal of Nuclear Sciences and Applications, 45()4-() or every observed value of, the corresponding values, as derived from Eq.() can be obtained from the Reichenberg table [8]. According to Eq.(), is a function of B and t only and independent on the concentration of the external solution. In the present study, the linearity test of versus t plots has been employed to distinguish between film and particle diffusion controlled rates of exchange and the experimental conditions were chosen for particle diffusion mechanism as a main rate determining step (as a limited batch technique). The energy of activation (E a ) for the self-diffusion of cations was determined by applying the Arrhenius equation: D i = D o e -Ea/RT (4) where D o is the self-diffusion coefficient. The entropy of activation (?S*) can be calculated from D o by substituting in the equation proposed by Barrer et al., [9]: KTd D o =.7 h e S*/R where K is the Boltzmann constant, T is the absolute temperature, d is the average distance between two successive positions in the process of diffusion which was taken as.5mm and h is the plank's constant. The above equations were used for calculation the values of D o, D i, E a and?s* for the exchanged systems and the obtained results are given in Tables and. Table (): Values of the diffusion coefficient of Cs +, Co + and Cd + on different particle diameters of tin vanadate at 5± o C. Particle diameters (± mm) D i (x 9 cm s - ) (5) Cs + Co + Cd Analysis of data To verify the particle diffusion mechanism of the metal ions Cs +, Co + and Cd + on SnV (Tin vanadate) under the conditions set in the present work, the following experimental results were obtained: (i) the rate of exchange of different metal ions on SnV is independent on metal concentrations in solution up to - M, (ii) straight line relationships passing through the origin were obtained between the function and t for the three metal ions under study, (iii) the exchange rate of Cs +, Co + and Cd + were found to increase with the decrease of the particle size of the prepared SnV exchanger and (iv) the plots of B [the slopes of straight lines ( versus t plots)] versus /r for Cs +, Co + and Cd + on SnV gave straight lines. All these results provide a good base supporting the particle diffusion mechanism under our experimental conditions. Similar findings were previously reported [-5]. A study of the concentration effect on the rate of exchange for Cd + /H + exchange at 5± o C shows that at concentration > - M, the initial rate of exchange is independent of concentration as shown in ig.(). The experimental conditions were set for particle diffusion mechanism only [6]. -6

4 Arab Journal of Nuclear Sciences and Applications, 45()4-() The values of () at different time intervals for the exchange of Cs +, Co + and Cd + ions on three different particle sizes of SnV were plotted as represented in igs.(-4), respectively.. - 5x Time.?.57 mm?.8 mm?. mm time,min ig. (): Effect of concentration of Cd on SnV at 5± o C. ig. (): Plots of and against time for exchange of Cs + ion on SnV at different particle diameters and 5± o C mm.8 mm. mm Time Effect ig.(): of Particle Plots of Size of and Tin Vanadate against on time Co + for exchange of Co + ion on SnV at different particle di ameters and 5± o C...75 mm 8 mm mm Time Time Effect of particle size of Tin Vanadate on Cd + ig.(4): Plots of and against time for exchange of Cd + ion on SnV at different particle diameters and 5± o C. -7

5 Arab Journal of Nuclear Sciences and Applications, 45()4-() These igs.(-4) show that the plots of () versus (t) for Cs+/H+, Co+/H+ and Cd+/H+ systems are linear and start from the origin proving a particle diffusion control mechanism [7]. Also, these figures indicate that the rate of exchange increases with the decrease in the particle sizes, which agrees with the fundamental conditions of particle diffusion. The average values of diffusion coefficients (Di) of Cs+, Co+ and Cd+ on SnV of different particle diameters were calculated from the relations of Vs. t, ig.(-4) and given in Table (). rom this table it is found that the values of diffusion coefficient (Di) which generally slightly increase with increasing the particle diameters of the prepared samples. The rate of exchange of Cs+, Co+ and Cd+ ions on Tin vanadate (SnV) increase with decreasing the particle diameter which agree with the fundamental conditions of particle diffusion, which support the particle diffusion mechanism. This trend was also reported for others ion-exchangers [, ]. The higher values obtained of Di may be attributed to the large particles formed from agglomerated of smaller compact unites. Therefore, a quicker diffusion took place through the channels between these unites. urthermore, the self diffusion coefficients of the metal ions studied are independent on the metal ion concentration in solution and dependent on the particle size of exchangers. This indicates that under these conditions, the rate determining step is mainly diffusion through the particle and film diffusion is nearly neglected and hence the mechanism of diffusion of these ions in our matrix is considered to be unchanged by the composition of the matrix [8]. Plots of (B) versus /r are straight lines indicating that the rate of exchange is inversely proportional to the square of the radius of particle as shown in ig.(5) which again verifies that diffusion through the exchanger particle is the rate-determining step. The relations between () and against time for the exchange of Cs+, Co+ and Cd+ ions for a particle diameter.85± mm and reaction temperatures 5, 45 and 65oC on SnV are given in igs.(6-8), respectively. These relations gave straight lines passing through the origin in all studies cases. ound from these results it was found that the rate of exchange reaction increased with rising the reaction temperature from 5 to 65± o C [9]. This trend may be due to the increase of the mobility of ions with increasing the reaction temperature from 5 to 65± o C. This agrees with the reported results for the rate of exchange of Na + and Co + on crystalline and amorphous sodium titanate at different reaction temperature []. The diffusion coefficient values (Di) of the investigated metal ions were calculated from the slopes of the previous relations at 5, 45 and 65±oC using Eq.(). The results are summarized in Table (). rom this table it is clear that the values of diffusion coefficient of the investigated metal ions increase in the order: Cs+ > Cd+ > Co+ This trend could attributed to the hydration energy and the hydrated ionic radii of the studied metal ions which take the order: Cs + > Cd + > Co +. Therefore, the mobility of these metal ions in the exchange particles increases in the reverse order[6]. -8

6 Arab Journal of Nuclear Sciences and Applications, 45()4-() B Cs + Co + Cd /r ig.(5): Plots of B against /r for exchange of Cs +, Co + and Cd + ions on SnV at 5± o C. x. 65 o C 45 o C 5 o C Time ig.(6): Plots of and against time for exchange of Cs + on SnV at particle diameter.85±mm and different reaction temperature.. 65 o C 45 o C 5 o C Time, min ig.(7): Plots of and against timefor exchange of Co + on SnV at particle diameter.85 ± mm and different reaction temperature.. 65 O C 45 o C 5 o C Time, min ig.(8): Plots of and against time for exchange of Cd + on SnV at particle diameter.85 ± mm and different reaction temperature. -9

7 Arab Journal of Nuclear Sciences and Applications, 45()4-() Table (): Thermodynamic parameters of Cs +, Co + and Cd + /H + systems on SnV at different reaction temperatures. Exchange system Drying temperature Particle diameter, mm. Reaction Temp., o k D i x 9 D o x 8 cm s - cm s - E a kjmol -?S* Jmol - k - Cs + /H Co + /H + 5 o C ± mm Cd + /H The effect of drying temperature of SnV (5, and 4 o C) on the rate of exchange of the investigated metal ions was studied as a relation between and against time as shown in igs.(9-)... 5 o C o C 4 o C Time, min ig.(9): Plots of and against time for exchange of Cs + on SnV at particle diameter.85± mm and different drying temperatures.. 5 o C o C 4 o C Time, min ig.(): Plots of and against time for exchange of Co + on SnV at particle diameter.85± mm and different drying temperatures.. 5 O C o C 4 o C Time, min ig.(): Plots of and against time for exchange of Cd + on SnV at particle diameter.85± mm and different drying temperatures. rom these figures, it is found that the rate of exchange decreases by increasing the drying temperature from 5 to 4 o C. It is clear that an appreciable decrease of self-diffusion of Cs +, Co + and Cd + with an increase in the drying temperature of SnV from 5 to 4 o C as shown in Table (). The lowering in the Di values for Cs +, Co + and Cd + with increasing the drying temperatures of the ion exchanger from 5 to 4 o C may be attributed to the decrease in the surface area and porosity of the dried exchanger as experimentally found [6]. The lower porosity means less free water inside the exchanger particles which hinders the diffusion of the metal ions []. -

8 Arab Journal of Nuclear Sciences and Applications, 45()4-() Table (): Values of diffusion coefficient of Cs +, Co + and Cd + on SnV at different drying temperature and at particle diameter.85± mm. Exchange system Drying temperature D i x 9 cm s - 5 o C Cs + /H + o C 4 o C 5 o C Co + /H + o C 4 o C 5 o C Cd + /H + o C 4 o C When log D i is plotted against /T for the exchange of Cs +, Co + and Cd + on SnV, straight lines were obtained (ig.). rom the slopes of these lines and applying Arrhenius equation, the energy of activation (E a ) and the self-diffusion coefficient (D o ) were calculated. On the other hand, the entropy of activation (?S*) can be calculated from D o using Eq.(5). Log D i Cs + Cd + Co /T, o K ig.(): Arrhenius plots for exchange of Cs +, Co + and Cd + on SnV at different reaction temperatures. It is of interest to compare the values of D i obtained for Cs +, Co + and Cd + on SnV with that previously reported using other inorganic exchangers. The D i values given by Shady and El-Gammal [] for Na + /H + and Cs + /H + exchange system on titanium(iv) antimonate were.7x - and.98x - cm s - respectively. The D i values given by El-Naggar, Mowafy and Abdel-Galil [6] for Cs + /H +, Co + /H + and Eu + /H + exchange system on SiTi were 4.x 7,.98x 7 and 4.5x 7 respectively. Also, the diffusion coefficient (D i ) for Cs + /*Cs + and Na + /*Na + exchanges on hydrous zirconium oxide was found to be 6.7x -9 and.4x -9 cm s - []. -

9 Arab Journal of Nuclear Sciences and Applications, 45()4-() order: urthermore, the activation energy E a for the investigated metal ions (at 5 o C) has the Cd + /H + (4.9) > Co + /H + (.894) > Cs + /H + (.9) This trend agrees with that already reported for Cs + and Co + on SiTi [6], which has the order: Co + /H + (.9) > Cs + /H + (.87) The relatively small activation energies values (E a ) obtained, Table, for Cs +, Co + and Cd +, indicated that the rate of exchange of particle diffusion []. On other word, these values of activation energy are relatively small compared to that reported for other organic and inorganic exchangers which confirm the particle diffusion mechanism. The negative?s* values obtained for all studies systems are given in table. The negative values obtained for the entropy of activation, suggest that no significant structure change occur in SnV. The entropy change normally depends on the extent of hydration of the exchangeable and exchanging ions a long with any change in water structure around ions that may occur when they pass through the channels of exchanger particles. The lowest values of?s* for Cs +, Co + and Cd + on SnV support the higher stability and hence the least steric difference of the system. These result are parallel to that reported for other inorganic ion exchanger []. The activation energy was found to decrease with the decrease in the entropy of the activation of the system (Table ). The same trend was observed for other ion exchange materials [5]. 4-CONCLUSIONS The exchange kinetics of Cs +, Co + and Cd + on the prepared Tin vanadate (SnV) were studied as a function of concentration, particle radius, drying temperature of SnV and reaction temperatures. The rate of exchange is independent on the metal ion concentration and this is a evident that the conditions are set in this work to be suitable for a particle diffusion mechanism for all investigated metal ions. The rate of exchange increases with decreasing the particle size and drying temperature of the exchange materials. While, the rate increases with increasing the reaction temperature. The diffusion coefficient values (D i ) of Cs +, Co + and Cd + ions on SnV at 5 o C decrease with increasing the drying temperatures from 5 to 4 o C. Negative values of entropy of activation (?S*) were obtained and this anticipate that the investigated metal ions are exchanged with H + of SnV in the unhydrated form. 5-REERENCES [] E.S. Zakaria, I.M. Ali, El-Naggar, Colloid Surf. A: Physicochem. Eng. Aspects, (). [] J. Marinsky, Y. Marcus, "Ion Exchange and Solvent Extraction", marcel dekker Inc., New York, (995). -

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