Study on preferential selectivity of nuclear grade resin Indion-223 towards some bivalent ions

In the present paper attempts are made to understand the selectivity of nuclear grade cation exchange resin Indion-223 in H + form towards Ca 2+ and Mg 2+ bivalent ions in the solution based on thermodynamic concept. It was observed that with rise in temperature the equilibrium constant K values for H + /Ca 2+ uni-bivalet ion exchange reactions increases from 0.000397 to 0.000639. Similarly for H + /Mg 2+ uni-bivalet ion exchange reactions the equilibrium constant K values increases from 0.000177 to 0.000333. The increase in equilibrium constant values with rise in temperature indicate endothermic ion exchange reactions having the enthalpy change values of 38.92 and 51.46 kJ/ mol respectively. The difference in K values and enthalpy values were used to predict the selectivity behaviour of the resin towards the Ca 2+ and Mg 2+ bivalent ions in the solution. The thermodynamic concept of the present study can be applied to understand the selectivity behaviour of different nuclear as well as non-nuclear grade resins towards wide range of ionic species present in the exchanging liquid medium.


INTRODUCTION
Since their development, synthetic organic ion exchange resins are mainly used for water treatment, for instance in the preparation of demineralized water. Ion exchange resins are also having many industrial applications like purification and separations. As far as nuclear industry is concerned, one specific application is the purification of coolant and / or moderator in nuclear reactors. Particularly for CANDU-type reactors, heavy water is used as a moderator, and it is kept in high purity conditions by ion exchange resins in a purification loop. The resins remove both radiological and non-radiological impurities [1]. In nuclear power plants these resins are widely applied in-primary coolant (water) purification, treatment of primary effluents and fuel storage pond water, steam generator blow-down demineralization, for treatments of liquid waste and drainage water, purification of boric acid for recycling, condensate polishing (for nuclear power plants with boiling water reactors) [2--6]. Inorganic ion exchangers often have the advantage of a much greater selectivity than organic resins for certain radiologically important species, such as caesium and strontium. These inorganic materials may also prove to have advantages with respect to immobilization and final disposal when compared with organic ion exchangers. However, in nuclear power plant operations the currently available inorganic exchangers cannot entirely replace conventional organic ion exchange resins, especially in high purity water applications or in operations in which the system chemistry must be controlled through the maintenance of dissolved species such as lithium ions or boric acid [7].
These organic ion exchange resins are very effective at transferring the radioactive content of a large volume of liquid into a small volume of solid and have proved to be reliable and effective for the control of both the chemistry and radiochemistry of water coolant systems at nuclear power plants and also for processing some liquid radioactive waste [7]. As a result organic ion exchange resins are developed extensively [8][9][10][11] and various aspects of ion exchange technologies have been continuously studied to improve the efficiency and economy in various technological applications [5,6,12,13]. Development and synthesis of new organic ion exchange resins is usually followed by their characterization . Generally the selected ion exchange materials must be compatible with the chemical nature of the liquid waste such as pH, type of ionic species present as well as the operating parameters, in particular temperature .

MATERIALS AND METHODS
Glasswares: All apparatus used in the study were made up of Pyrex or Coming glass. Microburette of 0.02 mL accuracy was used for the entire experimental work.
Analytical balance: For weighing the sample above 25 mg, analytical balance of 0.1 mg sensitivity was used. Metler balance was used for weighing the samples less than 25 mg.
Potentiometer: Digital potentiometer of Equiptronics make having saturated calomel electrode as a reference electrode and platinum electrode in contact with quinhydrone as an indicator electrode was used in the experimental work. All Chemicals used were of analytical reagent (AR) grade. Distilled deionised water was used throughout the experiments for solution preparation. Ion exchange Resin: The ion exchange resin Indion 223 as supplied by the manufacturer (Ion Exchange India Limited, Mumbai) was a strongly acidic gel type nuclear grade anion exchange resin in H + form having styrene divinyl benzene cross-linking. The resin was having

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ILCPA Volume 34 --SO 3ˉ functional group, having moisture content of 50-55 %. The operational pH range was 0-14 and maximum operating temperature was 120 °C. The soluble non-polymerized organic impurities of the resin were removed by repeated Soxhlet extraction using distilled deionised water and occasionally with methanol. In order to ensure complete conversion of resins in H + form, the resins were conditioned with 0.1 N HCl in a conditioning column. The resins were further washed with distilled deionised water until the washings were free from H + ions. The resins in H + form were air dried over P 2 O 5 and used for further studies. The were calculated. From the K values obtained at different temperatures, the enthalpy change values of the above uni-bivalent ion exchange reactions were calculated.
The apparent equilibrium constants (K app. ) calculated by the equation (3) were plotted versus the equilibrium concentrations of the bivalent ions in the solution (Figure 1 and 2). Lower the equilibrium concentration of the bivalent ion, lower would be its concentration in the resin and in the limiting case of zero equilibrium concentration of the bivalent ion in the solution, the resin would be in its standard state. Therefore on extrapolating the above curve to zero equilibrium concentration of bivalent ion in the solution, the equilibrium constant in the standard state, K std. was obtained. Having thus obtained the equilibrium constant in the standard state, the activity coefficient ratio of ions γR2Y/(γRH) 2 at any finite equilibrium concentration of bivalent ion in the solution was calculated as the ratio of K std. /K app (Tables 1  and 2). From the slope of the graph of log K std. against 1/T (in Kelvin), the enthalpy change of the ion exchange reactions 1 and 2 were calculated (Figure 3). The equilibrium constant K std. values for the reactions 1 and 2 were found to increase with rise in temperature indicating  Table 3. Thermodynamics of ion exchange reactions using Indion-223 resin Amount of the ion exchange resin in H + form = 0.500 g, Ion exchange capacity = 3.02 meq./0.5 g

CONCLUSION
From the results of present study, it appears that the experimental technique used here can be applied further to understand the ionic selectivity of different industrial grade ion exchange resins. It is expected that such studies will provide valuable information in order to decide about the selection of those resins for efficient separation of various ionic species present in the industrial waste water effluents.