Comparative Study of Strongly Basic Anion Exchange Resins Tulsion A-30 and Indion-820 by Application of 131I and 82 Br as a Tracer Isotopes

In the present investigation radioactive tracer isotopes 131 I and 82 Br were used to study the kinetics of iodide and bromide ion–isotopic exchange reaction. The kinetics study was performed to study the effect of ionic concentration and temperature on the specific reaction rate calculated in min. From the initial ionic concentration and knowing the value of specific reaction rate, the values of amount of ions exchanged in mmol and percentage ion exchange were calcu lated. It was also observed that for both the resins, there exists strong positive co-relat ionship between amount of ions exchanged and concentration of ionic solution, while a strong negative co-relationship was observed between amount of ions exchanged and temperature of exchanging medium. The values of specific reaction rate, amount and percentage of ions exchanged so obtained for the ion-isotopic exchange reactions using strong base, nuclear grade anion exchange resin Tulsion A-30 and non-nuclear grade anion exchange resin Indion-820 were compared. It was observed that the above values calculated were higher for Tulsion A-30 as compared to that for Indion-820, indicating superior performance of Tu lsion A-30 under identical experimental conditions. The similar study on application of radioactive tracer isotopes can be extended further for performance assessment of different nuclear as well as non-nuclear grade ion exchange resins.


Introduction
Although radioisotopes have been applied to the solution of p roblems in industry fo r over 50 years, research and development of the technology continues unabated [1,2]. The economic benefits that may be derived fro m the use of this technology are great, a fact that is recognized by the governments of developing countries. There are two ma in reasons for the continu ing interest. Firstly, it is industry driven . Because o f th eir u n ique p ropert ies , rad ioact ive isotopes can be used to obtain information about plants and processes that cannot be obtained in any other way. Often, the info rmat ion is obtained with the p lant on-stream and without disrupting the process in any way. This can lead to substantial economic benefits, fro m shutdown avoidance to process optimization. Secondly, the methodology is derived fro m man y fields o f scien ce and techno logy includ ing radioisotope production, radiation detection, data acquisition, treatment and analysis, and mathematical modelling [3][4][5][6][7][8][9]. Important characteristics of radioisotope for use as tracer are half life, type and energy of radiation and availability. Short lived isotopes (a few hours to a few days of half life) are preferred as they have high specific activity and the system becomes non-radioactive very soon. If larger amounts of radioactivity are needed for use, then it is preferred to have short lived isotopes. Also it is ideal to have a ga mma emitting isotope as gamma ray measurement is the simplest and also external mon itoring of the process is practicable.
The fundamental princip le in radiochemical investigations is that the chemical properties of a radio isotope of an element are almost the same as those of the other stable/radioactive isotopes of the element. When radioisotope is present in a chemical form identical to that of the bulk of the element in a chemical process, then any reaction the element undergoes can be directly traced by monitoring the radioisotope. Radiochemical work involves two ma in steps first is the sampling of chemical species to be studied and second is quantitative determination of the radiation emitted by the radioisotope in the sample [10]. In rad iotracer study, a short lived rad ioisotope in a physico-chemical form similar to that of the process material is used to trace the material under study. The radioisotopes in suitable physical and chemical forms are introduced in systems under study. By mon itoring the radioactivity both continuously or after sampling (depending on the nature of study), the movement, Pravin U. Singare: Comparative Study of Duolite A-378 and Duolite A-143 Anion Exchange Resins by Application of 131 I and 82 Br as a Tracer Isotopes adsorption, retention etc. of the tracer and in turn, of the bulk matter under investigation, can be followed. The tracer concentration recorded at various locations also helps to draw information about the dynamic behaviour of the system under study. The radioisotopes preferred for such studies are gamma emitters having half-life co mpatible with the duration of studies. The strength of radioactivity used varies depending on the nature of application. Applicat ions of radiotracers in chemical research cover the studies of reaction mechanis m, kinetics, exchange processes and analytical applications such as radiometric titrat ions, solubility product estimation, isotope dilution analysis and autoradiography. Radio isotope tracers offer several advantages such as high detection sensitivity, capability of in-situ detection, limited me mory effects and physic-chemical co mpatib ility with the material under study. The radioisotopes have proved as a tool to study many problems in chemical, bio logical and med icinal fields. Radiotracers have helped in identification of leaks in buried pipelines and dams. Process parameters such as mixing efficiency, residence time, flow rate, material inventory and silt movement in harbours are studied using radioisotopes [10].The efficiency of several devices in a wastewater treatment plant (primary and secondary clarifiers, aeration tank) is investigated by means of radiotracers [11].
Considering the above wide use of rad ioactive isotopes in various industrial and technical applicat ions, in the present investigation, they are applied to assess the performance of industrial grade anion exchange resins Tulsion A-30 and Indion-820 under different operational parameters like temperature and ionic concentrations. It is expected that the tracer technique used here can also be used for characterizat ion of other organic ion exchange resins which are widely synthesized for their specific technical applications [12][13][14]. The present technique can also be extended further to standardize the operational parameters so as to bring about the most efficient performance of those resins in their specific industrial applications.

Condi tioni ng of Ion Exchange Resins
Ion exchange resin Tulsion A-30 is a nuclear grade strong base anion exchange resin in hydro xide form (by Thermax India Ltd., Pune), wh ile Ind ion-820 is a macroporous strong base anion exchange resin in chloride form (by Ion Exchange India Ltd., Mu mba i). Details regarding the properties of the resins used are given in Table 1. These resins were converted separately in to iodide / bromide form by treat ment with 10 %KI / KBr solution in a conditioning column which is adjusted at the flow rate as 1 mL / min . The resins were then washed with double d istilled water, until the washings were free fro m iodide/bromide ions as tested by AgNO 3 solution. These resins in bromide and iod ide form were then dried separately over P 2 O 5 in desiccators at room temperature.

Radioacti ve Tracer Isotopes
The radioisotope 131 I and 82 Br used in the present experimental work was obtained fro m Board of Radiation and Isotope Technology (BRIT), Mu mba i. Details regarding the isotopes used in the present experimental work are given in Table 2.

Study on Kinetics of Iodi de Ion-Isotopic Exchange Reaction
In a stoppered bottle 250 mL (V) of 0.001 M iodide ion solution was labeled with diluted 131 I radioactive solution using a mic ro syringe, such that 1.0 mL of labeled solution has a radioactivity of around 15,000 cp m (counts per minute) when measured with γ -ray spectrometer having NaI (Tl) scintillat ion detector. Since only about 50-100 μL of the radioactive iodide ion solution was required for labeling the solution, its concentration will remain unchanged, which was further confirmed by potentiometer t itration against AgNO 3 solution. The above labeled solution of known initial activity (A i ) was kept in a thermostat adjusted to 30.0 °C. The swelled and conditioned dry ion exchange resins in iodide form weighing exact ly 1.000 g (m) were transferred qu ickly into this labeled solution which was vigorously stirred by using mechanical stirrer and the activity in cpm of 1.0 mL of solution was measured. The solution was transferred back to the same bottle containing labeled solution after measuring activity. The iodide ion-isotopic exchange reaction can be represented as: (1) Here R-I represents ion exchange resin in iodide form; I* -(aq.) represents aqueous iodide ion solution labeled with 131 I radiotracer isotope.
Similar experiments were carried out by equilibrat ing separately 1.000 g of ion exchange resin in iodide form with labeled iodide ion solution of four different concentrations ranging up to 0.004 M at a constant temperature of 30.0 °C. The same experimental sets were repeated for higher temperatures up to 45.0 °C.

Study on Kinetics of Bromi de Ion-Isotopic Exchange Reaction
The experiment was also performed to study the kinetics of bromide ion-isotopic exchange reaction by equilibrating 1.000 g of ion exchange resin in bromide form with labeled bromide ion solution in the same concentration and temperature range as above. The labeling of bro mide ion solution was done by using 82 Br as a radioactive tracer isotope for which the same procedure as explained above was followed. The bro mide ion-isotopic exchange reaction can be represented as: R-Br + Br* -(aq.) R-Br* + Br -(aq.) (2) Here R-Br represents ion exchange resin in bromide form; Br* -(aq.) represents aqueous bromide ion solution labeled with 82 Br radiotracer isotope.

Comparati ve Study of Ion-Isotopic Exchange Reactions
In the present investigation it was observed that due to the rapid ion-isotopic exchange reaction taking p lace, the activity of solution decreases rapidly initially, then due to the slow exchange the activity of the solution decreases slowly and finally re mains nearly constant. Preliminary studies show that the above exchange reactions are of first order [31,32]. Therefore logarith m of activity when plotted against time gives a co mposite curve in which the activity initially decreases sharply and thereafter very slowly giving nearly straight line (Figure 1), evidently rapid and slow ion-isotopic exchange reactions were occurring simu ltaneously [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Now the straight line was extrapolated back to ze ro time. The extrapolated portion represents the contribution of slow process to the total activity which now includes rapid p rocess also. The activity due to slow p rocess was subtracted from the total activity at various time intervals. The d ifference gives the activity due to rapid process only. From the activity exchanged due to rapid process at various time intervals, the specific react ion rates (k) of rap id ion-isotopic exchange reaction were calculated. The amount of iodide / bro mide ions exchanged (mmol) on the resin were obtained fro m the initial and final activ ity of solution and the amount of exchangeable ions in 250 mL of solution. Fro m the amount of ions exchanged on the resin (mmo l) and the specific reaction rates (min -1 ), the init ial rate of ion exchanged (mmo l/ min) was calcu lated.
Because of larger solvated size of bro mide ions as compared to that of iodide ions, it was observed that the exchange of bro mide ions occurs at the slower rate than that of iodide ions [33]. Hence under identical experimental conditions, the values of specific react ion rate (min -1 ), amount of ion exchanged (mmo l) and in itial rate of ion exchange (mmo l/ min) are calculated to be lower for bro mide ion-isotopic exchange reaction than that for iodide ion-isotopic exchange reaction as summarized in Tables 3  and 4. For both bromide and iodide ion-isotopic exchange reactions, under identical experimental conditions, the values of specific react ion rate increases with increase in concentration of ionic solution from 0.001M to 0.004M (Table 3). However, with rise in temperature fro m 30.0 °C to 45.0 °C, the specific reaction rate was observed to decrease (Table 4). Thus in case of Tulsion A-30 at 35.0℃ when the ionic concentration increases from 0.001M to 0.004M, the specific reaction rate values for iodide ion-isotopic exchange increases from 0.222 to 0.265 min -1 , wh ile for bro mide ion-isotopic exchange the values increases from 0.184 to 0.220 min -1 . Similarly in case of Indion-820, under identical experimental conditions, the values for iodide ion-isotopic exchange increases from 0.100 to 0.132 min -1 , while for bromide ion-isotopic exchange the values increases from 0.088 to 0.115 min -1 . However when concentration of ionic solution is kept constant at 0.002 M and temperature is raised fro m 30.0 ℃ to 45.0℃, in case of Tu lsion A-30 the specific reaction rate values for iodide ion-isotopic exchange decreases fro m 0.246 to 0.218 min -1 , while for bro mide ion-isotopic exchange the values decreases from 0.206 to 0.178 min -1 . Similarly in case of Indion-820, under identical experimental conditions, the specific reaction rate values for iodide ion-isotopic exchange decreases from 0.120 to 0.090 min -1 , wh ile for bro mide ion-isotopic exchange the values decreases from 0.107 to 0.080 min -1 . Fro m the results, it appears that iodide ions exchange at the faster rate as compared to that of bro mide ions which was related to the e xtent of solvation (Tables 3 and 4).
Fro m the knowledge of A i , A f , volume of the exchangeable ionic solution (V) and mass of ion exchange resin (m), the K d value was calculated by the equation Heu mann et al. [34] in the study of chloride distribution coefficient on strongly basic anion exchange resin observed that the selectivity coefficient between halide ions increased at higher electro lyte concentrations. Adachi et al. [35] observed that the swelling pressure of the resin decreased at higher solute concentrations resulting in larger K d values. The temperature dependence of K d values on cation exchange resin was studied by Shuji et al. [36]; were they observed that the values of K d increased with fall in temperature. The present experimental results also indicates that the K d values for bromide and iodide ions increases with increase in ionic concentration of the external solution, however with rise in temperature the K d values were found to decrease. Thus in case of Tulsion A-30 at 35.0℃ when the ionic concentration increases from 0.001M to 0.004M, the log K d values for iodide ions increases from 9.5 to 12.3, while for bro mide ions the values increases from 8.0 to 10.3. Similarly in case of Indion-820, under identical experimental conditions, the log K d values for iodide ions increases from 5.9 to 7.3, wh ile fo r bro mide ions the values increases from 2.4 to 3.9. Ho wever when concentration of ionic solution is kept constant at 0.002 M and temperature is raised from 30.0 ℃ to 45.0 ℃, in case of Tulsion A-30 the log Kd values for iodide ions decreases from 11.1 to 9.6, wh ile for bro mide ions the values decreases from 9.8 to 7.8. Similarly in case of Indion-820, under identical experimental conditions, the log K d values for iodide ions decreases from 7.0 to 5.2, while for bromide ions the values decreases from 4.0 to 2.0. It was also observed that the K d values for iodide ion-isotopic reaction were calcu lated to be higher than that for bromide ion-isotopic reaction (Tables 3 and 4).

Comparati ve Study of Anion Exchange Resins
Fro m the Table 3 and 4, it is observed that for iodide ion-isotopic exchange reaction by using Tulsion A-30 resin, the values of specific reaction rate (min -1 ), amount of iodide ion exchanged (mmo l), init ial rate of iodide ion exchange (mmo l/ min) and log K d were 0.234, 0.311, 0.073 and 10.6 respectively, which was higher than 0.112, 0.221, 0.025 and 6.2 respectively as that obtained by using Indion-820 resins under identical experimental conditions of 35.0℃, 1.000 g of ion exchange resins and 0.002 M labeled iodide ion solution. The identical trend was observed for the two resins during bromide ion-isotopic exchange reaction.
Fro m Table 3, it is observed that using Tulsion A-30 resins, at a constant temperature of 35.0℃, as the concentration of labeled iodide ion solution increases 0.001 M to 0.004 M, the percentage of iodide ions exchanged increases from 59.9 % to 66.3 %. While using Indion-820 resins under identical experimental conditions the percentage of iodide ions exchanged increases fro m 43.0 % to 45.5 %. Similarly in case of bromide ion-isotopic exchange reaction, the percentage of bromide ions exchanged increases from 48.7 %to 53.0 % using Tulsion A-30 resin, while for Indion-820 resin it increases from 35.4 % to 40.1 %. The effect of ionic concentration on percentage of ions exchanged is graphically represented in Figure 2.
Fro m Table 4, it is observed that using Tulsion A-30 resins, for 0.002 M labeled iodide ion solution, as the temperature increases 30.0 ℃ to 45.0℃, the percentage of iodide ions exchanged decreases from 63.3 % to 59.6 %. While using Indion-820 resins under identical experimental conditions the percentage of iodide ions exchanged decreases from 45.0 %to 42.5 %. Similarly in case of bromide ion-isotopic exchange reaction, the percentage of bromide ions exchanged decreases from 52.7 % to 46.7 % using Tulsion A-30 resin, while for Indion-820 resin it decreases from 39.4 %to 33.3 %. The effect of temperature on percentage of ions exchanged is graphically represented in Figure 3.
The overall results indicate that under identical experimental conditions, as compared to Indion-820 resins, Tulsion A-30 resins shows higher percentage of ions exchanged. Thus Tulsion A-30 resins show superior performance than Indion-820 resins under identical operational parameters.

Statistical Correlations
The results of present investigation show a strong positive linear co-relationship between amount of ions exchanged and concentration of ionic solution (Figures 4, 5). In case of iodide ion-isotopic exchange reaction, the values of correlation coefficient (r) were calculated as 0.9996 and 1.0000 for Tu lsion A-30 and Indion-820 resins respectively, while fo r bro mide ion-isotopic exchange reaction, the respective values of r was calcu lated as 0.9999 and 0.9996.
There also e xist a strong negative co-relationship between amount of ions exchanged and temperature of exchanging med iu m (Figures 6, 7). In case of iodide ion-isotopic exchange reactions the values of r calculated for Tulsion A-30 and Indion-820 resins were -0.9992 and -0.9959 respectively. Similarly in case of bro mide ion-isotopic exchange reactions the r values calculated were -0.9854 and -0.9981 respectively for both the resins.

Conclusions
The experimental work carried out in the present investigation will help to standardize the operational process parameters so as to improve the performance o f selected ion exchange resins. The radioactive tracer technique used here can also be applied for characterization of different nuclear as well as non-nuclear grade ion exchange resins.