Comparative Study of Indion-860 and Indion GS-400 Anion Exchange Resins by Application of 131 I and 82 Br as a Tracer Isotopes

The present investigation deals with the application of radioactive tracer isotopes I and Br for the performance assessment of two widely used industrial grade ion exchange resins Indion-860 and Indion GS-400. The study reveals that for both the resins, iodide ion-isotopic exchange reaction take place at the faster rate as compared to bromide exchange reaction. Also for both the resins the amount of ions exchanged increases with increase in ionic concentration and decreases with rise in temperature. The above resins were characterized based on kinetics of ion-isotopic exchange reactions. It was observed that the values of specific reaction rate, initial rate of exchange and distribution coefficient values calculated for Indion GS-400 was higher than that calculated for Indion-860 under identical experimental conditions, indicat ing the superior performance of Indion GS-400 over Indion-860.


Introduction
Rad ioactive tracers were firs t ap p lied to ind ustrial problem solving around the middle o f the last century. Since then their use has increased steadily so that , at p resent, radiotracer techniques are used extensively throughout the wo rld fo r t roub leshoot ing an d p ro cess op t imizat ion in industry. The economic benefits that may be derived from the use of this technology are great, a fact that is recognized by the govern men ts o f dev elo p ing cou nt ries. Tho ugh radioisotopes have been applied to the solution of problems in industry for over 50 years, research and development of the techno logy cont inues unabated . There are t wo main 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, treat ment and analysis, and mathemat ical modeling. The radioisotopes have proved as a tool to study many proble ms in chemical, biological 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 [1]. The efficiency of several devices in a wastewater treatment plant (primary and secondary clarifiers, aeration tank) is also investigated by means of radiotracers [2].
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 main 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 [1]. In radiotracer 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, 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 behavior 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.
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 Indion-860 and Indion GS-400 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 synthesized for their specific technical applications [3][4][5][6][7]. 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. Ion exchange resin Indion-860 is a weekly basic macroporous resin having -N + R 2 functional group; while Indion GS-400 is a gel type strongly basic anion exchange resin having -N + R 3 functional group. Both the resins as supplied by the manufacturer (Ion exchange India Ltd., Mumbai) were in chloride fo rm. Details regard ing the properties of the resins used are g iven in Table 1. These resins were converted separately in to iodide / bro mide form by treatment 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 distilled water, until the washings were free fro m iodide/bro mide ions as tested by AgNO 3 solution. These resins in bromide and iodide 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 mbai. 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 micro 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℃. 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: R-I + I* - Here R-I represents ion exchange resin in iodide form; I* -(aq.) represents aqueous iodide ion solution labeled with 131 I radiotracer isotope.
The activity of solution was measured at a fixed interval of every 2.0 min. The final act ivity (A f ) of the solution was also measured after 3h wh ich was sufficient time to attain the equilibriu m [9][10][11][12][13][14]. The activity measured at various time intervals was corrected for background counts.
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℃. The same experimental sets were repeated for higher temperatures up to 45.0℃.

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* - 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 remains nearly constant. Preliminary studies show that the above exchange reactions are of first order [15,16]. 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 [9][10][11][12][13][14]. Now the straight line was extrapolated back to zero 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 o f bro mide ions (310 p m) as compared to that of iodide ions (300 p m), it was observed that the exchange of bro mide ions occurs at the slower rate than that of iodide ions [17]. Hence under identical experimental conditions, the values of specific reaction rate (min -1 ), amount of ion exchanged (mmo l) and init ial rate of ion exchange (mmo l/ min) are calcu lated to be lower for bromide 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). Ho wever, with rise in temperature fro m 30.0℃ to 45.0℃, the specific reaction rate was observed to decrease (Table 4). Fro m the results, it appears that iodide ions exchange at the faster rate as co mpared to that of bro mide ions which was related to the extent 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. [18] 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. [19] 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. [20]; 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. It was also observed that the K d values for iodide ion-isotopic reaction were calculated to be higher than that for bro mide ion-isotopic reaction (Tables 3 and 4).

Comparati ve Study of Anion Exchange Resins
Fro m the Table 3, it is observed that for iodide ion-isotopic exchange reaction by using Indion GS-400 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 0.199, 0.132, 0.026 and 17.4 respectively, which was higher than 0.183, 0.129, 0.024 and 17.0 respectively as that obtained by using Indion-860 resins under identical experimental conditions of 30.0℃, 1.000 g of ion exchange resins and 0.001 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 Indion GS-400 resins, at a constant temperature o f 30.0 ℃ , as the concentration of labeled iodide ion solution increases 0.001 M to 0.004 M , the percentage of iodide ions exchanged increases from 52.7 % to 63.4 %. While using Indion-860 resins under identical experimental conditions the percentage of iodide ions exchanged increases from 51.6 % to 55.8 %. Similarly in case of bromide ion-isotopic exchange reaction, the percentage of bromide ions exchanged increases from 48.2 % to 52.4 % using Indion GS-400 resin, wh ile for Indion-860 resin it increases from 41.4 % to 47.1 %. 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 Indion GS-400 resins, for 0.001 M labeled iodide ion solution, as the temperature increases 30.0℃to 45.0℃ , the percentage of iodide ions exchanged decreases fro m 52.7 % to 52.4 %. While using Indion-860 resins under identical experimental conditions the percentage of iodide ions exchanged decreases from 51.6 % to 48.3 %. Similarly in case of bromide ion-isotopic exchange reaction, the percentage of bromide ions exchanged decreases from 48.2 % to 46.1% using Indion GS-400 resin, wh ile for Indion-860 resin it decreases fro m 41.4 % to 32.4 %. 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-860 resins, Indion GS-400 resins shows higher percentage of ions exchanged. Thus Indion GS-400 resins show superior performance than Indion-860 resins.

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 using Indion GS-400 and Indion-860 resins, the values of correlation coefficient (r) were found to be 0.9996 and 0.9987 respectively, while for bromide ion-isotopic exchange the respective values of r were calculated as 1.0000 and 0.9994.
There also exist a negative co-relat ionship between amount of ions exchanged and temperature of exchanging med iu m (Figures 6, 7). Fo r Indion GS-400 during iodide isotopic exchange reaction somewhat poor co rrelationship was observed having value of (r) -0.7746, however for Indion-860 resins, excellent correlat ionship exist having (r) value of -0.9778. Similarly in case of bro mide ion-isotopic exchange the (r) values calcu lated for Indion GS-400 and Indion-860 were -0.9898 and -0.9978 respectively.

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.