Radiotracer Technique in Study of Strongly Basic Anion Exchange Resins Dowex-SBR LC and Indion-454

The present paper demonstrates application of isotopic tracer technique in characterization of anion exchange resins Dowex-SBR LC and Indion-454 for which 131 I and 82 Br radio isotopes were used. The characterization was made based on iodide and bromide ion-isotopic exchange reaction kinetic data obtained for the two resins. It was observed that during iodide ion-isotopic exchange reaction performed at 35.0 °C, 1.000 g of ion exchange resins and 0.002 mol/L labeled iodide ion solution, the values of specific reaction rate (min -1 ), amount of ion exchanged (mmol), initial rate of ion exchange (mmol/min) and log K d were 0.379, 0.426, 0.161 and 16.2 respectively for Dowex-SBR LC resin, which was higher than the respective values of 0.156, 0.243, 0.038 and 13.4 as that obtained by using Indion-454 resins. The identical trend was observed for the two resins during bromide ion-isotopic exchange reaction. The results of present investigation also indicate that during the two ion-isotopic exchange reactions, for both the resins, there exists a strong positive linear correlation between amount of ions exchanged and concentration of ionic solution; and strong negative correlation between amount of ions exchanged and temperature of exchanging medium. Based on overall results it appears that under identical experimental conditions, as compared to Indion-454 resins, Dowex-SBR LC resins show superior performance. It is expected here that the present technique can be extended further for characterization of different ion exchange resins which will further help in the selection of those reins for the specific industrial application.


INTRODUCTION
Industrial applications of radioisotopes can be mainly categorized into two. The first one being the use of radiation from sealed sources of radioisotopes or from electron beam accelerators for industrial processing, non-destructive testing. The second major group of applications is the use of radiotracers in inventory control, study of process parameters, trouble shooting in industrial systems, flow measurements, leakage studies etc. Industrial applications of radioisotopes ensure good quality products and bring down the cost of manufacture by ways of sensitive non-destructive testing and efficient in-process control. 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  LTD, 2013 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 investigated by means of radiotracers [2]. Radioisotopes are also employed in certain manufacturing processes to induce desired chemical reactions [1]. Thus radioisotopes have become useful tool and almost every branch of industry uses them [1]. Radioisotope tracers offer several advantages such as high detection sensitivity, capability of in-situ detection, limited memory effects and physic-chemical compatibility with the material under study. Unlike many other advanced technologies, isotope applications have a very low gestation period. A technology developed in the laboratory is often successfully applied in the field within a very short time span. Generally the radioisotopes in suitable physical and chemical forms are introduced in systems under study. By monitoring 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. This make isotope technology one of the highly visible peaceful uses of nuclear energy and a major reason for continuing with nuclear research. Radiotracer methodology is described extensively in the literature [2][3][4][5][6][7][8][9]. Considering the extensive technological application of radioactive tracers, in the present investigation, attempts are made to assess the performance of Dowex-SBR LC (nuclear grade) and Indion-454 (non-nuclear grade) ion exchange resins under different experimental conditions by application of tracer technique.

1. Conditioning of ion exchange resins
Dowex-SBR LC is a strongly basic, quaternary ammonium Type I nuclear grade anion exchange resins in hydroxide form (by Dow Chemical Company, Midland, Michigan) while Indion-454 is a strongly basic anion exchange resin in chloride form (by Ion Exchange India Ltd., Mumbai). Details regarding the properties of the resins used are given in Table 1. These resins were converted separately in to iodide / bromide 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 from iodide/bromide 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.

Radioactive Tracer Isotopes
The radioisotope 131 I and 82 Br used in the present experimental work was obtained from Board of Radiation and Isotope Technology (BRIT), Mumbai, India. Details regarding the isotopes used in the present experimental work are given in Table 2.

3. Study on kinetics of iodide ion-isotopic exchange reaction
In a stoppered bottle 250 mL (V) of 0.001 mol/L 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 cpm (counts per minute) when measured with γ-ray spectrometer having NaI (Tl) scintillation 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 titration 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 exactly 1.000 g (m) were transferred quickly 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: 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 [19][20][21].
The final activity (A f ) of the solution was also measured after 3h which was sufficient time to attain the equilibrium [10][11][12][13][14].
The activity measured at various time intervals was corrected for background counts. Similar experiments were carried out by equilibrating 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 mol/L at a constant temperature of 30.0 °C.
The same experimental sets were repeated for higher temperatures up to 45.0 °C.
International Letters of Chemistry, Physics and Astronomy Vol. 18

4. Study on kinetics of bromide 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 bromide ion solution was done by using 82 Br as a radioactive tracer isotope for which the same procedure as explained above was followed. The bromide ion-isotopic exchange reaction can be represented as: here R-Br represents ion exchange resin in bromide form; Br*ˉ( aq.) represents aqueous bromide ion solution labeled with 82 Br radiotracer isotope [19][20][21].

1. Comparative study of ion-isotopic exchange reactions
In the present investigation it was observed that due to the rapid ion-isotopic exchange reaction taking place, 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 [10][11][12][13][14]. Therefore logarithm of activity when plotted against time gives a composite 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 simultaneously [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 process also. The activity due to slow process was subtracted from the total activity at various time intervals. The difference gives the activity due to rapid process only. From the activity exchanged due to rapid process at various time intervals, the specific reaction rates (k) of rapid ion-isotopic exchange reaction were calculated. The amount of iodide / bromide ions exchanged (mmol) on the resin were obtained from the initial and final activity of solution and the amount of exchangeable ions in 250 mL of solution. From the amount of ions exchanged on the resin (mmol) and the specific reaction rates (min -1 ), the initial rate of ion exchanged (mmol/min) was calculated.
Because of larger solvated size of bromide ions as compared to that of iodide ions, it was observed that the exchange of bromide ions occurs at the slower rate than that of iodide ions. Hence under identical experimental conditions, the values of specific reaction rate (min -1 ), amount of ion exchanged (mmol) and initial rate of ion exchange (mmol/min) are calculated to be lower for bromide ion-isotopic exchange reaction than that for iodide ionisotopic 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 reaction rate increases with increase in the concentration of iodide and bromide ions in solution from 0.001 mol/L to 0.004 mol/L (Table 3). However, with rise in temperature from 30.0 °C to 45.0 °C, the specific reaction rate was observed to decrease (Table 4). Thus in case of Dowex-SBR LC at 35.0 °C when the concentration of iodide and bromide ions in solution increases from 0.001 mol/L to 0.004 mol/L, the specific reaction rate values for iodide ion-isotopic exchange increases from 0.368 to 0.400 min -1 , while for bromide ionisotopic exchange the values increases from 0.320 to 0.355 min -1 . Similarly in case of Indion-454, under identical experimental conditions, the values for iodide ion-isotopic exchange increases from 0.145 to 0.182 min -1 , while for bromide ion-isotopic exchange the values increases from 0.125 to 0.156 min -1 . However when the concentration of iodide and bromide ions in solution is kept constant at 0.002 mol/L and temperature is raised from 30.0 °C to 45.0 °C, in case of Dowex-SBR LC the specific reaction rate values for iodide ion-isotopic exchange decreases from 0.389 to 0.360 min -1 , while for bromide ion-isotopic exchange the values decreases from 0.342 to 0.309 min -1 . Similarly in case of Indion-454, under identical experimental conditions, the specific reaction rate values for iodide ion-isotopic exchange decreases from 0.168 to 0.140 min -1 , while for bromide ion-isotopic exchange the values decreases from 0.143 to 0.121 min -1 . From the results, it appears that iodide ions exchange at the faster rate as compared to that of bromide ions which was related to the extent of solvation (Tables 3 and 4).
From 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

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ILCPA Volume 18 Previous studies [15,16] on halide ion distribution coefficient on strong and weak basic anion exchange resins indicate that the selectivity coefficient between halide ions increased at higher electrolyte concentrations. Adachi et al. [17] 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. [18]; 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 exchange reaction were calculated to be higher than that for bromide ion-isotopic exchange reaction (Tables 3 and 4).

2. Comparative study of anion exchange resins
From the Table 3 and 4, it is observed that for iodide ion-isotopic exchange reaction by using Dowex-SBR LC resin, the values of specific reaction rate (min -1 ), amount of iodide ion exchanged (mmol), initial rate of iodide ion exchange (mmol/min) and log K d were 0.379, 0.426, 0.161 and 16.2 respectively, which was higher than 0.156, 0.243, 0.038 and 13.4 respectively as that obtained by using Indion-454 resins under identical experimental conditions of 35.0 °C, 1.000 g of ion exchange resins and 0.002 mol/L labeled iodide ion solution. The identical trend was observed for the two resins during bromide ion-isotopic exchange reaction.
From Table 3, it is observed that at a constant temperature of 35.0 °C, as the concentration of labeled iodide ion solution increases from 0.001 mol/L to 0.004 mol/L, the percentage of iodide ions exchanged increases from 83.32 % to 88.44 % using Dowex-SBR LC resins and from 45.80 % to 51.56 % using Indion-454 resins. Similarly in case of bromide ion-isotopic exchange reactions under identical experimental conditions, the percentage of bromide ions exchanged increases from 75.64 % to 81.19 % using Dowex-SBR LC resin and from 39.90 % to 48.60 % using Indion-454 resin. The effect of ionic concentration on percentage of ions exchanged is graphically represented in Figure 2.
From Table 4, it is observed that for 0.002 mol/L labeled iodide ion solution, as the temperature increases from 30.0 °C to 45.0 °C, the percentage of iodide ions exchanged decreases from 86.70 % to 82.04 % using Dowex-SBR LC resins and from 50.60 % to 44.28 % using Indion-454 resins. Similarly under identical experimental conditions, in case of bromide ion-isotopic exchange reactions, the percentage of bromide ions exchanged decreases from 79.16 % to 73.90 % using Dowex-SBR LC resin and from 45.20 % to 38.70 % using Indion-454 resin. 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-454 resins, Dowex-SBR LC resins shows higher percentage of ions exchanged. Thus Dowex-SBR LC resins show superior performance over Indion-454 resins under identical operational parameters.

Statistical Correlations
The results of present investigation show a strong positive linear correlation 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) was calculated as 0.9999 for both the resins, while for bromide ion-isotopic exchange reaction, the values of r was calculated as 0.9998 and 0.9987 for Dowex-SBR LC and Indion-454 resins respectively. There also exist a strong negative correlation between amount of ions exchanged and temperature of exchanging medium (Figures 6, 7). In case of iodide ion-isotopic exchange reactions the values of r calculated for Dowex-SBR LC and Indion-454 resins were -0.9989 and -0.9968 respectively. Similarly in case of bromide ion-isotopic exchange reactions the r values calculated were -0.9996 and -0.9963 respectively for the two resins.

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