РАДИАЦИОННАЯ ХИМИЯ =
УДК 541.15
STUDY OF ION-ISOTOPIC EXCHANGE REACTION KINETICS AND THERMODYNAMICS BY APPLICATION OF SHORT LIVED RADIOISOTOPES © 2014 P. U. Singare
Department of Chemistry, Bhavan's College, Munshi Nagar, Andheri (West) Mumbai, 400058 India E-mail: pravinsingare@gmail.com Поступила в редакцию 08.05.2014 г. В окончательном виде 22.05.2014 г.
In case of Duolite A-143 resins during iodide ion isotopic exchange reaction, the enthalpy and energy of activation values calculated were 6082.33 and 8663.83 J mol-1, respectively, which were less than the respective values of 6478.20 and 9059.70 J mol-1 obtained during bromide ion-isotopic exchange reaction. The identical trend was observed for Purolite NRW-4000 resins during the two ion-isotopic exchange reactions. The thermodynamic data suggest that iodide ion-isotopic exchange reaction was more favorable than bromide ion-isotopic exchange reaction by using both the resins. This was further supported from the fact that for both the resins iodide ion-isotopic exchange reaction rates calculated were higher than bromide ion-isotopic exchange reaction under identical experimental conditions. In case of Purolite NRW-4000 resins during the bromide ion-isotopic exchange reaction, the enthalpy and energy of activation values calculated were 3784.81 and 6366.30 J mol-1, respectively, which were less than the respective values of 6478.20 and 9059.70 J mol-1 obtained for the same reaction using Duolite A-143 resins. The identical trend was observed for the two resins during the iodide ion-isotopic exchange reaction. The thermodynamic data suggest that Purolite NRW-4000 resins were more efficient than Duolite A-143. This was further confirmed from the fact that under identical experimental conditions higher amount of ions were exchanged using Purolite NRW-4000 resins as compared to those using Duolite A-143 resins. The overall results of the present investigation reveal superior performance of Purolite NRW-4000 over Duolite A-143 resins under identical experimental conditions.
DOI: 10.7868/S0023119714060131
In many cases ion exchange is the most appropriate and the most efficient method for the treatment of a variety of low and intermediate level liquid waste streams. With respect to economy and efficacy, ion exchange stands between the other two major liquid waste treatment processes of chemical precipitation and evaporation. Chemical precipitation is often less expensive but is not always effective in removing radionuclides from solution [1]. While evaporation may yield higher decontamination factors, it is also more costly than ion exchange. The development of new ion exchangers is narrowing the gap in decontamination factors between evaporation and ion exchange. Ion exchangers containing sulpho- and phospho-acidic groups and those containing tetra-ammonium basic groups are strong acidic and strong basic exchangers, respectively, whereas those containing phenolic and primary amino groups are weak acidic and weak basic exchangers, respectively. Exchangers with carboxy groups and tertiary amino groups take a medium position between strong and weak acidic and basic exchangers, respectively. Efforts to develop new organic ion exchangers for their specific industrial applications are continuing [2, 3] and various aspects of ion exchange technologies have been continuously stud-
ied to improve the efficiency and economy of their application in various technological applications [4-7]. However, for proper selection of ion exchange resin, it is essential to have adequate knowledge regarding their physical and chemical properties, which forms the complementary part of resin characterization study. 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.
Although number of techniques are available for the characterization of ion exchange resins [8—11], but the radiotracer technique offer several advantages such as high detection sensitivity, capability of in situ detection, limited memory effects and physico-chemical compatibility with the material under study [12, 13]. Considering the extensive technological application of radioactive tracers, in the present investigation, attempts are made to apply the same technique to study the kinetics of ion-isotopic exchange reactions in Purolite NRW-4000 (nuclear grade) and Duolite A-143 (non-nuclear grade) anion exchange resins. It is expected that the kinetics data obtained here will not only be used in characterization of these
Table 1. Properties of ion exchange resins
Ion exchange resin Matrix Functional group Mean particle size (mm) Moisture content (%) Operating pH Maximum operating temperature (°C) Total exchange capacity (mEq/mL)
Purolite Polystyrene crosslinked -N+R3 0.57 55 0-14 60 1.0
NRW-4000 with divinyl benzene
Duolite A-143 Crosslinked polystyrene -N+R3 0.75 65 0-14 100 1.0
Table 2. Properties of 131I and 82Br tracer isotopes [12]
Isotopes Half-life Radioactivity/mCi y-Energy/MeV Chemicalform Physical form
131I 8.04 d 5 0.36 Iodide* Aqueous
82Br 36 h 5 0.55 Bromide** Aqueous
* Sodium iodide in dilute sodium sulphite. ** Ammonium bromide in dilute ammonium hydroxide.
resins but also in standardization of the process parameters for their efficient application.
EXPERIMENTAL
Conditioning of ion exchange resins. Purolite NRW-4000 is a nuclear grade Type I porous gel strong base anion exchange resin in hydroxide form (supplied by Purolite International India Private Limited, Pune, India) while Duolite A-143 is a gel strongly basic anion exchange resin in chloride form (supplied by Auchtel Products Ltd., Mumbai, India). 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 AgNO3 solution. These resins in bromide and iodide form were then dried separately over P2O5 in desiccators at room temperature.
Radioactive tracer isotopes. The radioisotope 131I and 82Br 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.
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 131I 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 y-ray spectrometer having NaI (Tl) scintillation detector. Since only about 50—100 ^L 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 AgNO3 solution. The above labeled solution of known initial activity (A) 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 of1.0 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*q.) ^ R-I* + I-aq.). (1)
Here R-I represents ion exchange resin in iodide
form; I*q.) represents aqueous iodide ion solution labeled with 131I radiotracer isotope.
The activity of solution was measured at a fixed interval of every 2.0 min. The final activity (Af) of the solution was also measured after 3 h which was sufficient time to attain the equilibrium [14-18]. 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.
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 82Br 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:
R-Br + Br*-.) ^ R-Br* + BW (2)
Here R-Br represents ion exchange resin in bromide
form; Br*q.) represents aqueous bromide ion solution labeled with 82Br radiotracer isotope.
RESULTS AND DISCUSSION
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 [14-18]. 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 (Fig. 1), evidently rapid and slow ion-iso-topic exchange reactions were occurring simultaneously [14-18]. Now the straight line was extrapolated bac
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