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Separation of 134Cs and 152Eu using inorganic ion exchangers, zirconium vanadate and ceric vanadate
ARTICLE IN PRESS
Applied Radiation and Isotopes 63 (2005) 293–297 www.elsevier.com/locate/apradiso
Separation of 134Cs and 152Eu using inorganic ion exchangers, zirconium vanadate and ceric vanadate Susanta Lahiria,, Kamalika Roya, Soumya Bhattacharyab, Samir Majic, S. Basuc a
Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India b Indian Institute of Technology, Madras, Chennai 600 036, India c Department of Chemistry, The University of Burdwan, Burdwan 713 104, India Received 19 October 2004; received in revised form 11 January 2005; accepted 10 March 2005
Abstract Two inorganic ion exchangers, zirconium vanadate and ceric vanadate were synthesized and applied to confine and separate 152Eu and 134Cs from a synthetic mixture. The percentages of adsorption of the two radionuclides were studied for the two ion exchangers at varying pH conditions. At pH 3, zirconium vanadate adsorbs both Eu and Cs and a column chromatographic separation was achieved using 0.1 M EDTA as the eluant. The ceric vanadate ion exchanger showed an increased trend in adsorption for both the radionuclides with increase of pH value from 1 to 6. At pH 1, a column chromatographic separation of these radionuclides from a mixture was achieved, because at this pH only 134Cs was adsorbed to ceric vanadate bed in the column. r 2005 Elsevier Ltd. All rights reserved. Keywords: Inorganic ion exchanger; Zirconium vanadate; Ceric vanadate;
1. Introduction Inorganic ion exchangers, due to their resistance to radiation and chemical attack and their compatibility with potential immobilization matrices, find wide applications in industries, including waste treatment, hydrometallurgy, preparation of high purity materials, water purification and several environmental applications. Due to growing environmental awareness, chemists are trying to restrict the use of hazardous compounds or carcinogenic solvents as far as possible. The mandate of green chemistry is to reduce the use of such carcinogenic solvents. Separation with an inorganic ion exchanger does not involve organic solvents and in that way is cleaner than a conventional liquid–liquid Corresponding author.
E-mail address: [email protected] (S. Lahiri).
152
Eu;
134
Cs
extraction (LLX) process. The advantage of solid phase extractants to be used for practical purposes like ‘water purification’ and ‘decontamination’ lies in the fact that the extractant does not require further treatment after taking up the desired element from its matrix. For an effective application, the inorganic ion exchangers are supposed to contain ions that are exchangeable with others present in a solution in which it is considered to be insoluble. During the past few years, a wider application of inorganic ion exchangers in nuclear waste treatment has been investigated for fission and activation product elimination (Bortun et al., 1999). Keeping parity with the modern trends, we have recently synthesized two new inorganic ion exchangers, viz. zirconium vanadate (Roy et al., 2002; Roy et al., 2003) and ceric vanadate (Maji and Basu, 2004). The zirconium vanadate ion exchanger has already proved itself to be a potential adsorber for alkali metals,
0969-8043/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2005.03.007
ARTICLE IN PRESS 294
S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
specially 134,137Cs and to be useful as an exchanger for some heavy elements like Bi, Po, Pb, and Tl. The ceric vanadate ion exchanger has emerged as a potential exchanger for some hazardous species like 90Sr, 137Cs and isotopes of Pb, Tl, Bi, Po, etc. Again, 152,154,155Eu isotopes are produced primarily as fission products. 152Eu can also be produced by neutron activation of nuclear reactor control rods. It is therefore found at certain places, such as reactors and facilities that process spent nuclear fuel (Niese and Gleisberg, 1996). On the other hand radioactive 137Cs is produced in nuclear reactors and nuclear weapons. The largest single source of 137Cs is a fallout from atmospheric nuclear weapons tests in the 1950s and 1960s, which dispersed and deposited 137Cs worldwide. Cesium-137 undergoes radioactive decay with the emission of beta particles and relatively strong gamma radiation. The half-life of 137Cs is 30.17 yr. Because of the chemical nature of cesium, it moves easily through the environment. This makes the clean-up of 137Cs difficult. Among the different methods for decontamination of radioactive cesium, amide-type open chain crown ethers like N,N,N0 ,N0 -tetraphenyl-3,6-dioxaoctanediamide (TDD) were used by Wen et al. (2002). But the methodology involves the use of volatile and carcinogenic organic solvents like nitrobenzene and hazardous chemicals like picric acid. Thus, in the present investigation, an attempt has been made to achieve reasonable separation of the two radionuclides, 152Eu and 134Cs, using the two inorganic exchangers. This might be a cleaner process than the processes based on LLX.
2. Experimental 2.1. Synthesis of zirconium vanadate For the preparation of the zirconium vanadate exchanger, 0.1(M) sodium vanadate solution (100 ml) was added dropwise to a solution of 0.1(M) zirconium oxychloride solution (50 ml) (both Reidel AR) in 2(M) HCl with constant stirring. After the addition was complete, a fine yellow precipitate appeared. The reaction mixture was diluted to 1l and allowed to settle for 24 h. The precipitate was washed thoroughly with deionized water until chloridefree and then dried by gentle heating. 2.2. Synthesis of ceric vanadate For the preparation of the ceric vanadate exchanger, ammonium ceric nitrate and sodium vanadate (both Reidel AR) were used in equimolar concentrations. Sodium vanadate (150 ml) was added drop by drop in a solution of ammonium ceric nitrate (50 ml) in water with
constant stirring. After the addition was complete a fine yellow precipitate appeared. The reaction mixture was diluted to 2l of deionized water and allowed to settle for 24 h. The precipitate was washed thoroughly with deionized water by repeated decantation to make it chloride free and finally dried by gentle heating. 2.3. Characterization of zirconium vanadate The material was characterized by its elemental analysis, thermal, chemical and radiation stabilities, pH titration and ion exchange capacity (IEC) for different metals; results have been discussed thoroughly in our earlier publication (Roy et al., 2002). 2.4. Characterization of ceric vanadate Ceric vanadate was characterized by its elemental analysis, determination of thermal, radiation and chemical stabilities, pH titration and ion exchange capacity (IEC) for different metals. The IEC of the exchanger for sodium was determined by pH titration method; the IEC of other ions was determined by equilibrating nearly 0.1 g of the exchanger with 50 ml of 0.1 M solution of different metal salts. The liberated acid was measured by titration with standard alkali solution. Results are presented in Table 1. For the determination of thermal stability, thermogravimetric analysis was carried out using a thermal analyzer (Metlar Toledo) in dynamic air atmosphere. Radiation stability was measured by determining the ion exchange capacity of the solid before and after irradiation with 60Co gamma ray source at a dose rate of 2.5 Gy/min. For determining the chemical stability, about 0.1 g of the exchanger was dissolved in different solvents. The change in ion exchange capacity was measured after evaporating the solvent. 2.5. Determination of optimum ph for separation of 152Eu and 134Cs by batch extraction The required radioisotopes, 152Eu and 134Cs, were procured from the Board of Radiation and Isotope Technology (BRIT), Trombay, India.
Table 1 Ion exchange capacity of ceric vanadate for different metal ions Na+ K+ Cs+ Mg2+ Ca2+ Sr2+
2.88 meq/g 1.10 meq/g 0.13 meq/g 0.88 meq/g 0.76 meq/g 0.50 meq/g
ARTICLE IN PRESS S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
Different acid solutions of varying pH were prepared from HCl solution using proper dilutions. The pH values were measured using a digital pH meter (Sambros). Because we wanted to obtain the optimum pH conditions for successful separation of 152Eu and 134 Cs from a mixture, 0.20 g portions of ion exchangers were shaken with equal volumes of acid solution of varying pH from 1 to 6 along with 100 ml of radioactive solution containing 152Eu and 134Cs. All the sets were compared with a blank, which was prepared in a same way as above but without the exchanger. The solutions were shaken vigorously for 5 min and then centrifuged. The aqueous phase was taken for gamma spectroscopic analysis. An HPGe detector (EG&G ORTEC) having a resolution of 2 keV at 1.33 MeV was used. 2.6. Column chromatographic separation of 134 Cs
152
Eu and
After getting an idea of KD values of europium and cesium radionuclides in both the exchangers, we performed column chromatography to carry out the separation of the radionuclides from their mixture. 6 g of zirconium vanadate or ceric vanadate was weighed out and preconditioned with dilute HCl at pH 3 or pH 1, respectively. A column of 1 cm internal diameter was packed with this material; the length of the column was 5 cm. 100 ml of active solution was added to the column. The elution drop rate was maintained at 2 drop/min. In the case of zirconium vanadate, separation could be achieved using two different conditions. The first condition was to precondition the column at pH 1 with HCl; then elution of europium was done using the same pH solution. In the second method, both the radionuclides were allowed to get completely adsorbed by the column at pH 3, elution of europium was then carried out with EDTA solution. In case of ceric vanadate, elution was carried out with 0.1 M HCl solution.
3. Results and discussions
295
also stable in organic solvents like benzene, ethanol and cyclohexane. The IEC values of ceric vanadate are indeed very high for alkali metal cations, whereas adsorption was somewhat low for alkaline earth metal cations. It is interesting to note that the IEC of Na (2.88 meq/g) is even higher than that of the well-known amorphous zirconium phosphate (1.88 meq/g). The result shows that the material may be better than that of amorphous zirconium phosphate with respect to its ion exchange capacity. IEC values of the exchangers with different metal ions have been presented in Table 1. The radiation stability of the exchanger was measured by determining the IEC of the exchanger before and after irradiation with 2.5 Gy/min. The exchanger was found to retain its IEC up to a total dose of 7500 Gy, which is quite an appreciable amount of radiation dose. 3.2. Determination of optimum ph for separation of 152Eu and 134Cs for zirconium vanadate It has been observed that 134Cs was adsorbed by the exchanger at all the pH conditions in the acidic range. The best condition was at pH 3, at which 96.56% of Cs was adsorbed by the exchanger, the ion exchange capacity for Cs+ ions at this pH was found to be 0.192 meq/g. Cesium is exchanged as Cs+ ion with the exchangeable H+ ion of the ion exchanger. Hence adsorption for cesium has been observed at all the pH values of acidic range. 152Eu was also adsorbed above pH 1 and the maximum adsorption of 93.62% was observed at pH 3. In lower pH, Eu exists as [Eu(OH2)3]+3 and this hydrated structure does not remain in a more acidic medium. Thus, at pH 1 no adsorption of Eu has been observed (Greenwood and Earnshaw, 1989). With increase in pH, the hydrated species gradually forms and the cationic exchange with the exchangeable H+ ions of the exchanger occurs. This is reflected in the adsorption profile graph of Cs and Eu, which is shown in Fig. 1. The distribution coefficient values (KD) for the two radionuclides for the two exchangers are tabulated in Table 2 and Table 3.
3.1. Characterization of ceric vanadate The exchanger was found to contain 27% Ce and 25.25% V, which nearly corresponds to the ratio Ce:V ¼ 2:5. Thermal analysis shows that 12.25% of the mass was lost at nearly 120 1C, corresponding to 12 molecules of water. There is no further mass loss up to 225 1C. From elemental and thermal analysis, it may be concluded that the formula of the exchanger is 4CeO2, 5V2O5, 12H2O. The exchanger has appreciable stability in different acid, alkali and organic solvents. Ceric vanadate is stable in 2 M of HCl or H2SO4 solution and in 1 M of HNO3, HClO4, or NaOH solution. The exchanger is
3.2.1. Column chromatographic separation of 152Eu and 134 Cs A column preconditioned at pH 1 was loaded with a solution containing the radionuclides 152Eu and 134Cs. As indicated from the results of batch extraction, 152Eu was not at all adsorbed and could be eluted out easily using the same pH 1 solution, while 134Cs was retained completely in the resin bed. The elution curve for 152Eu is represented in Fig. 2. In another set of experiments, the column was preconditioned at pH 3 and loaded with a solution containing the radionuclides 152Eu and 134Cs. At this condition, both the isotopes were completely adsorbed inside the column bed. Eu was then eluted
ARTICLE IN PRESS S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
296
12000
100
Counts
Adsorption, %
80 60 Eu Cs
40
8000
4000
20
0 0
0 0
1
2
4
3
5
6
7
pH 152
Fig. 1. Adsorption profile of Eu and for zirconium vanadate ion exchanger.
5
134
Cs with varying pH
10 15 Volume of 0.1 M HCl
20
152
Fig. 2. Elution profile of zirconium vanadate.
Eu using 0.1 M HCl from
12000 152
Eu and
134
10000
Cs at different pH for zirconium
8000
pH
152
134
1 2 3 4 5 6
0 31.6 73.3 35.7 70.3 52.1
14.5 46.6 140.6 39.7 46.9 67.6
Eu
Cs
Counts
Table 2 KD values of vanadate
6000 4000 2000 0 0
5
10
15
20
Volume of 0.1M EDTA solution (mL)
Fig. 3. Elution profile of from zirconium vanadate. Table 3 KD values of 152Eu and 134Cs at different pH for ceric vanadate pH
152
134
1 2 3 4 5 6
0.01 104.2 104.9 53620 53620 53620
12.8 140.5 134.3 111.5 91.1 143.2
Eu
Cs
preferentially with 0.1 M EDTA at pH 9. Europium, being in +3 oxidation state, the complex formation constant is higher and is eluted as Eu-EDTA complex earlier in preference to a Cs-EDTA complex (Huheey, 1983). The elution profile with 0.1 M EDTA is shown in Fig. 3. The elution of Cs from the column material was found to be very difficult and could be done only after breaking down the column material using 6 M HNO3.
152
Eu using 0.1 M EDTA at pH 9
3.2.2. Determination of optimum pH for separation of Eu and 134Cs for ceric vanadate An increasing trend in the extent of adsorption with pH of the ceric vanadate exchanger was observed in the cases of both Eu and Cs. At pH 1, Cs was adsorbed to certain extent (72%), but there was no adsorption for Eu at this pH. At a higher pH (pH 4–6) Eu was 100% adsorbed, while Cs was adsorbed to an extent of 96%. The trend of Cs and Eu adsorption in the ceric vanadate exchanger is similar to that of the zirconium vanadate exchanger. This is because of the similar physical properties and the same mode of action of these two ion exchangers. The adsorption profile at different pH conditions is shown in Fig. 4.
152
3.2.3. Column chromatographic separation of 152Eu and Cs using ceric vanadate ion exchanger To achieve a condition of separation of the two radionuclides, we preconditioned ceric vanadate at pH 1 and packed it inside a glass column. The column was loaded with a mixture containing 152Eu and 134Cs and 134
ARTICLE IN PRESS S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
from each other using them. Both the exchangers have a strong binding ability with Cs and the metal can be extracted only after decomposing the exchanger. A clean separation between Eu and Cs has been achieved by using the column chromatographic method.
100
Adsorption, %
80 Eu Cs
60
297
40
Acknowledgement 20 0 1
0
2
3 pH
Fig. 4. Adsorption profile of 152Eu and for ceric vanadate ion exchanger.
4
5
6
One of the authors (Samir Maji) sincerely acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, for providing a necessary fellowship.
134
Cs with varying pH
References 10000
Counts
8000 6000 4000 2000 0 0
2
4
6 8 10 12 14 16 18 20 Volume of 0.1 M HCl
Fig. 5. Elution profile of
152
Eu from ceric vanadate.
eluted with the same pH 1 solution of HCl. Eu eluted out easily, while Cs remained inside the column material. The elution profile of Eu is shown in Fig. 5. At higher acid concentrations, the exchanger got destroyed. Cs could be eluted after destruction of the column material.
4. Conclusion The batch extraction data for the two exchangers show that 152Eu and 134Cs can be effectively separated
Bortun, A.I., Khainakov, S.A., Bortun, L.N., Jaimez, E., Garcı´ a, J.R., Clearfield, A., 1999. Synthesis and characterization of a novel layered tin(IV) phosphate with ion exchange properties. Mater. Res. Bull. 34 (6), 921. Greenwood, N.N., Earnshaw, A., 1989. Chemistry of the Elements, Maxwell Macmillan International Editions. Pergamon Press, Oxford. Huheey, J.E., 1983. Inorganic Chemistry, third ed. Harper and Row Publishers, Singapore. Maji, S., Basu, S., 2004. Synthesis of a novel ion exchanger: ceric vanadate and its application to separation of 90Sr-90Y pair. Discussion Meeting on Application of Radiotracers in Chemical, Environmental and Biological Sciences, Saha Institute of Nuclear Physics, Kolkata, April 15–16, p. 30. Niese, S., Gleisberg, B., 1996. Determination of radioisotopes of Ce, Eu, Pu, Am and Cm in low-level wastes from power reactors using low-level measuring techniques. Appl. Radiat. Isot. 47, 1113. Roy, K., Pal, D.K., Basu, S., Nayak, D., Lahiri, S., 2002. Synthesis of a new ion exchanger, zirconium vanadate, and its application to the separation of barium and cesium radionuclides at tracer levels. Appl. Radiat. Isot. 57, 471. Roy, K., Basu, S., Ramaswami, A., Lahiri, S., 2003. Application of tracer packet technique for multielemental uptake studies on the inorganic ion exchanger zirconium vanadate. Appl. Radiat. Isot. 59, 105. Wen, Y.H., Lahiri, S., Quin, Z., Wu, X.L., Liu, W.S., 2002. Decontamination of radioactive cesium from natural NaCl by amide-type open-chain crown ethers. J. Radioanal. Nucl. Chem. 253, 263.
Applied Radiation and Isotopes 63 (2005) 293–297 www.elsevier.com/locate/apradiso
Separation of 134Cs and 152Eu using inorganic ion exchangers, zirconium vanadate and ceric vanadate Susanta Lahiria,, Kamalika Roya, Soumya Bhattacharyab, Samir Majic, S. Basuc a
Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India b Indian Institute of Technology, Madras, Chennai 600 036, India c Department of Chemistry, The University of Burdwan, Burdwan 713 104, India Received 19 October 2004; received in revised form 11 January 2005; accepted 10 March 2005
Abstract Two inorganic ion exchangers, zirconium vanadate and ceric vanadate were synthesized and applied to confine and separate 152Eu and 134Cs from a synthetic mixture. The percentages of adsorption of the two radionuclides were studied for the two ion exchangers at varying pH conditions. At pH 3, zirconium vanadate adsorbs both Eu and Cs and a column chromatographic separation was achieved using 0.1 M EDTA as the eluant. The ceric vanadate ion exchanger showed an increased trend in adsorption for both the radionuclides with increase of pH value from 1 to 6. At pH 1, a column chromatographic separation of these radionuclides from a mixture was achieved, because at this pH only 134Cs was adsorbed to ceric vanadate bed in the column. r 2005 Elsevier Ltd. All rights reserved. Keywords: Inorganic ion exchanger; Zirconium vanadate; Ceric vanadate;
1. Introduction Inorganic ion exchangers, due to their resistance to radiation and chemical attack and their compatibility with potential immobilization matrices, find wide applications in industries, including waste treatment, hydrometallurgy, preparation of high purity materials, water purification and several environmental applications. Due to growing environmental awareness, chemists are trying to restrict the use of hazardous compounds or carcinogenic solvents as far as possible. The mandate of green chemistry is to reduce the use of such carcinogenic solvents. Separation with an inorganic ion exchanger does not involve organic solvents and in that way is cleaner than a conventional liquid–liquid Corresponding author.
E-mail address: [email protected] (S. Lahiri).
152
Eu;
134
Cs
extraction (LLX) process. The advantage of solid phase extractants to be used for practical purposes like ‘water purification’ and ‘decontamination’ lies in the fact that the extractant does not require further treatment after taking up the desired element from its matrix. For an effective application, the inorganic ion exchangers are supposed to contain ions that are exchangeable with others present in a solution in which it is considered to be insoluble. During the past few years, a wider application of inorganic ion exchangers in nuclear waste treatment has been investigated for fission and activation product elimination (Bortun et al., 1999). Keeping parity with the modern trends, we have recently synthesized two new inorganic ion exchangers, viz. zirconium vanadate (Roy et al., 2002; Roy et al., 2003) and ceric vanadate (Maji and Basu, 2004). The zirconium vanadate ion exchanger has already proved itself to be a potential adsorber for alkali metals,
0969-8043/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2005.03.007
ARTICLE IN PRESS 294
S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
specially 134,137Cs and to be useful as an exchanger for some heavy elements like Bi, Po, Pb, and Tl. The ceric vanadate ion exchanger has emerged as a potential exchanger for some hazardous species like 90Sr, 137Cs and isotopes of Pb, Tl, Bi, Po, etc. Again, 152,154,155Eu isotopes are produced primarily as fission products. 152Eu can also be produced by neutron activation of nuclear reactor control rods. It is therefore found at certain places, such as reactors and facilities that process spent nuclear fuel (Niese and Gleisberg, 1996). On the other hand radioactive 137Cs is produced in nuclear reactors and nuclear weapons. The largest single source of 137Cs is a fallout from atmospheric nuclear weapons tests in the 1950s and 1960s, which dispersed and deposited 137Cs worldwide. Cesium-137 undergoes radioactive decay with the emission of beta particles and relatively strong gamma radiation. The half-life of 137Cs is 30.17 yr. Because of the chemical nature of cesium, it moves easily through the environment. This makes the clean-up of 137Cs difficult. Among the different methods for decontamination of radioactive cesium, amide-type open chain crown ethers like N,N,N0 ,N0 -tetraphenyl-3,6-dioxaoctanediamide (TDD) were used by Wen et al. (2002). But the methodology involves the use of volatile and carcinogenic organic solvents like nitrobenzene and hazardous chemicals like picric acid. Thus, in the present investigation, an attempt has been made to achieve reasonable separation of the two radionuclides, 152Eu and 134Cs, using the two inorganic exchangers. This might be a cleaner process than the processes based on LLX.
2. Experimental 2.1. Synthesis of zirconium vanadate For the preparation of the zirconium vanadate exchanger, 0.1(M) sodium vanadate solution (100 ml) was added dropwise to a solution of 0.1(M) zirconium oxychloride solution (50 ml) (both Reidel AR) in 2(M) HCl with constant stirring. After the addition was complete, a fine yellow precipitate appeared. The reaction mixture was diluted to 1l and allowed to settle for 24 h. The precipitate was washed thoroughly with deionized water until chloridefree and then dried by gentle heating. 2.2. Synthesis of ceric vanadate For the preparation of the ceric vanadate exchanger, ammonium ceric nitrate and sodium vanadate (both Reidel AR) were used in equimolar concentrations. Sodium vanadate (150 ml) was added drop by drop in a solution of ammonium ceric nitrate (50 ml) in water with
constant stirring. After the addition was complete a fine yellow precipitate appeared. The reaction mixture was diluted to 2l of deionized water and allowed to settle for 24 h. The precipitate was washed thoroughly with deionized water by repeated decantation to make it chloride free and finally dried by gentle heating. 2.3. Characterization of zirconium vanadate The material was characterized by its elemental analysis, thermal, chemical and radiation stabilities, pH titration and ion exchange capacity (IEC) for different metals; results have been discussed thoroughly in our earlier publication (Roy et al., 2002). 2.4. Characterization of ceric vanadate Ceric vanadate was characterized by its elemental analysis, determination of thermal, radiation and chemical stabilities, pH titration and ion exchange capacity (IEC) for different metals. The IEC of the exchanger for sodium was determined by pH titration method; the IEC of other ions was determined by equilibrating nearly 0.1 g of the exchanger with 50 ml of 0.1 M solution of different metal salts. The liberated acid was measured by titration with standard alkali solution. Results are presented in Table 1. For the determination of thermal stability, thermogravimetric analysis was carried out using a thermal analyzer (Metlar Toledo) in dynamic air atmosphere. Radiation stability was measured by determining the ion exchange capacity of the solid before and after irradiation with 60Co gamma ray source at a dose rate of 2.5 Gy/min. For determining the chemical stability, about 0.1 g of the exchanger was dissolved in different solvents. The change in ion exchange capacity was measured after evaporating the solvent. 2.5. Determination of optimum ph for separation of 152Eu and 134Cs by batch extraction The required radioisotopes, 152Eu and 134Cs, were procured from the Board of Radiation and Isotope Technology (BRIT), Trombay, India.
Table 1 Ion exchange capacity of ceric vanadate for different metal ions Na+ K+ Cs+ Mg2+ Ca2+ Sr2+
2.88 meq/g 1.10 meq/g 0.13 meq/g 0.88 meq/g 0.76 meq/g 0.50 meq/g
ARTICLE IN PRESS S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
Different acid solutions of varying pH were prepared from HCl solution using proper dilutions. The pH values were measured using a digital pH meter (Sambros). Because we wanted to obtain the optimum pH conditions for successful separation of 152Eu and 134 Cs from a mixture, 0.20 g portions of ion exchangers were shaken with equal volumes of acid solution of varying pH from 1 to 6 along with 100 ml of radioactive solution containing 152Eu and 134Cs. All the sets were compared with a blank, which was prepared in a same way as above but without the exchanger. The solutions were shaken vigorously for 5 min and then centrifuged. The aqueous phase was taken for gamma spectroscopic analysis. An HPGe detector (EG&G ORTEC) having a resolution of 2 keV at 1.33 MeV was used. 2.6. Column chromatographic separation of 134 Cs
152
Eu and
After getting an idea of KD values of europium and cesium radionuclides in both the exchangers, we performed column chromatography to carry out the separation of the radionuclides from their mixture. 6 g of zirconium vanadate or ceric vanadate was weighed out and preconditioned with dilute HCl at pH 3 or pH 1, respectively. A column of 1 cm internal diameter was packed with this material; the length of the column was 5 cm. 100 ml of active solution was added to the column. The elution drop rate was maintained at 2 drop/min. In the case of zirconium vanadate, separation could be achieved using two different conditions. The first condition was to precondition the column at pH 1 with HCl; then elution of europium was done using the same pH solution. In the second method, both the radionuclides were allowed to get completely adsorbed by the column at pH 3, elution of europium was then carried out with EDTA solution. In case of ceric vanadate, elution was carried out with 0.1 M HCl solution.
3. Results and discussions
295
also stable in organic solvents like benzene, ethanol and cyclohexane. The IEC values of ceric vanadate are indeed very high for alkali metal cations, whereas adsorption was somewhat low for alkaline earth metal cations. It is interesting to note that the IEC of Na (2.88 meq/g) is even higher than that of the well-known amorphous zirconium phosphate (1.88 meq/g). The result shows that the material may be better than that of amorphous zirconium phosphate with respect to its ion exchange capacity. IEC values of the exchangers with different metal ions have been presented in Table 1. The radiation stability of the exchanger was measured by determining the IEC of the exchanger before and after irradiation with 2.5 Gy/min. The exchanger was found to retain its IEC up to a total dose of 7500 Gy, which is quite an appreciable amount of radiation dose. 3.2. Determination of optimum ph for separation of 152Eu and 134Cs for zirconium vanadate It has been observed that 134Cs was adsorbed by the exchanger at all the pH conditions in the acidic range. The best condition was at pH 3, at which 96.56% of Cs was adsorbed by the exchanger, the ion exchange capacity for Cs+ ions at this pH was found to be 0.192 meq/g. Cesium is exchanged as Cs+ ion with the exchangeable H+ ion of the ion exchanger. Hence adsorption for cesium has been observed at all the pH values of acidic range. 152Eu was also adsorbed above pH 1 and the maximum adsorption of 93.62% was observed at pH 3. In lower pH, Eu exists as [Eu(OH2)3]+3 and this hydrated structure does not remain in a more acidic medium. Thus, at pH 1 no adsorption of Eu has been observed (Greenwood and Earnshaw, 1989). With increase in pH, the hydrated species gradually forms and the cationic exchange with the exchangeable H+ ions of the exchanger occurs. This is reflected in the adsorption profile graph of Cs and Eu, which is shown in Fig. 1. The distribution coefficient values (KD) for the two radionuclides for the two exchangers are tabulated in Table 2 and Table 3.
3.1. Characterization of ceric vanadate The exchanger was found to contain 27% Ce and 25.25% V, which nearly corresponds to the ratio Ce:V ¼ 2:5. Thermal analysis shows that 12.25% of the mass was lost at nearly 120 1C, corresponding to 12 molecules of water. There is no further mass loss up to 225 1C. From elemental and thermal analysis, it may be concluded that the formula of the exchanger is 4CeO2, 5V2O5, 12H2O. The exchanger has appreciable stability in different acid, alkali and organic solvents. Ceric vanadate is stable in 2 M of HCl or H2SO4 solution and in 1 M of HNO3, HClO4, or NaOH solution. The exchanger is
3.2.1. Column chromatographic separation of 152Eu and 134 Cs A column preconditioned at pH 1 was loaded with a solution containing the radionuclides 152Eu and 134Cs. As indicated from the results of batch extraction, 152Eu was not at all adsorbed and could be eluted out easily using the same pH 1 solution, while 134Cs was retained completely in the resin bed. The elution curve for 152Eu is represented in Fig. 2. In another set of experiments, the column was preconditioned at pH 3 and loaded with a solution containing the radionuclides 152Eu and 134Cs. At this condition, both the isotopes were completely adsorbed inside the column bed. Eu was then eluted
ARTICLE IN PRESS S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
296
12000
100
Counts
Adsorption, %
80 60 Eu Cs
40
8000
4000
20
0 0
0 0
1
2
4
3
5
6
7
pH 152
Fig. 1. Adsorption profile of Eu and for zirconium vanadate ion exchanger.
5
134
Cs with varying pH
10 15 Volume of 0.1 M HCl
20
152
Fig. 2. Elution profile of zirconium vanadate.
Eu using 0.1 M HCl from
12000 152
Eu and
134
10000
Cs at different pH for zirconium
8000
pH
152
134
1 2 3 4 5 6
0 31.6 73.3 35.7 70.3 52.1
14.5 46.6 140.6 39.7 46.9 67.6
Eu
Cs
Counts
Table 2 KD values of vanadate
6000 4000 2000 0 0
5
10
15
20
Volume of 0.1M EDTA solution (mL)
Fig. 3. Elution profile of from zirconium vanadate. Table 3 KD values of 152Eu and 134Cs at different pH for ceric vanadate pH
152
134
1 2 3 4 5 6
0.01 104.2 104.9 53620 53620 53620
12.8 140.5 134.3 111.5 91.1 143.2
Eu
Cs
preferentially with 0.1 M EDTA at pH 9. Europium, being in +3 oxidation state, the complex formation constant is higher and is eluted as Eu-EDTA complex earlier in preference to a Cs-EDTA complex (Huheey, 1983). The elution profile with 0.1 M EDTA is shown in Fig. 3. The elution of Cs from the column material was found to be very difficult and could be done only after breaking down the column material using 6 M HNO3.
152
Eu using 0.1 M EDTA at pH 9
3.2.2. Determination of optimum pH for separation of Eu and 134Cs for ceric vanadate An increasing trend in the extent of adsorption with pH of the ceric vanadate exchanger was observed in the cases of both Eu and Cs. At pH 1, Cs was adsorbed to certain extent (72%), but there was no adsorption for Eu at this pH. At a higher pH (pH 4–6) Eu was 100% adsorbed, while Cs was adsorbed to an extent of 96%. The trend of Cs and Eu adsorption in the ceric vanadate exchanger is similar to that of the zirconium vanadate exchanger. This is because of the similar physical properties and the same mode of action of these two ion exchangers. The adsorption profile at different pH conditions is shown in Fig. 4.
152
3.2.3. Column chromatographic separation of 152Eu and Cs using ceric vanadate ion exchanger To achieve a condition of separation of the two radionuclides, we preconditioned ceric vanadate at pH 1 and packed it inside a glass column. The column was loaded with a mixture containing 152Eu and 134Cs and 134
ARTICLE IN PRESS S. Lahiri et al. / Applied Radiation and Isotopes 63 (2005) 293–297
from each other using them. Both the exchangers have a strong binding ability with Cs and the metal can be extracted only after decomposing the exchanger. A clean separation between Eu and Cs has been achieved by using the column chromatographic method.
100
Adsorption, %
80 Eu Cs
60
297
40
Acknowledgement 20 0 1
0
2
3 pH
Fig. 4. Adsorption profile of 152Eu and for ceric vanadate ion exchanger.
4
5
6
One of the authors (Samir Maji) sincerely acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, for providing a necessary fellowship.
134
Cs with varying pH
References 10000
Counts
8000 6000 4000 2000 0 0
2
4
6 8 10 12 14 16 18 20 Volume of 0.1 M HCl
Fig. 5. Elution profile of
152
Eu from ceric vanadate.
eluted with the same pH 1 solution of HCl. Eu eluted out easily, while Cs remained inside the column material. The elution profile of Eu is shown in Fig. 5. At higher acid concentrations, the exchanger got destroyed. Cs could be eluted after destruction of the column material.
4. Conclusion The batch extraction data for the two exchangers show that 152Eu and 134Cs can be effectively separated
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