ISSN 1066-3622, Radiochemistry, 2014, Vol. 56, No. 2, pp. 177–182. © Pleiades Publishing, Inc., 2014. Original Russian Text © S.A. Kulyukhin, L.V. Mizina, M.P. Gorbacheva, I.A. Rumer, V.A. Lomovskoi, E.I. Saunin, 2014, published in Radiokhimiya, 2014, Vol. 56, No. 2, pp. 151–155.

Removal of 131I and 137Cs from Aqueous and Aqueous-Organic Solutions with Porous Polyvinyl Formal S. A. Kulyukhin*, L. V. Mizina, M. P. Gorbacheva, I. A. Rumer, V. A. Lomovskoi, and E. I. Saunin Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, block 4, Moscow, 119071 Russia; * e-mail: [email protected] Received April 16, 2013

Abstract—The removal of 131I and 137Сs from aqueous and aqueous-organic solutions with porous polyvinyl formal (PPVF) was studied. The degree of radionuclide removal from water with a PPVF sample is determined by the amount of water absorbed by the polymer and is not influenced by the concentration of salts in solution. Porous polyvinyl formal allows recovering 137Сs from aqueous-organic solutions with 70 to 99% efficiency. Keywords: iodine-131, cesium-137, removal from solutions, porous polyvinyl formal DOI: 10.1134/S106636221402009X

Operation of nuclear fuel cycle enterprises is accompanied by accumulation of a large amount of organic wastes containing various radionuclides, including those of cesium and iodine. When treating such wastes, it is often necessary to remove not only radionuclides but also traces of water. The majority of radionuclides often occur in the aqueous fraction of these mixed solutions. Various methods are used today for removing iodine and cesium radionuclides from liquid radioactive waste (LRW) [1–3]. Unfortunately, the methods used do not allow simultaneous removal of water from organic LRW, which complicates reuse of organic solvents.

lution. Owing to the developed porous structure and hydrophilicity, PPVF exhibits high water-absorbing capacity (up to 98 vol % and 5000 wt %). In the wet state, it becomes soft and elastic, but on drying its initial shape is restored. PPVF rapidly absorbs and retains water, but water can be removed by squeezing. The material exhibits good chemical resistance, especially to organic solvents and alkaline solutions; it is insoluble in water and hydrocarbons. The presence of acetal, hydroxyl, and acetyl groups creates prerequisites for the localization of cations, anions, and neutral molecules via formation of various coordination compounds.

The reaction of polyvinyl alcohols with aldehydes yields polyvinyl acetals [4–6]. They contain acetal, hydroxyl, and acetyl groups in their structure.

Therefore, it was interesting to examine the possibility of using PPVF for the removal of 131I and 137Сs from both aqueous and aqueous-organic solutions. That was the goal of our study. EXPERIMENTAL 131

I and 137Сs were supplied by Izotop Public JointStock Company in the form of carrier-free Na131I and 137 СsNO3 solutions. The radioactivity of the nuclides was measured by γ-ray spectrometry with a semiconductor Ge–Li detector coupled with a multichannel pulse analyzer. The measurement accuracy was ±5%.

Polyvinyl acetal

One of representatives of this class of compounds is polyvinyl formal, from which lint-free porous polyvinyl formal (PPVF) is produced [7]. PPVF is a gasfilled polymeric material having three-dimensional open cellular structure and high hydrophilicity of the polymeric base. It is prepared by acetalization of polyvinyl alcohol with formaldehyde in acidic aqueous so-

Because 137Cs and 131I were used in this study as radioactive tracers for weighable amounts of I2, KI, 177

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KIO3, and CsNO3, the designations 131I2, K131I, K131IO3, and 137CsNO3 refer to labeled compounds and not to compounds of pure 137Cs and 131I radionuclides. We used 10–5–10–3 M radioactive solutions of 131I2, K131I, K131IO3, and 137CsNO3 with the 131I and 137Сs specific activity of 2 × 104 to 3 × 105 Bq mL–1. All the salts, alkalis, and acids used in the study were of chemically pure grade. The organic solvents (dodecane, toluene, hexane, ethanol) were of chemically pure grade and were used without additional purification. Polyvinyl formal was prepared by the reaction of a concentrated solution of polyvinyl alcohol with protonated formaldehyde in the presence of concentrated H2SO4 as catalyst:

The acetalization in the presence of acid catalysts mainly leads to intramolecular acetalization of 1,3-glycol groups with the formation of strong six-membered acetal rings. Thus, acetalization yielded concentrated metastable polymer solutions, from which droplets of dilute polymer solutions separated out with the subsequent formation of cellular structures of second kind. Experiments on PPVF synthesis were performed in the presence of a pore-forming agent, starch paste. The pore structure of the materials obtained includes three types of pores: macropores from 10 to 100 μm in diameter, mesopores (0.1–10 μm), and micropores (1– 100 nm). The macropore walls are pierced with mesoand micropores. Preliminarily we determined the PPVF capacity for water with sample squeezing and without it. An initially dry PPVF sample (0.157 g) was placed in H2O for 120 min. Then the sample was withdrawn, excess water was allowed to drain, and the sample was weighed. The wet sample weight was 1.865 g, i.e., the sample absorbed 1.708 mL of H2O. The calculated capacity of the material for water is 10.88 g of H2O per gram of PPVF. After that, the wet sample (1.865 g) was squeezed to dryness using filter paper and then dried in air at 25°С. After squeezing with filter paper the sample weight was 0.217 g, and after drying in air,

0.150 g. The 0.007-g weight loss of the dry sample relative to the initial sample may be due to partial washout of starch from PPVF. The calculated capacity of the squeezed material was 0.38 g of H2O per gram of PPVF. Batch experiments on the removal of 131I and 137Сs from aqueous solutions containing 131I2, K131I, K131IO3, and 137CsNO3 with PPVF were performed as follows. A polymer sample was added to an aqueous solution containing 131I and 137Сs in various chemical forms. The S : L ratio was varied from 1 : 50 to 1 : 200. The system was allowed to stand for 2 h, after which the polymer sample was separated from the solution and excess solution was allowed to freely drain. The mother liquor was combined with the solution that freely drained from the PPVF sample. In some experiments, the polymer sample was squeezed, and the squeezed solution was combined with the mother liquor. The polymer sample and the combined solution were transferred into vessels for measuring the nuclide activity by γ-ray spectrometry. Batch experiments on the removal of 137Сs from aqueous-organic solutions with PPVF were performed as follows. First we prepared radioactive aqueousorganic solutions containing 137Cs. In the first procedure, we first prepared an aqueous-organic solution and then added 137Cs (hereinafter, solution no. 1). In the second procedure, 137Cs was first added into water, and then the radioactive water was mixed with organic solvents (hereinafter, solution no. 2). The water concentration in organic solvents was varied from 0.1 to 10.0 vol %. The radioactive solution obtained was thoroughly mixed, and a polymer sample was added. The S : L ratio was varied in the range from 1 : 20 to 1 : 50. The system was allowed to stand for 3 h, after which the polymer sample was separated from the solution. The polymer without squeezing and the mother liquor were transferred into vessels for measuring the nuclide activity by γ-ray spectrometry. In addition, we performed batch experiments on I2 localization from aqueous and alcoholic solutions onto PPVF. For this purpose, PPVF samples were placed in an I2 solution, kept there for a definite time, withdrawn, and subjected to physicochemical studies. RESULTS AND DISCUSSION Figure 1 illustrates the kinetics of the removal of I , IO3–, CH3131I, and 131I2 from the corresponding aqueous solutions with PPVF at 25°С. In these experi131 – 131

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ments, PPVF samples were washed two times with an aqueous solution of the corresponding compounds without 131I. As can be seen, 131I is not noticeably removed from aqueous solutions of KI, KIO3, and CH3I. The degree of the 131I removal did not exceed 5%. For an aqueous solution of I2, the degree of removal of 131I was about 16%, and the PPVF sample became deep blue. At the contact time of PPVF and aqueous salt solution of 60 min and V/m = 200 mL g–1, the calculated values of Kd for 131I are ~0.2 for KI, ~7.2 for KIO3, ~0.6 for CH3I, and ~30.6 for I2. Thus, the PPVF performance in 131I removal from aqueous solutions is low. On the other hand, radionuclides can be removed from aqueous solutions not only via sorption, but also via water absorption by PPVF. To study this problem, we performed experiments on the removal of 131I and 137Cs from an aqueous KI solution with PPVF with and without squeezing the polymer sample. Data on the removal of 131I and 137Cs from 10–3 and 10–5 M aqueous KI solutions are given in Table 1. As seen from Table 1, the unsqueezed PPVF sample bore 10–14% of each radionuclide. As the PPVF capacity for water is 10.88 g of H2O per gram of PPVF, the samples should have absorbed 1.6–1.7 mL of water, i.e., 16–17% of the total solution volume taken in the experiment. Thus, the calculated radionuclide content of the unsqueezed PPVF sample virtually corresponds to the amount of the absorbed water. The radionuclide uptake by PPVF is not influenced by the carrier concentration in the solution. On the other hand, as seen from Table 1, 2–4% of the radionuclides remained in the material after squeezing. The water capacity of the squeezed PPVF sample is 0.38 g of H2O per gram of PPVF; therefore, the samples should have absorbed 0.06–0.07 mL of water, i.e., 0.6–0.7% of the total solution volume taken in the experiment. Thus, the radionuclide content in the squeezed PPVF sample is higher than the water content. Hence it follows that a minor fraction of radionuclides (about 1.5–3.5%) is sorbed onto PPVF. In this connection, we performed similar experiments but at lower L : S ratio. With a decrease in L : S from ~60 to ~15, the amount of the radionuclides taken up by the unsqueezed PPVF sample increased to 60– 70%, and the amount of the absorbed water was about 72%, i.e., the effect was the same as in the abovedescribed experiments. However, for the squeezed sample, the data were essentially different. After RADIOCHEMISTRY Vol. 56 No. 2 2014

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Fig. 1. Kinetics of 131I removal from aqueous solutions: (1) 10–3 M KI, (2) 3.2 × 10–5 M CH3I, (3) 10–3 M KIO3, and (4) 0.9 × 10–3 M I2. mPPVF = 0.025 g, V = 5 mL, T = 25°C.

squeezing, the PPVF samples contained ~0.7% of the water taken in the experiment but 16–17% of the total amount of the radionuclides in the system. This means that the fraction of the radionuclides sorbed onto PPVF increased by a factor of more than 5–10. Apparently, high hydrophilicity of the polymer is one of the factors determining its ability to sorb anions and cations from water. As noted above, in sorption of 131I2 from an aqueous solution onto PPVF, the polymer became deep blue. It was interesting to study this process. Batch experiments on I2 sorption from neutral aqueous solutions onto PPVF were performed as follows. Four samples were cut out from PPVF plates. The PPVF samples were poured over with a yellow solution containing 220 mg L–1 I2. Blue spots appeared on all the PPVF samples virtually immediately. The solution color changed from yellow to pale yellow. Approximately after 15-min contact of the solution with PPVF samples, the solution became green, and all the PPVF samples became blue. After 2-h contact, all the PPVF samples had very deep dark blue color. The Table 1. Data on removal of 131I and 137Cs from 10–3 and 10–5 M aqueous KI solutions with PPVF with and without squeezing the polymer sample (V = 10 mL, T = 25°C, contact time of the solution and PPVF 120 min) Content in PPVF, % 131 137 I Cs 0.159 Without squeezing 9.6 ± 0.5 10.8 ± 0.5 0.161 With squeezing 1.84 ± 0.09 3.85 ± 0.19 0.149 Without squeezing 13.3 ± 0.7 13.5 ± 0.7 0.185 With squeezing 3.27 ± 0.16 3.34 ± 0.17

[KI], mPPVF, g M 10–5 10–3

Experimental conditions

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and the color changed from dark blue to blue. After 7-day contact, the mother liquors in all the cases became colorless. The coloration of the PPVF samples was nonuniform, with white areas; the external layers were the most strongly colored. After 10 days, all the samples were withdrawn from solution and allowed to dry in air at room temperature. By the moment of withdrawal, the PPVF samples were pale blue. The samples dried in air were light blue.

Fig. 2. Photograph of the PPVF material after contact with aqueous I2 solution.

mother liquor was blue-green. After 2-h contact, the PPVF samples were withdrawn from the solution and thoroughly squeezed to remove excess liquid. In so doing, a blue solution separated from PPVF samples; it probably contained a compound of I2 with starch used in the synthesis as a pore-forming agent. After that, the samples were washed several times with water, and excess liquid was again removed by squeezing. The squeezed samples were poured over with distilled water. No changes in the color of PPVF samples (deep blue) and solution (colorless) were observed in 96 h. Simultaneously, fresh PPVF samples were placed into green-blue mother liquors. Two PPVF samples were taken dry, whereas the other two PPVF samples were preliminarily soaked in distilled water. The volume of PPVF samples increased virtually immediately on placing them into water. All the samples very quickly (within less than 1 min) acquired blue color after placing in an I2 solution. After 24-h contact with PPVF samples, the solution became light blue-green, i.e., a blue substance was partially released from the materials. In all the cases, the PPVF samples were deep blue. After 5 days, one of the PPVF samples was withdrawn from the solution and squeezed with a filter paper. The wet filter paper did not change color after squeezing, i.e., the squeezed solution contained no colored substances. The other PPVF samples were left in contact with water. The squeezed PPVF sample was cut into several pieces in longitudinal and transverse directions. We found that the PPVF sample coloration was nonuniform throughout the volume (Fig. 2). The external layers were blue, whereas the middle remained white. In 4 days, the coloration of all the samples kept in solution without protection from light became weaker,

A 0.1 M AgNO3 solution was added to the mother liquor from which the PPVF samples were withdrawn. Approximately in 2 h, all the solutions became yellow, and pale brown AgI precipitates formed. In the course of the experiments, we determined the PPVF capacity for I2 with sample squeezing and without it. The experiments were performed as follows. A PPVF sample (0.141 g) was placed in 10 mL of a 1.12 mg mL–1 I2 solution, kept there for 120 min, and withdrawn. In the process, the solution became bluegreen, and the sample became deep blue. The solution volume after withdrawing the PPVF sample was 7.3 mL. The amounts of I2 in the initial and final solutions were determined by colorimetry, and the amount of iodine in the material was determined from the difference. The unsqueezed material contained 8.63 mg (0.068 mmol) of I2. The calculated PPVF capacity without squeezing was 61.2 mg of I2 per gram of PPVF. To determine the PPVF capacity for I2 with squeezing of the material, the experiments were performed as follows. A PPVF sample (0.149 g) was placed in 10 mL of a 1.12 mg mL–1 I2 solution, kept there for 120 min, and withdrawn. As in the experiment without PPVF squeezing, the solution was blue-green and the sample was deep blue. The sample withdrawn from the solution was squeezed until the moisture ceased to separate out. The mother liquor was combined with the squeezed solution, and the I2 content of the combined solution was determined by colorimetry. We found that the squeezed material contained 3.55 mg (0.028 mmol) of I2. The calculated capacity of the squeezed PPVF sample was 51.5 mg of I2 per gram of PPVF. Along with determining the PPVF capacity for I2 from aqueous solutions, we examined the stability of the PPVF samples with the sorbed I2 in various media. To this end, PPVF samples in the form of thin plates were placed in an aqueous I2 solution. Because of small sample thickness, the samples were impregnated with the iodine solution virtually throughout the volume, i.e., they had deep blue color both in the external RADIOCHEMISTRY Vol. 56 No. 2 2014

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No changes in the solution and sample color were observed on keeping the blue PPVF sample in H2O for 1 and 24 h. However, pressing the PPVF sample under a water layer with a glass stick resulted in the release of a blue liquid imparting color to the solution.

15-min contact, the PPVF samples became pale yellow, whereas the solution color did not change noticeably. After 5-day contact, the pale brown PPVF samples were withdrawn from the solution and squeezed to remove excess solution. In the course of squeezing, blue spots appeared on the sample surface. After drying in air, the PPVF samples were pale yellow.

In a 1 M HNO3 solution, the initially blue PPVF sample floated on the solution surface and preserved color for 1 h, with the solution becoming light blue. After keeping for 24 h, the color of the PPVF sample was preserved, but the material lost elasticity and became loose. The 1 M HNO3 solution in contact with the PPVF sample became fully colorless.

In another series of experiments, dry PPVF samples were placed into an alcoholic I2 solution at V/m = 100 mL g–1. In so doing, the PPVF samples did not increase in volume and acquired pale brown color with blue spots. After 5-day contact, no apparent changes were observed in the color of PPVF samples and solutions.

After placing the blue PPVF sample into 3 M HNO3, the solution became pale brown, but the sample preserved the deep blue color. The solution and PPVF sample colors were preserved for 1 h, but the PPVF sample became considerably softer. After the 24-h contact, the PPVF sample and solution became fully colorless, i.e., all the colored I2 compounds decomposed.

Thus, in sorption of I2 from water and alcoholic solution, the PPVF samples, irrespective of their pretreatment (impregnated with water or dry samples), acquire deep blue and yellow color, respectively.

layers and inside. The blue PPVF samples were placed in H2O and HNO3 solutions (1, 3, and 6 M).

On placing the blue sample into 6 M HNO3, the solution rapidly (within less than 1 min) turned brown and the sample rapidly underwent softening and disintegration. Within 5 min, the PPVF sample virtually fully dissolved in 6 M HNO3 with the formation of a viscous brown solution. However, the brown color of the solution disappeared within 3 h. Thus, the blue PPVF samples with the sorbed I2 are stable in H2O and HNO3 (1 and 3 M) but dissolve in 6 M HNO3. The colored blue compounds formed in the course of I2 sorption onto PPVF are readily soluble and stable in water; they are preserved for 24 h in PPVF on its contact with 1 M HNO3, but are unstable in 3 and 6 M HNO3. Along with the experiments on I2 sorption onto PPVF from water, we performed similar experiments on sorption from alcoholic solutions. Preliminarily we prepared a 0.22 mg mL–1 solution of I2 in 96% ethanol. Then we prepared four PPVF samples, of which two samples were preliminarily placed into water and two samples were left dry. As in the previous experiments, the volume of PPVF samples increased virtually immediately on placing them into water. After 24 h, the PPVF samples were squeezed to remove excess water and placed into an alcoholic I2 solution at V/m = 100 mL g–1. After RADIOCHEMISTRY Vol. 56 No. 2 2014

Because the radionuclide uptake by PPVF from water is associated with the water absorption by the polymer, it was interesting to examine the possibility of treatment of radioactive organic solvents containing small amounts of water. We performed experiments on the 137Cs removal from hexane, ethanol, toluene, and dodecane containing 0.1–10.0 vol % water. The results are given in Table 2. As can be seen, irrespective of the way of introducing 137Cs into an aqueous-organic solution, the degree of removal of 137Cs is 75–82% for ethanol and 90–99% for toluene and dodecane. However, for hexane, the degree of removal of 137Cs varies by more than 10% depending on the procedure of preparing the radioactive aqueous-organic solution. Probably, in the case of solution no. 1, 137Cs is distributed between the aqueous and organic phases, with the distribution ratio in the systems being different. Probably, in the case of toluene and dodecane, 137Cs is mainly localized in the aqueous phase, whereas in hexane and ethanol a minor fraction of 137Cs passes to the organic phase. This fact may be responsible for the decrease in the 137Cs uptake by PPVF in these solvents. In the case of solution no. 2, 137Cs virtually fully remains in the aqueous phase in the course of stirring, except ethanol solutions. It was interesting to examine the possibility of increasing the degree of the 137Cs removal from aqueousorganic solution by increasing the time of contact of the solution with PPVF. For this purpose, dodecane solutions containing 137Cs and 0.1–1.0 vol % H2O were kept in contact with PPVF for 24 h. We found that an

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Table 2. Data on 137Cs removal from ethanol, toluene, hexane, and dodecane containing 0.1–10.0 vol % water with PPVF (V = 10 mL, mPPVF = 0.2–0.3 g, T = 25°C, contact time of the solution and PPVF 180 min) Experiment conditions

Solution no. 1

Solution no. 2

137

[H2O], M 0.1 0.5 1.0 5.0 10.0 0.1 0.5 1.0 5.0 10.0

ethanol – 81.8 ± 0.5 75.1 ± 0.4 78.5 ± 0.8 80.6 ± 0.4 – 82.0 ± 0.6 77.3 ± 0.4 79.2 ± 0.5 81.3 ± 0.8

increase in the contact time of the liquid phase and PPVF had virtually no effect on the degree of 137Cs removal, which was about 98.9% in 24 h. Thus, porous polyvinyl formal can be used for treatment of organic solvents to remove traces of water and radionuclides dissolved in water. REFERENCES 1. Kulyukhin, S.A., Usp. Khim., 2012, vol. 81, no. 10, pp. 960–982. 2. Ryabchikov, B.E., Ochistka zhidkikh radioaktivnykh otkhodov (Treatment of Liquid Radioactive Wastes), Moscow: DeLi, 2008.

Cs content in PPVF, % toluene hexane 98.5 ± 0.4 84.9 ± 0.8 97.6 ± 0.8 78.9 ± 0.5 97.0 ± 0.3 81.4 ± 0.4 98.0 ± 0.5 83.0 ± 0.7 97.3 ± 0.8 84.2 ± 0.6 97.5 ± 0.4 99.1 ± 0.4 91.3 ± 0.8 99.5 ± 0.2 99.8 ± 0.1 99.8 ± 0.1 98.5 ± 0.4 99.0 ± 0.1 98.8 ± 0.3 98.7 ± 0.4

dodecane 96.4 ± 0.8 94.4 ± 0.7 94.1 ± 0.8 97.9 ± 0.4 99.5 ± 0.2 97.7 ± 0.4 98.9 ± 0.3 97.0 ± 0.8 99.1 ± 0.2 99.4 ± 0.1

3. Milyutin, V.V., Physicochemical Methods for Removing Radionuclides from Low- and Intermediate-Level Liquid Radioactive Wastes, Doctoral (Chem.) Dissertation, Moscow: Frumkin Inst. of Physical Chemistry and Electrochemistry, Russian Acad. Sci., 2008. 4. Entsiklopediya polimerov (Polymer Encyclopedia), Moscow, 1972, vol. 1, pp. 227–231. 5. Rozenberg, M.E., Polimery na osnove vinilatsetata (Polymers Based on Vinyl Acetate), Leningrad: Khimiya, 1983. 6. Vlodavets, I.N., Abazrova, N.A., Sinitsyna, G.M., et al., Dokl. Akad. Nauk SSSR, 1972, vol. 204, no. 3, pp. 610– 611. 7. Site of the Research and Production Center for Structural Materials, http://www.niipcsm.ru.

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