SEPARATION OF COBALT-60, STRONTIUM-90,

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1 SEPARATION OF COBALT-60, STRONTIUM-90, AND CESIUM-137 RADIOISOTOPES BY COMPETITIVE TRANSPORT ACROSS POLYMER INCLUSION MEMBRANES Witalis PELLOWSKI*, Cezary A. KOZŁOWSKI**, and Władysław WALKOWIAK*, *Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wrocław University of Technology, Wrocław, Wybrzeże Wyspiańskiego 27 St., Poland **Department of Chemistry, Technical University of Częstochowa, Częstochowa, Armii Krajowej 19 Str., Poland, INTRODUCTION Recently, novel polymer inclusion membranes (PIMs) have been developed. The PIMs are formed by casting cellulose triacetate (CTA) from an organic solution to form a thin, stable film. The casting solution also contains an ionic carrier and a membrane plasticizer (e.g., o-nitrophenyl alkyl ether). The membrane is used to separate aqueous source and receiving phases, without any organic solvent. Therefore, PIMs are simpler to use than SLMs, and do not suffer from loss of the organic solvent. Polymer inclusion membranes with cellulose triacetate as a support was first used for metal ion removal by Sugiura et al. in Many researchers investigated metal ions transport process by PIM with differential carriers and plasticizer (Lamb, J.D.; Nazarenko, A.Y.; Hayashita, T.; Rais, J.; Bartsch, R. A..; Charewicz, W. A.; Walkowiak, W.; Kozłowski, C.; et al.). Also polyvinyl chloride (PVC) was applied as a support (Rais, J.; et al.). In this work, the competitive transport of radioactive isotopes, i.e. Cs- 137, Sr-90, and Co-60 from aqueous solutions through polymer inclusion membranes has been investigated. As the ionic carriers, crown ethers with 21-crown-7 cavity size, i.e. dibenzo-21-crown-7 and tertbutyl-dibenzo-21- crown-7 as well as 3,7-dinonyl-naphtalene-1-sulfonic acid were applied. EXPERIMENTAL Chemicals Dichloromethane, o-nitrophenyl pentyl ether (ONPPE), and cellulose triacetate (CTA) were purchased from Fluka and used without further purification. 3,7-dinonyl-naphtalene-1-sulfonic acid (DNNS), dibenzo-

2 21-crown-7 (DB21C7), and tertbutyl-dibenzo-21-crown-7 (tbudb21c7) were achieved from Aldrich. The gamma radioactive isotopes, i.e. Co-60, Sr-85 (instead of Sr-90), and Cs-137 were in the form of chloride acidic solutions (0.1 M HCl). They were or the carrier free (Cs-137) or sufficiently high specific activity to neglect the effect of carrier concentration (Co-60: 452 GBq/g, Sr-85: 675 GBq/g). These isotopes were purchased from the Atomic Energy Institute (Świerk). Polymer inclusion membrane preparation A solution of support (cellulose triacetate), the ion carrier (DB21C7, tbudb21c7 or DNNS), and the plasticizer (o-nitrophenyl pentyl ether ONPPE) in dichloromethane was prepared. A portion of this solution was poured into a membrane mold comprised of a 9.0 cm glass ring attached to a glass plate with CTA - dichloromethane glue. The organic solvent was than allowed to evaporate overnight and the resultant membrane was separated from the glass plate by immersion in cold water. Membrane transport experiments Transport experiments were carried out in a permeation cell in which the membrane film was tightly clamped between two cell compartments. The average thickness of the CTA membrane was 28 µm. The effective area of the membrane was 7.0 cm 2. Both, the source and receiving aqueous phases (45 ml each) were stirred at 600 rpm with synchronous motors. To determine metal ions concentration, a multichannel gamma radiation spectrometer with germanium detector (Canberra Ind.) was used. Transport kinetics The kinetics of the PIMs transport can be described as a first-order reaction in metal-ion concentration: ln( c / ci) = kt (1) where c is the time-dependent metal ion concentration in the source phase (M), c i is the initial concentration of metal ion in the source phase (M), k is the rate constant (s -1 ), and t is the time of transport (s). To calculate k values, the plots of ln(c/c i ) vs. time were prepared. The initial flux was calculated from the equation: J = (V / A) k (2) i c i where V is the volume of source phase (m 3 ), and A is the effective membrane area (m 2 ). Also, the recovery factor (RF) of metal ions from the source phase was calculated:

3 RF = ((n n ) / n ) 100% (3) i r i where n i the initial amount of metal in the source phase (moles), and n r the amount of metal in the source phase after process (moles), i.e. 72 hours for Cs(I) + Sr(II) + Co(II). The selectivity coefficient was defined as the ratio of initial fluxes for M1 and M2 metal ions, respectively. S = J / J (4) i,m1 i,m2 RESULTS AND THEIR DISCUSSION A series of experiments dealt with competitive transport of Sr 2+, Cs +, and Co 2+ cations from dilute aqueous solutions containing the metal ions species at concentrations of M and 0.10 M sodium nitrate through PIMs with 3,7-dinonyl-naphtalene-1-sulfonic acid, dibenzo-21-crown-7, or tertbutyl-dibenzo-21-crown-7 as the ionic carriers. We also used the mixtures of DNNS with crown ethers as the ionic carriers. The values of the initial fluxes, recovery factors, and selectivity orders with selectivity coefficients are shown in Fig. 1 and Table a) DB21C7 tbudb21c b) DNNS J i, pmol/m 2 s c) DB21C7+DNNS tbudb21c7+dnns 10 Cs(I) Sr(II) Co(II) Fig. 1. Cesium(I), strontium(ii), and cobalt(ii) initial fluxes for competitive transport across PIMs with DB21C7, tbudb21c7, and DNNS as ionic carriers. Source phase: M Cs +, Sr 2+ and Co 2+, 0.10 M NaNO 3, ph = 4.0; Receiving phase: 1.0 M HCl; Membrane: 4.0 cm 3 ONPPE / 1.0 g CTA, 0.10 M DNNS, and/or 0.30 M crown ethers (based on plasticizer)

4 Tab. 1. The recovery factors and selectivity orders with selectivity coefficients for competitive transport of Cs(I), Sr(II), and Co(II) through PIM. Conditions of experiments see Fig. 1 Ionic carrier DB21C7 tbu-db21c7 DNNS DB21C7+DNNS tbu- DB21C7+DNNS Metal ions RF, % Cs(I) 30.2 Sr(II) 13.1 Co(II) 8.20 Cs(I) 16.9 Sr(II) 14.3 Co(II) 7.85 Cs(I) 70.9 Sr(II) 99.0 Co(II) 55.4 Cs(I) 98.8 Sr(II) 96.2 Co(II) 19.3 Cs(I) 13.0 Sr(II) 36.8 Co(II) 12.3 Selectivity orders and selectivity coefficients Sr(II) > Cs(I) > Co(II) Sr(II) > Cs(I) > Co(II) For both crown ethers with cavity size of 21-crown-7, the initial fluxes for Cs +, Sr 2+, and Co 2+ are very low and form the following selectivity order:. Consequently, the removal of all metal cations from the source phase is low and does not exceed 30 % of particular metals. On the other hand, the DNNS as a typical acidic ionic carrier, prefers Sr(II) cations which results in high initial fluxes and selectivity order as follows: Sr(II) > Cs(I) > Co(II). The recovery factor in this case is very high for Sr(II) (99 %), while for Cs(I) and Co(II) is 71 and 55 %, respectively. The DB21C7 and DNNS mixture applied as ionic carrier gives the same selectivity order as DB21C7 alone but much higher initial fluxes and recovery factors of cesium(i) are observed. In this case, 99 % of Cs(I) and 96 % of Sr(II) is removed from source aqueous phase. The tbu-21c7 and DNNS mixture used as ionic carrier reverse the selectivity order in comparison with tbu-21c7 alone (Sr(II) > Cs(I) > Co(II)) but initial fluxes and recovery factors are not as high as observed for DB21C7 + DNNS mixture. The comparison of two crown ethers with the same, DB21C7 cavity size, shows that attachment of tertbutyl group into two benzo rings as a lipophilic part of crown ether decreases the cesium(i) transport, and increases the strontium(ii) transport through PIMs containing the mixture of crown ether and DNNS as an ionic carrier. CONCLUSIONS Both crown ethers, i.e. DB21C7, and tbu-db21c7 used as ionic carriers for competitive transport of cesium(i), strontium(ii), and cobalt(ii) give the preferential selectivity order: with low initial

5 metal fluxes and recovery factors. The 3,7-dinonyl-naphtalene-1-sulfonic acid applied as the ionic carrier gives very high fluxes for all metal cations (Fig. 1) with the selectivity order: Sr(II) > Cs(I) > Co(II) and the entire amount of Sr(II) is removed from the source aqueous phase. For the mixture of DB21C7 and DNNS applied as the ionic carrier practically entire removal of Cs(I) and Sr(II) is observed. In comparison to the DB21C7 + DNNS mixture, the application of tbu-db21c7 and DNNS mixture results in reverse selectivity order and much lower recovery factor. ACKNOWLEDGEMENTS Financial support of this work was provide by the Polish Science Foundation (C.K. BS /99, W.W W-3, and W.P. 3T09B03219) REFERENCES 1. Sugiura M., Kikkawa M., Urita S. 1987, Sep. Sci. Technol., 22, Hayashita T., Fujimoto T., Moriata Y., Bartsch R. A. 1994, Chemistry Letters, Lamb, J. D. Nazarenko A. Y., Uenishi J., Tsukube H. 1997, J. Membr. Sci., 134, Nazarenko A. Y., Lamb J. D. 1997, J. Inclusion Phenom. Mol. Recognit. Chem., 29, Rais J., Mason C.V., Abney K.D. 1997, Sep. Sci. Technol., 32, Walkowiak W., Bartsch R.A., Kozłowski C., Gęga J., Charewicz W.A., Amiri-Eliasi B. 2000, Journal of Radioanalitycal & Nuclear Chemistry, 246,