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Chem. Mater. 2000, 12, 3798-3804
Sorption Behavior of Radionuclides on Crystalline Synthetic Tunnel Manganese Oxides Alan Dyer,*,† Martyn Pillinger,† Jon Newton,† Risto Harjula,‡ Teresia Mo¨ller,‡ and Suheel Amin§ Chemistry Division, Science Research Institute, Cockcroft Building, University of Salford, Salford M5 4WT, U.K., Laboratory of Radiochemistry, Department of Chemistry, PO Box 55, FI-00014, University of Helsinki, Helsinki, Finland, and British Nuclear Fuels plc, Risley, Warrington, Cheshire WA3 6AS, U.K. Received July 26, 2000. Revised Manuscript Received September 19, 2000
Cryptomelane-type (potassium) and todorokite-type (magnesium and calcium) manganese oxides were prepared and studied for the removal of radionuclides from aqueous solutions containing inactive metal ions of interest in radionuclide-bearing nuclear waste effluents. Batch distribution coefficients were measured as a function of pH and sodium, potassium, magnesium, and calcium ion concentrations. Trace strontium (89Sr) and cesium (137Cs) ions were taken up by ion-exchange mechanisms. Selectivity coefficients were estimated as KCs/K ) 0.6 and KSr/K ) 1.0 for cryptomelane and KCs/Mg ) 7550, KSr/Mg ) 50, and KSr/Ca ) 10 for todorokite. The cryptomelane analogue was particularly effective for the separation of trace silver ions (110Ag) in low-pH solutions. Todorokite analogues were effective for a wider range of radionuclides, including 57Co across a wide pH range (1-10) and 137Cs in acidic solution. The affinity sequence for magnesium and calcium ion-extracted (acid-treated) todorokites in 0.1 HNO3 was 137Cs > 59Fe > 51Cr ≈ 57Co ≈ 241Am > 54Mn > 63Ni > 65Zn > 236Pu > 89Sr.
Introduction There is an ongoing research effort to develop highly selective ion-exchange materials for the removal of metal radionuclides from aqueous radioactive wastes.1,2 This is a challenging task as the radionuclides are usually present at trace concentrations in the presence of large excesses of inactive metal ions. Benefits include large reductions in the volumes of solidified waste and reductions in the radioactive discharges into the environment. Inorganic ion exchangers possess a number of characteristic advantages for these purposes, including superior chemical and thermal stability, good resistance to radiation damage, and good compatibility with final waste forms. Porous inorganic crystals with layered3-6 and tunnel7-11 structures are particularly promising as they often have high capacities and * Corresponding author. E-mail:
[email protected]. † University of Salford. ‡ University of Helsinki. § British Nuclear Fuel plc. (1) (a) Inorganic Ion Exchange Materials; Clearfield, A., Ed.; CRC Press: Boca Raton, FL, 1982. (b) Clearfield, A. Ind. Eng. Chem. Res. 1995, 34, 2865-2872. (2) Lehto, J.; Harjula, R. Radiochim. Acta 1999, 86, 65-70. (3) Clearfield, A.; Bortun, A. I.; Bortun, L. N.; Cahill, R. A. Solvent Extr. Ion Exch. 1997, 15, 285-304. (4) Bortun, L. N.; Bortun, A. I.; Clearfield, A. In Ion Exchange Developments and Applications; Greig, J. A., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1996; pp 313-320. (5) (a) Kodama, T.; Komarneni, S. J. Mater. Chem. 1999, 9, 533539. (b) Kodama, T.; Komarneni, S. J. Mater. Chem. 1999, 9, 24752480. (6) Lehto, J.; Harjula, R.; Girard, A.-M. J. Chem. Soc., Dalton Trans. 1989, 101. (7) Poojary, D. M.; Bortun, A. I.; Bortun, L. N.; Clearfield, A. Inorg. Chem. 1996, 35, 6131. (8) Bortun, A. I.; Bortun, L. N.; Clearfield, A. Solvent Extr. Ion Exch. 1996, 14, 341.
selectivities for certain monovalent and divalent cations. New materials are discovered regularly, some of which are analogues of natural minerals, while others have novel structures. Mixed-valence manganese oxides with tunnel structures are one class of potentially interesting sorbents that can be synthesized in the laboratory.12,13 Examples include analogues of the minerals todorokite (channels of 3 × 3 MnO6 units, tunnel size 6.9 Å, Mg2+ in the tunnels)14-17 and hollandite (2 × 2, tunnel size 4.6 Å, Ba2+ in the tunnels).18,19 To date, most of the ionexchange work on these materials has focused on the characterization of the ion-sieve properties of metal-ion(9) Behrens, E. A.; Sylvester, P.; Clearfield, A. Environ. Sci. Tecnol. 1998, 32, 101-107. (10) Dyer, A.; Pillinger, M.; Amin, S. J. Mater. Chem. 1999, 9, 24812487. (11) Dyer, A.; Pillinger, M. In Advances in Ion Exchange for Industry and Research; Williams, P. A., Dyer, A., Eds.; Royal Society of Chemistry: Cambridge, U.K., 1999; pp 261-269. (12) Brock, S. L.; Duan, N.; Tian, Z. R.; Giraldo, O.; Zhou, H.; Suib, S. L. Chem. Mater. 1998, 10, 2619-2628. (13) Feng, Q.; Kanoh, H.; Ooi, K. J. Mater. Chem. 1999, 9, 319333. (14) (a) Golden, D. C.; Chen, C. C.; Dixon, J. B. Science 1986, 231, 717-719. (b) Golden, D. C.; Chen, C. C.; Dixon, J. B. Clays Clay Miner. 1987, 35, 271-280. (15) (a) Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy, L.; Potter, D. I.; O’Young, C. L. Science 1993, 260, 511515. (b) Shen, Y. F.; Suib, S. L.; O’Young, C. L. J. Am. Chem. Soc. 1994, 116, 11020-11029. (16) Tian, Z. R.; Yin, Y. G.; Suib, S. L. Chem. Mater. 1997, 9, 11261133. (17) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Chem. Mater. 1995, 7, 1722-1727. (18) DeGuzman, R. N.; Shen, Y. F.; Neth, E. J.; Suib, S. L.; O’Young, C. L.; Levine, S.; Newsam, J. M. Chem. Mater. 1994, 6, 815-821. (19) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Chem. Mater. 1995, 7, 148.
10.1021/cm001142v CCC: $19.00 © 2000 American Chemical Society Published on Web 12/18/2000
Sorption Behavior of Radionuclides
extracted derivatives by pH titration and distribution coefficient measurement studies.13,17,19 This article presents a detailed examination of as-synthesized tunnel manganese oxides for the separation of fission product (137Cs, 89Sr), activated corrosion product (57Co, 51Cr, 54Mn, 59Fe, 63Ni, 65Zn, 110Ag), and actinide (236Pu, 241Am) radionuclides from aqueous solutions containing competing ions of interest in radionuclide-bearing waste effluents. Experimental Section Chemicals used were of reagent-grade quality and were obtained from commercial sources without further purification. Radioisotopes were supplied by Amersham International, Amersham, U.K. Physical Analyses. Powder X-ray diffraction (XRD) patterns were collected on a Siemens D5000 diffractometer with Cu K* radiation. Water contents in samples preequilibrated over saturated NaCl for 1 week were determined by thermogravimetry (TGA) using a Mettler TA3000 system at a heating rate of 5 K min-1 under a nitrogen atmosphere. Chemical Analyses. Elemental chemical analyses for sodium, potassium, magnesium, calcium, and manganese were performed by atomic absorption spectroscopy (AAS). About 50 mg of sample was dissolved with 20 mL of double-distilled water (DDW), 1 mL of concentrated nitric acid, and 2 mL of 30% hydrogen peroxide. The colorless solution was boiled for 2-5 min to remove excess peroxide, and the solution was diluted with DDW to 100 mL to make a stock solution. Average manganese oxidation states were determined using the standard oxalic acid-permanganate back-titration procedure. About 0.2 g of each sample was weighed out and transferred into a 250 mL Erlenmeyer flask containing a 50 mL solution of 10% H2SO4 mixed with 0.5 g of Na2C2O4. With mild heating (106 43 300
50 350 >106 83 500 45 900 2820 >106 593 >106
185 149 32 000
38 11 100 295 600 366 500 >106 716 200 1550 >106
45 >106 50
5 >106 11 12 550 >106 29 600 5570 14 550
127 2 24
20 700 1 1 970 290 24 300 9.4 58 400 54Mn > 63Ni > 65Zn > 236Pu. Radiolytic Stability. A sample of TOD-2(Mg) was exposed to 6.1 MGy in the Co-60 source at the University of Salford. Powder XRD indicated no change in the sample crystallinity or composition. As a further test of the materials resistance to radiation damage, the distribution coefficient of 89Sr in 0.1 M Mg(NO3)2 was measured on the sample before and after irradiation. Again, there was no appreciable change (246 mL g-1 before exposure, 349 mL g-1 after exposure). (23) Bengtsson, G. B.; Bortun, A. I.; Strelko, V. V. J. Radioanal. Nucl. Chem. 1996, 204, 75-82.
Figure 5. Distribution coefficients of radionuclides in 0.1 M HNO3 on acid-treated todorokites (V/m ) 600 mL g-1).
Concluding Remarks There are only a few industrial-scale applications of inorganic ion exchangers for radioactive waste volume minimization in operation. These mainly involve Csselective and Sr-selective materials. The aim of continued research in this area is to find cesium and strontium selective materials which work efficiently in highly acidic and alkaline media and also to find exchangers which are selective for strontium over calcium. In addition, selective exchangers are needed for the activated corrosion products, transuranium elements and anionic fission products (Tc, I). Manganese oxides with tunnel structures constitute a large class of selective ion exchangers and sorbents. In addition to the synthetic cryptomelane- (2 × 2) and todorokite-type (3 × 3) materials studied here, there are several other structure types such as romanechite (2 × 3) and RUB-7 (2 × 4).24 The selectivity of these exchangers toward metal ions depends on their structures. The cryptomelane analogue prepared in this work has a very high affinity for potassium ions but was also effective for the removal of radiosilver ions. The todorokite analogue was effective for a wider range of radionuclides, including cobalt across a wide pH range and cesium in acidic solution. The uptake of trace cesium and strontium on these materials was substantially governed by cation-exchange reactions. Uptake of cobalt is probably more complex, involving a combination of ion exchange and sorption on specific redox sites. Preparation of the todorokite analogue in the calcium form instead of the magnesium form resulted in a very large improvement in performance for cesium in acidic solution and also for strontium over calcium. Acknowledgment. This study was funded by the European Commission Framework 4 Nuclear Fission Safety Program (Contract FI4W-CT95-0016). The authors also wish to acknowledge all partners in this contract for helpful discussions. CM001142V (24) Rziha, T.; Gies, H.; Rius, J. Eur. J. Mineral. 1996, 8, 675686.
