Environ. Sci. Technol. 2001, 35, 626-629
Cesium and Strontium Ion Exchange on the Framework Titanium Silicate M2Ti2O3SiO4‚nH2O (M ) H, Na) SUSAN SOLBRÅ, NICOLA ALLISON, STEPHEN WAITE, AND SERGEY V. MIKHALOVSKY* School of Pharmacy and Biomolecular Sciences, University of Brighton, Lewes Road, Brighton BN2 4GJ, U.K. ANATOLIY I. BORTUN,† LYUDMILA N. BORTUN, AND ABRAHAM CLEARFIELD Department of Chemistry, Texas A&M University, College Station, Texas 77843
The ion exchange properties of the titanium silicate, M2Ti2O3SiO4‚nH2O (M ) H, Na), toward stable and radioactive 137Cs+ and 89Sr2+, have been examined. By studying the cesium and strontium uptake in the presence of NaNO3, CaCl2, NaOH, and HNO3 (in the range of 0.01-6 M) the sodium titanium silicate was found to be an efficient Cs+ ion exchanger in acid, neutral, and alkaline media and an efficient Sr2+ ion exchanger in neutral and alkaline media, which makes it promising for treatment of contaminated environmental media and biological systems.
Introduction An ongoing search for materials for selective removal and safe storage of radioactive elements from nuclear waste solutions shows that crystalline inorganic ion exchangers, especially those with an open and rigid framework structure, satisfy many of the requirements for such applications, because they possess high thermal, radiation, and chemical stability and unique selectivity for certain elements (1). Among these exchangers are transition metal ferrocyanides (2-4), some heteropolyacids (5-7) and hydrated polyvalent metal trisilicates (8-10) as selective cesium binders and crystalline antimonic acids, and silico-antimonates (11, 12) that are selective to strontium. Recently, hydrous crystalline sodium silicotitanate, TAM-5, with a molar ratio of Na:Si:Ti ) 0.76-0.91:0.72-0.75:1.0, TAM-5 doped with Nb, and sodium titanium silicate Na2Ti2O3SiO4‚2H2O, STS, were synthesized, and their ion exchange behavior was characterized (13-18). The chemical composition of TAM-5 was disclosed in (19). It was found that these ion exchangers exhibit a high affinity for cesium and strontium ions in the presence of an excess of other competitive species and, hence, could be regarded as promising materials for highly alkaline nuclear waste remediation (14, 16). The uptake of ions is controlled by the accessibility of at least two different ion exchange sites in the titanium silicate framework (20). Sodium titanium silicate has a framework structure enclosing tunnels that run parallel to the c-axis (15). There are two sites for the Na+ ions, half are in the tunnels and the * Corresponding author phone: +44 (0) 1273 642034; fax: +44(0) 1273 679333; e-mail:
[email protected]. † Present address: ASEC, Delphi Chassis & Energy System, P.O. Box 580970, Tulsa, OK 74158. 626
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 3, 2001
other half are held in openings into the tunnels within the framework atoms. These framework sites are too small to accommodate ions the size of potassium or larger. The larger ions must find sites within the tunnel by displacement of sodium or protons. Cesium ions fit into tunnels forming eight bonds to silicate oxygens that are equal to the sum of the Cs+ + O2- radii. This tight binding accounts for the high selectivity of the exchanger for this ion. Potassium ion also occupies this tunnel site, but the bond distances are less favorable (8). The selectivity for sodium is more complex. On titration of the proton form of the exchanger (2), the sodium enters the framework sites for which the selectivity is high. Sodium ions within the tunnels are loosely held and readily exchange with other ions. The removal of Cs+ and Sr2+ from nuclear waste solutions containing 5-7 M Na+ is explainable on the basis that the larger ions are capable of displacing a small amount of Na+ from the tunnels because of the high selectivity toward these ions even in such high concentrations of sodium ion. So far, no data have been available on STS application for selective Cs+ and Sr2+ removal from different environmental media and biological systems. Considering this fact, STS ion exchange affinity toward Cs+ and Sr2+ in groundwater and seawater simulants and artificial foodstuff fluid (AFF) has been studied.
Experimental Methods Reagents and Analytical Procedures. All reagents were of analytical grade (Aldrich). 89SrCl2, 35 MBq/mL, and 137CsCl, 40 MBq/mL (Amersham Int., U.K.), were used as radioactive labels. Synthesis. A framework sodium titanium silicate, STS, was prepared as follows: to 30 mL of a freshly prepared 2 M TiCl4 solution (HCl factor is 3.5-3.7) in a 1.0 L Teflon vessel were added quickly and under stirring 40 mL of 30% H2O2, 150 mL of distilled water and 40 mL of 10 M NaOH. Then 4.3 g of silicic acid dissolved in 200 mL of 1 M NaOH was added to the reaction mixture. The reaction system was treated hydrothermally (200 °C) for 10 days. The product obtained was filtered, washed with water, and dried at 7080 °C in air. According to the elemental analysis, the compound had the formula Na1.6H0.4Ti2O3SiO4‚2.0H2O. Sodium titanium silicate was converted into the protonated form, H2Ti2O3SiO4‚1.6H2O (STS-H), by treatment with 0.5 M HNO3. Ion Exchange Study. Cesium and strontium ions uptake in the presence of background electrolytes (NaNO3, HNO3, NaOH, CaCl2) was studied in batch experiments, with volumeto-mass ratio, V:m ) 200:1 (mL/g) at 25 °C. The contact time was 10-12 days under constant shaking. The pH of the solutions after equilibration with the exchanger was measured using a Corning-340 pH meter. The initial and final concentration of cesium and strontium in these solutions were measured using a Varian SpectrAA-300 atomic absorption spectrometer. Detection limits for Cs and Sr were 0.05-0.10 ppm depending on the background electrolyte. Uptake of cesium and strontium ions from groundwater simulant, seawater simulant, Ringer solution, and artificial foodstuff fluid (AFF) by the sodium titanium silicate was studied in batch experiments at V:m ratio ) (200-40,000):1 (mL/g), ambient temperature, and 4 days contact. The groundwater simulant was prepared from metal chlorides and contained (in mg/L) Ca2+ - 100, Mg2+ - 6.3, Na+ - 25, Sr2+ - 0.8, Cl- - 236, and/or Cs+ - 0.5. The seawater simulant contained (in mg/L) Na+ - 8,050, Mg2+ - 930, Ca2+ - 305, K+ - 295, Sr2+ - 7.5, Cs+ - 0.5, and Cl- - 14,410. The Ringer 10.1021/es000136x CCC: $20.00
2001 American Chemical Society Published on Web 01/06/2001
TABLE 1. The KdCs Values (mL/g) for Cesium Exchange on STSa electrolyte concn, M 5‚10-3 1‚10-2
5‚10-2 0.1 0.5 1 2 3 6 a
NaNO3
CaCl2
>200,000 >200,000 >200,000 >200,000 >200,000
>200,000 >200,000 >200,000 >200,000 160,000 57,000
53,000
Initial [Cs+] ) 1‚10-3 M.
b
NaOHb
>20,000 16,700 3450 995 320 150
HNO3 >200,000 165,000 126,000 94,000 35,100 10,300 3100
Initial [Cs+] ) 1‚10-4 M.
(C0 - Ce) V × Ce m
(1)
where Co and Ce are the ion concentrations in the initial solution and in the solution after equilibration with the exchanger, respectively; V/m is the volume-to-mass ratio. Decontamination factor, DF, showing the ability of the exchanger to purify the solution, is defined as
DF ) Co/Ce
electrolyte concn, M 5‚10-3 1‚10-2 5‚10-2 0.1 0.5 1 2 3
a
solution contained (in mg/L) NaCl - 8000, NaHCO3 - 200, KCl - 200, CaCl2 - 200, glucose - 1000, Sr2+ - 0.8, or Cs+ - 0.5 (21). The artificial foodstuff fluid contained (in mM) K+ - 110, Na+ - 25, Mg2+ - 8.5, Ca2+ - 0.5, H2PO4- - 60, SO42- - 16, Cl- - 11, lactate - 50, Sr2+ - 0.9 ppm, or Cs+ - 0.5 ppm, pH 5.5 (22). All simulants were spiked with 89Sr (160-450 Bq/mL) and 137Cs (136-250 Bq/mL). Activity of the supernatants after centrifugation was counted using a 1209 RackBeta Liquid Scintillation Counter (LKB Wallac). In these experiments only concentration of radioactive Sr or Cs was measured before and after the contact, which does not affect Kd results. The affinity of the exchangers for the ions of interest was expressed through the distribution coefficient (Kd, mL/g), calculated according to the formula
Kd )
TABLE 2. The Kdsr Values (mL/g) for Strontium Exchange on STSa
(2)
Results and Discussion It is known that the framework titanium silicate has a high affinity for cesium and strontium in neutral and alkaline media. In this paper data on the STS ion exchange performance in different media containing Na+, K+, Ca2+, and Mg2+ that compete for the adsorption sites with Cs+ and Sr2+ are presented. The importance of this study is due to the fact that in nuclear waste, biological systems, and the environment, cations of sodium, potassium, calcium, and magnesium are present in amounts exceeding those of radioactive strontium and cesium by several orders of magnitude. The KdCs and KdSr values for the STS sample as a function of the sodium nitrate, calcium chloride, sodium hydroxide, and nitric acid concentrations in solution are shown in Tables 1 and 2, respectively. The STS sample exhibits an extremely high affinity for cesium ion (KdCs > 100,000-200,000 mL/g) in the presence of NaNO3 or CaCl2 in a broad concentration range (Table 1). However, the KdCs values show a tendency to decrease at the electrolyte concentration above 0.5-1.0 M. This decrease is not so drastic as was found in the case of another framework titanium silicate, K3HTi4O4(SiO4)3‚ 4H2O, that has a tunnel structure with similar parameters (23). This indicates that in solutions with pH values close to neutral neither Na+ nor Ca2+ could compete with Cs+ ion for the tunnel ion exchange sites of Na2Ti2O3SiO4‚2H2O and that the exchanger is selective for cesium ion.
NaNO3
CaCl2
>200,000 >200,000 30,500 7800 450 150
65,000 26,500 3900 1300
Initial [Sr2+] ) 1‚10-3 M.
b
NaOHb
HNO3
>20,000 >20,000 >20,000 >20,000 >20,000 >20,000 >20,000
4500 25 1 ‚ 106 mL/g), and they do not decrease with the volume-to-mass ratio increase from 10,000 to 40,000 mL/g. The KdSr values found at V:m )10002500 are also high (>2 ‚ 105 mL/g), but they show a tendency to decrease as the V:m ratio increases. This could be caused by the presence of calcium and sodium ions competing for VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
627
TABLE 4. The Kd137Cs and Kd89Sr Values for the STS Sample in Seawater Simulant V:m, mL/g
Kd137Cs, mL/g
V:m, mL/g
Kd89Sr, mL/g
500 1000 2500 5000 7500 10,000
28,500 28,200 19,000 18,500 9800 9000
50 100 140 180 250 400 1000
22,600 12,300 8200 4300 1640 580 210
TABLE 5. The KdCs and KdSr Values for the STS Sample in Ringer Solution V:m
KdCs
V:m
KdSr
1000 2000 4000 5000 7500 10,000
260,000 175,000 110,000 62,000 37,000 27,800
200 500 1000 2000 4000
193,000 29,000 2200 950 720
TABLE 6. The KdCs and KdSr Values for the STS Sample in Artificial Foodstuff Fluid V:m
KdCs
V:m
KdSr
500 1000 5000 10,000 15,000
7500 9100 11,000 12,000 12,000
250 500 1000 2000 3000
25 15 7 1000 >1000 >1000 >1000 >1000 >1000
56 22 14 8.1 3.6 1.9 1.0
>1000 >1000 >1000 >1000 >1000 >1000 >1000
1 1 1 1 1 1 1
a
Initial [Cs+] ) 6.0 ppm, initial [Sr2+] ) 5.3 ppm.
in both cases formation of soluble strontium organic complexes resulted in significant reduction of Sr2+ removal. The efficiency of the stable cesium and strontium isotopes removal by the framework titanium silicates from Ringer solution was also tested (Table 7). Cesium decontamination factors are extremely high (DFCs > 1,000, with residual cesium concentration below detection limit) for both samples, and they do not depend on the titanium silicate form. These data are in agreement with data in Table 5 on radiocesium uptake and show that 1 g of Na2Ti2O3SiO4‚2H2O is able to remove cesium quantitatively from up to 4 L of the Ringer solution. The affinity of the titanium silicates for Sr2+ is much lower than for Cs+. No strontium uptake by the protonated titanium silicate is observed, which is in agreement with our previous data on the low affinity of H2Ti2O3SiO4‚2H2O for divalent cations. Titanium silicate in the sodium form is able to remove Sr2+ ion from Ringer solution. The strontium decontamination factor is relatively high (55-105) at low V:m ratio (100:1, mL/g), but it decreases drastically with the V:m ratio increase, which could be a result of cesium competition for the selective ion exchange sites. Comparison of the ion exchange behavior of the sodium titanium silicate in different media shows that the presence of competitive ions (Na+, K+, Ca2+) as well as an increase in alkalinity or acidity of the solution affects the affinity of Na2Ti2O3SiO4‚2H2O for Sr2+ stronger than its affinity for Cs+. This imposes limitations on the titanium silicate application for selective strontium removal from complex solutions starting from less than 1 M in HNO3 and up to 1-2 M in NaOH. Efficient Sr2+ uptake occurs in neutral and alkaline solutions that do not contain an excess of calcium ion. The presence of some organic compounds in biological liquors has a detrimental effect on strontium uptake by STS but does not deteriorate its affinity for cesium ion.
Acknowledgments The authors wish to acknowledge Dr. Peter Ballance, University of Sussex, for his advice on using radioisotope technique.
Literature Cited (1) Clearfield, A. Ind. Eng. Chem. Res. 1995, 34, 2865. (2) Tananaev, I. V. Chemistry of Ferrocyanides; Nauka: Moscow, 1971. (3) Lee, E. F. T.; Streat, M. J. Chem. Technol. Biotechnol. 1983, 33A, 80. (4) Ayrault, S.; Loos-Nescovic, C.; Fedoroff, M.; Gainier, E. Talanta 1994, 41, 1435. (5) Amphlett, C. B. Inorganic Ion Exchangers; Elsevier: New York, 1964. (6) Inorganic Ion Exchangers in Chemical Analysis; Qureshi, M., Varshney, K. G., Eds.; CRC Press: Boca Raton, FL, 1991. (7) Clearfield, A.; Nancollas, G. H.; Blessing, R. H. In Ion Exchange and Solvent Extraction; Marinsky, J. A., Marcus, Y.; Eds.; Marcel Dekker: New York, Vol. 5, 1973; pp 69-92.
(8) Poojary, D. M.; Bortun, A. I.; Bortun, L. N.; Clearfield, A. Inorg. Chem. 1997, 36, 3072. (9) Bortun, A. I.; Bortun, L. N.; Clearfield, A. Solv. Extr. Ion Exch. 1997, 15, 909. (10) Clearfield, A.; Bortun, A. I.; Bortun, L. N.; Poojary, D. M.; Khainakov, S. A. J. Mol. Struct. 1998, 470, 207. (11) Belinskaya, F. A.; Milizyna, E. A. Uspekhi Khimii 1980, 49, 1904. (12) Abe, M. In Ion Exchange and Solvent Extraction; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1995. (13) Anthony, R. G.; Philip, C. V.; Dosch, R. G. Waste Management 1993, 13, 503. (14) Anthony, R. G.; Dosch, R. G.; Gu, D.; Philip, C. V. Ind. Eng. Chem. Res. 1994, 33, 2702. (15) Poojary, D. M.; Cahill, R. A.; Clearfield, A. Chem. Mater. 1994, 6, 2364. (16) Zheng, Z.; Gu, D.; Anthony, R. G.; Klavetter, E. Ind. Eng. Chem. Res. 1995, 34, 2142. (17) Bortun, A. I.; Bortun, L. N.; Clearfield, A. Solv. Extr. Ion Exch. 1996, 14, 341. (18) Poojary, D. M.; Bortun, A. I.; Bortun, L. N.; Clearfield, A. Inorg. Chem. 1996, 35, 6131. (19) Anthony, R. G.; Dosch, R. G.; Robert G.; Philip C. V. Method of Using Novel Silico-Titanates; US Patent 6,110, 378, 2000.
(20) Zheng, Z.; Philip, C. V.; Anthony, R. G.; Krumhansl, J. L.; Trudell, D. E.; Miller, J. E. Ind. Eng. Chem. Res. 1996, 35, 4246. (21) Clinical Laboratory Methods and Diagnosis; Frankel, S., Reitman, S., Eds.; C. V. Mosby: Saint Louis, 1963; Vol. I. (22) Bengtsson, G. B.; Bortun, A. I.; Strelko, V. V. J. Radioanal. Nucl. Chem. 1996, 204, 75. (23) Behrens, E. A.; Poojary, D. M.; Clearfield, A. Chem. Mater. 1996, 8, 1236. (24) Lange’s Handbook of Chemistry, 12th ed.; Dean J. A., Ed.; McGraw-Hill: New York, 1979; Section 10. (25) Zheng, Z. Ph.D. Dissertation, Texas A&M University: College Station, 1996. (26) Marsh, S. F.; Svitra, Z. V.; Bowen, S. M. Effects of Soluble Organic Complexants and Their Degradation Products on the Removal of Selected Radionuclides from High-Level Waste; Technical Report LA-13000, Part III and IV; Los Alamos National Laboratory: Los Alamos, New Mexico, 1995.
Received for review June 19, 2000. Revised manuscript received October 13, 2000. Accepted November 1, 2000. ES000136X
VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
629
