Chapter 11
Structural Basis of Selectivity in Tunnel Type Inorganic Ion Exchangers
Downloaded by UNIV OF ARIZONA on June 13, 2017 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch011
Abraham Clearfield, Damodara M. Poojary, Elizabeth A. Behrens, Roy A. Cahill, Anatoly I. Bortun, and Lyudmila N. Bortun Department of Chemistry, Texas A&M University, College Station, TX 77843-3255 The crystal structures of two tunnel type inorganic ion exchangers are described. A knowledge of structure is necessary to understand the ion exchange properties of these compounds. The titanosilicate of composition Na H Ti O (SiO )•1.8H O has a square framework structure outlining a tunnel parallel to the c-axis. In addition, the faces outlining the tunnel have cavities in which Na fits snugly but alkali metal ions larger than Na are excluded. Cs fits within the tunnels forming eight bonds with oxygen atoms of the silicate having distances of 3.183(5) Åand 3.057(6) Å. Because of its large diameter, Cs can only occupy half of the tunnel sites for a maximum uptake of 25% of the total exchange capacity. The remaining charge is satisfied by Na and protons withinthetunnel. The affinity for Cs is much greater than for Na in the tunnel sites so that small amounts of Cs may be removed from concentrated sodium nitrate solutions making this exchanger useful for nuclear waste remediation. The second exchanger, K H(TiO) (SiO )•4H O, has a structure similar to the first but with a shorter c-axis. Thus, ions occupy the face centers but not the tunnels so that the selectivity series for alkali metal ions depends upon ion size. During the past 50 years, nuclear defense activities have produced large quantities of nuclear waste that now require safe and permanent disposal (i). The general procedure to be implemented involves the removal of Cs and Sr from the waste solutions for disposal in permanently vitrified media (2). These ions are present in the waste solutions at concentrations of 10" -10" M admixed with many other species and at high salt concentrations. Therefore, highly selective sorbents or ion exchangers are required. Further, at the high radiation doses present in the solution, organic exchangers or séquestrants are likely to decompose over time. Inorganic ion exchangers are resistant to radiation damage and can exhibit remarkably high selectivities (J). A promising group of compounds are those with tunnel structures in which the tunnel space is fit to the size of the ion of interest. There are literally hundreds of inorganic compounds with tunnel structures or a combination of tunnels and cavities. However, for our purposes, the atoms constituting the framework must not be affected by either strong alkali or acid. Therefore, zeolites are unsuitable as are many compounds of amphoteric metals. Phosphates may be useable in acid solution but many of them hydrolyse in basic solution. For our purposes, we will choose certain compounds of titanium and zirconium as silicates and germanates for use in nuclear 1.64
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©1999 American Chemical Society
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
169 waste treatments. However, the principles we develop will be quite general and can be applied to situations other than nuclear waste systems. For use in mild acid, alkali or neutral solutions a much wider range of elements may be suitable for construction of the framework and thus many more compounds can be utilized. Compounds With Tunnel Structures Sodium Titanium Silicate. A titanium silicate of ideal formula Na Ti 0 (Si0 )«2H 0 was discovered through a collaboration between Sandia National Laboratory and Texas A&M University (4). This compound was reported to be highly selective for Cs and Sr * in the presence of large amounts of sodium ion and NaOH and also for Cs in moderately strong acid solution. No structural details were given. We were able to synthesize this compound as a highly crystalline powder from a mixture of titanium isopropoxide, tetraethylorthosilicate and NaOH under hydrothermal conditions (5). The procedure is quite specific as small changes in experimental conditions yield related compounds such as ETS-4 (6). The structure was solved ab initio from X-ray powder data (5). This compound has a tetragonal unit cell with the arrangement of atoms forming a framework that encloses tunnels. The framework is formed from Ti0 octahedra and Si0 tetrahedra. There are four octahedra at the comers of a square with 4 symmetry, that is, two octahedra up and two below turned 90° from the first two. These octahedra are bridged by silicate tetrahedra in the a- and b-axis directions (Figure 1). In the c-axis direction, the octahedra are connected by bridging oxo groups. The unit cell dimensions are approximately a = b = 7.8Â, c = 12.0Â. Half the sodium ions are situated in the ac and be planes bonded by four oxygen atoms from silicate groups above and below the Na . The coordination sphere of die Na is completed by two water molecules in the axial positions (Figure 2). The remaining sodium ions are located in the tunnels but at an occupancy of 64%. The reason for this lowered occupancy is the limited space available within the tunnels. The water molecules bonded to the framework sodium ions also he within the tunnels accounting for one of the two waters in the formula. An additional 0.8 H 0 also resides in the tunnels bonded to the sodium ions within the tunnels but since there are four formula units per unit cell there are 7.2 H 0 in the tunnel of one unit cell. The charge balance is made up by protons so the real, as opposed to the ideal, formula is Na H Ti O (SiO )* 1.8H.O. The sodium ions in the face-centers are bonded to four silicate oxygens at a distance of 2.414(5) Â. The two bonds to the water molecules are somewhat longer 2.765(1) Â. For the sodium ions within the tunnel the Na-O bonds are longer, 2.74(2) Â to silicate oxygens and 2.79-3.17 Â to water, indicating much weaker bonding. Exhaustive treatment of die sodium titanosilicate with a 0.2 M CsN0 solution led to only partial exchange of the Na . The final composition was Na! Cs H Ti O (SiO )*H O. Solution of the crystal structure of this phase (5) revealed the reason for the low uptake. Cesium ion cannot fît in the space occupied by Na in the framework. This was shown conclusively by treating the acid form of the exchanger H Ti 0 (Si0 )»nH 0 with Cs . In both the sodium and proton phases, die Cs only occupied positions within the tunnel (Figure 3). There are two positions within the tunnels for Cs*. In both sites the Cs sits exactly in the center of the square but at either l/4c, 3/4c or 0.13c, 0.63c. The c-axis is approximately 12Â long so a Cs at l/4c is approximately 6Â away from its mate at 3/4c. However, the second Cs at 0.13c would be less than 1.5Â away from the one at l/4c. This is smaller than the radius of the Cs so only one of the two sites can be occupied in any one unit cell. Thus, the maximum capacity for Cs is only 25% of the total (theoretical) capacity. As a result, some Na and H 0 fill the remaining space in the tunnel. The Cs at 1/4 is eight coordinate with Cs-O bond distances of 3.183(5) Â whereas the Cs in the second site at 0.13c is six coordinate with four bonds of length 2
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Downloaded by UNIV OF ARIZONA on June 13, 2017 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch011
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Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by UNIV OF ARIZONA on June 13, 2017 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch011
>UL Figure 1. Schematic structure of sodium titanosilicate as viewed down the axis. The disordered sodium ions in the tunnel are represented by filled circles and the water molecules by open circles.
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by UNIV OF ARIZONA on June 13, 2017 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch011
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Figure 2. Representation of the titanosilicate structure as viewed down the baxis showing the arrangement of the atoms in the ac and be faces and the six coordinate sodium ions in the framework sites.
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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Figure 3. Schematic representation of the Cs exchanged phase of the titanosilicate as viewed down the c-axis. The tunnel also contains sodium ions and water molecules.
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
173 3.057(6) Â and two bonds to water molecules of length 2.95(2) Â. These bond distances are very close to the sum of the ionic radii for the observed coordination numbers of the cesium ion and are largely responsible for the high affinity for this ion. The crystal structure data explains very well the observed ion exchange behavior of the titanosilicate. This is illustrated by the titration curves (Figure 4) for Na and Cs using the acid form of the exchanger, H Ti 0 Si0 H 0. The theoretical capacity is 7.81 meq/g. Cesium ion is exchanged in acid solution below pH 2 and attains a total uptake of about 25% of the theoretical capacity. This uptake is in keeping with the X-ray data showing that no Cs resides in the framework and only half the sites in the tunnel can be occupied by Cs . The remainder of the sites are occupied by water and hydronium ions. In contrast, sodium ion begins to exchange at a pH near 2 and attains a total uptake of 6.1 meq/g, which is very close to that required by the sodium phase formula. Rietveld refinement of the structure at different levels of exchange showed that the sodium ion fills the preferred sites in the framework first and then begins to fill the tunnel sites (8). However, if an equimolar mixture of sodium and cesium ions is used as the titrant only about 0.5 meq/g of Cs is exchanged over the entire pHrangebut more than 4 meq/g of Na is exchanged. We interpret this as a competition for the several ion exchange sites. The sodium ion can diffuse into both the framework sites and the tunnel sites. The larger Cs ions can only move down the tunnels while the sodium ions can enter the tunnels from all three directions, down the tunnels and through the framework sites. Diffusion of Cs into the lattice is further retarded by the strong bonding it experiences within the tunnels. Thus, while the affinity of the exchanger for Cs is very high, kinetic and mainly steric factors determine that more Na will be exchanged. By the reverse token Cs is strongly retained by the exchanger and thus is easily separated from Na by mild acid treatment. Another factor that may be responsible for the slow diffusion of cesium ion may result from the significant electrostatic repulsive forces it must encounter when sodium is present in the framework sites, presenting a barrier to its diffusion. In contrast, the smaller sodium ion moving through the tunnels and faces could diffuse more rapidly, encountering less electrostatic repulsion, and, in competition with Cs , not only fills up the framework sites but is also found in the tunnels. This further reduces the Cs occupancy. Thus, while Cs is thermodynamically favored over Na as shown from K values, we attribute its low uptake to less favorable kinetic and steric factors. Additional studies to further develop these concepts are in progress. Many nuclear waste streams are alkaline and contain high levels of Na , lesser but significant amounts of K , and small amounts of Cs (10 -10" M). Thus, in spite of the low uptake of Cs , the high K values allow Cs to be removed from such solutions by the titanosilicate, albeit with a very low capacity. Figure 5 shows the potentiometric titration curves including those for K and I i ions. We note that initially K is sorbed from acid solution to a somewhat greater extent than Cs . The potassium uptake curve parallels that for Cs up to pH 7 where there is a change of slope in the curve. The amount of K* exchanged at this pH is -2.5 meq/g. The X-ray diffraction results show (8) that K preferentially occupies the tunnel sites at l/4c, 3/4c. There are four K-O bond distances (8-fold coordination) at 3.18(1) Â and four at 2.82(1) Â. The former value is somewhat large, but the latter is die expected value for the sum of the ionicradiifor eight coordination. Potassium ion is too large to fit into the framework sites. Instead, additional loading of K results in occupation of a second site close to the framework but halfway between the K ions at 1/4 c, 3/4 c (Figure 6). Because of the volume limitations of the tunnel, the maximum uptake of K is less than Na , and the formula derived by chemical analysis is Kj H Ti 0 (Si0 )*H 0. This value for K exchange compares favorably to K H as derived from the Rietveld refinement Because of the positioning of die K ions, their distribution is 0.5 moles within the tunnels at the centers of the ab plane and l/4c, 3/4c and about 0.85-0.88 moles in the second site near the framework at 0 and l/2c. +
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Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
174 8
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Downloaded by UNIV OF ARIZONA on June 13, 2017 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch011
I
K*>Na >Li based on the suitability of the bond lengths. This is in fact the case for the acid phase K values in ml/g at pH 2.5-3 were Li,
