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Ion exchange selectivity of phillipsite for Cs+: a structural investigation using the Rietveld method
Microporous and Mesoporous Materials 32 (1999) 319–329 www.elsevier.nl/locate/micmat
Ion exchange selectivity of phillipsite for Cs+: a structural investigation using the Rietveld method A.F. Gualtieri a, *, D. Caputo b, C. Colella b a Dipartimento di Scienze della Terra, Universita` di Modena e Reggio Emilia,Via S.Eufemia 19, 41100 Modena, Italy b Dipartimento di Ingegneria dei Materiali e della Produzione, Universita` Federico II, P.le V. Tecchio 80, 80125 Napoli, Italy Received 21 December 1998; received in revised form 18 March 1999; accepted for publication 9 June 1999
Abstract Different ion exchange selectivities for Cs of two phillipsites, one sedimentary and one synthetic, having similar framework compositions, were interpreted by means of Rietveld structural analysis. Both phillipsites were preexchanged to achieve Na forms throughout exhaustive elution using an NaCl solution and then enriched in Cs by contact with mixed Cs+ and Na+ solutions. X-ray diffraction data of the original and Cs-exchanged samples were refined using the Rietveld method. Four extra-framework sites labelled I, II, II∞, and II◊ were refined. Cation distribution in the four sites showed that, although sites I and II are readily occupied by Cs, some Na is prone to be retained in site II∞, which displays therefore a reduced selectivity for Cs. The new site II◊ is occupied by Cs. The synthetic sample retains much more Na than the sedimentary sample, and this is the reason for the higher selectivity of the sedimentary phillipsite for Cs compared with the synthetic one. The results of this work also suggest that phillipsite may be utilized as a selective cation exchanger for the removal of Cs+ radionuclide in nuclear waste. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Cs-exchange; Ion selectivity; Phillipsite; Structure Rietveld refinement
1. Introduction Zeolite phillipsite (PHI ) [1] has a the topological symmetry B2mb [2,3] and space group P2 /m 1 [4] with two extra-framework sites (I and II ). The position of the two sites is shown in Fig. 1 (after Ref. [5]). Site I usually concentrates most of the Ba and K. Site II, while being always populated, has a surprisingly low occupancy below 50%, although a full site occupancy is sterically possible [6,7]. This prevents an oversaturation of positive charges over the coordinated framework oxygens, * Corresponding author. Fax +39-59-417399. E-mail address: [email protected] (A.F. Gualtieri)
and the extra amount of cations may migrate to site II∞. Na and Ca, owing to their small size, cannot populate site I, whereas large cations such as K or Ba cannot migrate to site II∞ occupied by Na. Phillipsite shows a notable selectivity for Cs, i.e. a preference for this cation compared with other cations [8]. In a recent paper [9], the selectivity of phillipsite for Cs was examined as a function of the framework composition in three phillipsite samples: two natural (one sedimentary and one hydrothermal ) and one synthetic. Two points were evidenced: (a) phillipsite beforehand preexchanged into the Na form was able to exchange fully Na for Cs and (b) the selectivity for Cs,
1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 12 1 - 3
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and Co [12]. Clinoptilolite has already been positively tested for this purpose [13].
2. Experimental section 2.1. Materials
Fig. 1. Structure of phillipsite projected on the b–c plane showing the position of sites I and II. Site I is located in a peripheral position displaced along one of the larger channels in the type I octagonal prism, and site II is located in the cage formed by the type II octagonal prism and the adjacent type I octagonal prism, near an intersection of the two sets of channels running along a and b.
measured by the equilibrium constant K [10,11] a of Na+ to Cs+ exchange, was found to be a function of the Si/Al ratio. Surprisingly, the values of K were rather different for the sedimentary and a synthetic samples (26.2 and 16.8, respectively) in spite of a substantial identity of what appeared to be framework and extra-framework positions. This article intends to give an interpretation of these different selectivities. A structural investigation was undertaken of the samples in their original and the Cs-enriched forms. An understanding of the Cs selectivity of phillipsite is also very important in view of the potential application of this zeolite for the removal of 137Cs+ radionuclide from nuclear wastes. In the past, environmental contamination by radioactive substances from nuclear weapons tests, nuclear plants and accidental releases, and the problem of the generation of radioactive isotopes in nuclear reprocessing plants have prompted a search for substances capable of removing selectively, effectively and cheaply radionuclides such as Cs, Sr,
The sedimentary phillipsite sample was obtained from the parent material (Neapolitan yellow tuff from Marano, northern outskirts of Naples), using the separation methods described in Ref. [14]. The purity was checked by X-ray diffraction. Its chemical analysis, determined through EDS microanalysis using a LINK AN 10.000S apparatus, turned out to be (in wt.%): SiO =52.1, Al O =18.6, 2 2 3 Fe O =0.2, MgO=0.2, CaO=2.3, Na O=3.3, 2 3 2 K O=7.5, H O=15.7. The water content was 2 2 measured by thermogravimetry (Netzsch STA-409 thermoanalyzer). The Si/(Si+Al ) ratio was 0.70 [9]. The synthetic phillipsite was obtained hydrothermally from an aluminosilicate gel having a Si/Al ratio of 3.93 [9]. The gel was treated at 120°C for 3 days under continuous stirring, with a mixed NaOH–KOH solution (Na/K=3.99, OH− molality equal to 1.00). The solid-to-liquid ratio was 1/10. The chemical composition of the phillipsite crystals, the purity of which was checked by scanning electron microscopy (Cambridge Stereoscan TP 250), was (in wt.%): SiO =52.5, 2 Al O =17.9, Na O=5.3, K O=8.7, H O=15.5. 2 3 2 2 2 Its Si/(Si+Al ) ratio was 0.71. To obtain the Cs-enriched forms of the two phillipsites, the samples were initially transformed into their Na forms by exhaustive elution with NaCl solution and successively allowed to react with solutions containing different amounts of Cs and Na, prepared from CsCl and NaCl (reagent grade Carlo Erba). The Cs+ concentrations in both phillipsites were determined by chemically dissolving the zeolites and measuring the Cs+ in solution by AAS (Perkin Elmer AA2100). The calculated chemical analyses of the Cs-enriched phillipsite samples, considering the Cs and Na contents measured by AAS and the water loss determined by thermal analysis, were (in wt.%): SiO =40.7, Al O =14.5, Fe O =0.2, Na O= 2 2 3 2 3 2
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0.9, Cs O=35.6, H O=8.2 for the sedimentary 2 2 sample, and SiO =43.2, Al O =14.7, Na O=2.7, 2 2 3 2 Cs O=28.8, H O=10.6 for the synthetic sample. 2 2 2.2. X-ray data collection The powders of the samples were side-loaded on a flat Al holder and analysed with Cu radiation in a Bragg–Brentano (BB) geometry. Data were collected in the angular range 17–140° 2h with steps of 0.02° and 20 s per step, a divergence slit of 0.5° and a receiving slit of 0.1 mm. An internal standard (silicon NBS 640a) was added to the powder samples and the data were re-collected in the range 5–80° 2h using a step scan of 0.02° 2h and 5 s per step for the refinement of the absolute cell constants. 2.3. Refinement and structure analysis The refinement of the lattice constants and structures were both performed in the space group P2 /m using GSAS [15] with starting atomic co1 ordinates taken from Ref. [4]. The background profile was fitted with a Chebyshev polynomial function with three coefficients. Peak profiles were modelled using a pseudo-Voigt function with two Gaussian and two Lorentzian coefficients, a sample displacement parameter and an asymmetry coefficient. Soft constraints on the T–O distances were imposed and used as additional observations in the earlier stages of the refinement.
3. Results The statistics of the refinements are reported in Table 1 together with refined cell constants. Fig. 2
shows the observed (crosses), calculated (continuous line) and difference curve (bottom line) of the refined patterns. Table 2 gives the refined coordinates of the extra-framework cations, their populations, and thermal parameters. Coordinates of the framework are available upon request from the authors. None of the samples showed (Si,Al ) ordering in the tetrahedral sites given the very low spread of D(T– ˚ . The O) which ranges from 0.002 to 0.005 A ˚ average T–O distance increases from 1.654(8) A ˚ and 1.672(8) A ˚ in the and 1.659(8) to 1.679(9) A sedimentary and synthetic Cs-exchanged samples, respectively. Table 3 reports the chemical composition and refined occupancy of the extra-framework atoms of the samples showing a very good agreement between the number of refined cations and those from the chemical analysis. Table 4 shows the T– O distances and interactions of the extra-framework sites with the framework oxygens and water molecules.
4. Discussion The exchanged forms showed a complete exchange CsK in site I (see the variation of the ˚ in the average I–O distance from 3.00–3.02 A ˚ original samples to 3.08–3.22 A in the exchanged samples) with an 11-fold coordination (eight framework oxygens and three water molecules at ˚ ) in the sedimentary an average distance of 3.08 A phillipsite, and a 9-fold coordination (five framework oxygens and four water molecules at an ˚ ) in the synthetic average distance of 3.21 A phillipsite. In both exchanged forms, cations in site II were
Table 1 Refined unit cells and refinement statistics of the phillipsite samples Sample
˚) Space a (A group
˚) b (A
˚) c (A
b (°)
No. of Independent Refined R R R(F2) D p wp wd observations reflections parameters (%) (%) (%)
Sedimentary Cs-sedimentary Synthetic Cs-synthetic
P2 /m 9.9664(8) 1 P2 /m 10.051(5) 1 P2 /m 10.0386(8) 1 P2 /m 10.067(2) 1
14.195(1) 14.195(7) 14.183(1) 14.235(3)
8.696(1) 8.693(5) 8.6704(8) 8.709(2)
124.89(1) 125.16(3) 125.08(7) 125.06(2)
5969 6177 6140 6148
819 847 842 843
82 85 82 79
9.3 11.1 13.5 7.3 9.5 10.9 9.6 11.7 14.2 9.5 11.5 14.0
1.7 1.75 1.61 1.57
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(a)
(b) Fig. 2. Observed (crosses), calculated (continuous line) and difference curve (bottom line) of the refined patterns: (a) sedimentary phillipsite; (b) synthetic phillipsite; (c) Cs-exchanged sedimentary phillipsite; (d) Cs-exchanged synthetic phillipsite. The insets contain the 80–140° 2h region.
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(c)
(d) Fig. 2. (continued).
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Table 2 Coordinates, populations, and thermal parameters of the extra-framework cations. The figures in parentheses in the first column give the site multiplicity while the figures in parentheses in the other columns give the standard deviations Site
Sedimentary
Synthetic
Cs-sedimentary
Cs-synthetic
I(2) X Y Z Occ U ISO
0.8707(9) 0.250 0.2063(8) 1.0 0.08(1)
0.9482(8) 0.250 0.3169(8) 0.46(3) 0.05(1)
0.8562(7) 0.250 0.1619(7) 1.0 0.09(2)
0.8085(8) 0.250 0.0468(5) 1.0 0.07(2)
II(4) X Y Z Occ U ISO
0.6125(8) 0.6370(7) 0.4419(9) 0.17(2) 0.08(1)
0.606(2) 0.636(2) 0.474(1) 0.54(3) 0.08(2)
0.6557(6) 0.7232(7) 0.6194(7) 0.24(4) 0.06(2)
0.6982(5) 0.7028(6) 0.5703(5) 0.29(3) 0.07(3)
II∞(4) X Y Z Occ U ISO
0.366(1) 0.1248(9) 0.439(1) 0.31(3) 0.08(1)
0.508(2) 0.113(1) 0.674(1) 0.29(2) 0.08(2)
0.392(3) 0.142(4) 0.537(4) 0.16(3) 0.04(1)
0.520(2) 0.141(4) 0.386(4) 0.34(5) 0.06(2)
II◊(2) X Y Z Occ U ISO
– – – – –
– – – – –
0.7977(6) 0.750 0.5205(5) 0.51(5) 0.09(3)
– – – – –
W1(2) X Y Z Occ U ISO
0.815(2) 0.750 0.513(1) 1.0 0.06(2)
0.696(3) 0.750 0.395(3) 0.76(4) 0.10(3)
– – – – –
– – – – –
W2(2) X Y Z Occ U ISO
0.214(2) 0.750 0.489(1) 1.0 0.03(2)
0.207(4) 0.750 0.477(3) 1.0 0.04(1)
0.052(3) 0.750 0.442(5) 1.0 0.06(2)
−0.059(8) 0.750 0.062(7) 1.0 0.06(3)
W3(4) X Y Z Occ U ISO
0.267(2) 0.872(1) 0.106(2) 0.87(4) 0.05(2)
0.258(3) 0.844(3) 0.049(5) 1.0 0.05(2)
0.456(6) 0.76(1) 0.202(7) .63(6) 0.07(2)
0.351(6) 0.892(7) 0.29(1) 1.0 0.08(3)
W4(2) X Y Z Occ U ISO
0.512(2) 0.250 0.562(3) 0.82(4) 0.09(2)
0.511(3) 0.250 0.458(5) 0.73(5) 0.07(2)
0.544(4) 0.250 0.77(1) 1.0 0.104(4)
0.676(5) 0.250 0.614(7) 1.0 0.08(3)
W5(4) X Y Z Occ U ISO
0.500 0.500 0.500 1.0 0.10(1)
0.500 0.500 0.500 1.0 0.07(1)
0.500 0.500 0.500 0.80(5) 0.08(2)
0.500 0.500 0.500 1.0 0.07(2)
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Table 3 Chemical formulae of the samples and cation distributions. The numbers of refined atoms per unit cell for each site are reported in square brackets, and the esd’s in parentheses Sample
Site I
Site II
Site II∞
Site II◊
Chemical formula
Sedimentary Synthetic Cs-sedimentary Cs-synthetic
K[2.0] K[0.9(4)] Cs[2.0] Cs[2.0]
Ca+Na[0.68(4)] Na+K[2.16(6)] Cs[0.96(5)] Cs[1.16(7)]
Na[1.24(5)] Na[1.12(6)] Na[0.64(4)] Na[1.36(3)]
– – Cs[1.02(7)] –
K Ca Na (Al Si O ) · 11.33H O 2.08 0.54 1.38 4.72 11.3 32 2 K Na (Al Si O ) · 11.21H O 2.42 2.21 4.59 11.4 32 2 Cs Na (Al Si O ) · 11.33H O 4.20 0.47 4.72 11.3 32 2 Cs Na (Al Si O ) · 11.2H O 3.24 1.36 4.59 11.4 32 2
Fig. 3. Distribution of the extra-framework cations in the Cs-exchanged sedimentary phillipsite.
completely exchanged by Cs which coordinated five framework oxygens and three water molecules ˚ in the sedimentary at an average distance of 3.11 A one, and three framework oxygens and four water ˚ in the molecules at an average distance of 3.29 A synthetic sample. In the sedimentary sample, Cs with a 9-fold coordination (average distance of ˚ ) was also refined in site II◊ on the mirror 3.16 A plane close to mutually exclusive site II. The migration is a consequence of the short II–II ˚ ) which prevents the site populadistance (0.76 A tion to exceed 50%. In fact, in this configuration (see Fig. 3) only 24% Cs filled site II. The migration did not occur in the synthetic sample where the population of Cs in site II was only 29%. Although in both samples residual Na was found in site II∞, its population was lower in the
sedimentary sample (16%) with more Cs in site II and II◊ with respect to the population in the synthetic sample (34%). As discussed in Refs. [5– 7] Na in site II∞ is necessary because the population of site II is always less than 50%. This allows the prevention of an oversaturation of positive charges over the coordinated framework oxygens and a compensation of the negative framework density at the bottom of the cage. In both samples, the W1 site on the mirror plane which usually coordinates the cation in site II was empty, probably because there is not enough space for a water molecule if large Cs cations are adsorbed in the cage. Fig. 4 is an attempt to explain the difference in the ion exchange of Cs for the sedimentary and synthetic phillipsite and reports the cation exchange path from the starting materials to the Cs-exchanged forms via the Na-exchanged forms. The cation distribution of the intermediate Na-exchanged forms (shaded area) was assumed on the basis of structural constraints (the degree of exchange NaK in site I is 50% and the population of site II is less than 50% [6,7]) and the cation distributions in the original and final Cs-exchanged phillipsites. The following general picture can be drawn: in the sedimentary sample, site I, originally fully occupied by K, is half occupied after the NaK substitution, and then fully exchanged by Cs. The synthetic sample instead had less K in site I which is successively half exchanged by Na, and then fully exchanged by Cs. In addition, the occupancy of site I with K was originally very low, and more Na was captured in site II∞, if we assume that the population of site II is ca. 50% (2 a.f.u.). Thus, since the degree of CsNa substitution in site II∞ was low, the syn-
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Table 4 ˚ ) involving framework atoms and extra-framework cations. The average is calculated on the basis Significant interatomic distances (A of the coordinating anions at a bond distance Distance
Sedimentary
T1–Oa T2–Oa T3–Oa T4–Oa Average
1.653(8) 1.656(9) 1.653(8) 1.647(9) 1.652
I–O1×2 I–O3×2 I–O5×2 I–O8 I–O9 I–W1 I–W2 I–W3×2 I–W4 Average
Synthetic 1.655(8) 1.664(8) 1.655(8) 1.661(9) 1.659
Cs-sedimentary
Cs-synthetic
1.696(9) 1.669(9) 1.687(9) 1.666(8) 1.679
1.673(7) 1.671(7) 1.671(8) 1.673(8) 1.672
3.20(4) 3.18(5) 2.80(8) 3.38(4) >3.5 2.65(8) 3.20(5) 2.82(7) – 3.02
3.07(4) >3.5 3.02(5) >3.5 >3.5 2.96(5) 2.96(4) 2.93(5) – 3.00
3.23(6) 3.39(6) 2.73(7) 3.13(6) 3.19(7) – 3.01(4) 2.91(5) 2.90(6) 3.06
3.33(5) 3.65(7) 3.05(8) 3.36(6) 3.61(7) – 3.20(6) 3.20(8) 3.20(7) 3.21
II–O1 II–O1 II–O2 II–O6 II–O6 II–O7 II–O9 II–W1 II–W2 II–W3 II–W4 II–W5 Average
2.68(6) – 2.65(7) – – 2.44(5) – 2.36(5) – – 2.02(9) 2.44(3) 2.43
2.81(4) – 2.77(4) – – 2.60(8) – 2.15(5) – – 2.28(7) 2.27(7) 2.48
2.81(7) 3.36(7) >3.7 2.95(6) 3.38(9) – 3.04(7) – >3.7 2.99(8) 2.98(7) 3.42(7) 3.12
3.09(5) >3.7 3.39(7) >3.7 >3.7 3.26(4) >3.7 – 3.57(6) 3.57(7) 3.19(7) 3.36(6) 3.39
II∞–O1 II∞–O2 II∞–O3 II∞–O6 II∞–O7 II∞–W1 II∞–W3 II∞–W4 II∞–W5 Average
2.12(7) >2.85 – 2.67(9) 2.57(8) 2.73(8) >2.85 2.15(6) 2.09(8) 2.39
>2.85 2.43(4) – >2.85 >2.85 2.63(5) 2.27(7) 2.74(8) 2.17(6) 2.44
– 2.39(6) >2.85 – 2.41(7) – 2.41(5) 2.40(9) 2.40(7) 2.40
– >2.85 2.27(7) 2.82(6) >2.85 – 2.31(7) 2.29(6) 2.30(5) 2.40
II◊-O1×2 II◊-O2×2 II◊-O7×2 II◊-W2 II◊-W3 II◊-W4 Average
– – – – – – –
a Average of the four tetrahedral distances.
– – – – – – –
3.17(6) 3.28(7) 3.33(7) 3.00(8) 2.91(8) 2.92(8) 3.15
– – – – – – –
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Fig. 4. Hypothetical cation exchange path from the starting material to the Cs-exchanged forms via the intermediate Na-exchanged forms (shaded area) assumed on the basis of structural constraints (the degree of exchange NaK in site I is 50% and the population of site II is less than 50% [6,7]) and the cation distributions in the starting phillipsites and final Cs-exchanged forms.
thetic sample retained much more Na there and needed less Cs in site II to achieve charge balance with respect to the sedimentary sample. The sedimentary sample had a Si/(Si+Al ) ratio of 0.704 which is very similar to that of sample 2 (phillipsite from Perrier, France, with Si/(Si+Al )=0.72) described in [5–7] and exchanged with K, Na, Ca, Ba, and Sr. The exchange by Na, K and Ca (populating sites I and II ) was not complete, and some residual Na was refined in site II∞. The exchange by Ba and Sr (populating sites I and II ) was complete, and no Na was refined in site II∞. Given the nature of the exchanged cation (ionic radius and charge) and its coordination number on site II, it is possible to predict whether site II∞ is populated or not. Table 5 reports the atomic charge (c), the coordination number (CN ), the ionic radius (r) for this CN, and the average cation–anion distance (d ) for the
cations populating site II in the exchanged phillipsite sample with a Si/(Si+Al ) ratio of ca. 0.70– 0.72. If we define the parameter C=crd as an atomic sterical occupancy coefficient, when C is larger than 5.73 (as in the case of Sr2+ and Ba2+), site II∞ is not populated since the negative charge density at the bottom of the cage is already compensated by the presence of such large and doubly charged cations in the cage. When C is lower than 5.73, Na in site II∞ is needed to achieve a proper distribution of the positive charge in the cage. Concerning the unit cell, it was found that the Ba- and K-exchanged forms [5–7] showed b ort angles much closer to 90° with a clear indication that K and Ba on site I, given their ionic radii, had a tendency to coordinate oxygens O1 and O3 in such a way that the cavity becomes more symmetrical and consequently the cell symmetry
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Table 5 Atomic parameters necessary for the calculation of C=crd, the atomic sterical occupancy coefficient, for extra-framework cations on site II of phillipsite with Si/(Si+Al )=0.70–0.72, indicating if site II∞ is populated Cation in site II a
Atomic charge c
Coordination number CNa
˚) Ionic radius r (A for that CNb
Average cation–anion distance ˚ )a d (A
C=crd
Na in II∞a
Na+ K+ Cs+ Ca++ Sr++ Ba++
1 1 1 2 2 2
6 9 9 7 8 8
1.02 1.55 1.78 1.06 1.26 1.42
2.40 3.09 3.22 2.50 2.80 3.05
2.45 4.79 5.73 5.30 7.06 8.66
Yes Yes Yes Yes No No
a Data taken from Ref. [5]. b Data taken from Ref. [16 ].
is getting closer to orthorhombic. This work showed that in the sedimentary Cs-exchanged sample Cs plays a role in the distortion of the cavity around site I which is in concert with the behavior of K and Ba. The calculated angle in the pseudo-orthorhombic setting b is in fact 90.15°. ort 5. Conclusions The results of this work are: (1) Cs fully exchanges for K in site I, and for Ca, Na in site II, while Na is retained in site II’. (2) A new position occupied by Cs was located and labelled site II◊ on the mirror plane close to the mutually exclusive site II. (3) A parameter C, the atomic sterical occupancy coefficient, based upon the nature of the cation in site II and its population predicts whether site II∞ is populated: if C<5.73, Na in site II∞ is needed to achieve a proper distribution of the positive charge in the cage. (4) Site W1 is always empty because there is no space available in the cage of type II when the large Cs cations are present. In general, the more Cs is adsorbed, the less room is available for water molecules. (5) The difference in the degree of cation exchange for Cs in the sedimentary and synthetic phillipsites was mainly due to the starting distribution of the extra-framework cations. Despite their similar Si/(Si+Al ) ratios, the synthetic sample did not contain Ca which was instead detected in the
sedimentary sample. To template the structure, some K is needed in site II together with Na. Consequently, site I is half occupied. In the sedimentary sample with Ca and Na in site II, no K is required there, and a full occupancy of site I is possible. (6) Phillipsite is a very good potential cation exchanger for 137Cs+ in nuclear waste, like other commercially available zeolites such as clinoptilolite, although germane experiments should be done to support this statement, essentially because the selectivity may significantly differ with the pH and the nature and relative concentrations of the cations in the solution.
Acknowledgements Financial support is acknowledged from the Italian CNR and MURST. Prof. E. Passaglia is greatly ackowledged for critical reading of the manuscript. Thanks also go to the three anonymous referees for good suggestions and improvement of the paper.
References [1] G. Gottardi, E. Galli, in: Natural Zeolites, Springer, Berlin, 1985, p. 409. [2] R. Sadanaga, F. Marumo, Y. Take´uchi, Acta Crystallogr. 4 (1961) 1153. [3] H. Steinfink, Acta Crystallogr. 15 (1962) 644.
A.F. Gualtieri et al. / Microporous and Mesoporous Materials 32 (1999) 319–329 [4] R. Rinaldi, J.J. Pluth, J.V. Smith, Acta Crystallogr., Sect. B 30 (10) (1974) 2426. [5] A.F. Gualtieri, E. Passaglia, E. Galli, A. Viani, Microporous Mesoporous Mater. (1999) in press. [6 ] E. Passaglia, A.F. Gualtieri, E. Galli, Proc. of Zeolite 97 International Conference, Ischia (Naples, Italy), (1999) in press. [7] A.F. Gualtieri, E. Passaglia, E. Galli, Proc. of Zeolite 97 International Conference, Ischia (Naples, Italy), (1999) in press. [8] C. Colella, Mineral. Deposita 31 (1996) 554. [9] A. Adabbo, D. Caputo, B. de Gennaro, M. Pansini, C. Colella, Microporous and Mesoporous Mater. (1999) in press. [10] G.L. Gaines, H.C. Thomas, J. Chem. Phys. 21 (1953) 714.
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[11] D. Caputo, R. Dattilo, M. Pansini, Proc. III Convegno Nazionale di Scienza e Tecnologia delle Zeoliti, Cetraro, Italy, September 28–29, (1995). [12] D.T. Karamanis, X.A. Aslanoglou, P.A. Assimakopoulos, N.H. Gangas, A.A. Pakou, N.G. Papayannakos, Clays Clay Min. 45 (1997) 709. [13] E.A. Behrens, P. Sylvester, A. Clearfield, Environm. Sci. Technol. 32 (1) (1998) 101. [14] M. de’Gennaro, E. Franco, L’Industria Mineraria 30 (1979) 329. [15] A.C. Larson, von R.B. Dreele, in: Generalized Structure Analysis System, Los Alamos National Laboratory, Los Alamos, NM, 1998, p. 86. LAUR. [16 ] R.D. Shannon, C.T. Prewitt, Acta Crystallogr., Sect. B 25 (1969) 925.
Ion exchange selectivity of phillipsite for Cs+: a structural investigation using the Rietveld method A.F. Gualtieri a, *, D. Caputo b, C. Colella b a Dipartimento di Scienze della Terra, Universita` di Modena e Reggio Emilia,Via S.Eufemia 19, 41100 Modena, Italy b Dipartimento di Ingegneria dei Materiali e della Produzione, Universita` Federico II, P.le V. Tecchio 80, 80125 Napoli, Italy Received 21 December 1998; received in revised form 18 March 1999; accepted for publication 9 June 1999
Abstract Different ion exchange selectivities for Cs of two phillipsites, one sedimentary and one synthetic, having similar framework compositions, were interpreted by means of Rietveld structural analysis. Both phillipsites were preexchanged to achieve Na forms throughout exhaustive elution using an NaCl solution and then enriched in Cs by contact with mixed Cs+ and Na+ solutions. X-ray diffraction data of the original and Cs-exchanged samples were refined using the Rietveld method. Four extra-framework sites labelled I, II, II∞, and II◊ were refined. Cation distribution in the four sites showed that, although sites I and II are readily occupied by Cs, some Na is prone to be retained in site II∞, which displays therefore a reduced selectivity for Cs. The new site II◊ is occupied by Cs. The synthetic sample retains much more Na than the sedimentary sample, and this is the reason for the higher selectivity of the sedimentary phillipsite for Cs compared with the synthetic one. The results of this work also suggest that phillipsite may be utilized as a selective cation exchanger for the removal of Cs+ radionuclide in nuclear waste. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Cs-exchange; Ion selectivity; Phillipsite; Structure Rietveld refinement
1. Introduction Zeolite phillipsite (PHI ) [1] has a the topological symmetry B2mb [2,3] and space group P2 /m 1 [4] with two extra-framework sites (I and II ). The position of the two sites is shown in Fig. 1 (after Ref. [5]). Site I usually concentrates most of the Ba and K. Site II, while being always populated, has a surprisingly low occupancy below 50%, although a full site occupancy is sterically possible [6,7]. This prevents an oversaturation of positive charges over the coordinated framework oxygens, * Corresponding author. Fax +39-59-417399. E-mail address: [email protected] (A.F. Gualtieri)
and the extra amount of cations may migrate to site II∞. Na and Ca, owing to their small size, cannot populate site I, whereas large cations such as K or Ba cannot migrate to site II∞ occupied by Na. Phillipsite shows a notable selectivity for Cs, i.e. a preference for this cation compared with other cations [8]. In a recent paper [9], the selectivity of phillipsite for Cs was examined as a function of the framework composition in three phillipsite samples: two natural (one sedimentary and one hydrothermal ) and one synthetic. Two points were evidenced: (a) phillipsite beforehand preexchanged into the Na form was able to exchange fully Na for Cs and (b) the selectivity for Cs,
1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 12 1 - 3
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and Co [12]. Clinoptilolite has already been positively tested for this purpose [13].
2. Experimental section 2.1. Materials
Fig. 1. Structure of phillipsite projected on the b–c plane showing the position of sites I and II. Site I is located in a peripheral position displaced along one of the larger channels in the type I octagonal prism, and site II is located in the cage formed by the type II octagonal prism and the adjacent type I octagonal prism, near an intersection of the two sets of channels running along a and b.
measured by the equilibrium constant K [10,11] a of Na+ to Cs+ exchange, was found to be a function of the Si/Al ratio. Surprisingly, the values of K were rather different for the sedimentary and a synthetic samples (26.2 and 16.8, respectively) in spite of a substantial identity of what appeared to be framework and extra-framework positions. This article intends to give an interpretation of these different selectivities. A structural investigation was undertaken of the samples in their original and the Cs-enriched forms. An understanding of the Cs selectivity of phillipsite is also very important in view of the potential application of this zeolite for the removal of 137Cs+ radionuclide from nuclear wastes. In the past, environmental contamination by radioactive substances from nuclear weapons tests, nuclear plants and accidental releases, and the problem of the generation of radioactive isotopes in nuclear reprocessing plants have prompted a search for substances capable of removing selectively, effectively and cheaply radionuclides such as Cs, Sr,
The sedimentary phillipsite sample was obtained from the parent material (Neapolitan yellow tuff from Marano, northern outskirts of Naples), using the separation methods described in Ref. [14]. The purity was checked by X-ray diffraction. Its chemical analysis, determined through EDS microanalysis using a LINK AN 10.000S apparatus, turned out to be (in wt.%): SiO =52.1, Al O =18.6, 2 2 3 Fe O =0.2, MgO=0.2, CaO=2.3, Na O=3.3, 2 3 2 K O=7.5, H O=15.7. The water content was 2 2 measured by thermogravimetry (Netzsch STA-409 thermoanalyzer). The Si/(Si+Al ) ratio was 0.70 [9]. The synthetic phillipsite was obtained hydrothermally from an aluminosilicate gel having a Si/Al ratio of 3.93 [9]. The gel was treated at 120°C for 3 days under continuous stirring, with a mixed NaOH–KOH solution (Na/K=3.99, OH− molality equal to 1.00). The solid-to-liquid ratio was 1/10. The chemical composition of the phillipsite crystals, the purity of which was checked by scanning electron microscopy (Cambridge Stereoscan TP 250), was (in wt.%): SiO =52.5, 2 Al O =17.9, Na O=5.3, K O=8.7, H O=15.5. 2 3 2 2 2 Its Si/(Si+Al ) ratio was 0.71. To obtain the Cs-enriched forms of the two phillipsites, the samples were initially transformed into their Na forms by exhaustive elution with NaCl solution and successively allowed to react with solutions containing different amounts of Cs and Na, prepared from CsCl and NaCl (reagent grade Carlo Erba). The Cs+ concentrations in both phillipsites were determined by chemically dissolving the zeolites and measuring the Cs+ in solution by AAS (Perkin Elmer AA2100). The calculated chemical analyses of the Cs-enriched phillipsite samples, considering the Cs and Na contents measured by AAS and the water loss determined by thermal analysis, were (in wt.%): SiO =40.7, Al O =14.5, Fe O =0.2, Na O= 2 2 3 2 3 2
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0.9, Cs O=35.6, H O=8.2 for the sedimentary 2 2 sample, and SiO =43.2, Al O =14.7, Na O=2.7, 2 2 3 2 Cs O=28.8, H O=10.6 for the synthetic sample. 2 2 2.2. X-ray data collection The powders of the samples were side-loaded on a flat Al holder and analysed with Cu radiation in a Bragg–Brentano (BB) geometry. Data were collected in the angular range 17–140° 2h with steps of 0.02° and 20 s per step, a divergence slit of 0.5° and a receiving slit of 0.1 mm. An internal standard (silicon NBS 640a) was added to the powder samples and the data were re-collected in the range 5–80° 2h using a step scan of 0.02° 2h and 5 s per step for the refinement of the absolute cell constants. 2.3. Refinement and structure analysis The refinement of the lattice constants and structures were both performed in the space group P2 /m using GSAS [15] with starting atomic co1 ordinates taken from Ref. [4]. The background profile was fitted with a Chebyshev polynomial function with three coefficients. Peak profiles were modelled using a pseudo-Voigt function with two Gaussian and two Lorentzian coefficients, a sample displacement parameter and an asymmetry coefficient. Soft constraints on the T–O distances were imposed and used as additional observations in the earlier stages of the refinement.
3. Results The statistics of the refinements are reported in Table 1 together with refined cell constants. Fig. 2
shows the observed (crosses), calculated (continuous line) and difference curve (bottom line) of the refined patterns. Table 2 gives the refined coordinates of the extra-framework cations, their populations, and thermal parameters. Coordinates of the framework are available upon request from the authors. None of the samples showed (Si,Al ) ordering in the tetrahedral sites given the very low spread of D(T– ˚ . The O) which ranges from 0.002 to 0.005 A ˚ average T–O distance increases from 1.654(8) A ˚ and 1.672(8) A ˚ in the and 1.659(8) to 1.679(9) A sedimentary and synthetic Cs-exchanged samples, respectively. Table 3 reports the chemical composition and refined occupancy of the extra-framework atoms of the samples showing a very good agreement between the number of refined cations and those from the chemical analysis. Table 4 shows the T– O distances and interactions of the extra-framework sites with the framework oxygens and water molecules.
4. Discussion The exchanged forms showed a complete exchange CsK in site I (see the variation of the ˚ in the average I–O distance from 3.00–3.02 A ˚ original samples to 3.08–3.22 A in the exchanged samples) with an 11-fold coordination (eight framework oxygens and three water molecules at ˚ ) in the sedimentary an average distance of 3.08 A phillipsite, and a 9-fold coordination (five framework oxygens and four water molecules at an ˚ ) in the synthetic average distance of 3.21 A phillipsite. In both exchanged forms, cations in site II were
Table 1 Refined unit cells and refinement statistics of the phillipsite samples Sample
˚) Space a (A group
˚) b (A
˚) c (A
b (°)
No. of Independent Refined R R R(F2) D p wp wd observations reflections parameters (%) (%) (%)
Sedimentary Cs-sedimentary Synthetic Cs-synthetic
P2 /m 9.9664(8) 1 P2 /m 10.051(5) 1 P2 /m 10.0386(8) 1 P2 /m 10.067(2) 1
14.195(1) 14.195(7) 14.183(1) 14.235(3)
8.696(1) 8.693(5) 8.6704(8) 8.709(2)
124.89(1) 125.16(3) 125.08(7) 125.06(2)
5969 6177 6140 6148
819 847 842 843
82 85 82 79
9.3 11.1 13.5 7.3 9.5 10.9 9.6 11.7 14.2 9.5 11.5 14.0
1.7 1.75 1.61 1.57
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(a)
(b) Fig. 2. Observed (crosses), calculated (continuous line) and difference curve (bottom line) of the refined patterns: (a) sedimentary phillipsite; (b) synthetic phillipsite; (c) Cs-exchanged sedimentary phillipsite; (d) Cs-exchanged synthetic phillipsite. The insets contain the 80–140° 2h region.
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(c)
(d) Fig. 2. (continued).
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Table 2 Coordinates, populations, and thermal parameters of the extra-framework cations. The figures in parentheses in the first column give the site multiplicity while the figures in parentheses in the other columns give the standard deviations Site
Sedimentary
Synthetic
Cs-sedimentary
Cs-synthetic
I(2) X Y Z Occ U ISO
0.8707(9) 0.250 0.2063(8) 1.0 0.08(1)
0.9482(8) 0.250 0.3169(8) 0.46(3) 0.05(1)
0.8562(7) 0.250 0.1619(7) 1.0 0.09(2)
0.8085(8) 0.250 0.0468(5) 1.0 0.07(2)
II(4) X Y Z Occ U ISO
0.6125(8) 0.6370(7) 0.4419(9) 0.17(2) 0.08(1)
0.606(2) 0.636(2) 0.474(1) 0.54(3) 0.08(2)
0.6557(6) 0.7232(7) 0.6194(7) 0.24(4) 0.06(2)
0.6982(5) 0.7028(6) 0.5703(5) 0.29(3) 0.07(3)
II∞(4) X Y Z Occ U ISO
0.366(1) 0.1248(9) 0.439(1) 0.31(3) 0.08(1)
0.508(2) 0.113(1) 0.674(1) 0.29(2) 0.08(2)
0.392(3) 0.142(4) 0.537(4) 0.16(3) 0.04(1)
0.520(2) 0.141(4) 0.386(4) 0.34(5) 0.06(2)
II◊(2) X Y Z Occ U ISO
– – – – –
– – – – –
0.7977(6) 0.750 0.5205(5) 0.51(5) 0.09(3)
– – – – –
W1(2) X Y Z Occ U ISO
0.815(2) 0.750 0.513(1) 1.0 0.06(2)
0.696(3) 0.750 0.395(3) 0.76(4) 0.10(3)
– – – – –
– – – – –
W2(2) X Y Z Occ U ISO
0.214(2) 0.750 0.489(1) 1.0 0.03(2)
0.207(4) 0.750 0.477(3) 1.0 0.04(1)
0.052(3) 0.750 0.442(5) 1.0 0.06(2)
−0.059(8) 0.750 0.062(7) 1.0 0.06(3)
W3(4) X Y Z Occ U ISO
0.267(2) 0.872(1) 0.106(2) 0.87(4) 0.05(2)
0.258(3) 0.844(3) 0.049(5) 1.0 0.05(2)
0.456(6) 0.76(1) 0.202(7) .63(6) 0.07(2)
0.351(6) 0.892(7) 0.29(1) 1.0 0.08(3)
W4(2) X Y Z Occ U ISO
0.512(2) 0.250 0.562(3) 0.82(4) 0.09(2)
0.511(3) 0.250 0.458(5) 0.73(5) 0.07(2)
0.544(4) 0.250 0.77(1) 1.0 0.104(4)
0.676(5) 0.250 0.614(7) 1.0 0.08(3)
W5(4) X Y Z Occ U ISO
0.500 0.500 0.500 1.0 0.10(1)
0.500 0.500 0.500 1.0 0.07(1)
0.500 0.500 0.500 0.80(5) 0.08(2)
0.500 0.500 0.500 1.0 0.07(2)
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325
Table 3 Chemical formulae of the samples and cation distributions. The numbers of refined atoms per unit cell for each site are reported in square brackets, and the esd’s in parentheses Sample
Site I
Site II
Site II∞
Site II◊
Chemical formula
Sedimentary Synthetic Cs-sedimentary Cs-synthetic
K[2.0] K[0.9(4)] Cs[2.0] Cs[2.0]
Ca+Na[0.68(4)] Na+K[2.16(6)] Cs[0.96(5)] Cs[1.16(7)]
Na[1.24(5)] Na[1.12(6)] Na[0.64(4)] Na[1.36(3)]
– – Cs[1.02(7)] –
K Ca Na (Al Si O ) · 11.33H O 2.08 0.54 1.38 4.72 11.3 32 2 K Na (Al Si O ) · 11.21H O 2.42 2.21 4.59 11.4 32 2 Cs Na (Al Si O ) · 11.33H O 4.20 0.47 4.72 11.3 32 2 Cs Na (Al Si O ) · 11.2H O 3.24 1.36 4.59 11.4 32 2
Fig. 3. Distribution of the extra-framework cations in the Cs-exchanged sedimentary phillipsite.
completely exchanged by Cs which coordinated five framework oxygens and three water molecules ˚ in the sedimentary at an average distance of 3.11 A one, and three framework oxygens and four water ˚ in the molecules at an average distance of 3.29 A synthetic sample. In the sedimentary sample, Cs with a 9-fold coordination (average distance of ˚ ) was also refined in site II◊ on the mirror 3.16 A plane close to mutually exclusive site II. The migration is a consequence of the short II–II ˚ ) which prevents the site populadistance (0.76 A tion to exceed 50%. In fact, in this configuration (see Fig. 3) only 24% Cs filled site II. The migration did not occur in the synthetic sample where the population of Cs in site II was only 29%. Although in both samples residual Na was found in site II∞, its population was lower in the
sedimentary sample (16%) with more Cs in site II and II◊ with respect to the population in the synthetic sample (34%). As discussed in Refs. [5– 7] Na in site II∞ is necessary because the population of site II is always less than 50%. This allows the prevention of an oversaturation of positive charges over the coordinated framework oxygens and a compensation of the negative framework density at the bottom of the cage. In both samples, the W1 site on the mirror plane which usually coordinates the cation in site II was empty, probably because there is not enough space for a water molecule if large Cs cations are adsorbed in the cage. Fig. 4 is an attempt to explain the difference in the ion exchange of Cs for the sedimentary and synthetic phillipsite and reports the cation exchange path from the starting materials to the Cs-exchanged forms via the Na-exchanged forms. The cation distribution of the intermediate Na-exchanged forms (shaded area) was assumed on the basis of structural constraints (the degree of exchange NaK in site I is 50% and the population of site II is less than 50% [6,7]) and the cation distributions in the original and final Cs-exchanged phillipsites. The following general picture can be drawn: in the sedimentary sample, site I, originally fully occupied by K, is half occupied after the NaK substitution, and then fully exchanged by Cs. The synthetic sample instead had less K in site I which is successively half exchanged by Na, and then fully exchanged by Cs. In addition, the occupancy of site I with K was originally very low, and more Na was captured in site II∞, if we assume that the population of site II is ca. 50% (2 a.f.u.). Thus, since the degree of CsNa substitution in site II∞ was low, the syn-
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Table 4 ˚ ) involving framework atoms and extra-framework cations. The average is calculated on the basis Significant interatomic distances (A of the coordinating anions at a bond distance Distance
Sedimentary
T1–Oa T2–Oa T3–Oa T4–Oa Average
1.653(8) 1.656(9) 1.653(8) 1.647(9) 1.652
I–O1×2 I–O3×2 I–O5×2 I–O8 I–O9 I–W1 I–W2 I–W3×2 I–W4 Average
Synthetic 1.655(8) 1.664(8) 1.655(8) 1.661(9) 1.659
Cs-sedimentary
Cs-synthetic
1.696(9) 1.669(9) 1.687(9) 1.666(8) 1.679
1.673(7) 1.671(7) 1.671(8) 1.673(8) 1.672
3.20(4) 3.18(5) 2.80(8) 3.38(4) >3.5 2.65(8) 3.20(5) 2.82(7) – 3.02
3.07(4) >3.5 3.02(5) >3.5 >3.5 2.96(5) 2.96(4) 2.93(5) – 3.00
3.23(6) 3.39(6) 2.73(7) 3.13(6) 3.19(7) – 3.01(4) 2.91(5) 2.90(6) 3.06
3.33(5) 3.65(7) 3.05(8) 3.36(6) 3.61(7) – 3.20(6) 3.20(8) 3.20(7) 3.21
II–O1 II–O1 II–O2 II–O6 II–O6 II–O7 II–O9 II–W1 II–W2 II–W3 II–W4 II–W5 Average
2.68(6) – 2.65(7) – – 2.44(5) – 2.36(5) – – 2.02(9) 2.44(3) 2.43
2.81(4) – 2.77(4) – – 2.60(8) – 2.15(5) – – 2.28(7) 2.27(7) 2.48
2.81(7) 3.36(7) >3.7 2.95(6) 3.38(9) – 3.04(7) – >3.7 2.99(8) 2.98(7) 3.42(7) 3.12
3.09(5) >3.7 3.39(7) >3.7 >3.7 3.26(4) >3.7 – 3.57(6) 3.57(7) 3.19(7) 3.36(6) 3.39
II∞–O1 II∞–O2 II∞–O3 II∞–O6 II∞–O7 II∞–W1 II∞–W3 II∞–W4 II∞–W5 Average
2.12(7) >2.85 – 2.67(9) 2.57(8) 2.73(8) >2.85 2.15(6) 2.09(8) 2.39
>2.85 2.43(4) – >2.85 >2.85 2.63(5) 2.27(7) 2.74(8) 2.17(6) 2.44
– 2.39(6) >2.85 – 2.41(7) – 2.41(5) 2.40(9) 2.40(7) 2.40
– >2.85 2.27(7) 2.82(6) >2.85 – 2.31(7) 2.29(6) 2.30(5) 2.40
II◊-O1×2 II◊-O2×2 II◊-O7×2 II◊-W2 II◊-W3 II◊-W4 Average
– – – – – – –
a Average of the four tetrahedral distances.
– – – – – – –
3.17(6) 3.28(7) 3.33(7) 3.00(8) 2.91(8) 2.92(8) 3.15
– – – – – – –
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327
Fig. 4. Hypothetical cation exchange path from the starting material to the Cs-exchanged forms via the intermediate Na-exchanged forms (shaded area) assumed on the basis of structural constraints (the degree of exchange NaK in site I is 50% and the population of site II is less than 50% [6,7]) and the cation distributions in the starting phillipsites and final Cs-exchanged forms.
thetic sample retained much more Na there and needed less Cs in site II to achieve charge balance with respect to the sedimentary sample. The sedimentary sample had a Si/(Si+Al ) ratio of 0.704 which is very similar to that of sample 2 (phillipsite from Perrier, France, with Si/(Si+Al )=0.72) described in [5–7] and exchanged with K, Na, Ca, Ba, and Sr. The exchange by Na, K and Ca (populating sites I and II ) was not complete, and some residual Na was refined in site II∞. The exchange by Ba and Sr (populating sites I and II ) was complete, and no Na was refined in site II∞. Given the nature of the exchanged cation (ionic radius and charge) and its coordination number on site II, it is possible to predict whether site II∞ is populated or not. Table 5 reports the atomic charge (c), the coordination number (CN ), the ionic radius (r) for this CN, and the average cation–anion distance (d ) for the
cations populating site II in the exchanged phillipsite sample with a Si/(Si+Al ) ratio of ca. 0.70– 0.72. If we define the parameter C=crd as an atomic sterical occupancy coefficient, when C is larger than 5.73 (as in the case of Sr2+ and Ba2+), site II∞ is not populated since the negative charge density at the bottom of the cage is already compensated by the presence of such large and doubly charged cations in the cage. When C is lower than 5.73, Na in site II∞ is needed to achieve a proper distribution of the positive charge in the cage. Concerning the unit cell, it was found that the Ba- and K-exchanged forms [5–7] showed b ort angles much closer to 90° with a clear indication that K and Ba on site I, given their ionic radii, had a tendency to coordinate oxygens O1 and O3 in such a way that the cavity becomes more symmetrical and consequently the cell symmetry
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Table 5 Atomic parameters necessary for the calculation of C=crd, the atomic sterical occupancy coefficient, for extra-framework cations on site II of phillipsite with Si/(Si+Al )=0.70–0.72, indicating if site II∞ is populated Cation in site II a
Atomic charge c
Coordination number CNa
˚) Ionic radius r (A for that CNb
Average cation–anion distance ˚ )a d (A
C=crd
Na in II∞a
Na+ K+ Cs+ Ca++ Sr++ Ba++
1 1 1 2 2 2
6 9 9 7 8 8
1.02 1.55 1.78 1.06 1.26 1.42
2.40 3.09 3.22 2.50 2.80 3.05
2.45 4.79 5.73 5.30 7.06 8.66
Yes Yes Yes Yes No No
a Data taken from Ref. [5]. b Data taken from Ref. [16 ].
is getting closer to orthorhombic. This work showed that in the sedimentary Cs-exchanged sample Cs plays a role in the distortion of the cavity around site I which is in concert with the behavior of K and Ba. The calculated angle in the pseudo-orthorhombic setting b is in fact 90.15°. ort 5. Conclusions The results of this work are: (1) Cs fully exchanges for K in site I, and for Ca, Na in site II, while Na is retained in site II’. (2) A new position occupied by Cs was located and labelled site II◊ on the mirror plane close to the mutually exclusive site II. (3) A parameter C, the atomic sterical occupancy coefficient, based upon the nature of the cation in site II and its population predicts whether site II∞ is populated: if C<5.73, Na in site II∞ is needed to achieve a proper distribution of the positive charge in the cage. (4) Site W1 is always empty because there is no space available in the cage of type II when the large Cs cations are present. In general, the more Cs is adsorbed, the less room is available for water molecules. (5) The difference in the degree of cation exchange for Cs in the sedimentary and synthetic phillipsites was mainly due to the starting distribution of the extra-framework cations. Despite their similar Si/(Si+Al ) ratios, the synthetic sample did not contain Ca which was instead detected in the
sedimentary sample. To template the structure, some K is needed in site II together with Na. Consequently, site I is half occupied. In the sedimentary sample with Ca and Na in site II, no K is required there, and a full occupancy of site I is possible. (6) Phillipsite is a very good potential cation exchanger for 137Cs+ in nuclear waste, like other commercially available zeolites such as clinoptilolite, although germane experiments should be done to support this statement, essentially because the selectivity may significantly differ with the pH and the nature and relative concentrations of the cations in the solution.
Acknowledgements Financial support is acknowledged from the Italian CNR and MURST. Prof. E. Passaglia is greatly ackowledged for critical reading of the manuscript. Thanks also go to the three anonymous referees for good suggestions and improvement of the paper.
References [1] G. Gottardi, E. Galli, in: Natural Zeolites, Springer, Berlin, 1985, p. 409. [2] R. Sadanaga, F. Marumo, Y. Take´uchi, Acta Crystallogr. 4 (1961) 1153. [3] H. Steinfink, Acta Crystallogr. 15 (1962) 644.
A.F. Gualtieri et al. / Microporous and Mesoporous Materials 32 (1999) 319–329 [4] R. Rinaldi, J.J. Pluth, J.V. Smith, Acta Crystallogr., Sect. B 30 (10) (1974) 2426. [5] A.F. Gualtieri, E. Passaglia, E. Galli, A. Viani, Microporous Mesoporous Mater. (1999) in press. [6 ] E. Passaglia, A.F. Gualtieri, E. Galli, Proc. of Zeolite 97 International Conference, Ischia (Naples, Italy), (1999) in press. [7] A.F. Gualtieri, E. Passaglia, E. Galli, Proc. of Zeolite 97 International Conference, Ischia (Naples, Italy), (1999) in press. [8] C. Colella, Mineral. Deposita 31 (1996) 554. [9] A. Adabbo, D. Caputo, B. de Gennaro, M. Pansini, C. Colella, Microporous and Mesoporous Mater. (1999) in press. [10] G.L. Gaines, H.C. Thomas, J. Chem. Phys. 21 (1953) 714.
329
[11] D. Caputo, R. Dattilo, M. Pansini, Proc. III Convegno Nazionale di Scienza e Tecnologia delle Zeoliti, Cetraro, Italy, September 28–29, (1995). [12] D.T. Karamanis, X.A. Aslanoglou, P.A. Assimakopoulos, N.H. Gangas, A.A. Pakou, N.G. Papayannakos, Clays Clay Min. 45 (1997) 709. [13] E.A. Behrens, P. Sylvester, A. Clearfield, Environm. Sci. Technol. 32 (1) (1998) 101. [14] M. de’Gennaro, E. Franco, L’Industria Mineraria 30 (1979) 329. [15] A.C. Larson, von R.B. Dreele, in: Generalized Structure Analysis System, Los Alamos National Laboratory, Los Alamos, NM, 1998, p. 86. LAUR. [16 ] R.D. Shannon, C.T. Prewitt, Acta Crystallogr., Sect. B 25 (1969) 925.