Ion
exchange
kinetics
and
thermodynamics
of
hydrosodalite, a narrow pore zeolite P. Aprea, D. Caputo, N. Gargiulo, B. de Gennaro, F. Iucolano, B. Liguori, C. Colella* Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy
Abstract The ion-exchange properties of a synthetic hydrosodalite (Na-hS) have been investigated by kinetic and thermodynamic analysis of exchange reactions of the original sodium form for lithium, potassium and calcium forms. Kinetic curves, modelled by a Langmuir-type equation, revealed that exchange rate for lithium and for potassium are of the same order, whereas they are two order faster than for calcium. Thermodynamic analysis of the cation exchange isotherms pointed out that sodalite is selective for sodium over the other three cationic forms examined, which is consistent with the preference exhibited by the sodalite type for sodium environments, either in natural or in laboratory crystallization. Na/Li and Na/Ca exchanges are incomplete, whereas unexpectedly Na/K exchange turns out to be complete, even though K+ dimension exceeds the width of the access window to sodalite cages. The obtained results have been discussed in terms of Eisenman-Sherry theory, pointing out agreements and discrepancies.
Keywords: Hydrosodalite; Hydroxysodalite; Cation exchange kinetics; Cation exchange isotherms
1. Introduction The name sodalite is representative of an interesting group of molecular-scale engineered composite materials. Due to the regular close-packed arrangement of its building units (see
*
Corresponding author. Tel.: +39 081 7682390
fax: +39 081 7682394 E-mail address:
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below), sodalite framework can serve, in fact, as “template” to house a regular array of well defined nanometre-size clusters with potential optic, electronic and magnetic properties [1,2]. Guest species, such as anions, cations or molecules, may be hosted in the sodalite structure by synthesis, ion exchange or adsorption and may be modified, if necessary, by thermal treatment. The well-known property of some sodalite materials to give rise to colour centres, if exposed to sodium vapours, is an example of this peculiar activity [3]. A thorough description of sodalite framework structure, also from a historical perspective, has recently been published [4, 5]. From a chemical point of view sodalite presents a wide variety of compositions, with particular reference to the included extra-framework species. The type material is an anhydrous mineral, having formula Na6[SiAlO4]6·2NaCl and belonging to the subclass of tectosilicates, group of feldspathoids. Its framework consists of a three-dimensional array of truncated octahedra, usually referred to as “sodalite-cages” or “βcages”, fused together in accordance with a BCC symmetry [6]. Besides Na+ cations, necessary to balance the negative charge of the alumino-silicate framework, sodalite structure includes also 2 Na+Cl− pairs per formula-unit. Sodalite analogues can be obtained by synthesis in a wide range of physical-chemical conditions [2,7,8]. Synthetic types usually retain the Na:Al:Si framework composition: there are a few exceptions, such as pure Na-Al or Si frameworks and a tetramethylammoniumsodalite, having a higher Si/Al ratio [2]). On the contrary, sodalite analogues present a series of variants as regards included species. The generic chemical composition can conveniently be written as Na6+x[SiAlO4]6Ax/z·yH2O, where A is an anion of valence z (reported structures with two anions [2] are here disregarded for the sake of simplicity), 0 ≤ x ≤ 2, 0 ≤ y ≤ 8. More in detail, depending on the content of the cage, the synthetic materials can be divided in three classes: (1) sodalite (where x = 0 or 2, y = 0 and A is an anion of various nature, such as alogenide, cyanide, sulphide, sulphite, perchlorate, chlorate, oxalate and others [2,7,8]); (2)
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basic hydrosodalite, also called hydroxysodalite, (where A is OH−, x = 2 and y = 3 or 4 [8]) and (3) hydrosodalite (where x = 0 and y = 8 [8]). Sodalite synthesis can be obtained also in non-aqueous environments, typically with organic solvents, having general formula X-CH2-CH2-Y (X and/or Y being hydrophilic groups), which proved to act as templates [9 and references therein]. Salt- or hydroxide-bearing sodalites are generally not considered as zeolites, since entrapped species, obstructing the microporous network, prevent from any adsorption or ion exchange activity. On the contrary, hydrosodalite, due to its ability to reversibly desorb water upon heating and to exchange its extraframework cations (Na+), is usually regarded as a zeolite [2]. Actually hydrosodalite has to be considered as a narrow-pore zeolite, seeing that its maximum window opening is a 6-ring, (usually in the zeolite literature structures with 8ring pores or less are considered small-pore zeolites). Literature reports that Na+ ions may be partially or totally replaced in hydrosodalite by other extraframework cations, such as, e.g., Li, K, Mg, Ca, Sr, Ag, Tl [10,11]. Structural modifications arisen from these replacements have also been investigated [10,11], but information on the thermodynamics and kinetics of ion exchange is still lacking, although this kind of investigation may present interest from two different points of view. First, ion exchange studies of narrow-pore zeolites are infrequent, because of kinetics constraints connected to cation diffusion through pore openings; second, sodalite structure is an uncommon example of simple model for ion exchange, because of its highly symmetric structural arrangement of sodalite cages (truncated octaedra). With the aim to further contribute to the knowledge of this class of microporous materials, this paper provides a deeper investigation of the ion-exchange properties of a synthetic hydrosodalite, studying its exchange kinetic and equilibrium curves for the Na+/Li+, Na+/K+ Na+/Ca2+ pairs. Chemical, thermal and structural analyses of the original Na-form and the three exchanged derivatives: Li+-, K+- and Ca2+-rich forms were also performed. The selection
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of the ingoing cations has suitably been made to study the implications of both cation dimension and charge on ion exchange: accordingly, Li+ and K+ were selected because they are smaller and larger of Na+, respectively (K+, whose ionic diameter is 2.66 Å, is even a little larger than the access window to sodalite cage, which averages 2.4 Å [2]); Ca2+ was selected because it is as large as Na+, but divalent.
2. Materials and methods 2.1. Preparation and modification of hydrosodalite samples Hydrosodalite (Na-hS) was prepared using a multi-step procedure [2]. Hydroxysodalite was first synthesized by a hydrothermal treatment of silico-aluminate magma having the following molar batch composition: 16.5 Na2O·Al2O3·2 SiO2·113 H2O. Accordingly, mixtures of 1 g of kaolin, USP (China clay; hydrated aluminum silicate), provided by Sigma-Aldrich, and 8 ml of a 16 M NaOH solution (solid-to-liquid ratio equal to 0.125 g·ml–1) were reacted at 140 °C for 3 days, in teflon-lined stainless steel reactors, rotated in oven at 35 rpm. After one day, synthesis was interrupted for a few minutes and the parent liquor replaced by a fresh NaOH solution, in order to remove possible metal impurities present in the raw material. At the end of the run, the synthesis product was washed with bidistilled water and then submitted to an extraction process with water in a Soxhlet apparatus for 3 days, with the intent to remove entrapped NaOH and to yield hydrosodalite (Na-hS). The final product was lastly dried at 80 °C and stored over a Ca(NO3)2 saturated solution (relative humidity close to 50%). The exchanged forms of hydrosodalite (Liex-hS, Kex-hS and Caex-hS) were obtained by putting the solid in contact with 1N aqueous solutions of Carlo Erba reagent grade lithium,
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potassium or calcium nitrates (solid-to-liquid ratio equal to 1/50 g·ml–1), according to the following procedure. A sealed polypropylene (PP) test tube containing the suspension was held at 25 ± 0.1 C under continuous stirring for one day. The liquid phase was then separated from solid and replaced by a fresh solution. This operation was repeated several times until the Na+ concentration in the liquid after equilibration was equal to Na+ content present as impurity in the fresh solution. At the end of treatment the obtained solids were washed with bi-distilled water, dried at 80 °C and stored at controlled humidity as described above. The collected liquids of every equilibration were mixed together and analyzed for cation concentration by ICP atomic emission spectrophotometry, using a Perkin-Elmer Optima 2100 DV ICP-OES apparatus. In the case of complete exchange (Na+ K+), the total Na+ amount, released by the sample until saturation by ingoing cation (K+), served to calculate the cation exchange capacity (CEC) of Na-hS.
2.2. Materials characterization Hydrosodalite samples (as synthesized or after ion exchange) were examined by X–ray diffractometry (XRD, Philips PW 1730 apparatus, rad. CuKα1) for phase identification and lattice parameters calculation (software GSAS II). Chemical analyses were performed by ICP atomic emission spectrophotometry, after dissolution, under microwave-induced heating (Perkin-Elmer Multiwave 3000 oven), of weighted amounts of the samples in a mixed HCl, HNO3 and HF solution with a further addition of H3BO3 to attain fluoride complexation. Water content was estimated by thermal analysis (see below). Simultaneous differential thermal analysis (DTA) and thermogravimetry (TG) were performed on the original sample and the various ion exchanged products, using a Netzsch
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thermoanalyser, mod. 409ST Luxx (weight of the sample: 20 mg; reference material: α-Al2O3; heating rate: 10 °C·min–1). Scanning electron microscopy (SEM) analyses were performed on selected samples, using a Cambridge S-440 instrument, to observe crystal morphology and to check crystallinity.
2.3. Ion exchange runs The kinetics of Na-hS exchange for the selected ingoing cations was investigated with the purpose to evaluate the time necessary to reach equilibrium in the three investigated systems. Accordingly, a series of weighted zeolite samples was put in contact, at 25 ± 0.1 °C and under continuous stirring, with solutions of each cation at 0.1 total normality and keeping the solid– to–liquid ratio equal to 1/100 g·ml1. Runs were stopped at different times and the parent solutions were analyzed by ICP spectrophotometry (see above) for the outgoing cation (Na+). Equilibrium runs were carried out by allowing samples of Na-hS to react at 25 ± 0.1 °C in sealed PP test tubes with solutions containing the said cation pairs at 0.1 total normality. The solid–to–liquid ratio was normally fixed at 1/100 g·ml1, but runs were sporadically performed at 1/200 and 1/500 g·ml1 ratios. Reversibility ion exchange tests were performed following the recommendations of Fletcher and Townsend [12]. Cation concentrations in solution were measured by ICP spectrophotometry, whereas cation concentration in solids was calculated by mass balance. The obtained data were arranged under the form of ion exchange isotherms, reporting the equilibrium concentration of the ingoing cation in the zeolite as a function of the equilibrium concentration of the same cation in the liquid phase.
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2.4. Processing ion exchange isotherms The exchange reaction involving a sodium zeolite and a solution containing a generic cation An+ can be written as follows: n + n Na (z) A (s)
n + n Na (s) A (z)
(1)
where subscripts “s” and “z” denote solution and zeolite, respectively. In order to estimate the main thermodynamic quantities measuring the selectivity of the zeolite for the ingoing cation, i.e. the equilibrium constants (KA) and the standard free energy of exchange (G0), each cation exchange isotherm was processed utilizing a standard procedure [13]. Accordingly, the selectivity coefficients Kc at the given temperature T, were computed with the relationships: Kc
E A a nNa , E nNa a A
(2)
where EA and ENa are the equivalent fractions of the cations An+ and Na+ in zeolite, respectively, and aA and aNa are the An+ and Na+ activities in solution, respectively. The former quantities were obtained from the “smoothed” isotherms, whereas the latter were calculated according to a procedure due to Ciavatta [14] and Glueckauf [15]. According to Gaines and Thomas [16], KA and G0 were at last computed as follows: 1
log K A 0.4343 (n 1) log KcdEA
(3)
0
ΔG0
RT lnK A , n
(4)
where R is the gas constant.
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3. Results and Discussion 3.1. Materials characterization Figure 1 shows two representative SEM images of (a) as-synthesized hydroxysodalite and (b) Na-hS (hydrosodalite), obtained, as mentioned above, by extraction of included NaOH in Soxhlet apparatus. Both materials are constituted by round submicron aggregates and appear to be highly crystalline. The measured average size of the aggregates was about 2μm. Figure 2 (a-e) reports, in comparison with each other, the XRPD patterns of the originally synthesized hydroxysodalite (a) and of the four hydrosodalite samples: Na-hS (b), Liex-hS (c), Caex-hS (d) and Kex-hS (e). The sharpness of the peaks is a confirmation of the high crystallinity of the samples. No extra peaks were found in any powder pattern besides those of hydroxy- or hydro-sodalite [17]. Inspecting the XRPD traces in Fig. 2 points out that, with exception of the Kex-hS pattern (e), cation exchange procedure results in modest changes in peak position and relative peak intensities. Table 1 reports the cubic cell parameters of the (a)-(d) samples in Fig. 2, which are in fact rather close to each other and in agreement with literature data [6]. The Kex-HS pattern (Fig. 2e) presents, on the contrary, remarkable peak shifts and changes in peak intensity, as a result of a possible triclinic symmetry [18]. Figure 3 (a-d) reports the thermal analysis profiles (TG and DTA) of Na-hS and its exchanged forms. The XRPD patterns of the ignited products of each sample at the end of the thermal analysis are shown in Figure 4 (a-d). The DTA curve of Na-hS (Fig. 3a) shows two endothermal effects with minima at about 102 °C and 245 °C, due to dehydration and an exothermal effect at about 895 °C connected to recrystallization to carnegieite (Fig. 4a). Water loss turned out to be 13.40%. Figure 3b shows that Liex-hS dehydrates at about 115 °C. A further weak endothermic effect at 380 C might be due to dehydroxylation. The exothermal effect with maximum at
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about 815 °C is connected to recrystallization to β-eucryptite (Fig. 4b). Water loss was equal to 18.72%. Caex-hS presents two dehydration effects with minima at about 90 °C and 260 °C, respectively (Fig. 3c). Starting from temperatures slightly higher than 800 °C the sample undergoes melting or structural breakdown, followed by recrystallization (thermal effect not shown in figure) to nepheline (Fig. 4c). Note that this sample is extremely rich in sodium, because it saved most of its original Na+ upon exchange for Ca2+ (see at the end of this section). Water loss amounted to 14.86%. The thermal analysis profile of Kex-HS shows a complex dehydration effect with four minima at about 80 °C, 145 °C, 195 °C and 320 °C (Fig. 3d). No other evident effects are recorded at higher temperatures, although the sample proved to undergo a gradual breakdown with a possible recrystallization of kaliophilite (Fig. 4d). The measured water content was 13.06%. Tables 2 and 3 report the results of the chemical analysis performed on the four sodalite samples and the relevant chemical formulas, based on 24 oxygens [6], respectively. Four points should be pointed out: (a) in all formulas the sum of the extraframework cations, in equivalents, matches that of Al moles, within the experimental errors (ENa + EMe ~ 1); (b) the Si/Al ratio is close to 1, within the experimental errors, as in the parent mineral; (c) Na+ Li+ and 2Na+ Ca2+ exchanges proved to be largely incomplete; (b) Na+ K+ exchange was unexpectedly either complete or conservative as regards crystallinity, notwithstanding K+ diameter, as mentioned before, is a little larger of the access window to sodalite cage. The Na/K exchange enabled the estimation of the CEC, which turned out to be 5.88 meq·g1, compared to the calculated value of 6.02 meq·g1 for the idealized Na6[SiAlO4]6·8H2O formula. The discrepancy might be due to some residual silica-rich
9
amorphous gel present in the sample. The experimental value has been used for the evaluations of the thermodynamic quantities.
3.3. Ion exchange runs Figure 5A reports the kinetics curves of Na-hS cation exchange for K+, Li+ and Ca2+, respectively. Here C is the equivalent amount of the ingoing cation per gram of zeolite, whereas C/Cex is the fraction of the CEC utilized. Each curve was conveniently modelled with a Langmuir type equation:
C
at , 1 bt
(5)
where t is time, and a and b are two constants. a, in particular, is the slope of the curve at t = 0, i.e., a measure of the rate of the exchange reaction in its initial stages, and a/b = Cmax, is instead the maximum amount of ingoing cation exchanged at equilibrium (t → ∞). In order to calculate a and Cmax, experimental data were processed using the following rearranged form of Eq. 5: t 1 t C a C max
(6)
Figure 5B reports the linearized kinetic curves. The obtained relevant parameters are reported in Table 4. Modelling appears to be reliable, as indicated by the r2 values in the third line of Table 4. Inspecting both Fig. 5 and Table 4 points out that Na-hS exchanges readily Li+, more than K+ and much more than Ca2+. At equilibrium, the cation more largely taken up is K+, followed by Li+ and Ca2+. Figure 5A also shows that time needed to reach equilibrium ranged from 2 days for the Na+/Li+ pair, through 4 days for the Na+/K+ pair, to 8 days for the Na+/Ca2+ pair. That is why the reaction time to delineate the exchange isotherms was set at 10 days for all the runs.
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Figures 6-9 show the exchange isotherms relative to the reactions Na+ → Li+, 2Na+ → Ca2+ (at total normality 0.1N and 0.01N) and Na+ → K+, respectively. The plots report the equivalent fraction of the ingoing cation versus the equivalent fraction of the same cation in solution at equilibrium. The exchange reaction appears to be reversible for all the three cationic couples, as the points relative to the direct and reverse reactions can be fitted by single curves. In the case of the Na+ → Li+ reaction (Fig. 6) the CEC of the material is not completely available for Li+ exchange, as only about 88% of Li+ equivalents can be hosted in the zeolite framework (see the chemical formula in Table 3). The isotherm is totally under the diagonal, demonstrating a substantial unselectivity of the ingoing cation over the whole composition range. According to the Eisenman-Sherry theory [19, 20], this behavior may be due to the high value of the hydration energy of Li+ and therefore to his greater affinity for water. Two isotherms were obtained for the Na+ → Ca2+ reaction at 0.1 (Fig. 7) and 0.01 (Fig. 8) total normality. In both cases the curves lie clearly under the diagonal (high unselectivity). In agreement with the theory [19, 20], Na-hS exhibits, however, a little higher affinity for Ca2+ at the lower total normality. The exchange is not complete, because only roughly 1/8 of the extraframework cation sites appears to be available for Ca2+ (maximum achievable ECa(z) approaches 0.24, see chemical formula in Table 3). This finding apparently disagrees with an outcome of the Eisenman-Sherry theory [19, 20], for which a low-silica zeolite should prefer the divalent cation in a uni-divalent exchange. In the hypothesis that Ca2+, having a ionic radius very close to that of Na+, goes to occupy its same position in the framework [21], i.e., at the window of the hexagonal ring with a distorted six-fold coordination [17], the above deviation from the theory may be interpreted as due to the difficulty of the double-charged calcium ion, compared with the single-charged sodium ion, to match the electroneutrality constraints.
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Figure 9 shows the exchange isotherm for the cationic couple Na+ → K+ at 25 °C and 0.1 total normality. The S-shaped isotherm shows in the left part of the plot (EK(s) ranging from 0 to roughly 0.5) a strong unselectivity for K+, which, after an inflexion point, is reverted to a strong selectivity for the same cation. This is a very uncommon behaviour, because exchange isotherms generally display (i) complete selectivity (curve totally above the diagonal), (ii) complete unselectivity (curve totally below the diagonal) (iii) inversion of selectivity (curve initially above the diagonal and then, after a plateau, under the diagonal) [22]. In order to verify that the collected data (empty circles in Fig. 9) refer to equilibrium conditions, cation exchange experiments were repeated starting from Kex-HS (reverse isotherm). The new set of points, plotted in the same figure (full circles in Fig. 9), proved to be well fitted by the same curve, pointing out both the achievement of equilibrium and the reversibility of the reaction. The uncommon shape of the isotherm can be explained, considering that K+ with its large size (K+ ionic diameter is equal to 2.66 Å), is hardly accepted by the sodalite framework (the access window dimension of sodalite cage averages 2.4 Å) at low cation concentration in the liquid phase (first part of the isotherm showing unselectivity). Increasing the equivalent fraction of K+ in solution (and, therefore, the cation exchange driving force), results in an anisotropic distortion of the sodalite framework [18], making it easier for the cation to enter the structure (last part of the isotherm showing selectivity). As a results, the mean selectivity, (i.e. the cation exchange selectivity extended to the whole Na+/K+ composition range) of hydrosodalite for K+ is not far from that for Na+. Table 5 summarizes the thermodynamic data obtained by processing the isotherms in Figs. 6, 7 and 9. Inspecting these data confirms that Na-hS is a zeolite decidedly selective for Na+, in agreement with its strong preference for sodium environments, either in natural or in laboratory crystallization. Accordingly, the cation exchange selectivity sequence for hydrosodalite is: Na+ ≈ K+ > Ca2+ > Li+.
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4. Conclusions The performed investigation on ion exchange features of hydrosodalite pointed out that sodalite is selective for sodium over lithium, potassium and calcium in accordance with its preferential crystallization in pure Na (hydroxide plus possible salts) environments. Exchange with Li and Ca is largely incomplete and unselective, whereas with the larger K exchange is unexpectedly complete, with a selectivity reversal at equivalent fraction of K+ in solution close to 0.5. It is to be observed that the unfavourable exchange for Li+ and Ca2+ and the less unfavourable exchange for K+ may be interpreted in terms of charge density, which is high for the former cations (high preference for water) and low for the latter (intermediate preference for zeolite lattice).
References [1] Ozin GA, Kuperman A ,Stein A (1989) Angew Chem Int Ed Engl 28:359-376. [2] Stein A, Ozin GA, in T. E. Mallouk (ed.) Advances in the Synthesis and Reactivity of Solids, Vol. 2, JAI Press Inc., Greenwich, pp. 93-154. [3] Barrer RM ,Cole JF (1968) J Phys Chem Solids 29:1755-1758. [4] Depmeier W (2005) Reviews in Mineralogy and Geochemistry 57:203-240. [5] Baur WH ,Fischer RX (2008) Micropor Mesopor Mat 116:1-3. [6] Baerlocher C, McCusker LB ,Olson D. Atlas of zeolite framework types. Elsevier, Amsterdam, 2007. [7] Barrer R. Hydrothermal chemistry of zeolites. Academic Press, London, 1982. [8] Engelhardt G, Luger S, Buhl JC, Felsche J (1989) Zeolites 9:182-186. [9] Caputo D, Pansini M, Adabbo M ,Colella C (1996) In: Colella C (ed) Atti 3° Congresso Naz. AIMAT (Italian Association of Materials Engineering). De Frede, Napoli, 885-894. [10] Latturner SE, Sachleben J, Iversen BB, Hanson J ,Stucky GD (1999) J Phys Chem B 103:7135-7144 [11] Werner S, Barth S, Jordan R ,Schulz H (1996) Z Kristallogr 211:158-162 [12] Fletcher P ,Townsend RP (1981) J Chem Soc, Faraday Trans 77:497-509 [13] Caputo D, de Gennaro B, Aprea P, Ferone C, Pansini M ,Colella C (2005) Stud Surf Sci Catal 155:129140 [14] Ciavatta L (1980) Ann Chim 70:551-567 [15] Glueckauf E (1949) Nature 163:414-415 [16] Gaines GLJ, Thomas HC (1953) Journal of Chemical Physics 21:714-718 [17] Felsche J, Luger S, Baerlocher C (1986) Zeolites 6:367-372 [18] Gualtieri AF, Aprea P (2006) Micropor Mesopor Mat 96:276-286 [19] Eisenman G (1962) Biophys J 2:259-323
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[20] Sherry HS (1969). In: J. A. Marinsky (ed.) Ion exchange, Vol II, Marcel Dekker, New York, pp. 89133. [21] Mortier WJ, Compilation of extra framework sites in zeolites, Butterworths Scientific Ltd., Guildford, UK, 1982, p. 62-63. [22] Colella C (1996) Min Dep 31:554-562
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Tables Table 1 Lattice parameters of the investigated sodalite samples. Sample
System
Space group
ao = 8.8964 Å
Hydroxysodalite Na-hS
Cell parameters
Cubic
ao = 8.8555 Å
P-43n
Liex-hS
ao = 8.7799 Å
Caex-hS
ao = 8.8415 Å ao = 9.1982 Å bo = 9.1937 Å
Kex-hS*
Triclinic
co = 9.2035 Å
P1
α = 89.709° β = 90.380° γ = 89.857°
* Data from Ref. 18.
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Table 2 Chemical analyses (%) of the investigated samples.* Oxide
Na-hS
Liex-hS
Caex-hS
Kex-hS
Li2O
-
8.13
-
-
Na2O
18.23
2.62
13.67
0.09
K2O
-
-
-
24.73
CaO
-
-
4.08
-
Al2O3
30.05
31.69
30.26
27.76
SiO2
38.36
38.87
37.12
34.39
H2O
13.40
18.72
14.86
13.06
Total
100.04
100.03
99.98
100.03
* Standard deviations are constantly under 0.02.
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Table 3 Chemical formulas of the investigated samples and some relevant chemical parameters. Sample
XNa
XMe
(XNa + XMe)
Si/Al
Na5.75Al0.96Si1.04O4.006·7.3H2O
1.00
-
1.00
1.08
Liex-HS
(Li5.15Na0.80)Al0.98Si1.02O4.006·9.8H2O
0.14
0.88
1.02
1.04
Caex-HS
(Na4.37Ca0.72)Al0.98Si1.02O4.006·8.1H2O
0.74
0.24
0.98
1.04
-
0.96
0.96
1.05
Na-HS
Kex-HS
Formula
K5.67Al0.98Si1.03O4.006·7.8H2O
X = ratio of extraframework cations (in equivalents) to Al moles. Me = generic ingoing cation.
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Table 4. Langmuir parameters from the kinetic curves. Na-K
Na-Li
Na-Ca
Cmax
0.71
0.22
0.16
a
32.06
41.94
0.65
r2
0.9997
0.9996
0.9999
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Table 5 Thermodynamic parameters of exchange reactions at 25 °C and 0.1 total normality. Cationic couple Exchange extent (%) Ka
G0 (kj eq-1)
Na+ → Li+
88
0.066
6.691
Na+ → ½Ca2+
24
0.163
2.233
100
0.745
0.725
+
+
Na → K
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Captions for figures Figure 1 – SEM images of (a) as-synthesized hydroxysodalite and (b) Na-hS (hydrosodalite) Figure 2 – XRPD patterns of the originally synthesized hydroxysodalite (a), Na-hS (b), Liex-hS (c), Caex-hS (d), Kex-hS (e). Figure 3 - Thermal analysis profiles (TG and DTA) of Na-hS (a), Liex-hS (b), Caex-hS (c), Kex-hS (d). Figure 4 - XRPD patterns of the ignited products of Na-HS (a), Liex-hS (b), Caex-hS (c), Kex-hS (d) at the end of the thermal analysis. Figure 5 – A: cation exchange kinetics curves of Na-hS for K+, Li+ and Ca2+, respectively. B: linearized kinetic curves. Figures 6 – Exchange isotherm of Li+ for Na+ into Na-hS at 25 °C and 0.1 total normality. Empty circles = forward points, full circles = reverse points. ELi(z) = Li+ equivalent fraction in zeolite, ELi(s) = Li+ equivalent fraction in solution. Figures 7 – Exchange isotherm of Ca2+ for Na+ into Na-hS at 25 °C and 0.1 total normality. Empty circles = forward points, full circles = reverse points. ECa(z) = Ca2+ equivalent fraction in zeolite, ECa(s) = Ca2+ equivalent fraction in solution. Figures 8 – Exchange isotherm of Ca2+ for Na+ into Na-hS at 25 °C and 0.01 total normality. Empty circles = forward points, full circles = reverse points. ECa(z) = Ca2+ equivalent fraction in zeolite, ECa(s) = Ca2+ equivalent fraction in solution. Figures 9 – Exchange isotherm of K+ for Na+ into Na-hS at 25 °C and 0.1 total normality. Empty circles = forward points, full circles = reverse points. EK(z) = K+ equivalent fraction in zeolite, EK(s) = K+ equivalent fraction in solution [18].
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Figures
Figure 1
21
Figure 2 22
Figure 3
23
Figure 4
24
Figure 5
25
Figure 6
26
Figure 7
27
Figure 8
28
Figure 9
29
