Ion exchange behaviour of two synthetic phillipsite-like phases

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1841

Ion e x c h a n g e b e h a v i o u r o f two synthetic phillipsite-like phases C. Colella a, B. de Gennaro a, B. Liguori a and E. Torracca b aDipartimento di Ingegneria dei Materiali e della Produzione, Universith Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy bDipartimento di Ingegneria Meccanica e Industriale, Universit~ di Roma 3, Via della Vasca Navale 79, 00146 Roma, Italy

Two phillipsite-like phases have been synthesized at 80~ treating a rhyolitic pumice in a mixed Na+-K+ alkaline environment, having different Na+/K+ molar ratios. The synthesized samples, displaying distinct X-ray diffraction patterns, have been characterized as regards their ion exchange properties. Ion exchange isotherms for the cation pairs NaJNH4, Na/Ba, N a ~ and Na/Ca have been obtained and the relevant thermodynamic equilibrium constants calculated with the help of a computer program. Both phillipsite-like phases revealed an excellent to good selectivity for Ba, NH4 and K and a substantial unselectivity for Ca, in substantial accordance with previous results obtained with a sedimentary phillipsite. Nevertheless, the different shapes of some isotherms and therefore the different occupancy of peculiar ion exchange sites indicate that the two synthetic phillipsites are structurally distinct, even if this supposition should be confirmed by a structural analysis.

1. INTRODUCTION Phillipsite is one of the most common natural zeolites, if we consider in general minerals of both hydrothermal and sedimentary origin. The sedimentary phillipsite-rich formations of economic interest are, however, concentrated in a few locations in the world, especially in Italy, Germany and, to a lesser extent, in Spain (Canary Islands) [ 1]. Phillipsite-bearing rocks (either tufts or ignimbrites), which often present as accompanying zeolitic phases chabazite and/or analcime, have proved to be employable in dozen environmental, industrial or agricultural applications [2], mostly in consideration of the good ion exchange selectivity for several toxic or noxious cations [3]. Phillipsite is readily obtained by synthesis. Amorphous alumino-silicate systems (for instance natural glasses) with medium to high silica content are prone to give phillipsite by hydrothermal treatment at low temperatures (<100~ in alkaline media, provided both Na + and K + are present in the reaction environment [4,5]. Some years ago two phases, both resembling phillipsite, but displaying apparently different X-ray diffraction patterns, were obtained by reacting in the same conditions similar reaction mixtures having, however, different K+/Na+ molar ratios [6]. Having in mind that isotypical phases with different chemistry may exhibit distinct ion exchange properties [7], this paper aims at (1) characterizing from a chemical point of view

1842 the two synthesized phillipsite-like phases, (2) investigating some ion exchange equilibria involving them, (3) correlating their ion exchange behaviour with chemical and/or structural features, also in the light of the known ion exchange properties of an analogous mineral phase [3].

2. EXPERIMENTAL SECTION

2.1. Materials Synthetic phillipsite was obtained by reacting for 7 days, at 80~ rhyolitic pumice from Lipari island (Messina, Italy) in 1 molal mixed NaOH-KOH solutions with a pumice-to-water ratio (w/w) equal to 1/10 [6]. The selected samples, designated as Phl and Ph2 (M and Ph in the original paper [6]), were obtained from systems characterized by K+/Na+ molar ratios equal to 0.09 and 0.67, respectively. Their X-ray diffractograms (Philips PW 1730 apparatus) revealed the presence of a phillipsite-like phase as the only crystalline component of the synthesized products. No attempts were made to ascertain the purity of the samples, which might therefore contain some unreacted amorphous material. The two phillipsite-like samples were pre-exchanged in their Na+-form (see below) and stored at room temperature over saturated Ca(NO3)2 solution (R.H. near 50%). Selected chemical analyses were performed using standard methods (see below). Water content was estimated by thermogravimetry (Netzsch STA 409 thermoanalyzer). 2.2. Cation exchange capacity Cation exchange capacity (CEC) of the two phillipsite-like samples was determined using the cross-exchange method [8]. Accordingly, two 1-g zeolite samples, placed on gooch filters, after extensive washing with distilled water, were percolated at about 60~ up to exhaustion by 0.5 M NaC1 or KC1 solutions, prepared by using reagent-grade Carlo Erba RPE chemicals (purity 99.5%). The obtained monocationic forms (Na + or K +) were then re-exchanged under the same conditions with potassium or sodium, respectively. Na + and K + concentrations in the eluates of the second exchange cycle, evaluated by atomic absorption spectrophotometry (AAS, Perkin Elmer AA 2100 apparatus), were used to calculate the CEC values, taking also into consideration the A1 content of the samples. It is to be observed that the CEC value can be ascribed to phillipsite only, because the contribution of the non-crystalline exchanging phases (essentially unreacted pumice) may be considered negligible [8]. 2.3 Ion exchange runs The two synthesized samples were used for the equilibrium studies in the presence of the Na/K, Na/Ca, NaJNH4 and Na/Ba cation pairs. Sodium forms of the two phillipsite samples, obtained through an analogous exhaustive procedure, like that used in the estimation of the CEC, were allowed to react at 25 • 0.1 ~ in sealed teflon test tubes with solutions, containing varying amounts of Na § and one of the cations K § Ca 2§ NH4§ or Ba 2§ at 0.1 total normality, prepared starting from the relevant reagent-grade Carlo Erba RPE chlorides. Reversibility ion exchange tests were performed following the recommendations of Fletcher and Townsend [9]. The reaction time was fixed at 3 days, which was beforehand proved to be sufficient to attain equilibrium. At equilibrium, the liquid phase was analyzed for the cation concentrations, whereas the concentration of the exchangeable cations in solids was calculated by mass balance.

1843

2.4. Analytical procedures Alcali metals were determined by AAS using the standard addition method; ammonium concentrations were measured colorimetrically using the Nessler's reagent with an AQUAMATE UV-Vis spectrophotometer (Spectronic Unicam); A1 and Fe (coming from an acid attack of the zeolite samples), Ca and Ba were estimated titrimetrically with EDTA [ 10].

2.5. Computation of the thermodynamic parameters The measured equilibrium data were plotted under the form of ion exchange isotherms, reporting the equivalent fraction of the ingoing cations in the solid phase as a function of the equivalent fraction of the same cations in solution. From the same data were the thermodynamic exchange parameters calculated with the help of a computer program following a standard procedure [11 ]. A critical step in this procedure is the computation of the cation activity coefficients in solution. In this study these coefficients have been estimated following two different methods, proposed by Ciavatta [12] and Pitzer [13], and compared with each other.

3. RESULTS

3.1. Characterization of the synthetic phillipsite-like samples Table 1 reports the main reflections of the X-ray diffraction patterns of the two synthesized phillipsite-like phases (Phl and Ph2), compared with those of a typical diagenetic phillipsite (NP), coming from the huge formation of Neapolitan yellow tuff (Campi Flegrei, NE Naples, Italy) [ 15]. Indexing is purely indicative; no attempts were made to refine the structures of the two synthetic phases. The diversity of the three patterns reported in Table 1 is evident, although a closer similarity between the Ph2 and NP patterns is undeniable. Differences in Phl and Ph2 patterns may certainly arise from a possibly different framework and extraframework composition. It is to be observed, however, that Na +- or K+-exchanged samples, obtained by extensive exchange of the two original samples with Na + or K +, respectively, gave rise to similar but not identical X-ray diffraction patterns, which accounts for the fact that the two samples might be structurally distinct. This is not surprising, considering that minor differences in structure are possible in isotypical phases, made homoionic through ion exchange procedures [ 16]. Selected chemical analyses were carried out on Na+-exchanged Phl and Ph2 samples (Table 2). From these data it can be seen that exchanged Na + is equal or slightly lower than total Na (in Phl this might account for the presence of minor amounts of impurities in the sample). The fact that Na + released is higher than A1 but lower than A1 + Fe content might be connected to the possible partial incorporation of Fe in the aluminosilicate framework. In this case the reduced amount of K + uptaken following to exchange with Na +, could be explained supposing that some Na + is replaced by hydronium ions. It is in any case noteworthy that in both zeolites A1 content is nearly coincident with K + uptaken. This suggests that A1 content may be considered as the maximum available CEC in the selected experimental conditions. On these bases, the calculated CEC values, which have been used throughout this study, are 3.48 mequiv./g and 3.15 mequiv./g for Phl and Ph2, respectively. Accordingly, neglecting the possible presence of Fe in the framework, the unit cell formulas of the Na-exchanged samples turned out to be Na4.5[Ala.sSill.5032]'12.8H20 and Na4.0[A14.0Si12.0032]'12.5H20,

1844 respectively. The Si/A1 ratio in the two samples, i.e., 2.6 and 3.0, respectively, accounts for their higher acidicy compared with the most common natural phillipsites formed by diagenesis [ 15].

Table 1 X-ray diffraction patterns of synthetic and natural phases connected to phillipsite Phl hkl

d

Ph2 I

101 8.17 8 020,200 7.09 67 210 121 5.37 6 220 5.00 43 002 301 4.29 4 131,311 4.09 58 202,022 212 321 410 141 103 3.240 24 420,240,331 3.167 100 113 123 2.949 12 042,402 2.889 10 501,341 2.744 18 422,242,151,511 133,313 2.676 62 * NP = Natural phillipsite from sedimentary

NP*

d

I

d

I

8.15 7.13

19 60

5.37 5.03 4.97 4.29 4.11 4.08

26 26 38 14 17 24

8.17 7.07 6.33 5.35 5.02

6 58 5 13 19

4.28 4.10

3 28

3.675

5

3.258 3.231 3.188 3.145 2.941

50 52 100 31 52

3.93 3.665 3.440 3.265 3.245 3.174

4 6 2 20 22 100

2.746 2.687 2.678 deposits

2.966 2.920 36 2.737 33 2.684 24 2.675 in Marano (Napoli, Italy) [ 14].

Table 2 Analytical data for the Na forms of the two synthetic phillipsite-like phases Measured quantity

Phl

Ph2

Total Na § (mequiv./g) Na § released (mequiv./g)* K § uptaken (mequiv./g)* A1 content (mequiv./g) A1 + Fe content (mequiv./g) Weight loss (%) * Following to a Na § -~ K § exchange.

3.95 3.67 3.48 3.50 3.82 17.9

3.24 3.26 3.15 3.15 3.45 17.6

14 3 19 19 19

1845 3.2. Ion exchange equilibria Figures 1 and 2 report the profiles of the ion exchange isotherms obtained for the two zeolites Phi and Ph2, respectively. A close inspection and comparison of the various curves enable to make a number of observations that are reported in the following. 9 Na§ NH4 § and N a § exchanges. Phi and Ph2 evidence distinct behaviours, in that the relevant isotherms exhibit a regular shape (convex curve) for the former zeolite and a very distinct plateau with an inversion of selectivity at about 90% of the equivalent fraction of the ingoing cation for the latter zeolite. The latter behaviour is practically identical to that found for a sedimentary phillipsite [3], demonstrating the close similarity between these two phases. 9 2 N a + . ~ - B a 2+ exchange. The isotherms are convex for both synthetic phillipsite-like phases, demonstrating a good selectivity for Ba 2+, in agreement with the results obtained for the natural phillipsite [3].

1

1

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

/

9

~

U~

0.2

0.4 E

0.6

0 0

0.8

0.2

0.4 E

NH4(s)

0.6

0.8

0.6

0.8

1

Ba(~

0.8

0.(

0.6 ,3

0.4

0.4

0.2

0.2

0

0

0.2

0.4

0.6 E

K(s)

0.8

1

0 0

/ 0.2

0.4 E

Ca(s)

Figure 1. Isotherms at 25~ for the exchange of various cations X z§ (top left: NH4§ top right: Ba2+; bottom left: K +; bottom right: Ca 2+ ) into Na-Phl at 0.1 total normality. Ex(s): X equivalent fraction in solution; Ex(z)" X equivalent fraction in the zeolite. Open circles: forward points; filled circles: reverse points.

1846

0.8

J

f

0.6

0.4

0.2

1

0 0

0.2

0.4

0.6

E

0.8

0

0.2

0.4

0.6 E

NH4(s)

0.8

Ba(s)

(

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

%''

'0'.2

'014

' 016 E

018

1

03

0.2

0.4

0.6 E

K(s)

0.8

1

Ca(s)

Figure 2. Isotherms at 25~ for the exchange of various cations X z+ (top left: NH4+; top right: Ba2+; bottom left: K+; bottom right: Ca 2+) into Na-Ph2 at 0.1 total normality. Ex(s): X equivalent fraction in solution; Ex(z)" X equivalent fraction in the zeolite. Open circles: forward points; filled circles: reverse points.

Table 3 Ka values of exchange reactions at 25~ in two synthetic phillipsites Cation pairs Phi Ph2 Ciavatta Pitzer Ciavatta Na + ~ K + 8.94 9.00 10.50 Na + ~ NH4 + 7.15 7.26 8.98 2Na + ~

Pitzer 10.57 8.85

Ca 2+

0.11

0.11

0.14

0.14

2 N a + --~ Ba 2+

30.03

29.02

29.46

28.47

9

2+ e x c h a n g e . Also in this case the isotherms present a similar behaviour: the curves are S-shaped for the presence of a selectivity reversal in the range of 58-60% of the ingoing cation substitution in the framework. This behaviour is dissimilar to that of the

2Na + ~-Ca

1847 mineral [3], which presents a very distinct limit (close to 50%) to the replacement o f N a + for Ca 2+. Table 3 reports the equilibrium constants, Ka, calculated with the help of a computer program following a standard procedure [11]. It is to be observed the excellent agreement between the values obtained following the two procedures indicated by Ciavatta [12] and Pitzer [ 13], which can therefore been used indifferently. The data confirm, from a qualitative point of view the trend already found for the sedimentary phillipsite, i.e., elevated selectivity for Ba 2+, K + and NH4 +, in this order, and a substantial unselectivity for Ca 2+ [3]. The selectivity series for both zeolites will be therefore: Ba > K > NH4 > Na > Ca.

4. DISCUSSION AND CONCLUSION An attempt can be made to interpret the collected ion exchange data in the light of the phillipsite structure [17,18], which, according to a recent investigation [19], is reported to contain three cation extraframework sites: site I, located on the mirror plane (001); site II, located in a general position near the intersection of the two sets of channels; site II' located in a position near to site II. Site I is preferentially populated by large-size cations; site II, which has a low occupancy (below 50%), is usually populated by small-size cations, but may be occupied also by large-size cations; site II' is usually empty, but, if necessary, it may be partially populated, preferentially by small-size cations, only if the nearest site II is unoccupied. Coherent interpretations may be worked out from the behaviours of the two synthetic phillipsites with the Na/Ba and Na/Ca cation pairs. Ba 2+, in fact, confirming its affinity for the phillipsite framework, already observed in the natural sample [3], presents an easy accessibility either to site I or to site II. As regards Ca 2+, it is apparent that this cation can readily substitute for Na + only in roughly half the sites of the synthetic phillipsites, presumably in type II sites, whereas occupancy of site I is manifestly difficult. It is interesting to point out that a distinct, but still coherent, behaviour is shown by the natural phillipsite [3], which presents a clear limit to the replacement of Na + for Ca z+ at about 50%. In this case the apparent higher selectivity of site II for Ca 2+ results in the change from an inversion point (in samples Phl and Ph2) to a plateau in the isotherm curve of the mineral [3]. As regards K + and NH4 + the observed differences in the behaviour of the samples Phl and Ph2 are more hardly explainable without the help of the structural analysis. Ph2, which behaves very closely as the natural counterpart [3], presents a plateau in the isotherm at about 90% conversion. This might be interpreted in terms of difficulties in populating the less selective site II (or site II' if site II is saturated, i.e., occupied at about 50%). The anomalous behaviour of Phl, compared to that of Ph2 and the natural phillipsite, which may account also of its distinct X-ray diffraction pattern, can not be interpreted easily. It may be supposed a poomess of cristallinity or a partial structure distortion, presumably due to the not completely favourable synthesis conditions in which it has been produced (scarce presence of K +, compared to Na +) [4,6,19]. It may be concluded that the two synthetic phillipsites, which have been demonstrated by the present ion exchange studies to be similar but not identical, confirm the selectivity sequence, presented by the sedimentary mineral [3], pointing out the favourable perspective of use of this zeolite for the removal of Ba and NH4 from wastewaters, as laboratory experiments have already proved [20,21 ].

1848

REFERENCES

1.

.

,

.

5. .

7. 8. 9. 10. 11. 12. 13. 14. 15.

16.

17. 18. 19. 20. 21.

C. Colella, M. de' Gennaro and R. Aiello, in Natural Zeolites: Mineralogy, Occurrence, Properties, Applications, D. L. Bish and D. W. Ming, Eds., Reviews in Mineralogy & Geochemistry, Mineralogical Society of America, Vol. 45, Washington, D.C., 2001, p. 551. C. Colella, in Handbook of Porous Solids, F. Schtith, K. Sing and J. Weitkamp, Eds., Wiley-VCH, Weiheim, Germany, 2002, in press. C. Colella, E. Torracca, A. Colella, B. de Gennaro, D. Caputo and M. de' Gennaro, in Zeolites and Mesoporous Materials at the Dawn of the 21 st Century, A. Galameau, F. Di Renzo, F. Fajula and J. Vedrine, eds., Studies in Surface Science and Catalysis, No. 135, Elsevier, Amsterdam 2001, p. 148 ( paper 01-O-05 in the CD-Rom). C. Colella and R. Aiello, Rend. Soc. Ital. Min. Petr., Milan, 31 (1975) 641. M. de' Gennaro, C. Colella, E. Franco and D. Stanzione, N. Jb. Miner. Mh. No. 4 (1988) 149. C. Colella, R. Aiello and V. Di Ludovico, Rend. Soc. Ital. Min. Petr., Milan, 33, No.2, (1977) 511. M. Adabbo, D. Caputo, B. de Gennaro, M. Pansini and C. Colella, Microporous and Mesoporous Materials, 28 (1999).315 C. Colella, M. de' Gennaro, E. Franco and R. Aiello, Rend. Soc. Ital. Min. Petr. (Milan), 38 (1982-83) 1423. P. Fletcher and R.P. Townsend, J. Chem. Soc. Faraday Trans. I, 77 (1981) 497. G. Schwarzenbach and H. Flaschka, Complexometric Titration, Methuen, London, 1969, 490 pp. D. Caputo, R. Dattilo and M. Pansini, Proc. III Conv. Naz. Scienza e Tecnologia delle Zeoliti, R. Aiello (Ed.), AIZ (Associazione Italiana Zeoliti), Napoli, Italy, 1995, p. 143. L. Ciavatta, Annali di Chimica, 70 (1980) 551. K.S. Pitzer, in Activity Coefficients in Electrolyte Solutions, K.S. Pitzer, ed., CRC Press, Boca Raton, Florida, 1991, p. 75. A. Colella, Laurea Thesis in Geological Sciences, Earth Science Department, Federico II University, Naples, Italy, 1997, 126 pp. M. de' Gennaro, M. Adabbo and A. Langella, in Natural Zeolites '93, D.W. Ming and F.A. Mumpton, eds., I.C.N.Z. (Int. Comm. Natural Zeolites), Brockport, New York, 1995, p. 51. A. Martucci, A. Alberti, M. Sacerdoti, G. Vezzalini, P. Ciambelli and M. Rapacciuolo, in Natural Zeolites for the Third Millennium, C. Colella and F.A. Mumpton, eds., De Frede, Napoli, 2000, p. 45. H. Steinfink, Acta Cryst., 15 (1962) 644. R. Rinaldi, J.J. Pluth and J.V. Smith, Acta Cryst., B30 (1974) 2426. A.F. Gualtieri, E. Passaglia and E. Galli, in Natural Zeolites for the Third Millennium, C. Colella and F.A. Mumpton, eds., De Frede, Napoli, 2000, p. 93. C. Colella and R. Aiello, Occurrence, Properties and Utilization of Natural Zeolites, D. Kallb and H.S. Sherry, eds., Akad6miai Kiadb, Budapest, 1988, p. 491. B. de Gennaro and A. Colella, Proc. EUROMAT 2001 (CD Rom), AIM (Ass. Ital. Metallurgia), Milano 2001, (Abstract in "Conference Abstracts", p. 353).