A simple method for the determination of clinoptilolite in natural zeolite rocks

F~UTTERWORTH ~rlE I N E M A N N

A simple method for the determination of clinoptilolite in natural zeolite rocks Zolffm Kriv~tcsy

Hungarian Academy of Sciences, Research Group of Analytical Chemistry, University of Veszpr~m

JOzsef Hlavay

Department of Analytical Chemistry, University of Veszpr~m, Veszpr~m,Hungary A reliable and apparent method based on diffuse reflectance FT i.r. spectroscopy (DRIFTS) for the quantitative determination of clinoptilolite in natural zeolites has been developed. The technique provides a simple and time-saving analysis of the minerals when compared with the powder X-ray diffraction (XRD) method and/or the conventional pellet preparation i.r. technique. Because of the complex structure of the natural minerals, the reliability of the analysis is improved further by combining the XRD and DRIFTS methods. In this study, the clinoptilolite concentration of natural zeolites mined in the US and Hungary was determined by both techniques. The results were compared, and good agreement was found. Keywords: Clinoptilolite;quantitativeanalysis; diffuse reflectanceinfrared analysis

INTRODUCTION The use of natural zeolites, in particular clinoptilolite, to remove ammonium ions from water is drawing increased interest. T h e ion-exchange capacity of zeolite rocks depends greatly on the clinoptilolite content of natural minerals. Recently, powder X-ray diffractometry (XRD) and thermal analyses have been applied widely to determine the individual components of natural rocks. However, it has been recognized increasingly that i.r. spectrometry can yield extra infermation and can serve as a rapid and useful structural analytical technique. It is also known that the i.r. spectra are influenced mainly by short-range order of the crystal structure, providing the possibility of a more reliable quantitative analysis compared with XRD measurements. ~ Structural studies of zeolites based on transmission i.r. spectroscopy have been discussed in detail. ~-~ A systematic investigation of the framework structures of many synthetic zeolites has been carried out in the mid i.r. region by Flanigen et al. 5 Interpretations o f these spectra were based on assigning the i.r. bands to certain structural groups in various zeolite frameworks. For the assignment, it is necessary to know the basic zeolite structure. So, i.r. spec-

Address reprint requests to Dr. Hlavay at the Department of Analytical Chemistry, University of Veszpr~mr, P.O. Box 158,

H-8201 Veszpr6m, Hungary.

Received 6 September 1994; accepted 9 November 1994 Zeolites 15:551-555, 1995 © Elsevier Science Inc. 1995 655 Avenue of the Americas, New York, NY 10010

t r o s c o p y is c o m p l e m e n t a r y to X-ray s t r u c t u r a l analysis. The fundamental vibrations of the framework TO4 (T = A 1 or Si) tetrahedra can be found in the 200 to 1,300 c m - ~ region. The spectra of zeolites can generally be divided into two regions: (1) those due to internal vibrations of the TO4 tetrahedron, which is the primary unit of structure and is not sensitive to other structural variations; and (2) vibrations that may be related to the linkages between tetrahedra. Recently, a method has been developed for the quantitative determination of mordenite in different natural zeolite rocks using synthetic mordenite as a standard by transmission i.r. spectroscopy. 6 Since 1978 d i f f u s e r e f l e c t a n c e FT i.r. s p e c t r o s c o p y (DRIFTS) has also been applied for the rapid investigation of minerals including zeolites. 7-9 One important advantage of DRIFTS over the transmission i.r. technique is that the sample is more likely investigated in its original physical-chemical state. In transmission i.r. spectroscopy the recrystaUization of the minerals due to the high pressure applied duringpellet preparation can frequently be observed. 10' l l Most of the investigation of geological samples by DRIFTS has focused on qualitative structural information. Recently, a simple and reliable m e t h o d based on DRIFTS for the quantitative determination of inorganic compounds in environmental samples has been developed. 12 15 In this paper, the application of the DRIFTS method for the quantitative determination of c l i n o p t i l o l i t e in n a t u r a l z e o l i t e r o c k s is d i s c u s s e d . -

0144-2449/95/$10.00 SSDI 0144-2449(94)00050-3

Determination of clinoptilolite in zeolite rocks: Z. Kriv~csy and J. Hlavay

EXPERIMENTAL

Materials Natural zeolite rocks were collected in the US and Hungary. T h e American samples produced by Minerals Research, Clarkson, NY, were as follows: Barstow (California), B u c k h o r n (New Mexico), Fort Leclede (Wyoming), and Horseshoe (Idaho). T h e Hungarian samples (Z1, Z2, and Z3) were obtained from Tokaj Mountain in northeast Hungary from drill cores. T h e rocks were ground in agate mortar for a particle size < 10 Ixm. The ground samples were investigated directly by XRD. For DRIFTS measurements, samples were diluted with spectroscopic grade KBr (Fluka). The particle size of the KBr was < 15 p.m with an average size of 9.7 p,m. First, 5 m/m% mixtures were prepared; then an additional 10-fold dilution was carried out, and the 0.5 m/m% aliquots were analyzed. The samples were mixed thoroughly at each dilution step for homogeneity. T h e standard mixtures applied for univariate and multivariate calibrations were also prepared by the same method, The purity of the standards of quartz, cristobalite, K-feldspar, montmoriUonite, and mordenite (Department of Mineralogy, University of Veszpr~m) was >95% measured by XRD. Analysis The diffractograms were obtained by a model 1051 Philips diffractometer at 154-pm wavelength (CuK~). For the DRIFTS measurements the samples were held overnight at 105°C in an oven before the analysis. The weight of the sample was 60 -+ 1 mg, and spectroscopically pure KBr was used as a reference. For reproducible measurements the sample holders were filled by applying 1 MPa pressure for 1 min on the surface of the sample using a simple, homemade filling device, l~ T h e spectra were recorded by a BioRad Digilab FTS60A Fourier transform spectrometer using a DTGS detector and a diffuse reflectance accessory designed specifically for the instrument. The resolution was 4 cm -1, and 64 scans were coadded, The spectra were converted to Kubelka-Munk (K-M) format to obtain a linear relationship between the reflection and the concentration of the sample. For univariate calibration the experimentally obtained K-M values were corrected by the exact value of the reference reflectance according to the method of Reinecke et al. 16 For multivariate calibration and spectral search the PCP-JPLS and spectral search software packages of Digilab, respectively, were used.

zeolite should be chosen based upon the XRD measurements. From the diffractograms, clinoptilolite mined in Horseshoe (Idaho) proved to be suitable as a standard and was used in further studies. In this sample neither other crystalline mineral phases nor amorphous phases were identified. In other American zeolites, two major mineral phases, clinoptilolite and mordenite, were found. In the Hungarian sampies, five crystalline phases were identified: clinoptilolite, quartz, cristobalite, K-feldspar, and clay raineral assumed to be montmorillonite. No cristobalite was found in sample Z1 above the detection limit. For the quantitative determination of mineralogical phases by XRD, single peaks of the individual compounds were selected. The intensities of the peaks were compared with those of the standards, and the results obtained for the single peaks were averaged. Fourteen peaks for clinoptilolite was chosen. Along with clinoptilolite, concentrations of the other phases were also estimated. The results for clinoptilolite concentrations are listed in Table I. The diffractogram of the Buckhorn sample was found to be very similar to that of the standard, and only traces of mordenite could be detected. T h e mordenite content of the Fort Leclede and Barstow samples were estimated to be about 10 and 30 m/m%, respectively. The amount of cristobalite in samples Z2 and Z3 was calculated around 20 m/m%; the quartz concentration of sample Z 1 was also valued at about 20 m/m%. The amount of the clay minerals was assigned between 5 and 10 m/m%, and the concentration of K-feldspar was estimated close to 5 m/m% in every Hungarian sample.

D R I F T S measurements First, calibration for the clinoptilolite in the 0.2-1 m/m% range using the Horseshoe (Idaho) sample as a standard was carried out. Three calibration curves were prepared based on the intensities of the 1,214c m - l, 1,063-cm- l (TO4 asymmetric stretching vibrations), and 609-cm- 1 absorption bands of clinoptilolite, which was assigned as a T - O bending vibration. 17 This absorption band was observed at a higher wavenumber than that of the T - O bending of the other zeolites. Nevertheless, this band should be characteristic of the heulandite group of zeolites because it can also be found in the i.r. spectrum of heulandire.iS The slope, the intersection, and the linear reClinoptilolite concentration of the zeolites determined by XRD and DRIFTS measurements

Table 1

Sample

RESULTS A N D D I S C U S S I O N For reliable determination of any mineral phase from natural matrix using DRIFTS, the calibration curve has to be prepared by an appropriate standard. Since there is no synthetic clinoptilolite available, the purest

552

Zeolites 15:551-555, 1995

Buckhorn Fort Leclede Barstow Zl

Z2

z3

XRD (m/m%)

DRIFTS (m/m%)

91 ± 6

92 - 4

77 -+ 6 61 --- 6

79 - 4 57 --- 5

47 ± 5 64 __ 3

55 ± 4

50 _+5

62 ± 6

56 ± 4

Determination of clinoptilolite in zeolite rocks: Z. KriwJcsy and J. Hlavay

stronger ones. Nevertheless, the regression coefficient of the calibration curve is still acceptable (see data in Table 2). In the Hungarian zeolite rocks, five different mineral phases were detected by the XRD measurements. The complexity of the samples resulted in spectral interferences over the whole spectral region. None of

"Fable 2 Parameters of the calibration curves of clinoptilolite Bands of clinoptilolite

[cm- 11

1214 0.580 0.0261 0.9994

Slope [(m/m%) -11 Intersection Regression coefficient

1063 1.350 0.0087 0.9997

609 0.313 0.0026 0.9990

the absorption bands o f the clinopti]olite could be

used directly for quantitative analysis. Therefore, multivariate calibration methods, PLS and PCR, were chosen to determine the clinoptilolite content of the Hungarian samples by DRIFTS. However, for performing the multivariate calibration properly and obraining reliable results, the identity of the major mineral phases and their approximate concentrations have to be known. So, it was necessary to confirm and/or to complete the XRD measurements by means of i.r. analysis. The DRIFTS spectra of the Hungarian samples are plotted in Figure 2. It can be seen that spectra of the Z2 and Z3 samples are very similar but that of the sample Z 1 differs in some ways from the others. For a detailed picture the 950-750 cm-1 range of the spectra is enlarged in Figure 3. In the spectrum of sample Z 1 a doublet band of quartz at 799 and 780 c m - x was identified, but in the two other spectra no clear evidence of the doublet was found. This agrees with the results of the XRD study; that is, sample Z1 contains only quartz, whereas the other two have both quartz and cristobalite (797 c m - i). Furthermore, it has been estimated by XRD that cristobalite is present in about a four times higher concentration compared with quartz in samples Z2 and Z3. This is also c o n f i r m e d by DRIFTS, since in the spectra of samples Z2 and Z3 the 797 c m - 1 band of cristobalite dominates over the quartz doublet. The highest concentration o f both quartz and cristobalite was estimated to be about 20

gression coefficient of the calibration curves are summarized in Table 2. Good linearity between the K-M intensities of the bands and the sample concentration was found. This permits the application o f DRIFTS for the quantitative analysis of clinoptilolite in natural zeolite rocks, Because o f spectral interferences, however, any of the bands with univariate calibration can seldom be used to analyze individual minerals. Since these minerals consist mainly of different silicates, spectral interference can be very strong around S i - O - Si stretching vibrations (1,300-700 cm-~), T h e American zeolites contained clinoptilolite and mordenite. The spectra of 1 m/m% mixtures of standard clinoptilolite and mordenite are plotted in Figure 1. The shapes of the two spectra are very similar except in the wavenumber region of 650-500 cm -1. This means that the bands at 1,214 and 1,063 cm-~ cannot be used directly for quantitative analysis of clinoptilolite. However, the T - O bending vibration band of clinoptilolite at 609 cm-~ is free from the interference o f mordenite. Therefore, this band was appropriate for univariate calibration for these sampies. On the other hand, the intensity of this band is relatively weak, and the calibration curve related to this band is consequently not as sensitive as for the 1.15 -

.85 -

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.55

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.

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Rgure I Diffuse reflectance spectra of 1 m/m% mixtures (*) of the clinoptilolite standard, Horseshoe sample (curve 1), end mordenite standard (curve 2). The band at 609 cm -1 of clinoptilolite, free of mordenite interference, was used for the conventional univariate calibration.

Zeolites 15:551-555, 1995

553

Determination of clinoptilolite in zeofite rocks: Z. Kriv~csy and J. Hlavay 1.15

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~

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I

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w a v e n u m b e r s [cm -~ ] Rgure 2 Diffuse reflectance spectra of the Hungarian zeolites.

rodm% by comparing the spectra with those of the 1 m/m% mixtures of the pure standards. Identification of the cristobalite can also be accomplished in the asymmetric stretching region (1,300-950 c m - 1) of the spectra (Figure 2). Montmorillonite was detected in the Hungarian samples by DRIFTS using the band at 915 cm - l in the spectrum of sample Z1 where a shoulder-like band characteristic of O - H vibration of montmorillonite can be observed. The concentration of the clay mineral calculated from the intensity of their band was estimated to be around 12 m/m%. On the basis of the results of the XRD and DRIFTS measurements, a set o f standards for the multivariate calibration was prepared as follows. Ten mixtures

containing clinoptilolite, quartz, cristobalite, montmorillonite, and K-feldspar in a relative concentration of 30-80, 0-30, 0-30, 0-15, and 0-7 m/m%, respectively, were prepared. The total concentration of the five standards in each mixture was 0.5 m/m%. Seven out of the 10 mixtures together with the 1 m/m% mixture of the pure standards were selected for the calibration. The three other mixtures were applied to validate the calibration method. The spectral range of 900-550 c m - 1 was chosen to perform the analysis with the highest reliability based on the validation procedure and residual spectrum analysis. Residual spectrum analysis is a powerful tool to control the reliability of the determination, providing a value called an F ratio, which is the proportion of the

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2

.08

z2

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w a v e n u m b e r s [ern -1 ] Figure 3 The 950-750 cm -1 region of the diffuse reflectance spectra of the Hungarian zeolites. T h e . is for the 915 cm -1 band of montmorillonite.

554

Z e o l i t e s 15:551-555, 1995

Determination of clinoptilolite in zeolite rocks: Z. KriwJcsy and J. Hlavay

variance o f the residual spectrum of the sample and the average variance of the residual spectra in the calibration set. The closer this value is to 1, the more accurate the determination. Using this region for the analysis both the standard deviations obtained for the validation standards and the values of F ratio were found to be the smallest. The clinoptilolite concentration of two parallels of each sample was determined by PCR and PLS. The clinoptilolite contents for the American sampies (Buckhorn, Fort Leclede, Barstow) and the Hungarian ones (Z1, Z2, Z3) are listed in Table 1. It can be seen that the values obtained by DRIFTS are in good agreement with those of the XRD method. In the American samples the increase of mordenite content correlates well with the decrease of the clinoptilolite concentration. Note also that the spectra of the American zeolites were run further in a spectral search program using a h o m e m a d e spectrum library of the most frequently occurring minerals. At the end of each run the values of the hit quality index (HQI) were calculated for each standard mineral found in the spectrum library. The strategy of the mathematical process is that the nearer the H Q I value obtained for a standard is to zero, the higher the similarity between the spectra of the sample and the standard. The spect r u m o f the clinoptilolite standard was c o m p a r e d with

the spectra of the samples in all cases, and the H Q I values w e r e f o u n d to be 0.09, 0.14, and 0.19 f o r the B u c k h o r n , Barstow, and F o r t Leclede samples, re-

spectively. T h e c o r r e l a t i o n between the H Q I values and the clinoptilolite concentrations o f the samples is not perfeet. T h e H Q I value o b t a i n e d f o r the Barstow sample is u n e x p e c t e d l y low c o m p a r e d w i t h the clinoptilolite concentration (see Table 1). However, if the secondbest fit, w h i c h was m o r d e n i t e in every case, was also

taken into consideration it could be observed that the H Q I value of mordenite for the Barstow sample was significantly smaller (0.29) than the same value of the other two samples (0.38-0.40). This indicates that the Barstow sample contains more mordenite and consequently, less clinoptilolite than the others do, as has already been proven by both XRD and DRIFTS measurements. I t can be assumed that the h i g h similarity

of the spectra of the two minerals is responsible for the smaller H Q I value calculated for clinoptilolite. Nevertheless, it should be concluded that spectral search in i.r. spectroscopy could also be useful for obtaining valuable information on the quality and rel-

ative quantity of the inorganic phases of the natural minerals. CONCLUSION On the basis of these results it can be concluded that DRIFTS is a powerful method for the quantitative determination of clinoptilolite in natural zeolite rocks. The method can be used by either univariate or multivariate calibration depending on the complexity of the sample. The advantages of DRIFTS over the conventional transmission i.r. technique are that less time and less embedding material are necessary to perform the total analysis. Heulandite shows an i.r. spectrum similar to that of clinoptilolite, so it can interfere in the determination. ACKNOWLEDGMENTS We thank L. Mer~nyi for XRD measurements and for valuable discussions on the interpretation of the rillfractograms and G. Barrios for help in sample preparation and DRIFTS measurements. T h e research was supported by the Hungarian National Research Foundation O T K A Project 2546. REFERENCES 1 Farmer, V.C. (Ed.) The Infrared Spectra of Minerals, Mineralogical Society, London, 1974 2 Flanigen, E.M. ACSMonogr. 1976, 171, 80 3 Milkey, R.G. Am. Miner. 1960, 45, 990 4 Karge, H.G. ACS Symp. Set. 1977, 40, 584 5 Flanigen, E.M., Khatami, H. and Szymanski, H.A. Molecular Sieve Zeolites: Advances in Chemistry Series 1971, 101,201 6 Hlavay, J., Vassztnyi, I. and Incz~dy, J. Spectrochim. Acta 1985, 41A, 1457 7 Fuller, P. and Griffiths, P.R. Am. Lab. 1978, 10, 69 8 Krishnan, K., Hill, S.L. and Brown, R.H. Am. Lab. 1980, 12, 104 9 Chalmers, J.M. and Mackenzie, M.W.Appl. Spectrosc. 1985,

39, 634 10 Duyckaerts, G. Analyst 1959, 84, 201 11 Smith, A.L. Applied Infrared Spectroscopy, Wiley, New York, 1979 Ch. 4 12 Kriv~csy, Z. and Hlavay, J. Spectrochim. Acta 1994, 50A, 49 13 Kriv~csy, Z. and Hlavay, J. Talanta 1994, 41, 1143 14 Kriv~csy, Z. and Hlavay, J. Spectrochim. Acta 1994, 50A, 2197 16 Kriv~csy, Z. and Hlavay, J. Talanta 1995 16 Reinecke, D., Jansen, A., Fister, F. and Schernau, U. Anal. Chem. 1988, 60, 1221 17 Goryainov, S.V., Stolpovskaya, A.N., Likhacheva, A.Y., Belitsky, I.A. and Fursenko, B.A. Proceedings of Zeolite 93, June 20-28, 1993 Boise, ID, p. 105 18 Breck, D.W. Zeolite MolecularSieveWiley, New York, 1974,

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