Interpretation of the infrared spectrum of the NH]-clays: application to the evaluation of the layer charge

Transcription

1 Clay Minerals (t 999) 34, Interpretation of the infrared spectrum of the NH]-clays: application to the evaluation of the layer charge S. PETIT, D. RIGHI, J. MADEJOVA* AND A. DECARREAU Universitk de Poitiers, CNRS UMR 6532 "HydrASA ; 40, avenue du Recteur Pineau, Poitiers Cedex, France, and *Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, Slovakia (Received 10 October 1998; revised 15 June 1999) ABSTRACT: The IR spectra of NH4-saturated + smectites were examined in terms of their charge characteristics. The u4 NH~ band near 1440 cm -1, observed in the DRIFTS spectra (obtained without use of a KBr matrix), was assigned to the vibrations of NH~ ions compensating the negative charge of the clays. When KBr was used as a diluting matrix, the u4 NH~ band was located at 1400 and/or 1440 cm -1. The band at 1400 cm -1, related to NH4Br, originated from the replacement of NH~ in the clay by K + from the KBr. For swelling clay minerals this band indicates that layers have permanent low charge density and/or variable charge. For non-swelling clay minerals, the 1400 cm I band characterizes the presence of variable charges only. The u4 NH~ band at 1440 cm -1 suggests that NH~ in the clay was not replaced by K + from KBr and remains in the interlayer space of the clay minerals. This absorption is due to NH~ compensating only permanent charge in the interlayers, or part of the interlayers with a high charge density. The presence of both bands at 1400 cm 1 and 1440 cm ~ in the IR spectrum suggests that the clays studied have a heterogeneous interlayer charge. Previous studies show that NH~ bearing 2:1 clays have two types of ammonium vibrational spectra. The first one, mainly observed for KBr pellets of NH4-smectites, gives absorption bands at 3150, 3020, 2840 and 1400 cm -~ (Petit et al., 1998). These NH~ bands, observed in the 1R spectra of many NH4 salts, correspond to vibrations of NH~ ions in the tetrahedral symmetry group (Nakamoto, 1963). According to Nakamoto (1963), Sherman & Smulovitch (1970) and Ryskin (1974), the intense absorption bands at 3150 cm 1 and 1400 cm -1 are due to u3 (stretching) and u4 (deformation) vibrations of the NH~ ion, respectively. The 2840 cm -a and the 3020 cm 1 bands are assigned to the overtone 2 ~2 and the (u4 + u2) combination vibration, respectively. The second type of ammonium vibrational spectrum shows bands in the cm t region and near 1440 cm -1. Chourabi & Fripiat (t981) assigned these bands to the adsorbed NH~ ion in tetrahedral symmetry as well, but due to H bonding of NH~ with the clay structure, the bands are in slightly different positions compared to those observed for NH4 salts (i.e. at 3150 and 1400 cm-1). This second type of ammonium vibrational spectrum is observed in the case of: (1) self-supporting films of clays (Mortland et al., 1962; Mortland & Raman, 1968; Chourabi & Fripiat, 1981; Srasra et al., 1994) and (2) nonexpandable NH4-clay minerals such as illites or micas (Vedder, 1965; Sucha et al., 1998). Moreover, this second type of ammonium vibrational spectrum is also present as a superimposed component on the first type of spectrum and was observed in the spectra of undried or dried KBr pellets of some smectites (Petit et al., 1998). No detailed discussion of IR spectra of NH4-clay minerals is available in the literature, therefore the purpose of this paper is to explain the occurrence of these two types of IR ammonium vibrational The Mineralogical Society

2 544 S. Petit et al. spectra for NH4-saturated smectites and to interpret this occurrence. The effect of layer charge on exchangeability of NH~ ions in KBr-clay mixtures is also discussed. MATERIALS AND METHODS Three synthetic saponites and one smectite obtained from the Source Clay Repository of the Clay Minerals Society (SAz-1) were used (Table 1). The sample SAz-1 is a Mg-rich montmorillonite with the majority of layer charge in the octahedral sheet. The samples SAP0.35, SAP0.5, and SAP0.8 are synthetic saponites with layer charge arising from tetrahedral substitutions only. The range of their tetrahedral substitution varies from 0.35 to 0.8 atoms of A1 substituted for Si per half unit-cell. These were the samples used by Petit et al, (1998). Suspensions of the samples in 1 M ammonium acetate solution were obtained by agitation and were left to stand for 2 h. After centrifugation, this process was repeated once to achieve complete NH~ exchange. The samples were then washed until free of ammonium salts (until negative reaction with the Nessler reagent). The NH~-saponites were further re-saturated with Ca 2+ using solutions of 1 M CaCI2. This exchange permits identification of the irreversibly collapsed layers of high-charged smectites by XRD (Righi et al., 1998). In the present work it was used to distinguish between fixed and exchangeable NH~ ions by Fourier transform infrared (FTIR) spectroscopy. A Nicolet 510 FTIR spectrometer was used to record FTIR spectra at 4 cm 1 resolution in the cm -1 range. The spectrometer was continuously purged with dry air during scanning of the transmission spectra of KBr pellets. The pellets of 2 cm diameter were prepared by mixing 3 mg of sample with 300 mg of KBr. Diffuse reflectance infrared spectra (DRIFTS) with and without dispersion in a KBr matrix were recorded using a DRIFTS accessory 'Collector' from Spectra-Tech. The samples, analysed at room temperature, were placed loosely into a sample cup of -1 mm depth and 3 mm diameter. Diluting the sample in a non-absorbing matrix, the proportion of the IR beam that is diffusely reflected by the sample increases. However, the DRIFTS spectra of saponites and SAz-1 obtained without dilution were of good quality in the regions of NH4 absorption, and dilution was used to study whether KBr affects position of NH4 bands in the spectra. To compare samples quantitatively, the spectra were normalized using an internal reference band, either the main Si-O band near 1000 cm -1 or the OH-stretching band depending on which was more appropriate for the transmission or reflectance measurements. RESULTS AND DISCUSSION The study was focused on the ua NH~ band near 1400 cm l only, because the intensity of this NH~ band is linked quantitatively to the N content of the analysed samples (Ferriso & Hornig, 1959; Vedder, 1965; Petit et al., 1998). Unlike the NH~- stretching region, where the bands are overlapped with OH-stretching vibrations of water (near 3400 cm i), the NH~ deformation region is free of other absorptions, and no decomposition is necessary to extract the u 4 NH~ signal (Petit et al., 1998). The important point is that the u4 NH~ band characterizes unambiguously the type of NH~ vibrational spectrum. If the u4 NH4 band is at TABLE 1. Mineralogy, source, and cation exchange capacities (CEC) of the samples used. Samples SAz-I SAP0.35 SAP0.5 SAP0.8 Mineralogy Montmorillonite, minor silica phase Source Apache County, Arizona, USA CEC 125 (cmol(+).kg i) References Jaynes & Bigham (1987) Synthetic saponites 9 + S14_xAlxMg3010(OH)eNax CRSCM, Orl6ans, France O.-L. Robert) Bergaoui et al. (1995); Petit et al. (1998)

3 IR spectra of NH~4-smectites cm 1, the other bands are located at 3150 cm 1, 3020 cm -I and 2840 cm -1, while if it is near 1440 cm 1, the other main band is near cm -1. Attribution of the 1400 cm -1 and 1440 Cm -1 bands The transmission spectrum of the NH~ saturated SAz-1, obtained from a KBr pellet, is presented in Fig. 1, curve f. The u4 NH~ band at 1400 cm -1 indicates the first type of ammonium spectrum which is typical of NH4 salts. Since the clay samples used were carefully washed until free of any NH4 salts, the band at 1400 cm -1 suggests an exchange of interlayer NH] for K + from KBr according to the reaction: NH4-clay + KBr --, K-clay + NH4Br The DRIFTS spectra of SAz-1 montmorillonite and the saponites were used to confirm this hypothesis. SAz-1 v 4 NH cm -1 O i Wavenumber (em -1) FI~. 1. The evolution of the u4 NH~ band for the NH4-saturated SAz-1 sample. (a) DRIFTS measurement, undiluted sample; (b) DRIFTS measurement, sample diluted with KBr and slight grinding; (c) DRIFTS measurement, 2 h later than for b; (d) DRIFTS measurement, 12 h later than for c and second grinding; (e) DRIFTS measurement, 2 h later than for d; (f) undried KBr pellet in transmission mode.

4 546 S. Petit et al. A DRIFTS spectrum of sample NH4-SAz-1, recorded without any dilution in KBr, (Fig. la) shows a broad band at 1440 cm -1. After mixing the sample with KBr and gentle grinding to homogenize it, the band shifted slightly to lower wavenumbers (Fig. lb). No change occurred after storing the clay/kbr mixture for several hours (Fig. lc). After overnight storage and supplementary mild grinding of the sample, the maximum of the band shifted again and a new band at 1400 cm -] appeared (Fig. ld). After two additional hours, the intensity of the band at 1400 cm 1 clearly increased, and the DRIFTS spectrum (Fig. l e) is comparable to that obtained in the transmission mode from a KBr pellet (Fig. lf). This experiment provides direct evidence for the exchange reaction between NH~ present in the interlayers of the clay and K + from the KBr. The 1400 cm ~ band is attributed to the vibrations of NH~ present in the pellets as NH4Br as a result of the ion exchange, while the 1440 cm ] band is assigned to the NH~ not replaced by K +, i.e. to the NH~ still remaining in the interlayers of the clay. Petit et al. (1998) found that after overnight drying of KBr pellets of various clay samples at ll0~ including SAz-1, no more changes were observed in the intensities ratio of the 1400 and 1440 cm -1 bands, and the replacement of NH~ by K + was completed. Thus, after drying the pellet overnight, the 1400 cm -1 band is due to the NH~ ions replaced by K +, while that at 1440 cm -j corresponds to the NH~ in the clay structure which is not replaceable by K + under the experimental conditions used. Influence of clay layer charge To investigate the influence of clay layer charge on the possible exchange of NH~ and K +, a series of synthetic saponites with a gradual increase of the tetrahedral charge was used (Table 1). The Na-saponites were NH~ saturated, and in a second step, these NH4-samples were back-saturated with Ca 2+. The KBr pellets of both NH4- and Casaturated samples were prepared. The transmission spectra of KBr pellets dried overnight are given in Fig. 2. The 1400 cm -l band is present for all NH4- saponites with almost the same intensity, while the intensity of the broader band at 1440 cm -I increases with the layer charge of the sample, as was previously reported (Petit et al, 1998). For,.Q <!! Wavenumber (cm -1) FIG. 2. The FTIR spectra in the u4 NH~ region, obtained in transmission mode of KBr pellets, dried overnight, of the saponite samples described in Table 1. (a) NH4-saturated samples; (b) Ca backsaturated samples. the NHa/Ca-saponites, no NH~ absorption is shown for the samples of lower layer charges (SAP0.35 and SAP0.5). The 1440 cm -1 band is observed only for that with the highest charge (SAP0.8, Fig. 2b). The absence of the NH~ band in the IR spectra of the two lower charged NH4/ Ca-saponites indicates that all exchangeable NH~ was removed by the Ca 2+ exchange. Therefore, the 1440 cm -1 band observed for the NH4-forms of SAP0.35 and SAP0.5 (Fig. 2, spectra a) corresponds to the NH] not replaced by K + from KBr, but replaced by Ca 2+, as indicated by the absence of this band in spectra b of Fig. 2. For SAP0.8, some NH~ ions remained as interlayer cations even after Ca 2+ saturation (Fig. 2, spectra b). The remaining NH~ in NH4/Ca-SAP0.8 is considered to be non-exchangeable, i.e. fixed, as in vermiculite (Mermut, 1994; Shen et al., 1997). The NH~ signals of NH4-SAP0.8 and NH4/Ca- SAP0.8 samples obtained by DRIFTS and KBr

5 IR spectra of NH~4-smectites 547 SAP cm -1 signal is due to NH; occurring as an interlayer cation in the clay. CONCLUSIONS ID o r~,.q < I Wavenumber (cm -1) Fie. 3. The FTIR spectra in the u4 NH~ region, of the SAP0.8 saponite. (a) DRIFTS measurement, undiluted NH4-saturated sample; (b) transmission mode of a KBr pellet, dried overnight, of the NH4-saturated sample; (c) DRIFTS measurement, undiluted NH4/Ca-saturated sample; (d) transmission mode of a KBr pellet, dried overnight, of the NH4/Ca-saturated sample. pellet methods are shown in Fig. 3. The spectra of NH4-samples are similar to those of SAz-1 montmorillonite (Fig. 1), i.e. a sharp band at 1400 cm -1 with a shoulder near 1440 cm 1 occurs in the KBr pellet spectrum (Fig. 3b), and a single broader band near 1440 cm -1 in DRIFTS (Fig. 3a). However, no difference was observed between these two types of spectra for the NH4/Casamples (Fig. 3c,d), thus confirming the presence of NH~ ions even after Ca 2 exchange. If the 1440 cm -1 band appears in the spectra collected using KBr as the diluting matrix, either by KBr pellet or the DRIFTS technique, it shows that some NH~ ions remain fixed in the clay and that they cannot be replaced by Ca 2+ or K +. It confirms that the a b C From the present study, it becomes possible to interpret the published data on the ammonium signal in NH~-bearing clay minerals. If FTIR measurements are performed without a diluting matrix (clay films, DRIFTS, IR-spectroscopy), the D 4 NH~ band observed is rather broad and located near 1440 cm 1. This band is due to NH~ ions present as compensating cations of the permanent and/or variable charges. In FTIR spectra obtained using KBr as a diluting matrix, either for transmission measurements using pellets or for DRIFTS, the ammonium signal is usually more complex, frequently with two superimposed u4 NH~ bands at 1400 and 1440 cm -1. The position and the shape of the u4 NH~ bands are clearly indicative of the exchangeability of the NH~ present in the clay. After complete replacement of the NH~ in the clay by K from KBr, the following three situations can occur. (1) The u4 NH~ band is sharp and appears at 1400 cm -1 without any other band in this region. This means that all the NH~ in the clay was replaced and is now present as NH4Br only. In the case of swelling clay minerals, such an exchange indicates that the layers have a low charge density. Conceivably, ammonium ions compensating the variable charge are also (and probably more readily) replaced. In the case of non-swelling clay minerals (kaolinite, mica, etc.), the u4 NH~ band at 1400 cm 1 represents the ammonium compensating the variable charge only, because there is either no permanent charge (e.g. kaolinite), or the permanent charge is compensated by non-exchangeable cations other than NH~ (e.g. mica). The ammonium saturation can then be used to measure quantitatively by IR, the variable charge of these minerals. Fialips et al. (1999) used this method to study the surface charge of synthetic kaolinites. (2) The presence of only a broad band near 1440 cm 1 means that ammonium from the clay is not replaced by K from KBr, and is still present in the interlayer space of clay minerals. This band suggests that NH~ is saturating permanent charges in the interlayers (or some of the interlayers) of clays with high charge density (e.g. Ca/NH 4- SAP0.8). This is in good agreement with spectra obtained from KBr pellets of natural NH4-illites and

6 548 S. Petit et al. micas which showed only this band at 1440 cm -1 when variable charges were compensated by other cations (Vedder, 1965; Lindgreen, 1994; Sucha et al., 1998). (3) Both bands at 1400 and 1440 cm -1 are present in the spectrum. This situation reflects heterogeneity in the clay charge (variable charge/ permanent charge; low/high layer charge or heterogeneous layer charge) compensated by NH~ ions. The integrated intensities of the bands at 1400 and 1440 cm -1 can be used to measure quantitatively the proportions of the different types of charge. The NH~ bands were previously used to quantify ammonium in phyllosilicates (Lindgreen, 1994), illite crystallization ratio (Sucha et al., 1998), surface acidity of smectites (Mortland & Raman, 1968), and also tetrahedral and octahedral charges (Chourabi & Fripiat, 1981; Ben Hadj-Amara et al., 1987; Srasra et al., 1994; Petit et al., 1998). This study showed that, using KBr pellets, the ammonium signal can, in addition, be used to indicate NH~ fixation which generally occurs on clay layers with high charge density (Sawhney, 1972). This is due to the extent of exchange between cation from the diluting salt matrix and cations compensating the permanent and/or variable charges of clay minerals. The present study showed that K + from KBr can easily replace NH~ in some clays, and that Ca 2+ is able to replace even those NH~ ions which are not replaceable by K +. By comparing various FTIR techniques (transmission, DRIFTS, and microscopy), Pelletier et al. (1999) proved that Na + from saponite samples could be replaced by K + from KBr. All these phenomena are closely connected to different hydration energies of exchangeable cations (Giiven, 1992) and to distributions of the interlayer charge (Ci~el & Machajdik, 1981). The use of various salts as matrices and/or back-saturating cations, as well as different techniques for studying ammoniumexchanged clay minerals by IR spectroscopy, promises to be a very useful way to characterize the charge of clay minerals. ACKNOWLEDGMENTS The authors wish to thank Dr J.L. Robert for supplying the synthetic saponite samples. M. Garais and D. Paquet are also acknowledged for their help in preparing the samples. Critical comments by P. Komadel helped to improve the paper. We also acknowledge Drs G. Lagaly and A.R. Mermut for their critical reviews. The financial support of the CNRS (project No. 4474) and of the Slovak Grant Agency (grant No. 2/4042/98) is gratefully acknowledged. REFERENCES Ben Hadj-Amara A., Besson G. & Tchoubar C. (1987) Caract~ristiques stmcturales d'une smectite diocta6- drique en fonction de l'ordre-d~sordre dans la distribution des charges ~lectriques: I. Etudes des r~flexions OOl. Clay Miner. 22, Bergaoui L., Lambert J.F., Vicente-Rodriguez M.A., Michot L.J. & Villi~ras F. (1995) Porosity of synthetic saponites with variable layer charge pillared by Al13 polycations. Langmuir, 11, Chourabi B. & Fripiat J.J. (1981) Determination of tetrahedral substitutions and interlayer surface heterogeneity from vibrational spectra of ammonium in smectites. Clays Clay Miner. 29, Ci~el B. & Machajdik D. (1981) Potassium- and ammonium-treated montmorillonites. I. Interstratified structures with ethylene glycol and water. Clays Clay Miner. 29, Ferriso C.C. & Hornig D.F. (1959) Absolute infrared intensities of the ammonium ion in crystals. J. Chem. Phys. 32, Fialips C.I., Petit S., Decarreau A. & Beaufort D. (1999) Influence of synthesis ph on kaolinite "crystallinity" and surface properties. Clays Clay Miner. (in press). Giiven N. (1992) Molecular aspects of clay-water interactions. Pp in: Clay-water Interface and its Rheological Applications, (N. Giiven & R.M. Pollastro, editors). The Clay Minerals Society, Boulder, Colorado. Jaynes W.F. & Bigham J.M. (1987) Charge reduction, octahedral charge, and lithium retention in heated, Li-saturated smectites. Clays Clay Miner. 35, Lindgreen H. (1994) Ammonium fixation during illitesmectite diagenesis in upper Jurassic shale, North Sea. Clay Miner. 29, Mermut A.R. (1994) Problems associated with layer charge characterization of 2:1 phyllosilicates. Pp in: Layer Charge Characteristics of 2:1 Silicate Clay Minerals, (A.R. Mermut, editor). CMS Workshop Lectures, 6, The Clay Minerals Society, Boulder, CO. Mortland M.M. & Raman K.V. (1968) Surface acidity of smectites in relation to hydration, exchangeable cation, and structure. Clays Clay Miner. 16, Mortland M.M., Fripiat J.J., Chaussidon J. & Uytterhoeven J. (1962) Interaction between ammo-

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