Jana Madejová

FTIR techniques in clay mineral studies

Transmission and reflection FTIR techniques were used to distinguish between different types of clay minerals and to derive information concerning their structure, composition and structural changes upon chemical modification. No special accessories were used; all reported sampling techniques are commonly available in the infrared laboratories. The selection of the sampling technique used depended on the purpose of the study and on the physical state of the sample. KBr pressed disk technique was found to be very useful for a routine characterization of both di- and trioctahedral clay minerals and for examination of structural modification of smectite during acid treatment. Transmission measurements of self-supporting films within a heated cell allowed for the study of in situ smectite dehydration. Attenuated total reflection (ATR) technique was successfully applied to obtain information on the wet clay samples. Diffuse reflectance (DRIFT) technique in the middle IR region was suitable for the study of organo-clays prepared by adsorption of alkyammonium cations on smectites. This technique was used in the near-IR region as well, where fixation of Li in smectites was investigated.

BASELINE STUDIES OF THE CLAY MINERALS SOCIETY SOURCE CLAYS: INFRARED METHODS

Infrared (IR) spectroscopy has a long and successful history as an analytical technique and is used extensively (McKelvy et al., 1996; Stuart, 1996). It is mainly a complementary method to X-ray diffraction (XRD) and other methods used to investigate clays and clay minerals. It is an economical, rapid and common technique because a spectrum can be obtained in a few minutes and the instruments are sufficiently inexpensive as to be available in many laboratories. An IR spectrum can serve as a fingerprint for mineral identification, but it can also give unique information about the mineral structure, including the family of minerals to which the specimen belongs and the degree of regularity within the structure, the nature of isomorphic substituents, the distinction of molecular water from constitutional hydroxyl, and the presence of both crystalline and non-crystalline impurities (Farmer, 1979). The interpretation of the absorption spectra of the Source Clays in the middle-IR (MIR) region (4000–400 cm−1) given here follows those of Farmer and Russell (1964), Farmer (1974a, 1979) and Russell and Fraser (1994). In addition, reflectance spectra in the near-IR (NIR) region (11,000–4000 cm−1), where overtones and combination vibrations occur, are included. These spectra provide information on structural OH groups and H2O in clay minerals (Bishop et al., 1994; Frost and Johansson, 1998; Petit et al., 1999a) which may not be clearly observed in the MIR spectra. Small changes in stretching and bending band positions are additive in the combination bands, thereby making them more readily differentiated (Post and Noble, 1993). Dispersive IR spectrometers are slowly being replaced by quicker and more sensitive Fourier transform (FT) instruments (Rintoul et al., 1998). The greater sensitivity of the FTIR spectrometers is related to the continuous detection of the entire transmitted energy …

Comparative FT-IR study of structural modifications during acid treatment of dioctahedral smectites and hectorite

Abstract Seven dioctahedral smectites of various composition and one trioctahedral smectite (hectorite) were leached in the HCl under different treatment conditions. Hectorite is much more susceptible to acid attack than the dioctahedral smectites. The rate of smectite dissolution increases with increasing octahedral Mg or Fe content as well as with the treatment temperature and acid concentration. The mechanism of smectite dissolution in HCl is independent of layer composition. The final reaction product obtained from acid dissolution of both smectite types is a hydrous amorphous silica phase. FT-IR spectroscopy is a sensitive method for characterization of the starting mineral, identification of some admixtures, such as quartz or kaolinite, and can be useful to assist in controling the extent of acid dissolution.

FTIR analyses of water in MX-80 bentonite compacted from high salinary salt solution systems

Abstract The effect of the exchangeable cations on the infrared (IR) spectra of water in MX-80 bentonite compacted from high salinary salt solution systems was studied using self-supporting film and attenuated total reflection (ATR) techniques. Na-bentonite MX-80 was saturated with homo-cationic (NaCl, KCl, CaCl 2 or MgCl 2 ) or hetero-cationic (mixtures of Na–, K–, Ca– and Mg–chlorides) solutions. The specimens for IR spectroscopy were prepared as self-supporting films (ssf) or compacted pastes. Differences in the wavenumbers and intensities of the structural OH group vibrations in relation to the type of the interlayer cation were found in the spectra of heated ssf. The most pronounced changes were observed for Mg-ssf, while only negligible changes occurred for K-ssf. The absorptions of water in heated Na- and K-ssf showed displacement of the stretching and bending bands to higher and lower wavenumbers, respectively, which indicates decreasing strength of H-bonding between water molecules. In contrast, for Mg-ssf the position of the stretching band of water substantially decreased on heating up to 90 °C followed by an increase upon further heating above 100 °C. The origin of these differences was discussed in terms of variations in the polarising ability of the interlayer cations influencing their hydration number. The ATR spectra of homo-cationic clay-pastes showed that the interlayer cations modify both the position and the intensity of the complex water band near 3400 cm −1 . The position decrease and the intensity increase followed the same order: K + , Na + , Ca 2+ , Mg 2+ . Good correlation between water band position and polarising power of the cations confirmed their influence on the strength of hydrogen bonds between water molecules. Similarly, a systematic shift of the H 2 O-stretching band to lower frequencies with the increasing Mg 2+ content in the samples was observed in the spectra of clay-pastes saturated with hetero-cationic chloride solutions. The intensity of the stretching band of water of both homo- and hetero-cationic pastes correlated very well with the water content obtained gravimetrically.

Dissolution of hectorite in inorganic acids

The effect of acid type and concentration on the reaction rate and products of dissolution of hectorite in inorganic acids was investigated. The dissolution of hectorite in hydrochloric (HCl), nitric (HNO3) and sulphuric (H2SO4) acids was characterized using quantitative chemical analysis, infrared (IR) and multinuclear MAS NMR spectroscopies. The rate of dissolution increased with acid concentration and decreased in the order HCl ≥ HNO3 = H2SO4 at the same molar concentration. No differences were found in the reaction products of hectorite treated with the three acids. The rate of Li dissolution was slightly greater than that of Mg at lesser acid concentrations (0.25 M), indicating that protons preferentially attack Li octahedra. The gradual changes in the Si-O IR bands reflects the extent of hectorite dissolution. The analysis of 29Si MAS NMR spectra relative peak intensities with dissolution time and acid concentration provided direct dissolution rates for tetrahedral (Q3) Si. After acid dissolution, most Si was bound in a three dimensional framework site (Q4), but a substantial part also occurred in the Si(OSi)3OH (Q31OH) and Si(OSi)2(OH)2 (Q220H) environments. These three sites probably occur in a hydrous amorphous silica phase. Both AlJV and AlVt rapidly disappeared from 27Al MAS NMR spectra of the dissolution products with acid treatment. The changes in IR and MAS NMR spectra of hectorite due to acid dissolution are similar to those of montmorillonite.

Characterisation of moderately acid-treated, size-fractionated montmorillonites using IR and MAS NMR spectroscopy and thermal analysis

The < 2 µm fractions of Cheto (Arizona, USA) and Jelsový Potok (Slovakia) montmorillonites have been treated with 1 mol dm–3 H2SO4 for periods up to 6 h at 95 °C. The materials obtained were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), infrared (IR) spectroscopy, and 29Si and 27Al magic-angle spinning (MAS) NMR spectroscopy. Both clays were partially decomposed under the conditions utilized, but the Cheto montmorillonite, which contained a higher proportion of octahedral magnesium, was more susceptible to acid attack. XRD was insensitive to this level of acid attack but XRF, IR and 29Si MAS NMR revealed the depopulation of the octahedral sheet. The number of acid sites was determined from the thermal desorption of cyclohexylamine and the catalytic activity was evaluated by reacting 2,3-dihydropyran with methanol to yield the tetrahydropyranyl ether. The test reaction was a more sensitive indicator of changes in clay acidity than the values determined by desorption of cyclohexylamine from acid sites.

Effect of non-swelling layers on the dissolution of reduced-charge montmorillonite in hydrochloric acid

Abstract A series of reduced-charge montmorillonites (RCM), prepared from the same parent Li-montmorillonite (Jelšový Potok, Slovakia) by heating at various temperatures (105-210°C) for 24 h, was treated with 6 M HCl at 95°C for periods up to 30 h. Reaction solutions obtained were analysed for Al, Fe, Mg and Li and the solid reaction products were investigated by FTIR spectroscopy. Both analyses provided evidence that the extent of dissolution decreased with increased amounts of Li fixed within the montmorillonite structure, i.e. with increased heating temperature. Differences in the acid dissolution process were reflected in the structural changes which occurred within the RCM samples, due presumably to different positions of fixed Li. The ethylene glycol monoethyl ether (EGME) surface areas, and XRD and HRTEM analyses of the RCM series revealed an increased amount of non-swelling layers in the samples prepared at higher temperatures, which caused a substantially slower decomposition of M7 and M8 in HCl. The [calculated XRD patterns of M6 and M7 confirmed the presence of 20% and 45% pyrophyllite-like layers, respectively, in these samples. Mixed-layer pyrophyllite-like-smectite and pyrophyllite-like crystals, containing only non-swelling layers, were found in sample MB. The results confirmed that the amount of swelling layers in RCM significantly affects their dissolution rate in HCl.

Alteration of smectites by treatments with hydrochloric acid and sodium carbonate solutions

Fine fractions of bentonites from Jelsový Potok (JP), Blsany (BL) (Czechoslovakia) and Mittelherwigsdorf (MI) (German Democratic Republic) were repeatedly treated with HCl and Na2CO3 solutions. The products were investigated by X-ray diffraction and infrared spectroscopy. The octahedral layers of smectite were partly dissolved whereas amorphous SiO2 was produced by the acid treatments. This SiO2 could be dissolved in Na2CO3 solution. The clay structure decomposition by acid increased with the octahedral Fe content and the total octahedral substitution of Fe and Mg for Al in the order: JP<MI<BL. Similarity of the materials before and after combined HCl + Na2CO3 treatment depended on the degree of the clay structure decomposition by the acid.

REDUCTION AND REOXIDATION OF NONTRONITE: QUESTIONS OF REVERSIBILITY

Redox cycles are common in nature and likely have a profound effect on the behavior of soils and sediments. This study examined a key component of redox cycles in smectites, namely, the reoxidation process, which has received little attention compared to the reduction process. Unaltered (oxidized) and reoxidized ferruginous smectites (nontronites) were compared using infrared and Mössbauer spectroscopies, and thermal gravimetric analysis. The infrared and thermal gravimetric data revealed that the structural OH content of reduced-reoxidized clay is about 15 to 20% less than in the original (oxidized) sample, indicating that the structure remains partially dehydroxylated even after reoxidation. Mössbaner spectra of reoxidized samples consisted of larger quadrupole splitting for Fe(III) doublets than in the unaltered samples, suggesting that the environment of Fe(III) is more distorted after the reduction-reoxidation treatment.

Characterization of octahedral substitutions in kaolinites using near infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy investigations in the near infrared (N1R) region of synthetic and natural kaolinites with various octahedral substitutions have been carried out in order to elucidate the relationships between the substituted cations and specific features of the NIR spectra. The combination modes of the OH stretching and bending vibrations characterizing Fe(III), Ga(III) and Cr(III) octahedral substitutions are identified in the NIR region at 4466, 4498 and 4474 cm-1, respectively, and the first overtones of the OH stretching vibrations at 7018, 7018 and 6986 cm-1, respectively. As far as we know, the bands of kaolinites containing Ga(III) or Cr(III) have not been reported yet. For both Ga(III) and Cr(III), the NIR observations explain why the bending vibration bands of AlGaOH and AlCrOH groups are not observed in the middle infrared (MIR) region.