M. Janek

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.

METHYLENE BLUE INTERACTIONS WITH REDUCED-CHARGE SMECTITES

The objective of this work was to prepare series of reduced-charge materials from different parent Li-saturated dioctahedral smectites, to investigate the effects of temperature, chemical composition and charge location in smectites on the charge reduction and to characterize reduced-charge smectites (RCSs) using methylene blue (MB) adsorption. The layer charge decrease, induced by Li fixation, is correlated with the trends in the spectra of MB-RCS dispersions in the visible region (VIS) spectra. Distribution of the negative surface charge of the clay minerals controls the distance between the adsorbed MB cations and thus affects the formation of MB dimers and higher agglomerates. Because each form of MB (monomer, dimer, higher agglomerate, J-aggregates) absorbs light at a different wavelength, the VIS spectra of MB depend sensitively on the charge density at the clay surface. Both cation exchange capacity (CEC) values and spectra of MB-clay dispersions clearly detect extensive reduction of the layer-charge density in reduced-charge montmorillonites (RCMs) upon Li-thermal treatment. The extent of charge reduction depends on the temperature of the thermal treatment, as well as on the octahedral charge of the montmorillonite. Reduction of the layer charge proceeds to a much lesser extent for smectites with mainly tetrahedral charge and high Fe content (Fe-rich beidellite and ferruginous smectite). Both CEC data and MB spectra detect only a slight decrease of the layer charge density, which relates to the low octahedral charge of these minerals. Following heating at higher temperatures (120-160°C), slightly higher Li fixation is indicated by CEC values; however, no charge reduction is confirmed by MB spectra. Release of protons accompanying Li+ fixation in Fe-rich smectites heated at 180 or 200°C was detected in the spectra of MB-clay dispersions and confirmed by Potentiometrie titrations.

Influence of the layer charge density of smectites on the interaction with methylene blue

Six Na-saturated smectites were isolated from bentonites of various deposits. Their H-forms were prepared using H–OH–H resins. The interaction of methylene blue (MB) (0.05 mmol g-1) with Na-saturated untreated and H-autotransformed clay samples was investigated by the VIS spectroscopy of their dispersions. The spectra provided information about the formation of protonated dye cations, dimers, and higher agglomerates on the clay surface. The amounts of various dye forms were compared with the distributions of negative layer charge of the used clays, obtained by the alkylammonium method. Formation of MB dimers and higher agglomerates reflected sensitively the layer charge density of the smectites. MB cations formed predominantly higher agglomerates at the surface of those smectites with high charge density. Suppressed dye agglomeration was observed for the samples of lower layer charge density and was caused by greater distances between the adsorbed MB cations. Due to the high sensitivity of MB agglomeration on the layer charge and the simplicity of the method, MB–clay interaction can potentially be utilized for the layer charge characterization of smectites.

Proton saturation and rheological properties of smectite dispersions

Proton-saturated dispersions of the < 2-μm fractions of five smectites were prepared. Potentiometric titration data of freshly saturated and autotransformed samples were analysed by calculating proton affinity distribution curves to distinguish different proton interacting sites. Sites with pK values of ∼ 2.8 and 11.3 were assigned to protons exchanged for sodium ions and deprotonisation of silanol groups, respectively. Hydrated aluminum ions in freshly proton-saturated dispersions were characterized by pK ∼ 6. This group of weakly acidic centers also included oligomeric hydroxoaluminum cations because the amount of these sites increased during autotransformation and was accompanied by a shift to pK ∼ 5.5. The freshly prepared proton-saturated dispersions showed low pH values (∼ 2.6), and the particles interacted by edge(+)/face(-) contacts. This increased the viscosity in comparison to the sodium forms at pH ∼ 7, and, depending on the smectite, yield values between 10 and 400 mPa were observed. Autotransformation removed all sites with pK ∼ 2.8, reduced the viscosity, the yield value (mostly < 10 mPa), and the flow behaviour approached that of the sodium smectite dispersions at pH ∼ 7. The cause is enrichment of aluminum and/or oligomeric hydroxoaluminum cations in the solution and Stem layer, which leads to fragmentation of the networks into smaller stacks of more densely packed particles.

Effect of autotransformation on the layer charge of smectites determined by the alkylammonium method

Abstract Hydrogen-forms of <2 μm fractions of six bentonites of various Fe contents were prepared by H+→OH-→H+ ion exchange using resins. Potentiometric titration curves revealed that the number of strong acid sites varied and accounted for 60-95% of the total acidity in the freshly prepared H-forms. The number of strong acid sites decreased and that of the weak acid sites increased on ageing. The process of autotransformation in aqueous dispersion at 90°C was completed within four days. Layer-charge distributions of all samples were inhomogeneous with layer charges from 0.25-0.39 Eq/unit O10(OH)2. Oxalate pretreatment of the samples resulted in changes in the layer-charge distribution due to the removal of readily soluble phases which may have blocked exchange sites. After autotransformation, the alkylammonium exchange method revealed inhomogeneous charge density distributions; the fraction of layers of the highest charge decreased. Comparison of total CEC obtained from potentiometric curves and interlamellar CEC calculated from the mean layer charge confirmed attack of protons from particle edges. However, for several samples the structural attack may also occur from the interlayer space. Autotransformation of the H- smectites decreased the mean layer charge. Protons probably attack the Mg(O,OH)6 octahedra preferentially during the autotransformation.

Acidity and catalytic activity of mildly acid-treated Mg-rich montmorillonite and hectorite

XRD, FTIR spectroscopy, cyclohexylamine desorption, potentiometric titrations and catalytic activity in an ether-forming test reaction have been used to investigate mildly acid-treated montmorillonite (SAz-1) and hectorite. The number of acid sites obtained from the cyclohexylamine desorption agreed with the accepted cation exchange capacity values for all samples. No evidence of any free protons was found in the potentiometric titration curves, which proved that complete autotransformation of the acid-treated samples had occurred over a much shorter period than previously recorded. Acid sites associated with Al3+ (Fe3+) released from the SAz-1 structure during acid treatment and subsequent autotransformation were of sufficient strength to catalyse the test reaction and produce tetrahydropyranyl ether in 80% yield. The complete absence of catalytic activity in the acid-treated hectorite samples confirmed that acid sites associated with interlayer Mg2+ and Li+ were unable to catalyse the test reaction.

Intercalation and Retention of Carbon Dioxide in a Smectite Clay promoted by Interlayer Cations

A good material for CO2 capture should possess some specific properties: (i) a large effective surface area with good adsorption capacity, (ii) selectivity for CO2, (iii) regeneration capacity with minimum energy input, allowing reutilization of the material for CO2 adsorption, and (iv) low cost and high environmental friendliness. Smectite clays are layered nanoporous materials that may be good candidates in this context. Here we report experiments which show that gaseous CO2 intercalates into the interlayer nano-space of smectite clay (synthetic fluorohectorite) at conditions close to ambient. The rate of intercalation, as well as the retention ability of CO2 was found to be strongly dependent on the type of the interlayer cation, which in the present case is Li+, Na+ or Ni2+. Interestingly, we observe that the smectite Li-fluorohectorite is able to retain CO2 up to a temperature of 35°C at ambient pressure, and that the captured CO2 can be released by heating above this temperature. Our estimates indicate that smectite clays, even with the standard cations analyzed here, can capture an amount of CO2 comparable to other materials studied in this context.

TERAHERTZ TIME-DOMAIN SPECTROSCOPY OF SELECTED LAYERED SILICATES

Micaceous layer silicate clay minerals are attractive materials for applications involving non-linear optics because of their low cost and ability to form well ordered, platy aggregates, but such applications require precise knowledge of the dielectric behavior of the clay. The purpose of the present study was to use Terahertz time-domain spectroscopy (THz-TDS) to determine the dielectric properties of certain cleavable layered clay minerals, including muscovite, vermiculite, phlogopite, and biotite. The samples were characterized by X-ray diffraction and Fourier transform infrared spectroscopy as well as chemical analysis by Energy dispersive X-ray spectroscopy. The THz frequency window investigated was the far-infrared region of 3.3 to ∼40.0 cm−1 corresponding to 0.1 and 1.2 THz, respectively. The samples were selected so that the hydrated form of the interlayer cation, e.g. Mg2+ present in the interlayer gallery of vermiculite, could be compared to species such as phlogopite, biotite, and muscovite with the dehydrated form of interlayer cations such as K+ or Na+. The frequency-dependent complex index of refraction of these natural materials was determined to vary between 2.50 and 2.80. The presence of water in the interlayer space of vermiculite was reflected in the detection of increased values of the absorption index in comparison with the muscovite, phlogopite, and biotite.