Iron impurities in the Hohen Hagen nontronite (NG-1) were identified as maghemite and goethite. The phase identified as maghemite was magnetically ordered at both room temperature and 87 K, with hyperfine magnetic fields of 48.6 and 50.7 tesla, respectively. Due to the magnetic properties of this phase, it was easily separated from aqueous dispersions of the clay using a hand magnet. X-ray powder diffraction analysis revealed maghemite, quartz, and phyllosilicate in the magnetically separated phase. The impurity identified as goethite remained in the non-magnetic <2-μm fraction of the clay, displayed magnetic order only at 87 K (47.44 tesla hyperfine field), and accounted for about 8% of the total area of the Mössbauer spectrum.

The reduction and reoxidation of three nontronite samples, GAN (API H-33a, Garfield, Washington), SWa-1 (ferruginous Washington smectite), and NG-1 (Hohen Hagen, Federal Republic of Germany) were studied with visible absorption and Mössbauer spectroscopy. The intensity of the intervalence electron transfer (IT) band at 730 nm in these nontronites was monitored during reduction and reoxidation at 277, 294, and 348 K. The results showed that the intensity of the band followed the number of Fe(II)-O-Fe(III) groups in the clay crystal, increasing to a maximum at about Fe(II): total Fe = 0.4; upon complete reduction, the band decreased to about the intensity of the unaltered, oxidized sample. With reoxidation of the sample with O2, the intensity of the band increased sharply, followed by a gradual decay back to the original, oxidized intensity. The ultimate level of Fe reduction achieved was at least 92%. Concomitantly, the color changed from yellow through green, blue-green, dark blue, light blue, and light gray as the Fe(II) content increased. The GAN nontronite was more difficult to reduce than the SWa-1 or NG-1 samples. The rate and level of reduction increased with the amount of reducing agent added.

Partial stabilization of Fe(II) in chemically reduced smectite, which normally readily undergoes reoxidation in air, was achieved. The purpose of this study was to determine if Fe(II) can be stabilized in reduced smectites by Li fixation upon heating. More than 80% of total Fe in ferruginous smectite SWa-1 was reduced to Fe(II) using the citrate-bicarbonate-dithionite (CBD) method while purging the clay dispersion with N2. The reduced smectite was Li-saturated, washed free of excess ions, freeze-dried, and heated in N2 atmosphere at 260°C for 24 h to produce Li-fixation. This treatment caused partial stabilization of Fe(II) in the clay structure. Chemical analysis, Mössbauer spectroscopy, and Fourier transform infrared (FTIR) spectroscopy proved that <20% of total Fe was Fe(II) after reoxidation with oxygen in a water dispersion, a treatment which causes complete reoxidation of Fe(II) in reduced Na-smectites. Decomposition of the OH-stretching band evident in the IR spectra indicated migration of Li into the vacant octahedra. Some of the OH groups in the reoxidized smectite were found in local trioctahedral configurations, associated with the AlFe(II)Li or Fe(III)Fe(II)Li groupings of central atoms in the octahedral sheet.

Reduction of structural Fe in Na-exchanged dioctahedral smectites decreases swellability in water, but because clay interlayers also collapse in the process the concomitant effect on surface hydration energy is uncertain. This study examined the hydration behavior of oxidized and reduced dioctahedral smectite clays exchanged with polar (Na) and weakly-polar (organic) cations to determine the nature of the surface before and after Fe reduction, and to determine if clay surfaces are hydrophilic or hydrophobic. The H2O content in various dioctahedral smectites decreased if Na was replaced by tetramethylammonium (TMA), trimethylphenylammonium (TMPA), or hexadecyltrimethylammonium (HDTMA). Among the organo-clays, H2O adsorption decreased with increasing complexity of the cation. For oxidized smectites, those exchanged with TMPA retained less H2O than those exchanged with Na at all pressures. The extent of this difference depended on the clay and decreased with increasing applied pressure. Reduction of Fe(III) to Fe(II) in the octahedral sheets decreased the swelling of Na-saturated smectites, apparently causing some previously swelling interlayers to collapse. If the Na interlayer cation was exchanged to alkylammonium after reduction, but prior to swelling-pressure measurements, the swelling increased or remained near constant, suggesting that the organo-cation disrupted the collapse process of the interlayers associated with the reduced smectite layers. Reduced TMPA-saturated smectite surfaces are more strongly hydrated if the octahedral sheet is reduced than if oxidized. Thus, reduction of structural Fe increases the hydration energy of smectite basal surfaces, but swellability could decrease or increase depending on the extent of interlayer collapse occurring with different exchangeable cations.

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.

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 potentiometric titrations.

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 (Farmel, 1979).

The potential use of near-infrared (NIR) spectroscopy as a characterization tool for organo-clays would be a great asset but little work has been done in this regard because the application of NIR to clay mineral studies is a relatively new phenomenon. The purpose of this study was to use NIR spectroscopy to investigate the effect of alkylammonium cations on the acid dissolution of a high-charge montmorillonite (SAz-1). Detailed analysis of the spectra of Li+-, TMA+- (tetramethylammonium), and HDTMA+- (hexadecyltrimethylammonium) saturated SAz-1 montmorillonite in the NIR region was achieved by comparing the first overtone (2ν) and combination (ν+δ) bands of XH groups (X = O, C) with the fundamental stretching (ν) and bending (δ) vibrations observed in the mid-infrared (MIR) region. Comprehensive analysis of the vibrational modes of CH3-N, CH3-C, and -CH2-C groups of TMA+ and HDTMA+ cations detected in the MIR and NIR regions was also performed. Both MIR and NIR spectra demonstrated that exchange of Li+ by TMA+ only slightly improved the resistance of SAz-1 layers to dissolution in 6 M HCl at 80°C, while exchange by the larger HDTMA+ cations almost completely protected the montmorillonite layers from acid attack. Use of NIR spectra in reaching these conclusions was crucial. Only in the NIR region could the creation of SiOH groups be monitored, which is an important indicator of the acidification of the montmorillonite surface. The OH-overtone region in the spectra of Li-SAz-1 and TMA-SAz-1 revealed that the SiOH band near 7315 cm−1 increases in intensity with enhanced acid treatment. In contrast, no SiOH groups were identified in the NIR spectra of HDTMA-SAz-1 treated in HCl, indicating that HDTMA+ completely covers the inner and outer surfaces of the montmorillonite and hinders access ofprotons to the Si-O− bonds created upon acid treatment.

Enhancement of the physico-chemical properties of elastomers can be achieved by the addition of fillers, such as silica, but the search for less expensive alternative materials continues. The objective of this study was to investigate natural or organically modified clay minerals as such an alternative. Organo-clays modified by quaternary ammonium cations with three methyl groups and longest alkyl chains of different lengths were prepared by ion-exchange reaction of the commercial product JP A030 (Envigeo, Slovakia) based on Jelšový Potok bentonite with organic salts: tetramethylammonium (TMA) chloride, octyltrimethylammonium (OTMA) bromide, and octadecyltrimethylammonium (ODTMA) bromide. Physico-chemical characterizations of the organo-clays used as fillers in rubber nanocomposites and their mechanical properties were measured using Fourier transform infrared (IR) spectroscopy, which provided information on the chemical composition of the mineral and on the amount of organic moieties adsorbed. X-ray diffraction analysis (XRD) was used to monitor the arrangement of organic chains in galleries of montmorillonite and showed that the longest-chain alkylammonium ODTMA+ ions were intercalated between layers, adopting a pseudotrimolecular conformation, while OTMA+ and TMA+ were in monomolecular arrangement. Surface areas were measured by sorption of N2 and ethylene glycol monoethyl ether. Natural rubber-clay nanocomposites were prepared by melt intercalation, in some cases also with addition of silica, a conventional reinforcing filler. The microstructure of montmorillonite in these composites was characterized by XRD analysis. The effect of clay and organo-clays loading from 1 to 10 phr (parts by weight per hundred parts of rubber) on stress at break, strain at break, and Modulus 100 (M100) was investigated by tensile tests. Filler ODTMA-JP A030 appears to be the most effective among the organoclays; surprisingly similar values of composite elongation and strength were obtained with unmodified bentonite JP A030.

Layer charge reduction of selected homoionic swelling clay minerals upon heating is reviewed. This phenomenon is known for Li+-montmorillonites as the Hofmann-Klemen effect. Aspects covered in the review include: mechanism of the charge reduction caused by the irreversible migration of small cations into the mineral layers; final sites of the fixed cations in the octahedral and/or tetrahedral sheets, as deduced on the basis of structural and spectroscopic data obtained in several studies; effects of octahedral and tetrahedral components of the layer charge; properties of the exchangeable cations; and the chemistry and structure of the mineral on charge reduction. Current knowledge has been summarized on the changes of various mineral properties connected with charge reduction, including the loss of swelling and the decrease in the cation exchange capacity, the most important change upon cation fixation. Also discussed are the preparation, properties, and advantages of uses of series of reduced-charge montmorillonites (RCMs) in research; interactions of RCMs with alkylammonium cations and organic cationic dyes, with some examples on the interactions with organic polymers and other organic compounds. Properties of organo-complexes and composite materials prepared from RCMs are also included in this review.