Katja Emmerich

Dehydroxylation behavior of heat-treated and steam-treated homoionic cis-vacant montmorillonites

Li+, Na+, Ca2+, Sr2+, Cu2+, or Zn2+-saturated samples of a cis-vacant montmorillonite from Linden, Bavaria, were heated to temperatures between 200–700°C. Half of each heated sample was subsequently autoelaved under steam at 200°C (∼1.5 MPa) to promote rehydroxylation. The smectites were characterized by cation-exchange capacity (CEC), determination of exchangeable cations, infrared (IR) spectroscopy, and thermoanalytical investigations of evolved water in a thermobalance linked with a mass spectrometer.Changes in the montmorillonite structure and dehydroxylation behavior are related to three respective mechanisms: type of the interlayer cation, interlayer cation radius, and the movement of the interlayer cation. The migration of the smaller Li+, Cu2+, and Zn2+ ions after heating produces a strong reduction of CEC due to the Hofmann-Klemen effect before the initiation of dehydroxylation. Thereafter, the CEC of these smectites remains constant over a large temperature interval during dehydroxylation. After rehydroxylation, Cu2+ and Zn2+-rich samples release 16–23 meq/100 g of Mg2+ from the structure. No Mg2+ release is observed for the Li+-rich montmorillonite. Also the dehydroxylation behavior after rehydroxylation differs between the Cu2+, Zn2+, and Li+-rich samples. The mass curves of the evolved water during thermoanalysis of the rehydroxylated Cu2+ and Zn2+-rich smectites show a peak doublet between 480–700°C. For the Li+, Na+, Ca2+, and Sr2+-rich montmorillonites, the second peak disappeared and a third peak at ∼760°C developed after rehydroxylation. The resulting structure after rehydroxylation of all samples is celadonite-like.

Clay profiling: the classification of montmorillonites

Montmorillonites, with the general composition

Experimental Investigations on the Frequency- and Temperature-Dependent Dielectric Material Properties of Soil

M_{\rm{x}}^ + \left( {{\rm{S}}{{\rm{i}}_{4 - y}}{\rm{A}}{{\rm{l}}_y}} \right)\left[ {{{\left( {{\rm{Al}},{\rm{F}}{{\rm{e}}^{3 + }}} \right)}_{2 - z}}{{\left( {{\rm{Mg}},{\rm{F}}{{\rm{e}}^{2 + }}} \right)}_z}} \right]{{\rm{O}}_{10}}{\left( {{\rm{OH}}} \right)_2}

A COMPREHENSIVE CHARACTERIZATION OF DIOCTAHEDRAL SMECTITES

Mx+(Si4−yAly)[(Al,Fe3+)2−z(Mg,Fe2+)z]O10(OH)2 where x = ξ = 0.2–0.6, x = y+z, and y≪z, vary widely in composition and structure. The commonly used classification into five montmorillonite and two beidellite groups for the solid-solution sequence does not allow an unambiguous classification with respect to structural features and the resulting properties.The smectite structure reveals five features that allow an unambiguous description of a sample: (1) identification as either a dioctahedral or a trioctahedral smectite; (2) layer charge; (3) charge distribution between tetrahedral and octahedral sheets; (4) cation distribution within the octahedral sheet; and (5) Fe content. In addition, the nature of interlayer cations should be given as they influence certain properties of montmorillonites. Analytical methods are now available to measure and determine these structural features. Therefore, a precise classification for montmorillonites requires determination of layer charge and exchangeable cations, analysis of chemical composition, and thermal analysis (to determine the octahedral structure), in addition to X-ray diffraction analysis.A comprehensive classification of montmorillonites based on these parameters is proposed. Ninety-six structural variations (expressed by systematic names) theoretically exist within the montmorillonite-beidellite series. Descriptive names can be used to elucidate the macroscopic properties of the montmorillonite samples in question.

SPONTANEOUS REHYDROXYLATION OF A DEHYDROXYLATED CIS-VACANT MONTMORILLONITE

Frequency- and temperature-dependent complex permittivity or conductivity of a silty clay loam were examined in a broad saturation and porosity range with network analyzer technique (1 MHz-10 GHz, 5 °C-40 °C, coaxial transmission line and open ended coaxial cells). An advanced mixture model based on the well-known Lichtenecker-Rother model (ALRM) was developed and used to parameterize complex permittivity or conductivity at a measurement frequency of 1 GHz under consideration of a dependence of the so-called structure parameter as well as the apparent pore water conductivity on saturation and porosity. The ALRM is compared with frequently applied mixture models: complex refractive index model, Looyenga-Landau-Lifschitz model, Bruggeman-Hanai-Sen model, and Maxwell-Garnet model as well as empirical calibration functions. Comparison of ALRM applied to the investigated frequency and temperature range with sophisticated broadband relaxation models indicates the potential and the limitation to predict the high-frequency electromagnetic material properties.

Microscopic structure and properties of discrete water layer in Na-exchanged montmorillonite.

The term ‘montmorillonite’ encompasses a wide range of chemical compositions and structures. Comprehensive and reliable characterization is essential for unambiguous classification. Twenty eight purified, Na-exchanged smectites (<0.2 µm) were characterized by layer-charge measurement using the alkylammonium method, by cation exchange capacity (CEC) measurement with Cu-triethylenetetramine, by determination of the chemical composition using X-ray fluorescence analysis, by calculation of the structural formula following determination of the octahedral structure (trans-vacant vs. cis-vacant) by simultaneous thermal analysis, and by X-ray diffraction analysis. Mössbauer spectroscopy was applied to determine the oxidation state and coordination of Fe and changes thereof during purification of the source materials.The charge derived from chemical composition was considerably greater (by up to 30%) than the measured layer charge. The independently measured layer charge was used to calculate the structural formula. The measured CEC values, comprising the permanent charges and the pH-dependent edge charges, were consistent with measured layer charge but not with layer charge derived from the chemical composition. Therefore, the structural formula of smectites should be calculated using the measured layer charge.The dehydroxylation temperature, which conveys information about the structure of the octahedral sheet, was correlated to the amount of Mg and Fe3+ and the location of charges. No relationship was found among the dehydroxylation temperature and the mean layer charge or the Mg content. In contrast, a clear relationship was observed between the Fe content and the dehydroxylation temperature. Montmorillonites with an Fe content <0.3/f.u. are cis-vacant and those containing Fe3+ > 0.3 mol/f.u. are trans-vacant, mostly with additional cis-vacancies. Tetrahedral substitution also appeared to be a function of the number of trans-vacancies.The parameters analyzed provide the basis for a new descriptive classification system.

THE FATE OF SMECTITE IN KOH SOLUTIONS

In general, montmorillonite and other dioctahedral 2:1 layer silicates are characterized by dehydroxylation temperatures between 500–700°C ( e.g., Mackenzie, 1957; Grim and Kulbicki, 1961; Schultz, 1969; Guggenheim, 1990). Differences in dehydroxylation temperature are primarily related to the kind of octahedrally coordinated cations present and their distribution and movement in dioctahedral 2:1 layer silicates (Drits et al., 1995), although the interlayer cation may have an effect also ( e.g., Guggenheim and Koster van Groos, 1992). Trans -vacant (tv) smectites and micas are characterized by dehydroxylation temperatures which are 150–200°C lower than those for the same minerals consisting of cis -vacant (cv) 2:1 layers. Most montmorillonites consist of a mixture of cv and tv 2:1 layers and lose their hydroxyls in two steps near ~550 and ~700°C (Drits et al., 1995). Hence, the investigation of the structure of dehydroxylated montmorillonite is of great interest to understand the dehydroxylation process ( e.g., Jonas, 1954; Heller et al., 1962; Drits et al., 1995). Dioctahedral 2:1 layer silicates are expected to produce well defined dehydroxylates after heating for a short time at temperatures between 500–700°C and cooling under laboratory atmosphere ( e.g., Grim and Bradley, 1948; Heller-Kallai and Rozenson, 1980; Drits et al., 1995). However, the heating rate (Hamilton, 1971) and duration of heating (Horvath, 1985) are important in determining if an anhydrous state is achieved. A slow heating rate lowers the apparent dehydroxylation temperature, which is a well known but often neglected phenomenon. Emmerich et al. (1999) found that a completely dehydroxylated state of montmorillonites that …

Natural mixture of silica and smectite as a new clayey material for industrial applications

In this work, we focus on the atomic structure of the water interlayer of Na-exchanged montmorillonite. For two different surface charge densities, namely -0.086 and -0.172 C/m(2), the adsorption process in the presence of water is described by first principles calculations. We describe the interactions and forces for every water molecule entering the interlayer during the swelling process. In particular, the dielectric permittivity of the water interlayer is calculated. Finally, we confirm our results performing ab initio thermodynamics calculations leading to a wide range of realistic experimental scenarios.

The cis- and trans-vacant variety of a montmorillonite: an attempt to create a model smectite

Abstract The aim of the present study was to investigate the detailed evolution of the SAz-1 smectite in 1 M KOH at 80 °C at a solid/liquid ratio of 1/80. AFM observations indicated no change in crystal size or shape. XRD measurements at 40% relative humidity revealed changes in expandability of the smectite. The (001) reflection profile of smectite was modeled using a trial-and-error approach. The results indicate that with increasing run time, the number of non-expandable layers with zero or one water layer increases and that the coherent scattering domain size of the smectite decreases. IR spectroscopy of the reacted smectite suggests that there is no change from the initial clay products. The dehydroxylation temperature showed a slight decrease from 619 to 605 °C. STA measurements demonstrated that the cis-vacant character of the octahedral sheet remained nearly unchanged throughout the experiment. Determination of the average layer charge showed a continuous increase from 0.32 to 0.42 eq/(Si/Al)4O10, whereas the layer charge distribution indicated the appearance of high charged smectite layers with a charge of ~ 0.6 eq/(Si/Al)4O10 and the disappearance of the low charged layers. XPS and SEM measurements indicate an increase of Al in the smectite samples. Isotope data support the theory of an internal diffusion mechanism by gradual changes in δ18O values. From these data it appears that KOH solutions provoke a mineralogical change in the 2:1 layer of the smectite minerals that increases the layer charge by increasing the Al content. This mineralogical change does not involve dissolution/crystallization processes and then must show solid-state transformation of the clays at 80 °C.

Low-frequency dielectric properties of three bentonites at different adsorbed water states

Abstract The main physico-chemical properties of a new smectitic clay containing large amounts of amorphous material are reviewed and potential industrial applications of this type of clay are discussed. Due to a 34% amorphous material content (natural silica gel), the investigated clay has very high porosity and can be used as it is or in acid-impregnated form for oil bleaching or phosphate reduction in edible oil. In the field of biodiesel purification, the new clay can be used to remove, in particular, mono-, diglycerides and glycerol. The natural silica-smectite mixture is also suitable as a carrier for liquid ingredients, for example in animal feeds, and might serve as a partial or complete substitute for synthetic precipitated silicas. In the field of bioseparation processes, the clay can be used as an adsorbent for protein separation by means of cation exchange. Due to the suppressed swelling (compared with smectite alone), it can be packed in columns which can be regenerated.