Conduction and diffusion in exchanged montmorillonite clays

The temperature dependences for conductivities and ¹H NMR relaxation times are reported for homionic (exchanged) montmorillonite clays. Ionic conductivity in the interlayer region is primarily protonic (H⁺), arising from the Brønsted acidity of hydrated interlayer cations. Hydrogens outside the hydration shell appear to undergo translation by chemical exchange.     

Mössbauer studies of the Umbria archeological clays

We report an analysis of two different clays from the region of Umbria (Italy): 1. potteries found one inside and the other outside the town of Todi, dating from the end of the IV century B. C., 2. bond clays of Bagnara (Nocera Umbra) and Colfiorito (Foligno) dating V century B.C.     

Pure and impure clays and their firing products

Mössbauer spectroscopy is highly suited for the study of clays whose industrial uses depend on the iron content. Reactions that take place during clay firing can be readily monitored by Mössbauer spectroscopy. Following dehydroxylation of clay minerals, the quadrupole splitting of octahedrally coordinated iron (III) increases abruptly, but reverts to lower values upon the formation of new, better ordered phases at higher temperatures. It is also shown that iron oxides may account for a considerably higher proportion of the total iron content of many clays than is commonly recognized, and their existence must be taken into consideration for a correct interpretation of the Mössbauer spectra of clays.     

Mössbauer study of illitic clays containing iron-rich impurities

The iron mineralogy of nineteen illitic clays from eastern Bavaria was studied by Mössbauer spectroscopy and X-ray powder diffraction. Mössbauer spectra of the <2 μm fraction were taken at RT, 120 K and 4.2 K. The clays contain both paramagnetic Fe³⁺ and Fe²⁺. Superparamagnetic oxides are frequently present. The Fe²⁺ quadrupole splitting and the ratio of Fe³⁺ at 4.2 K to Fe²⁺ at 120 K are correlated and define two groups distinguished by their mineral content. The samples were heated systematically for 48 h up to 1250°C in steps of 50°C. One clay which is rich in chlorite and Fe(II) was studied in detail after firing in air and following a reduction for 3 h at 800°C with charcoal. The transformations of the mineral phases with temperature as shown by X-ray diffraction are also evident in the Mössbauer spectra.     

Silver atoms and clusters in molecular sieves and clays

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) studies on small paramagnetic silver clusters generated radiolytically in molecular sieves and clays are reviewed. Special attention is paid to the role of framework structure with respect to cluster nuclearity and stability. In general, geometrical size constraints play a crucial role and molecular sieves with cage-like structures with small openings between the cages such as A and rho zeolites stabilize cationic silver clusters more efficiently than channel-like zeolite structures. In addition, other factors like total cation capacity, type of cocation and/or molecular adsorbate in the cages can affect the silver agglomeration process to a great extent. The mechanism of silver agglomeration in irradiated molecular sieves and clays based on the ESR results is described as well as the reactivity of tetrameric and hexameric silver clusters with various molecular adsorbates.     

Nitrate removal using natural clays modified by acid thermoactivation

Groundwater pollution by nitrates is a widespread problem in many locations in the world. The underground aquatic mantle of the Peninsula of Yucatan is highly vulnerable due to its karstic nature. Adsorption methods are a good choice for nitrate elimination. In this work, a natural calcium bentonite was modified by acid thermoactivation with HCl and H₂SO₄, and tested as a media for nitrate removal in an aqueous solution. The nitrate concentration in the solution was measured by FT-IR, using the Lambert-Beer law. Clay characterization was carried out by X-ray diffraction and FT-IR spectroscopy; surface area was measured by the BET method.     

Physical and chemical stabilities of pillared clays containing transition metal ions

Chromium(III) and iron (III) ions exchanged both into the pillars and into the micropore structure of pillared clays are compared during thermal treatments from 120°–480°C for various times and in different atmospheres (air and nitrogen). Thermal gravimetric analysis (TGA) is used to monitor the %H₂ O lost from 20°–720°C and X-ray diffraction (XRD) is used to elucidate structural information. Both untreated and heat-treated clays are then examined for resistance to acid-attack using solutions of HCl having a concentration range of 1.25 × 10⁻⁴ N to 1N. Transition-metal leaching during acid treatment is monitored via atomic absorption (AA), and found to increase as the concentration of HCl increases. The % loadings of transition metal (TM) is seen to be an important factor with respect to stability of the clays. Since iron-containing materials in most cases contain higher concentrations of TM than chromium-containing clays, these materials have undergone more decomposition of the clay lattice and are less stable to acid attack. The exception is Fe + ACH-BL where iron(III) ions have been incorporated into the pillars of an alumina pillared clay. When this material is heat-treated it becomes more resistant to 0.05N HCl than ACH-BL without TM ions.     

The characterization of soils, clays, and clay firing products

Mössbauer spectroscopic studies of soils are generally centered around the characterization of Fe³⁺ oxides, making use of distinctive magnetic properties of the different oxides and the high sensitivity of the technique for magnetically ordered phases. Most work on clays and clay minerals, in contrast, focuses on the determination of the oxidation state of iron, iron structural site occupancies and distortions, and the transformations that take place during clay firing.     

Novel clays: Solid-state synthesis,characterization and cation exchange selectivity

Several novel swelling mica-type clays have been synthesized by solid-state processes. Synthetic clays of ideal compositions such as Na₂Si₆Al₂Mg₆O₂₀F₄ · xH₂O (Na-2-mica), Na₃Si₅Al₃Mg₆O₂₀F₄ · xH₂O (Na-3-mica) and Na₄Si₄Al₄Mg₆O₂₀F₄ · xH₂O (Na-4-mica) have been prepared and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and ²⁷Al and ²⁹Si solid-state magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Powder XRD showed that all syntheses yielded water swollen micas with c-axis spacing of ∼1.2 nm except in the case of Na-2-mica which also showed a small peak of anhydrous mica phase with a c-axis spacing of 0.96 nm. Solid-state ²⁷Al MASNMR spectroscopy revealed that almost all the Al is present in the tetrahedral environment of the different micas. Solid-state ²⁹Si MASNMR spectroscopy revealed different Si (Al) nearest neighbor environments depending upon the composition of the various micas. Selective cation exchange studies were performed on the various micas using 0.5 N NaCl solution containing 12.9 ppm Sr²⁺ or 8.12 ppm of La³⁺. The results showed, for the first time, that Na-3-mica has a high selectivity for the trivalent cation tested. The previously reported high selectivity of these synthetic micas for the divalent cations has been confirmed. These selective cation exchange studies are of relevance in cation separations from drinking and waste waters.     

On the thermodynamic driving force for polymer intercalation in smectite clays

The view that the intercalation of a polymer in an unmodified smectite clay is driven by the entropic increase that results from displacement of adsorbed water is re-assessed in the light of experiments that show rapid melt intercalation into a clay that has been heat-treated to remove gallery water. Dehydrated smectite clays with collapsed layers take up poly(ethylene glycol) from the melt in only 1?ks or from aqueous solution in under 18?ks, re-establishing the basal plane spacing to that for intercalation in untreated clay, 1.8?nm. Differential scanning calorimetry showed that the intercalation of poly(ethylene glycol) into montmorillonite is exothermic with an enthalpy change of ?153?J?g^?1 based on the intercalated polymer and the heat of wetting for the internal surfaces of montmorillonite by poly(ethylene glycol) is ?0.08?J?m^?2. These results confirm the observation of re-expansion of heat-treated clays and imply that the reduction in free energy on intercalation results from a significant enthalpic change as well as an entropic change for clays with interlayer water, and primarily from an enthalpic change for clay in the absence of water.     

Tin-119 Mössbauer spectroscopic studies of novel tin oxide pillared clays

¹¹⁹Sn Mössbauer spectroscopy has shown that attempts to intercalate three aryltin compounds (Ph₃SnCl, (Ph₃Sn)₂O, Ph₂SnCl₂) into the synthetic smectite clay laponite under ambient conditions result in the formation of tin(IV) oxide pillared clays. The Mössbauer data indicate that the effectiveness of conversion to tin(IV) oxide pillars is in the order Ph₃SnCl> (Ph₃Sn)₂O>Ph₂SnCl₂. The organic product of the pillaring reaction has been identified by¹³C m.a.s. n.m.r. spectroscopy as benzene trapped within the pillared clay. The pillaring in these new materials is achieved via neutral precursors rather than by sacrificial reaction of the exchanged cation. Measurements by A.C. conductivity measurements (12 Hz–100 kHz) show significant precursor related increases in conductivity for the new pillared materials as compared with sodium-exchanged laponite.     

New hybrid nanostructures based on oxacillin-hydrotalcite-like anionic clays and their textural properties

New hybrid nanostructures based on inorganic matrices of hydrotalcite-like anionic clays (HT) incorporated with oxacillin are obtained by using calcinations-restructure method. XRD and TEM analyses are used to study the structural and textural characteristics of the clay containing hybrids. When nanoparticles of iron oxides are loaded on the layered anionic clay matrix a more effective delivery system of the drug is obtained. The results can be used to reduce the toxic side effects of oxacillin (e.g. upset stomach, diarrhea, cholestastic hepatitis), its aggregation process in aqueous solutions and also can open new perspectives for targeted the drug delivery.     

Mineralogical characterization of Greda clays and monitoring of their phase transformations on thermal treatment

The mineralogical characterization of two clay samples from the Central Andean Region of Peru, denominated White Greda and Red Greda, is reported. These clays contain the clay minerals mica and illite respectively. Both clays were treated thermally in an oxidising atmosphere under controlled conditions up to 1,100°C with the purpose of obtaining information about structural changes that may be useful for pottery manufacture. X-ray fluorescence was used for the elemental characterization of the samples and X-ray diffractometry was used to determine the collapse and formation of the mineral phases present in the samples caused by thermal treatment. At temperatures above 1,000°C it is observed the formation of spinel in the case of White Greda and of hematite, corundum and cristobalite in the case of Red Greda. Room temperature transmission Mössbauer spectroscopy allowed the monitoring of the variation of the hyperfine parameters with the thermal treatment temperature; In the case of the evolution of the quadruple splitting of the paramagnetic Fe^3?+? sites with temperature, in both clays, the analyses reproduced results such as the “camel back” curve shape, found by other workers (Wagner and Wagner, Hyperfine Interact 154:35–82, 2004; Wagner and Kyek, Hyperfine Interact 154:5–33, 2004).     

Some aspects concerning the characterization of iron oxides and hydroxides in soils and clays

A review of the systematic Mössbauer studies on the most encountered iron oxides and hydroxides is given in which the qualitative and quantitative aspects, helpful in the characterization of natural smaples, are emphasized. The present possibilities of Mössbauer spectroscopy in soil characterization are further illustrated from some examples of natural soils.Senior Research Associate at the National Fund for Scientific Research, Belgium.     

Al-pillared clays supported rare earths and palladium catalysts for deep oxidation of low concentration of benzene

Al-pillared clays supported rare earths (RE/Al-PILC) are prepared and used as supports of palladium catalysts for deep oxidation of low concentrations of benzene (130-160 ppm). The supports and catalysts are characterized by X-ray powder diffraction (XRD), FT-IR, BET, transmission electron microscopy (TEM) and temperature-programmed reduction (H₂-TPR). The results show that Al-pillaring results in a strong increase in the basal spacing (d_0 0 1) from about 1.2 to 1.8 nm, and an increase in the BET surface area from 63.6 (±3.2) to 238.8 (±11.9) m²/g. Activity tests of deep oxidation of low concentration benzene show catalysts supported on Al-PILC and RE/Al-PILC are obviously more active than that on raw clay. Pd/6% Ce/Al-PILC, in particular, can catalyze the complete oxidation of low concentration benzene at a temperature as low as about 290 °C.     

Using Flow Relaxography to Elucidate Flow Relaxivity

We have investigated the theoretical and experimental linear dependence of the reciprocal of the apparent longitudinal relaxation time [(T^*₁)⁻¹] of the NMR signal from spins in a flowing fluid on the volume flow rate,F_v, the so-calledinflow effect.We refer to the coefficient of this dependence as the longitudinalflow relaxivity, r_1F. A very simple model predicts that, under a range of conditions pertinent to modern flow studies and perfusion imaging experiments,r_1Fis controlled by the volume of the fluid in which the magnetization is perturbed by pulsed RF inversion or saturation, not the detection volume, and that it can be approximated as the reciprocal of half of the inversion volume. Phantom sample experiments, using a new, quantitative approach that we callflow relaxography,confirm the general predictions of this simple model. There are two intriguing implications of these findings for general NMR flow studies as well as for medical applications. It should be possible to vary the value ofr_1Fby simply (noninvasively) adjusting the inversion slice thickness, and thus measure the value of (blood¹H₂O, for example)F_vin a vessel without changingF_v, from the resultant varyingT^*₁values. Also, it should be possible to extrapolate to the intrinsicT₁value of the fluid signal (as if it were stationary), without altering or stopping the flow. Again, these are quite successful in phantom sample studies. Imaging versions of the flow relaxographic experiments are also possible. The twin goals of flow studies in medical MRI are the quantitative discrimination of the signals from flowing and nonflowing spins, and the accurate measurement of the flow rate of the former.     

Hydration of clays at the molecular scale: the promising perspective of classical density functional theory

We report here how the hydration of complex surfaces can be efficiently studied, thanks to recent advances in classical molecular density functional theory. This is illustrated on the example of the pyrophyllite clay. After presenting the most recent advances, we show that the strength of this implicit method is that: (1) it is in quantitative or semi-quantitative agreement with reference all-atom simulations (molecular dynamics here) for both the solvation structure and energetics, and (2) the computational cost is two to three orders of magnitude less than in explicit methods. The method remains imperfect in that it locally overestimates the polarisation of water close to hydrophylic sites of the clay. The high numerical efficiency of the method is illustrated and exploited to carry out a systematic study of the electrostatic and van der Waals components of the surface–solvent interactions within the most popular force field for clays, CLAYFF. Hydration structure and energetics are found to weakly depend upon the electrostatics. We conclude on the consequences of such findings on future force-field development.