High Layer Charge Research Articles

Characterization of the frayed edge site of micaceous minerals in soil clays influenced by different pedogenetic conditions in Japan and northern Thailand

Radiocesium interception potential (RIP), a quantitative index of a frayed edge site of micaceous minerals, was investigated on test clay minerals (illite, kaolinite, montmorillonite and vermiculite) and soil clays formed under different pedogenetic conditions in the Kinki district of Japan and in northern Thailand to elucidate the effect of pedogenetic weathering on the amount of frayed edge site of micaceous minerals. A frayed edge site is defined as the wedge-shaped intermediate zone between non-expansible (1.0 nm) and hydrated (1.4 nm) interlayers, on which Cs is selectively adsorbed. The test clay minerals demonstrated unique RIP values closely associated with their charge characteristics. Very small RIP in non-micaceous test minerals (0.006 mol kg−1 in kaolinite; 0.1 mol kg−1 in montmorillonite) supported the fact that negative charges in these minerals do not show selective adsorption of 137Cs. Large RIP values in vermiculite (25.9 mol kg−1) compared with illitic minerals (4.3 mol kg−1 in silt size; 11.8 mol kg−1 in clay size) suggested that the frayed edge site increased as the expanded layer formed in the micaceous minerals. In northern Thailand soil clays, the absence of a vermiculitic expanded layer was indicated. Periodic wet–dry conditions may restrict K depletion from micaceous minerals in the region. It is concluded that micaceous minerals were weathered without layer expansion and, therefore, the amount of the frayed edge site was regulated by the content of micaceous minerals in northern Thailand. In Japanese soil clays, large RIP values were found in the EB and Bs horizons of a Podzodic profile (SW1), which had large vermiculitic expanded layers like the test vermiculite. However, the RIP values drastically decreased from the Bs to E horizon, maintaining a high vermiculitic layer charge. Thus, intensive weathering in highly acidic soils in Japan might reduce the frayed edge site.

Spectral properties of TMPyP intercalated in thin films of layered silicates

The objective of this study was to investigate the spectral characteristics of tetracationic porphyrin dye (TMPyP), intercalated into films of three smectites. The smectites represented the specimens of high (Fluorohectorite; FHT), medium (Kunipia F montmorillonite; KF), and low layer charge (Laponite; LAP). Intercalation of TMPyP molecules was proven by XRD measurements. The molecular orientations of the dye cations were studied by means of linearly polarized ultraviolet–visible (UV–vis) and infrared (IR) spectroscopies. Both the UV–vis and the IR spectroscopy proved the anisotropic character of the films. The spectral analysis of the polarized UV–vis spectra and consequent calculations of tilting angles of the transition moments in the region of Soret band transitions were in the range of 25–35°. The determined angles indicated that the molecular orientation of the dye cations was almost parallel to the surface of the silicates. Slightly higher values, determined for a FHT film, indicated either a slightly more tilted orientation of the dye cations or the change of molecular conformation after the intercalation of the dye. Quenching of TMPyP fluorescence was observed, resulting from the formation of bimolecular layer arrangements with sandwich-type assemblies of the dye molecules.

Deintercalation of carbonate ions and anion exchange of an Al-rich MgAl-LDH (layered double hydroxide)

A novel method to deintercalate carbonate ions (CO 3 2−) from a layered double hydroxides (LDHs) with NaCl–HCl solutions was applied to well-crystallized Al-rich LDH2 (LDH with Mg/Al ≈ 2) prepared by using hexamethylenetetramine (HMT). Enhanced deintercalation of the carbonate ions was observed, and the reaction occurred without any morphological changes at 25 °C under proper HCl/NaCl concentration conditions. The optimum decarbonation conditions were the molar ratio [H +] solution/[CO 3 2−] LDH = 1.5–2.0, HCl concentration at around 0.005N, and NaCl concentrations more than 4 mol/L. The decarbonation was affected by the Mg/Al ratio of LDH. LDH2 was more difficult to decarbonate than LDH3 (Mg/Al ≈ 3) probably due to its higher layer charge. The obtained Cl − form of LDH2 (Cl −LDH2) could be converted into organo-LDH2s by normal anion exchange with long chain carboxylates, retaining the crystal morphology.

P.1.e.026 A functional MRI study of the brain and interpersonal behaviour in schizophrenia. A preliminary study

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry (a = b = 2.83 Å), and an average structural formula of Mg0.1612+Ca0.0102+K0.016+[Mn0.9024+Vac0.083Ni0.0152+]O2·nH2O. The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg2+ cations. The average structural formula of 10Å-vernadite is Mg0.1872+Ca0.0162+K0.013+[Mn0.8644+Vac0.074Ni0.0622+]O2·nH2O if the layers contain vacancy sites, or alternately Mg0.2022+Ca0.0182+K0.014+[Mn0.6134+Mn0.3203+Ni0.0672+]O2·nH2O, if they contain Mn3+. The average formula of todorokite is Mg0.1782+Ca0.0132+K0.019+[Mn0.6124+Mn0.3563+Ni0.0322+]O2·nH2O.A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg2+ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg2+ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg2+ (i.e., high layer charge), or Mn3+ and Mn4+ cations ordered following the Mn3+–Mn4+–Mn4+ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3,n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3,n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.

P.1.e.025 Altered white matter tracts in subjects at high genetic risk of schizophrenia by diffusion tensor tractography

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry (a = b = 2.83 Å), and an average structural formula of Mg0.1612+Ca0.0102+K0.016+[Mn0.9024+Vac0.083Ni0.0152+]O2·nH2O. The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg2+ cations. The average structural formula of 10Å-vernadite is Mg0.1872+Ca0.0162+K0.013+[Mn0.8644+Vac0.074Ni0.0622+]O2·nH2O if the layers contain vacancy sites, or alternately Mg0.2022+Ca0.0182+K0.014+[Mn0.6134+Mn0.3203+Ni0.0672+]O2·nH2O, if they contain Mn3+. The average formula of todorokite is Mg0.1782+Ca0.0132+K0.019+[Mn0.6124+Mn0.3563+Ni0.0322+]O2·nH2O.A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg2+ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg2+ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg2+ (i.e., high layer charge), or Mn3+ and Mn4+ cations ordered following the Mn3+–Mn4+–Mn4+ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3,n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3,n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.

Formation of todorokite from vernadite in Ni-rich hemipelagic sediments

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry ( a = b = 2.83 Å), and an average structural formula of Mg 0.161 2 + Ca 0.010 2 + K 0.016 + [ Mn 0.902 4 + Vac 0.083 Ni 0.015 2 + ] O 2 · n H 2 O . The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg 2+ cations. The average structural formula of 10Å-vernadite is Mg 0.187 2 + Ca 0.016 2 + K 0.013 + [ Mn 0.864 4 + Vac 0.074 Ni 0.062 2 + ] O 2 · n H 2 O if the layers contain vacancy sites, or alternately Mg 0.202 2 + Ca 0.018 2 + K 0.014 + [ Mn 0.613 4 + Mn 0.320 3 + Ni 0.067 2 + ] O 2 · n H 2 O , if they contain Mn 3+. The average formula of todorokite is Mg 0.178 2 + Ca 0.013 2 + K 0.019 + [ Mn 0.612 4 + Mn 0.356 3 + Ni 0.032 2 + ] O 2 · n H 2 O . A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg 2+ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg 2+ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg 2+ (i.e., high layer charge), or Mn 3+ and Mn 4+ cations ordered following the Mn 3+–Mn 4+–Mn 4+ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3, n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3, n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.

Spectral study on the molecular orientation of a tetracationic porphyrin dye on the surface of layered silicates

Abstract Tetracationic porphyrin dyes TMPyP and ZnPyP were intercalated into hydrophobized layered silicate films of three smectites. The smectites represented the layered silicate specimens of high (Fluorohectorite, Corning; FHT), medium (Kunipia F montmorillonite; KF) and low layer charge (Laponite, Laporte; LAP). The molecular orientations of the dye cations were studied by means of linearly-polarized ultraviolet-visible (UV-VIS) spectroscopy. The spectral analysis and consequent calculations of tilting angles of the transition moments at the wavelengths of Soret band transitions were in the range of 25°-35°. The determined angles indicated molecular orientation of the dye cations being almost parallel to the surface of the silicates. Slightly higher values (above 35°), determined for a FHT film, indicated either a slightly tilted orientation of the dye cations or the change of molecular comformation after the intercalation of the dye.

Factors influencing the hydration of layered double hydroxides (LDHs) and the appearance of an intermediate second staging phase

The hydration of layered double hydroxides (LDHs) was investigated by changing the interlayer anion species, the Mg/Al ratio of the LDH hosts and the relative humidity (RH). The anions were CO 3 2−, Cl −, Br −, NO 3 −, I −, SO 4 2−, and ClO 4 − (listed in the order of ion size, small to large) and LDHs with Mg/Al = 1.90 (LDH2) and 2.91 (LDH3) were used. Their XRD profiles were measured by an XRD diffractometer while controlling the RH in the range 0–95% at 25 °C. Only I −, SO 4 2−, and ClO 4 − LDH2s and SO 4 2− LDH3 showed a large step-wise basal-spacing expansion, 0.24–0.28 nm, under high RH conditions (> ca. 60%) probably due to the insertion of one water layer into the interlayer space. Such hydration occurred more favorably for the LDHs with larger anions and those with a higher layer charge (LDH2). Among them, I − and ClO 4 − LDH2s exhibited the second staging – alternate stacking of hydrated (H) and non-hydrated (NH) interlayers – in the intermediate RH region.

Hydrothermal Reactivity of Na‐n‐Micas (n = 2, 3, 4).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Modification of layer charge in smectites by microwaves

The layer charge reduction of two Li-saturated montmorillonites is referred to application of microwave radiation at 2.45 GHz for dispersions and 30.0 GHz for the solid powders. Efficiency of these treatments was compared to the same conditions applying conventional heating. The samples were heated in the temperature and time windows corresponding to 190–270 °C and 30–120 min, respectively. Changes in the mean layer charge were monitored by the determination of cation exchange capacity values using exchange of triethylene tetraamino copper ions. The charge reduction of the montmorillonites in aqueous dispersions was rather low (<30%) despite the fact, that high Li+ concentration dissolved in solution was selected (1 mol L−1). This behaviour was attributed to the very high water content in used dispersions and high hydration enthalpy of Li+ cations. Nevertheless, the microwave heated dispersions (2.45 GHz) showed detectable layer charge reduction as compared to conventionally heated dispersions, where no significant changes were found.Solid powders with different content of exchangeable lithium ions were prepared with solutions having different molar fraction of Li+ and Na+ cations (0%, 20%, 40%, 60%, 80%, 100% of Li+). Final composition of interlayer cations was analysed with ICP-OES. In contrary to dispersions, the microwave treatment of the prepared powders revealed high layer charge reduction, which was much higher than for conventionally heated powders. The efficiency was enhanced with increasing interlayer lithium content but reached a limiting value depending on the montmorillonite used. Migration of the lithium into the structure of the montmorillonite supported with microwaves was extremely fast, finished probably within the few minutes of the treatments. The exchangeable Li+ cations are accelerated through alternating electric field of microwaves and highly efficient layer charge neutralisation occurs. Infrared spectroscopy showed that the products obtained with microwave treatments correspond to the materials heated conventionally for much longer periods of time. Hence, applying microwaves the time and energy requirement can be significantly reduced. The X-ray diffraction showed that montmorillonite layers were able to swell in ethyleneglycol upon charge reduction, if the cation exchange capacity was not reduced more than 20–40%.

Enhanced sorption and fixation of radiocaesium in soils amended with K‐bentonites, submitted to wetting–drying cycles

SummaryThe use of bentonites as soil amendment has met with little success in reducing plant uptake of radiocaesium. However, bentonites exchanged with K+ have pronounced Cs+ binding capacity when subjected to wetting–drying cycles. Fifty‐four different bentonites were collected and characterized for cation exchange capacity and chemical composition. The radiocaesium interception potential (RIP) increased up to 160‐fold (mean 25) when the bentonites were converted to the K‐form and subjected to wetting–drying cycles. This increase in radiocaesium sorption was ascribed to a collapse of the clay sheets into an illite‐like structure, and was most pronounced in bentonites with a high layer charge. The RIP values of K‐bentonites subjected to 25 wetting–drying cycles ranged from 0.22 to 44.3 mol kg−1. The RIP yields, i.e. the RIP in soil–bentonite mixtures expressed per unit bentonite added, were even higher and ranged up to 99 mol kg−1. This upper limit is about 10‐fold higher than the RIP value of illite (∼ 10 mol kg−1), the principal 137Cs sorbent in soils of temperate climates. Wetting–drying also promoted fixation of radiocaesium in soils amended with K‐bentonites. About 30% of added 137Cs could be desorbed with 1 m ammonium acetate (NH4Ac) from an unamended soil after 25 wetting–drying cycles, while only between 8 and 21% of 137Cs could be desorbed from a soil amended with bentonite and a K‐salt. These findings support the proposition that addition of K‐bentonite may be effective in reducing availability of 137Cs in soils.

Alkaline earth cation exchange with novel Na-3-mica: kinetics and thermodynamic selectivities

The cation-exchange kinetics and thermodynamic equilibria for alkaline-earth metals by Na-3-mica [chemical composition of Na2.94(Mg5.55Ti0.07Fe0.04)oct(Al2.07Si5.83)tetO20F4·1.72H2O] with a high layer charge were investigated at room temperature. The Ba, Sr, Ca and Mg uptake kinetics were determined and first-order, modified Freundlich, parabolic diffusion and Elovich kinetic models were examined for the experimental data. The first-order model gave a relatively good fit throughout for these cations and indicated a kinetic selectivity order of Ca2+ < Mg2+ < Sr2+ < Ba2+. The cation exchange isotherms for Ba, Sr, Ca and Mg uptakes with the Na-3-mica were also determined. The Kielland plots evaluated from the cation exchange isotherms showed a thermodynamic selectivity order of Ba2+ ≫ Sr2+ ≫ Ca2+ > Mg2+. The above basic studies on the selective cation exchange of the highly charged ion exchanger are of relevance in the separation, immobilization and disposal of 90Sr from aqueous nuclear wastes.

Molecular orientation of methylene blue cations adsorbed on clay surfaces

AbstractCationic dye, methylene blue (MB), was intercalated into films of expandable clay minerals with low, moderate and high layer charges (saponite, montmorillonite and synthetic expandable mica-fluorohectorite, respectively) . Tilted orientation of the dye cations on the silicate surface was observed by polarized ultraviolet-visible (UV-Vis) transmission spectroscopy. The tilting angle of the dye cations increased with the formation of high-order H-aggregates that absorb light at 525 nm. These dye species were formed only on the surface of fluorohectorite, which has the highest layer charge, and exhibited dichroism. Models for the orientation of MB cations on a clay surface were proposed based on MB orientation models previously reported, and compared with the basal spacings determined by X-ray diffraction.

Effect of layer charge density on orientation and aggregation of a cationic laser dye incorporated in the interlayer space of montmorillonites

Effect of layer charge density of clay on the orientation and aggregation state of a laser dye, oxazine 4, in dye/clay complexes was investigated using a series of layer-charge-controlled montmorillonites as host materials. By the combination of polarized UV–vis spectroscopy and powder X-ray diffraction methods, it was revealed that the higher layer charge caused the formation of higher-order H-aggregates with the molecular axis nearly perpendicular to the silicate layer, and that the basal spacing was mostly governed by the degree of dye aggregation.

Porphyrin intercalation into a layered niobate derived from K4Nb6O17

The incorporation of guest species into two-dimensional inorganic structures can lead to materials with interesting chemical, catalytic, electronic, optical or mechanical properties. Concerning porphyrins and metalloporphyrins intercalation compounds, nanostructured materials have been obtained and evaluated in studies about photoprocess and catalytic reactions in confined media. The intercalation of bulky species such porphyrins into layered niobates is not easy to perform due to their high layer charge densities when compared to other layered materials. In this work we describe a method for TMPyP [5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H, 23H-porphyrin] intercalation into a layered niobate derived from K4Nb6O17. The potassium precursor was converted into the acidic-exchanged form and then intercalated with n-butylamine to produce an expanded material that was later used in the production of a dispersion containing exfoliated niobate sheets. The niobate dispersion was dropped into a porphyrin solution originating an organic-inorganic hybrid composite of formula TMPyP0.35H0.6K2Nb6O17·3H2O. XRD data suggest a tilted arrangement of the TMPyP ring with respect to the layers. Spectroscopic data (uv-visible absorption, fluorescence and resonance Raman) showed that TMPyP is intercalated in a non protonated form and that the interaction with the niobate layers surface is weak, corroborating with the proposed tilted orientation in the interlayer region.

Dicamba adsorption–desorption on organoclays

The adsorption–desorption of the herbicide dicamba (p K a=1.9, water solubility, 6.5 g kg −1) by organoclays (OCls) was studied at different concentrations and pH levels. Two smectites (SAz and SWy), varying in surface properties, were reacted with amounts of different alkylammonium cations [octadecyl(C18)-, hexadecyl-trimethyl(HDT)- and dioctadecyldimethyl(DOD)ammonium] equal to 50% or 100% of the clays' CEC. Adsorption isotherms of dicamba on diverse OCl were some of L-type and others of S-type, but both resulted in sigmoid form when extended to higher concentration. Adsorption was greater for OCls with high-layer charge, basal spacing, alkylammonium size and organocation saturation close to CEC. Dicamba adsorption by OCls seems to involve hydrophobic and polar interactions for which the availability of interlayer room between organocations is very important. Adsorption data at different pH levels and two different concentrations (0.05 and 1 mM) indicated that molecular dicamba is the main adsorbing species, especially at high concentration. Desorption isotherms were reversible, except in OCls with primary alkylammonium (C18) and largest quaternary (DOD), for which there were moderate hysteresis as a result of stronger polar contribution in the primary alkylammonium and the difficulty for diffusion in the case of the quaternary, bulky OCl. The treatment of an artificially dicamba-contaminated soil with highly adsorptive OCls rendered a dramatic decrease in the CaCl 2-released- or mobile dicamba, suggesting these OCls as potential immobilising agents. The amount of herbicide immobilised by the OCl was partially extractable with methanol/CaCl 2 solution, suggesting its biovailability and hence, its possible combination with bioremediation technique.

Intercalation of alkylammonium cations into expandable fluorine mica and its application for the evaluation of heterogeneous charge distribution

The interstratified layer nano-hybrids between n-alkylammonium and fluorine mica are prepared by ion-exchanging the interlayer Na+ ions in the synthetic fluorine mica, Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96, with n-alkylammonium cations, CH3(CH2)n − 1NH3+ (Cn; n = 6, 8, 10, 12, 14, 16, and 18). According to the X-ray diffraction profiles of nano-hybrids and their computer simulation results, their interstratified structural features could be clearly distinguished depending upon the number of carbon atoms in the alkyl chain (Cn). The C6–C8 and C14–C16 derivatives exhibit normal intercalation phases with the basal spacings of ∼13.8 A and ∼18.0 A, indicating the parallel mono- and bilayer arrangements of the intercalants between silicate layers, respectively. On the other hand, the C10–C12 and C18 hybrids show distinct superlattice lines in the X-ray diffraction patterns due to the interstratification between parallel monolayer–bilayer (d001 = 31.6 A) and parallel bilayer–pseudotriple layer (d001 = 39.6 A) of alkylammonium molecules, respectively, in the interlayer space of mica. The origin of such interstratification is found to be due to the charge heterogeneity of silicate interlayers in the Na+-fluorine mica; a high layer charge with 0.37 e−/Si4O10 and a low one with 0.28 e−/Si4O10 charge densities, respectively. The charge heterogeneity of silicate layers is also confirmed through the step-wise deintercalation of intercalants during their thermolysis under a nitrogen atmosphere.

Fenuron sorption on homoionic natural and modified smectites

The adsorption isotherms of fenuron (l, l‐dimethyl‐3‐phenylurea) on three smectites (SWy and SAz montmorillonites and SH hectorite) differing in their layer charge (SH<SWy<SAz) and saturated with several inorganic and organic cations were determined. The isotherms and sorption parameters from Freundlich equation indicate low adsorptivity on inorganic clays, but medium sorption in organoclays (OCls). Fenuron adsorption on homoionic smectites increases with decreasing layer charge and hydratation power of the inorganic exchangeable cation (except Fe3+), indicating that fenuron adsorbs as neutral molecule on uncharged siloxane surface by hydrophobic bonding, with some contribution of polar bond (fenuron C=O group and water associated to exchangeable cation). In the case of Fe3+‐saturated smectite fenuron protonation, provided by the interlayer acidic environment, promotes further sorption of fenuron as cationic form. The sorption on organoclays is enhanced via hydrophobic interaction with organocations, which is favoured for high layer charge and basal spacing and organocation saturation close to CEC. Quaternary alkylamonium is more efficient in high layer charge smectite, whereas primary alkylammonium is more efficient in medium charge smectite. The low values of the maximum sorption obtained with homoionic inorganic and organic smectites (100 and 5000 μmol/Kg) represent one fenuron molecule for each 2000–200 exchange sites and indicate that fenuron sorption is mainly associated to the outer exchange sites. This low adsorptivity of fenuron, as consequence of its high water affinity (high water solubility) would suggest high mobility of fenuron in natural soil and water systems.

Characterization and Catalytic Activity of Aluminum- and Aluminum/Tetramethylammonium-exchanged Bentonites

Three montmorillonites SWy-2 (Wyoming, USA), STx-1 (Texas, USA) and SAz-1 (Cheto, Arizona, USA) and a saponite Sap-Ca (California, USA) were activated with different amounts of Al3+ and tetramethylammonium (TMA+) cations to obtain materials with a combined Al3+ and TMA+ content equal to their CECs. The catalytic activity was evaluated by reacting α-pinene at 120 °C for 1 h to yield camphene, limonene, and other minor products and compared to clays prepared without TMA+. The total conversion over Sap-Ca, STx-1, SWy-2, and SAz-1 was 90%, 80%, 65%, and 25%, respectively, in samples treated with Al3+ alone. Similar yields were obtained over Al3+/TMA+-exchanged forms when the TMA+ content was less than 20% CEC, but in general the presence of TMA+ significantly reduced the total conversion. The activity of the Al3+-exchanged clays for this test reaction is attributed to their ability to remain expanded at the temperature of the activation process thus permitting easy access of the reagent. The comparatively low conversion over catalysts derived from SAz-1 was attributed to its high layer charge which restricted access to the active centers in the clay gallery.

Methods for Performing Atomic Force Microscopy Imaging of Clay Minerals in Aqueous Solutions

AbstractThree methods were developed that allow for the imaging of any clay mineral in aqueous solutions with atomic force microscopy (AFM). The methods involve fixing the particles onto special substrates that do not complicate the imaging process, but hold the particles sufficiently so that they do not move laterally or float away during imaging. Two techniques depend on electrostatic attraction under circumneutral pH conditions, between the negatively charged clay particles and the high point of zero charge substrate (either aluminum oxide or polyethyleneimine-coated mica) whereas the third technique depends on adhesion to a thermoplastic film. The first electrostatic technique involves a polished single crystal α-Al2O3 (sapphire) substrate. This was used successfully as a substrate for clay-sized minerals with high permanent layer charge localized on the basal planes (phlogopite and vermiculite) and when the AFM was operated in TappingMode to limit the lateral forces between the probe tip and the particles. However, electrostatic attraction between the sapphire surface and clay minerals such as smectite and kaolinite (low or no permanent layer charge) is not sufficiently strong to adequately fix the particles for imaging. The second electrostatic technique involves a polyethyleneimine-coated mica surface designed to immobilize a larger variety of clay minerals (phlogopite, vermiculite, montmorillonite, and kaolinite), and in this technique weak bonding between the clay and the organic film is also a factor. The third technique, which does not depend on electrostatic attraction, fixes clay particles into the surface of a thermoplastic adhesive called Tempfix. This has proven useful for fixing and imaging relatively large clay particles with well-defined morphology. The Tempfix mount also requires imaging in TappingMode because the Tempfix is relatively soft.