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Aflatoxins can cause intoxication and poisoning in animals and humans. Among these molecules, aflatoxin B1 (AFB1) is the most dangerous because of its carcinogenic and mutagenic properties. To mitigate these effects, clay adsorbents are commonly included in the diet of animals to adsorb the carcinogens and prevent their absorption in the gastrointestinal tract. In this study, four clays, three smectites (C-1, C-2, and C-3), and one zeolite (C-4), were compared as adsorbents of AFB1 and trace inorganic nutrients using an in vitro gastrointestinal model for poultry. Characterization of the clays using Fourier transform infrared spectroscopy revealed characteristic bands of smectites in C-1, C-2, and C-3 (stretching vibrations of Si-O, Al-O-Si, and Si-O-Si). The C-4 presented bands related to the bending vibration of structural units (Si-O-Si and Al-O-Si). X-ray diffraction analysis showed that C-1 is a montmorillonite, C-2 is a beidellite, C-3 is a beidellite-Ca-montmorillonite, and C-4 is a clinoptilolite. The elemental compositions of the clays showed alumina, silica, iron, calcium, and sodium contents. The cation exchange capacity was higher in C-3 clay (60.2 cmol(+)/kg) in contrast with the other clays. The AFB1 adsorption of C-1 was the highest (98%; p ˂ 0.001), followed by C-2 (94%). However, all the clays also sequestered trace inorganic nutrients (Fe, Mn, Zn, and Se). Both smectites, montmorillonite and beidellite, were the most suitable for use as adsorbents of AFB1. [ABSTRACT FROM AUTHOR]

Aflatoxins can cause intoxication and poisoning in animals and humans. Among these molecules, aflatoxin B1 (AFB1) is the most dangerous because of its carcinogenic and mutagenic properties. To mitigate these effects, clay adsorbents are commonly included in the diet of animals to adsorb the carcinogens and prevent their absorption in the gastrointestinal tract. In this study, four clays, three smectites (C-1, C-2, and C-3), and one zeolite (C-4), were compared as adsorbents of AFB1 and trace inorganic nutrients using an in vitro gastrointestinal model for poultry. Characterization of the clays using Fourier transform infrared spectroscopy revealed characteristic bands of smectites in C-1, C-2, and C-3 (stretching vibrations of Si-O, Al-O-Si, and Si-O-Si). The C-4 presented bands related to the bending vibration of structural units (Si-O-Si and Al-O-Si). X-ray diffraction analysis showed that C-1 is a montmorillonite, C-2 is a beidellite, C-3 is a beidellite-Ca-montmorillonite, and C-4 is a clinoptilolite. The elemental compositions of the clays showed alumina, silica, iron, calcium, and sodium contents. The cation exchange capacity was higher in C-3 clay (60.2 cmol(+)/kg) in contrast with the other clays. The AFB1 adsorption of C-1 was the highest (98%; p ˂ 0.001), followed by C-2 (94%). However, all the clays also sequestered trace inorganic nutrients (Fe, Mn, Zn, and Se). Both smectites, montmorillonite and beidellite, were the most suitable for use as adsorbents of AFB1.

Cenozoic basin sediments on the Tibetan Plateau are commonly characterized by frequent changes in sediment facies and coarse lithologies, which largely limits our understanding of the relationship between the Tibetan Plateau uplift and its environmental impacts. The monomineral index derived from those sediments can effectively eliminate the effects of grain size and sedimentary facies changes. Here, we present detailed mineralogical, spectroscopic, and micromorphological investigations of smectites collected from the Hongliugou section of the Qaidam Basin to explore the relationship between climate change and the Tibetan Plateau uplift in the early–middle Eocene. Multiple methods involving X–ray diffraction, transmission electron microscopy, 27Al nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy collectively show that the smectites in the Hongliugou section are mixtures of montmorillonites and beidellites. The origin of the beidellites was likely the weathering product of illite, as indicated by the presence of illite–beidellite mixed layers (an intermediate product of the alteration of illite to beidellite). The long–term decreasing trend of beidellite/smectite ratios from the highest values (∼1.1–82.0%, averaging 14.8%) in the Early Eocene Climate Optimum (∼52.0–49.5 Ma) to generally <2% at 49.5–43.9 Ma suggested a long–term climatic drying. This climate change history deduced from the variation of beidellite content is consistent with other climate records and supports that global climate change was the driving factor regulating regional climate change. Our study thus suggests that the monomineral index of smectite may serve as an effective method for reconstructing regional climate change, especially within complex sedimentary environments. • Both beidellite and montmorillonite were identified in the Qaidam Basin. • A continuous drying trend is shown by the beidellite content at ∼53–43.9 Ma. • Global climate change mainly regulated regional climate change during ∼53–43.9 Ma. • Smectite monomineral index reveals climate change in complex sedimentary settings. [ABSTRACT FROM AUTHOR]

The understanding of the structural formula of smectite minerals is basic to predicting their physicochemical properties, which depend on the location of the cation substitutions within their 2:1 layer. This implies knowing the correct distribution and structural positions of the cations, which allows assigning the source of the layer charge of the tetrahedral or octahedral sheet, determining the total number of octahedral cations and, consequently, knowing the type of smectite. However, sometimes the structural formula obtained is not accurate. A key reason for the complexity of obtaining the correct structural formula is the presence of different exchangeable cations, especially Mg. Most smectites, to some extent, contain Mg2+ that can be on both octahedral and interlayer positions. This indeterminacy can lead to errors when constructing the structural formula. To estimate the correct position of the Mg2+ ions, that is their distribution over the octahedral and interlayer positions, it is necessary to substitute the interlayer Mg2+ and work with samples saturated with a known cation (homoionic samples). Seven smectites of the dioctahedral and trioctahedral types were homoionized with Ca2+, substituting the natural exchangeable cations. Several differences were found between the formulae obtained for the natural and Ca2+ homoionic samples. Both layer and interlayer charges increased, and the calculated numbers of octahedral cations in the homoionic samples were closer to four and six in the dioctahedral and trioctahedral smectites, respectively, with respect to the values calculated in the non‐homoionic samples. This change was not limited to the octahedral sheet and interlayer, because the tetrahedral content also changed. For both dioctahedral and trioctahedral samples, the structural formulae improved considerably after homoionization of the samples, although higher accuracy was obtained the more magnesic and trioctahedral the smectites were. Additionally, the changes in the structural formulae sometimes resulted in changing the classification of the smectite. [ABSTRACT FROM AUTHOR]

Atrazine (A) is one of the most applied herbicides and has a negative impact on the environment and health. Density functional theory (DFT) and experimental methods were used in the study of the immobilization of atrazine in two smectites, montmorillonite (Mt) and beidellite (Bd), as well as in their organically modified structures. Four systems were examined: A-Mt and A-Bd, as well as the structures modified by tetramethylphosphonium cation (TMP), A-TMP-Mt and A-TMP-Bd. The calculations revealed a flat arrangement of the atrazine in the interlayer space of both smectites with higher stability of beidellite structures. The presence of the TMP cation increased the fixation of atrazine in both organically modified smectites. The calculated vibrational spectra allowed a detailed analysis of the overlapping bands observed in the experimental FTIR spectra and their correct assignment. Further, selected FTIR bands unambiguously assigned to atrazine and both smectites served for the estimation of the adsorbed amount of atrazine. It was shown that the adsorption capacity of both TMP-modified smectites did not increase in comparison to the adsorption capacity of unmodified smectite samples.

A review and a synthesis of the geological, mineralogical, and crystal chemical data available in the literature on active Spanish bentonitic exploitations were done, and at the same time, new data are provided from a set of representative samples from these deposits. They were located in three different areas with different geological origins: (1) Miocene sedimentary deposits from the Tajo Basin (Madrid−Toledo provinces) in the center of the Iberian Peninsula, where bentonites appear in two different units named for their colors (Green Clays and Pink Clays); (2) samples from Tamame de Sayago (Zamora province) originating from the hydrothermal alteration of granitic Variscan rocks; and 3) Miocene deposits originating from the hydrothermal alteration of volcanic or subvolcanic rocks from the Cabo de Gata volcanic area (Almería Province) in the southern part of Spain, where the three main deposits (Cortijo de Archidona, Los Trancos, and Morrón de Mateo) were studied. The bentonites from the Tajo Basin were formed mainly by trioctahedral smectites, and there were significant mineralogical differences between the Green and Pink Clays, both in terms of the contents of impurities and in terms of smectite crystallochemistry and crystallinity. The smectites from Tamame de Sayago were dioctahedral (montmorillonite−beidellite series), and they appeared with kaolinite, quartz, and mica in all possible proportions, from almost pure bentonite to kaolin. Finally, the compositions of the bentonites from the three studied deposits in Cabo de Gata were quite similar, and zeolites and plagioclases were the main impurities. The structural formulae of the smectites from Cortijo de Archidona and Los Trancos showed a continuous compositional variation in beidellite−montmorillonite, while in Morrón de Mateo, the smectites were mainly montmorillonite, although there was continuous compositional variation from Al montmorillonites to Fe−Mg-rich saponites. The variation in the smectite composition is due to the intrusion of a volcanic dome, which brings new fluids that alter the initial composition of the smectites.

Natural aluminosilicates can contain Fe in tetrahedral or octahedral coordination. Amongst smectites, tetrahedral iron is known to occur in Fe-rich nontronites but few indications exist for the presence of tetrahedral Fe in smectites of the montmorillonite--beidellite series. A set of 38 different bentonites showed a correlation of tetrahedral charge and Fe content in their smectites. All materials with large tetrahedral charge were rich in Fe. This could be explained by a general tendency of Fe to enter the tetrahedral sheet. To investigate this correlation, nine materials were selected and investigated by Mössbauer, UV-Vis, Fe K pre-edge and EXAFS spectroscopy with respect to tetrahedral Fe (Fe[IV]). The latter two methods were at the detection limit but Mössbauer and UV-Vis spectroscopy provided consistent results indicating the significance of both methods in spite of some scatter caused by the overall small amount of tetrahedral Fe. The results indicate the absence of any relation between Fe content and tetrahedral Fe. Tetrahedral Fe can be present in Fe-poor smectites and absent in the case of Fe-rich materials. This means that Fe-rich montmorillonites have a larger tetrahedral charge which is not caused by Fe[IV] but by Al[IV]. A possible explanation for this indirect relation is based on: the coordination of Al3+ in the weathering/smectite-forming solutions determines the coordination in the precipitates; and the Al[IV/VI] ratio increases with increasing pH. The correlation could thus be explained if the pH of weathering solutions generally was higher in Fe-rich parent smectite rocks than in more acidic smectite parent rocks. The relation between tetrahedral charge and Fe content can probably be explained by different geochemical contexts throughout the formation of smectites which affect the coordination of dissolved Al. [ABSTRACT FROM AUTHOR]

Além dos baixos teores normalmente encontrados na fração argila dos solos sob clima tropical e subtropical, o tamanho reduzido e a baixa cristalinidade dos minerais 2:1 secundários dificultam sua identificação por difratometria de raios X (DRX). Este estudo objetivou avaliar métodos químicos e físico de concentração de minerais 2:1 secundários na fração argila para facilitar a identificação por DRX, incluindo a natureza dos minerais quanto ao local de formação de cargas permanentes (lâmina tetraedral ou octaedral). Coletaram-se amostras de dois Cambissolos originados de argilito da Formação Guabirotuba na Bacia Sedimentar de Curitiba (PR): horizontes A, Bi, C1 (1,2 a 1,5 m), C2 (2,2 a 2,5 m), C3 (3,2 a 3,5 m) e C4 (4,2 a 4,5 m). Após remoção da matéria orgânica e dispersão da terra fina seca ao ar, a fração argila foi submetida a tratamentos sequenciais com ditionito-citratobicarbonato (DCB) (amostra desferrificada - remoção de óxidos de Fe pedogenéticos) e com soluções de NaOH a quente, em diferentes concentrações (0,5; 1,0; 1,5; 2,5; 3,5; 4,0; 4,5 e 5,0 mol L-1), para extração de gibbsita e caulinita, em diferentes graus. A fração argila desferrificada também foi submetida à separação física (centrifugação) em argila grossa (0,2 a 2 m) e fina (< 0,2 m). Foram realizados tratamentos auxiliares para identificar as espécies minerais 2:1 na fração argila: saturação com Mg e solvatação com etilenoglicol; saturação com K e secagem ao ar e aquecimento a 550 ºC; e saturação com Li (teste de Greene-Kelly). Os resultados mostraram que o método clássico de extração da caulinita, com solução de NaOH 5,0 mol L-1 a quente, não deve ser aplicado para concentração de minerais 2:1 secundários, pois também removeu grande parte desses minerais. O tratamento com DCB e com solução de NaOH 3,5 mol L-1 possibilitou, com maior eficiência, a concentração e identificação de minerais 2:1 secundários por DRX nas amostras dos horizontes A, Bi e C1. Nas amostras tomadas em maiores profundidades (horizontes C2, C3 e C4), devido aos maiores teores desses minerais e ao menor tamanho dos cristais (argila fina), a solução menos concentrada de NaOH (1,5 mol L-1) foi mais eficiente para esse propósito. No horizonte A, os minerais 2:1 concentraram-se na fração argila grossa, compatível com o maior grau de intemperismo desse horizonte. Identificou-se esmectita com hidroxi-Al entrecamadas nos horizontes mais superficiais (A e Bi) e esmectita nas amostras do horizonte C. A saturação com Li permitiu a identificação das esmectitas dioctaedrais montmorilonita e beidelita/nontronita. As adaptações ao métodopadrão (NaOH 5 mol L-1) favoreceram a concentração de minerais 2:1 secundários na fração argila dos solos; a concentração da solução de NaOH deve ser maior para horizontes com menor teor do mineral.Besides the typically low levels of secondary 2:1 minerals found in the clay fraction of soils in tropical and subtropical climates, the small mineral size and low crystallinity make their analyses by X-ray difractometry (XRD) difficult. This study aimed to evaluate physical and chemical methods for concentrating secondary 2:1 minerals in the clay fraction to facilitate their identification by XRD. Also, it was sought to analyze the nature of the minerals regarding the sites of origin of permanent charges (tetrahedral or octahedral sheets). Samples were collected in two Cambisols originated from argillite of the Guabirotuba formation in the sedimentary basin of Curitiba (PR), Brazil, in the horizons: A, Bi, C1 (1.2 to 1.5 m), C2 (2.2 to 2.5 m), C3 (3.2 to 3.5 m) and C4 (4.2 to 4.5 m). After removing the organic matter and dispersing the airdried fine earth, the clay was sequentially treated with dithionite-citrate-bicarbonate (DCB) (iron-free sample - removal of pedogenetic iron oxides) and with hot NaOH solutions at different concentrations (0.5, 1.0, 1.5, 2.5, 3.5, 4.0, 4.5, and 5.0 mol L-1) for kaolinite and gibbsite extraction at different degrees. The iron-free clay fraction was also physically separated (centrifugation) in coarse (0.2 to 2 µm) and fine clay (< 0.2 µm). Secondary treatments were performed to identify the 2:1 mineral species in the clay fraction: saturation with Mg and solvation with ethylene glycol, K saturation, air-drying and heating to 550 ºC; and Li saturation (Greene-Kelly test). Results showed that the classical method of kaolinite extraction with hot 5.0 mol L-1NaOH solution should not be used to concentrate secondary 2:1 minerals because the solution also removed a large portion of these minerals. The treatment with DCB and with 3.5 mol L-1NaOH solution was more efficient to concentrate and favored the identification of secondary 2:1 minerals by XRD in samples of the A, Bi and C1 horizons. In samples from greater soil depths (C2, C3 and C4 horizons), due to the higher levels of these minerals and the smaller crystal size (fine clay), a less concentrated NaOH solution (1.5 mol L-1) was more efficient for this purpose. In A horizon, 2:1 minerals were concentrated in the coarse clay fraction, what is compatible with the higher weathering degree of this horizon. Smectite with Al-hydroxy interlayers was identified in most surface horizons (A and Bi) and smectite was found in the C horizon samples. Li saturation made the identification of dioctaedral minerals (montmorillonite and beidellite/nontronite) possible. The modification of the standard method (5 mol L-1NaOH) favored the concentration of secondary 2:1 minerals in the soil clay fraction; the concentration of the NaOH solution must be higher for horizons with lower mineral content.

Greece is a major bentonite producer, with Milos Island being one of the largest bentonite mining centres in the world. In addition to the bentonite deposits which are well known, various other occurrences exist in the islands of Chios and Samos, Eastern Aegean and in some areas of Thrace, in NE Greece. These bentonites are associated with volcanic activity and their age ranges from Lower Oligocene in Thrace, to Lower-Middle Miocene in Chios and to Upper Miocene in Samos. Although some of these materials have been utilized since ancient times (e.g. the Samian Earth), these bentonites have been mined only at a local scale. In this study the first data on the mineralogical and geochemical characteristics of these bentonites are presented. The Samos bentonites of Upper Miocene age are medium-low grade (40-70% smectite) and crop out in the SE margin of the Karlovasi Basin formed at the expense of acidic pyroclastic flows mainly in a subaerial-lacustrine environment. They consist mainly of Ca-rich medium-high-charge montmorillonite, opal-CT and sanidine with minor quartz and locally mordenite. The Chios bentonites of Lower- Middle Miocene age were formed at the expense of trachyandesitic tuffs in a lacustrine environment in the Neogene Basin at the SE part of the island. They consist mainly of Ca,Mg-rich high-charge montmorillonite (33-75%), similar to the SAz-1, and opal-CT, minor plagioclase, chlorite, plagioclase, carbonates and traces of talc and serpentine. In Thrace, Lower Oligocene pyroclastic flows, tuffs and lavas of acidic-intermediate composition were altered to zeolites and bentonites in the sedimentary basins of the Feres-Pefka, Skaloma and Sappes areas. The parent pyroclastic rocks were altered to Ca-rich and in places, Na-rich, dioctahedral montmorillonite or Fe-rich beidellite (Sappes area) and with minor opal-CT, mordenite, quartz, plagioclase and, in places, illite. The bentonites are medium-high grade (50-58% smectite in Feres-Pefka, 74-86% in Skaloma and 29-40% in Sappes). The bentonites have moderate cation exchange capacity and moderate/low swelling index and viscosity and they are not suitable for the drilling and foundry industries. However, preliminary results show that after processing, most of the deposits might be utilized in high added-value applications such as bleaching earths or in the synthesis of clay-based polymer nanocomposites. [ABSTRACT FROM AUTHOR]

Swelling pressure of dioctahedral smectites in the montmorillonite – beidellite series was investigated by molecular dynamics simulations. The pressure was found to correlate inversely with the magnitude of the layer charge in the range of −0.5 to −1.0 per unit cell. The beidellite type smectites were found to have lower swelling pressure than the montmorillonite type smectites. A clear effect of the type of interlayer cations on the swelling pressure was found. The sodium smectites sustained significant pressure even at longer interlayer distances, while in calcium smectites the pressure decreased soon after the initial swelling. The simulation results are in good agreement with experimental observations and provide a tool for predicting macroscopic swelling behavior in smectites. [ABSTRACT FROM AUTHOR]

Five-membered cyclic organic carbonates (COC) are of interest for their ability to modify the surface properties of smectites and enhance the hydraulic resistance of bentonites to saline leachates. The mechanism of interaction of glycerol carbonate (GC) and several other hydroxyl containing cyclic organic carbonates (generally having progressively greater molecular masses) with sodium montmorillonite (Na + -Mt) was studied using powder X-ray diffraction and infrared spectroscopy. The 001 reflection for GC/Na + -Mt intercalates varied with the amount COC added, and the measured d 001 value increased from 1.29 nm to as large as 2.22 nm at equal-mass coverage of the COC to Na + -Mt. In general, when intercalated, the cyclic carbonyl (C O) stretch and the fundamental hydroxyl (O–H) stretch bands of COC derivatives were red-shifted with respect to these bands for neat COC, indicating strong ion-dipole interaction of the carbonyl group with interlayer Na + , and H-bonding of the OH group with both interlayer water and Mt surfaces. A stable and highly ordered intercalate was produced at a 1:1 mass loading with Mt in which about 6 GC molecules per unit cell (~ 7 molecules per Na + ion) replaced most of the interlayer water. [ABSTRACT FROM AUTHOR]

Smectites are clay minerals of interest due to their natural availability, extraordinary surface properties, and high swelling capacity in the presence of water. The behaviour of smectites and other layered phyllosilicates is largely affected by the appearance of isomorphous substitutions with a lower nuclear charge in their layer structures. These substitutions are the origin of negative net layer charge, and their distribution in the mineral layer impacts on the electrostatic properties of the mineral surfaces. The present density functional theory study shows that octahedral Mg(II) and tetrahedral Al(III) substitutions in aluminium-rich dioctahedral smectites tend to avoid each other and disperse in the layer structure. This behaviour is explained and demonstrated to originate from geometry deformations around the substitutions which are larger than the unsubstituted species. [ABSTRACT FROM AUTHOR]

In this study, an Al-pillared smectite was synthesized and changes in its colloidal properties were investigated. The pillaring solution was prepared by mixing 0.4 mol L−1NaOH and 0.2 mol L−1AlCl3.6H2O solutions. Intercalated clays were heated to obtain the pillared clay, and X-ray diffractometry (XRD), X-ray fluorescence (XRF), Fourier transform infrared (FTIR) spectroscopy and N2sorption/desorption isotherms analysis were done to characterize the changes in clay properties. Moreover, adsorption experiments were carried out in order to evaluate the capacity of the pillared clays to remove Cu2+from an aqueous solution and to characterize the interaction between adsorbent and adsorbate. The results indicate that the natural clay has a basal spacing of 1.26 nm, whereas the pillared clays reached 1.78 nm (500°C) and 1.80 nm (350°C) after calcination. XRF analysis revealed an increase in the Al3+in the pillared clay as compared to the natural clay. The surface area and pore volume (micro and mesoporous) were higher for the pillared clays. Experimental data from the adsorption experiment were fit to Langmuir and Freundlich and Temkin adsorption models, and the former one was the best fit (highestr2value) for all the clays and lower standard deviation (Δg%) for the natural clay. On the other hand, the Temkin model exhibited Δg% value lower for the pillared clays. Thermodynamics parameters demonstrate that the Cu2+adsorption process is spontaneous for all the clays, but with higher values for the pillared materials. In addition, application of the Dubinin–Radushkevich model revealed that the bond between the metal and the clay are weak, characterizing a physisorption. [ABSTRACT FROM PUBLISHER]

Abstract: Ab initio Molecular dynamics have been performed to study the free energy of adsorption of alkali atoms onto smectite clay surfaces and to identify the most favourable region in the interlayer for the cations. This is achieved by potential of mean force calculations using a constraint method to determine the lowest free energy configurations of lithium, potassium and sodium beidellite and sodium montmorillonite clays with a monolayer and bilayer of water present in the interlayer region. The constraint method has allowed us to examine the changes in the lowest free energy configuration for each ion with increasing hydration. From this, we can interpret the likelihood of clay swelling from the monolayer to bilayer coverage and compare with experimental observations. We find, that with a bilayer of water present, both lithium beidellite and sodium montmorillonite have their free energy minimum in the centre of the interlayer. For monolayer coverage, the free energy minimum for lithium, sodium and potassium beidellite is approximately the mid-point of the interlayer. Na-beidellite has a lowest free energy region at 6.1Å from the centre of the clay layer for both mono- and bi-layer coverage, while for the potassium counter-ion, commonly used as swelling inhibitor, the free energy profile for K-beidellite shows peaks close to both surfaces at approximately 6.2 and 8.6Å. We find that for systems where the free energy minimum remains in the middle of the interlayer when the hydration levels increase from monolayer to bilayer, it is known experimentally that these systems will swell in contact with water. The move to the middle of the interlayer with increasing hydration is associated with the full hydration sphere of the ion being composed purely of water oxygen atoms, and no clay surface oxygen atoms. [Copyright &y& Elsevier]

Rehydroxylation of the previously dehydroxylated dioctahedral 2:1 layer clay mineral occurs preferentially in specific sites within the former octahedral sheet. The rehydroxylation of dehydroxylated A1-rich and A1,Mg-rich 2:1 layers occurs as trans-vacant (tv) structural arrangements, regardless of whether the initial structure was tv or cis-vacant (cv). In nontronite (Fe-rich 2:1 layer clay), the dehydroxylate pseudo-cv structure is probably directly reconstructed into the rehydroxylated cv structure without migration of octahedral cations. Rehydroxylation occurs preferentially in the R3+-Or-R3+ former octahedral structural arrangements (Or = residual oxygen) over R2+-Or-R (R = R3+ or R2+ = A13+, Fe3+ or Mg2+, Fe2+). In the case of the R2+ octahedral substitution, the interlayer cation is attracted to the electrostatically undersaturated residual oxygen of the R2+-Or-R arrangement, which blocks the ability of water molecules to pass through the ditrigonal cavity and rehydroxylate the previously dehydroxylated local arrangement. The pyrophyllite-like type of octahedral R3+-Or-R3+ arrangements, formed due to the lack of tetrahedral substitution and resulting in the absence of interlayer cations, is thus favored for rehydroxylation over the mica-like R3+-Or-R3+ arrangements where Al occurs in the tetrahedral sheet. The valence of the interlayer cation and the charge density of the 2:1 layer clay mineral, which controls the interlayer cation content, also affect the degree of rehydroxylation.Dehydroxylated 2:1 layer minerals with a high-rehydroxylation potential, including beidellite and illite, use all the adsorbed water molecules that persist above 200 °C for rehydroxylation; the water vapor from the ambient environment also becomes a source of H2O molecules for rehydroxylation. The high demand for water molecules to use for rehydroxyltion results in a noticeable gain of mass in the temperature interval between 200 and 350 °C even during heating. [ABSTRACT FROM AUTHOR]

Thermal analysis experiments in the environment of an extremely low water vapor concentration provide insight into the first steps of the rehydration mechanism in smectite when completely dehydrated and the interlayer region is collapsed. The relative structural and compositional controls on dehydration and rehydration reactions are compared from a well-characterized suite of samples that vary with respect to chemical composition, octahedral and tetrahedral substitution, octahedral cation site vacancy, and degree of dehydroxylation. Techniques including multi-cycle heating-cooling thermogravimetric analysis and nitrogen gas adsorption on various smectite samples preheated at different temperatures followed by rehydration at ambient conditions were used to characterize the interaction of water molecules with completely dehydrated montmorillonite, beidellite, and nontronite smectite types. Beidellite with high-Al3+ tetrahedral substitution results in electrostatically undersaturated basal oxygen atoms that exert strong repulsion between the tetrahedral sheets of adjacent 2:1 layers. The interlayer region of dehydrated or dehydroxylated beidellite is capable of being spontaneously rehydrated even in low water vapor environments. In completely dehydrated montmorillonite and nontronite, the external surface area of the crystallites is a primary control on water adsorption at low humidity when the molecules form a shell around the exchangeable cations present on external surfaces. The potential of montmorillonite and nontronite to reopen a collapsed interlayer is significantly lower than beidellite because of their crystal-chemical features that result in 2:1 layer and interlayer cation attraction. With increasing water vapor partial pressure, the hydration potential of interlayer cations provokes a reopening of the interlayer. In a dehydroxylated nontronite, the undersaturated residual oxygen atom strongly bonds the interlayer cation within the ditrigonal ring of the tetrahedral sheet, resulting in a permanent interlayer collapse. The specific surface area calculated from a conventional N2 gas adsorption measurement using the BET model represents the external surface area of a dehydrated smectite crystallite and can be converted into the mean crystallite thickness. The mean crystallite thickness of a completely dehydrated smectite increases with an increase in preheating temperature. [ABSTRACT FROM AUTHOR]

This study is focused on the basic mineralogical and chemical characteristics of some selected samples from Jianping (JP) and Ningcheng (NC) area, China. These were studied using X-ray diffraction (XRD), expandability, Greene-Kelly test, specific surface area, cation exchange capacity (CEC), differential thermal analysis (DTA), and chemical analysis. The bulk samples consist of mainly montmorillonites along with appreciable amounts of cristobalite and with small amounts of quartz, feldspar and calcite. The basal spacings of montmorillonites in all these examined samples show at about 1.5 nm, indicating that predominant cations were divalent. Mean crystallite thickness of the present sample lies from 0.449 to 0.710 nm (4.58-7.24 layers). As Greene-Kelly test, samples from Ningcheng area confirmed appreciable amount of beidellite component but samples from Jianping area did not contain anything about it. Surface area of samples varies from 671 to 766 m g depending on the amounts of impurities. Analyzed CEC ranges from 70.30 to 82.75 meq 100g in < 2 μm size samples. The DTA curves are considered abnormal type. Samples from Ningcheng area show relatively higher amount of SiO than those from Jianping area, possibly due to difference of parent rock type. Trace element (Y, Nb, Ce and Zr) data are plotted on acidic to intermediate composition. It is apparent that no large chemical transfer would have been necessary during alteration. [ABSTRACT FROM AUTHOR]

Abstract: The surface energy of some clays belonging to the smectite group has been calculated starting from crystal structures and combining a partial charge model with the computation of the lattice energy. The dioctahedral smectites studied here include montmorillonite; beidellites; and nontronite. One of the differences between these clays is the location of the substitution in the octahedral sheet or in the tetrahedral one. Another is the possibility of vacancies in cis- or trans-octahedral positions. These locations and vacancies have an effect on the distortion of the crystal framework and therefore on the surface energy. Calculated surface energies of the solid samples increase in the order beidellites > montmorillonite > nontronite. The bond energy between the interlayer cation and the layer appears to follow the same order and to depend both on the nature of the most electropositive elements of the layer and on their location. The trends obtained provide elements for an analysis of data related to interlayer enlargement. [Copyright &y& Elsevier]

Abstract: Amine-terminated hyperbranched poly(amine-ester)(HBP) dendrimer synthesized in our laboratory was used as a complex agent of the lanthanum(La3+) particles, and montmorillonite (MMT) as a stabilizer to gain HBP/La/MMT ternary nanocomposites. The results of TEM showed that HBP/La composites were deposited on the surface of the MMT layers within nanoscales. The sizes of clusters become larger and larger with the increase of MMT content. The information of DSC indicated that the T g of HBP/La/MMT nanocomposites with 5–30 wt.% MMT were increased significantly. [Copyright &y& Elsevier]

Abstract: Nanocomposites of polyaniline (PANI) with organophilic montmorillonite (O-MMT) and hydrophilic montmorillonite (Na-MMT) were prepared. The nanocomposites were characterized using FT-IR, D.C. electrical conductivity measurement and cyclic voltammetry techniques. It was found that PANI/Na-MMT nanocomposite has lower (5.8%) and PANI/O-MMT nanocomposite has higher (29.4%) conductivity compared to pure polyaniline. Cyclic voltammetry experiments showed that both nanocomposites are electroactive. The anticorrosive properties of a 100μm thickness coating of nanocomposites on iron coupons were evaluated and compared with pure polyaniline coating. According to the results PANI/MMT nanocomposites have enhanced corrosion protection effect in comparison to pure polyaniline coating. Results showed also that the PANI/Na-MMT and PANI/O-MMT nanocomposites have considerably different corrosion protection efficiencies in various corrosive environments. [Copyright &y& Elsevier]

Abstract: In the current work, a sensitive, rapid and convenient electrochemical method was developed for the determination of Sudan I utilizing the excellent properties of montmorillonite calcium (MMT-Ca). Compared with the unmodified carbon paste electrode (CPE), MMT-Ca modified CPE not only significantly enhances the oxidation peak current of Sudan I but also lowers the oxidation overpotential, suggesting that the MMT-Ca modified CPE can remarkably improve the determining sensitivity of Sudan I. The experimental conditions such as determining medium, the content of MMT-Ca and accumulation time were optimized for the determination of Sudan I. The oxidation peak current is proportional to the concentration of Sudan I over the range from 0.05mgL−1 (2.01×10−7 mol L−1) to 1.0mgL−1 (4.03×10−6 mol L−1), and the limit of detection is 0.02 mg L−1 (8.06×10−8 mol L−1) for 2-min accumulation. Finally this newly-proposed sensing method was successfully employed to detect Sudan I in practical samples and good recovery was achieved. [Copyright &y& Elsevier]

Abstract: Tri-octahedral Li–Mg smectites (hectorites) were synthesized at temperatures ranging from 25 to 250°C, in the presence of solutions highly enriched in lithium. After removing all the exchangeable lithium from the synthesized clays, Li isotope fractionation (Δ7Liclay-solution) was determined. This fractionation was linked to Li incorporation into the structural octahedral site, substituting for Mg2+. As predicted, experimental Δ7Liclay-solution inversely correlates with temperature, and ranges from −1.6‰±1.3‰ at 250°C to −10.0‰±1.3‰ at 90°C, and then stays relatively constant down to 25°C. The relatively constant isotope fractionation factor below 90°C may be due to high concentrations of edge octahedra in low crystallinity smectites. The isotopic fractionation factor (α), for a given temperature, does not depend on the solution matrix, nor on the amount of structural Li incorporated into the clay. Empirical linear laws for α as a function of 1/T (K) were inferred. Smectite Li contents and smectite-solution distribution coefficients (D Li/Mg) increase with temperature, as expected for a substitution process. The fractions of dissolved Li incorporated into the smectite octahedral sites are small and do not depend on the duration of the experiment. In a seawater-like matrix solution, less Li is incorporated into the smectites, probably as a result of competition with dissolved Mg2+ ions for incorporation into the octahedral sites. The high Li contents observed in marine smectites are therefore best explained either by a significant contribution from basalts, by adsorption processes, or by the influence of seawater chemical composition on distribution coefficients. We also calculate, using present-day estimates of hydrothermal water and river fluxes, that a steady-state ocean would require a relatively large global clay-water Li isotope fractionation (−12‰ to −21‰). This study demonstrates the ability of laboratory experiments to quantify the impact of secondary phases on the Li geochemical cycle and associated isotope fractionations. [Copyright &y& Elsevier]

Abstract: Two synthetic smectites (montmorillonite and beidellite) are studied by a water adsorption technique in order to assess their specific surface areas under atmospheric conditions. A route recently proposed for extracting the thermodynamic data from experimental adsorption isotherms is used. The variation of the specific surface area during water adsorption is obtained, which can be linked to the enlargement of the interlayer space determined using X-ray diffraction. This variation is compared to an idealized specific surface area obtained from TEM and X-ray measurements in agreement with crystallographic models. All these results are also compared with those obtained previously for a natural montmorillonite. A simple view of swelling is proposed. [Copyright &y& Elsevier]

Abstract: Toluene isopropylation is carried out over modified montmorillonite clays. Effect of ion exchange and pillaring on the catalytic activity and selectivity is noted. Elemental analysis results give the extent of metal incorporated. Enhanced porosity is evident from X-ray diffraction as well as surface area – pore volume measurements. 29Si NMR shows the resonance for T (3Si, 1Al) units which shifts proportional to the amount of pillar metal present. Among the different modifications mixed metal oxide pillaring is found to be most effective in catalytic activity, p-cymene selectivity and lowering deactivation. [Copyright &y& Elsevier]

Abstract: Platinum sub colloids (nm range) were synthesized in the presence of a surfactant (CTAB) by micellar technique in which the metal particles ensured long-term stability in the aqueous dispersion and were prevented from agglomeration. When this surfactant stabilized Pt hydrosol was allowed to react with aqueous suspension of Na+ montmorillonite or Li+ hectorite, the cationic exchange reaction between Na+/Li+ and organic cation (CTA+) took place with simultaneous incorporation of the reduced Pt particles into the organoclay host. Finely divided and well-dispersed Pt particles (nm range) were generated by this method in the interlamellar space of the montmorillonite and hectorite. The catalyst samples with different metal loadings were synthesized and characterized with UV–Vis spectroscopy, XRD, TEM and AAS techniques. From the TEM micrographs it was confirmed that the diameter of the Pt particles intercalated was in the nanometer range. The catalytic activity of the synthesized catalysts was studied towards the selective hydrogenation of C cinnamaldehyde using a batch type high pressure reactor at different temperatures and hydrogen pressures. The catalytic activity of the catalysts and their selectivity towards cinnamyl alcohol were discussed in detail. The activity and selectivity of 1wt.% Pt loaded montmorillonite catalyst was found to be appreciable. [Copyright &y& Elsevier]

The crystal structure and crystal chemistry of dioctahedral smectites in high-enthalpy geothermal systems were investigated through samples collected in two wells drilled in the Bouillante geothermal area to understand the factors that control their vertical variation. Smectites were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and oxygen-isotope analysis. Smectites predominate within the upper part of the drill holes (up to 260 m depth; temperature range: 67-163 °C). The XRD, FTIR, and chemical microanalyses clearly demonstrate a transition from montmorillonite to beidellite with increasing depth and temperature that proceeded through interstratification of beidellite-like and montmorillonite-like layers. Montmorillonite predominates at temperatures below 100 °C, whereas beidellite predominates between 110 and 163 °C. However, this transition is not explained by a thermally controlled beidellitization process but appears to be related to hydrothermal fluids from which these smectites precipitated. The δ18O values of the fluids that equilibrated with smectites (-3.3 to 0.4‰) indicate that beidellitic smectite precipitated from the hot geothermal fluid associated with minor amounts of residual solutions resulting from local boiling. In the same way, montmorillonitic smectite precipitated from reacting solutions whose origin lies in the phreatic water table (±seawater contribution) associated with minor amounts of liquids resulting from the condensation of vapors escaped from the boiling zones. The mixing rate of geothermal fluid with meteoric waters exerted a major influence on the montmorillonite vs. beidellite ratio of the smectite material as underlined by the irregular depth-related smectite transition. [ABSTRACT FROM AUTHOR]

Abstract: Proton uptake on montmorillonite edge surfaces can control pore water pH, solute adsorption, dissolution kinetics and clay colloid behavior in engineered clay barriers and natural weathering environments. Knowledge of proton uptake reactions, however, is currently limited by strong discrepancies between reported montmorillonite titration data sets and by conflicting estimates of edge structure, reactivity and electrostatics. In the present study, we show that the apparent discrepancy between titration data sets results in large part from the widespread use of an erroneous assumption of zero specific net proton surface charge at the onset of titration. Using a novel simulation scheme involving a surface chemistry model to simulate both pretreatment and titration, we find that montmorillonite edge surface chemistry models that account for the “spillover” of electrostatic potential from basal onto edge surfaces and for the stabilization of deprotonated Alng sites through bond-length relaxation at the edge surface can reproduce key features of the best available experimental titration data (the influence of pretreatment conditions on experimental results, the absence of a point of zero salt effect, buffer capacity in the acidic pH range). However, no combination of current models of edge surface structure, reactivity and electrostatics can quantitatively predict, without fitted parameters, the experimental titration data over the entire range of pH (4.5 to 9) and ionic strength (0.001 to 0.5 mol dm−3) covered by available data. [Copyright &y& Elsevier]

Abstract: Two montmorillonites, an inorganic sodium montmorillonite (NaMMT) and an organo-modified montmorillonite (OMMT), were used for the preparation of montmorillonite/polypyrrole (MMT/PPy) composites. MMT particles were modified by the in situ polymerization of pyrrole in water, in aqueous solution of dodecylbenzenesulfonic acid (DBSA) used as anionic surfactant, and in water/methanol. Ferric chloride was used as oxidant in each case. Wide angle X-ray scattering (WAXS) measurements proved the intercalation of PPy into the galleries of NaMMT regardless the reaction media. In contrast, for OMMT/PPy composites, the increase of interlayer spacing depends on the preparation conditions, the highest increase in interlayer spacing was achieved in water/DBSA solution. The WAXS patterns of OMMT/PPy composites synthesized in methanol/water showed no change in interlayer spacing and the electrical conductivity of these composites was low, similar to that of NaMMT/PPy composites prepared under the same conditions. Conductivity about 1.1Scm−1 was reached for OMMT/PPy composites containing 13.3wt% PPy prepared in the presence of DBSA. The NaMMT/PPy composite containing 15.6wt% PPy and prepared under the same conditions showed a conductivity of 0.26Scm−1. X-ray photoelectron spectroscopy (XPS) proved that the surface of NaMMT/PPy composites is rich in MMT, whereas more PPy was found on the surface of OMMT/PPy composites. The conductivity of composites correlated with the N/Si atomic ratio determined from XPS results, which was taken as a semi-quantitative measure of the PPy surface fraction. [Copyright &y& Elsevier]

Abstract: The thermal stability of seven organically modified montmorillonites (‘organoclays’) has been investigated using differential thermal analysis, differential scanning calorimetry, and thermogravimetry in conjunction with X-ray diffractometry. Six organoclays were synthesised by replacing the interlayer inorganic cations, initially present, with quaternary phosphonium and ammonium surfactant cations. The samples modified with tetrabutylphosphonium (TBP), and butyltriphenylphosphonium (BTPP) ions have an appreciably higher thermal stability than the octadecyltrimethylammonium (ODTMA)-modified clays. Thus, in the case of TBP- and BTPP-modified montmorillonites, the onset temperature of decomposition is close to 300 °C. Samples modified with hexadecyltributylphosphonium (HDTBP) ions have a lower onset temperature of decomposition of 225 °C. In comparison, the onset temperature for ODTMA-montmorillonites (obtained at different concentrations of ODTMA-bromide) ranges from 158 to 222 °C, being highest where the concentration of intercalated surfactant is lowest. The onset temperature for a commercial alkylsilane-treated quaternary ammonium-modified organoclay (S-BEN N-400FP) is 207 °C. The basal spacing of the TBP- and BTPP-modified clays is 1.7–1.8 nm, indicating a monolayer arrangement of quaternary phosphonium ions in the interlayer space, while the value of 2.5 nm for HDTBP-montmorillonite indicates a more open structure. The ODTMA-modified samples have basal spacings ranging from 1.9 to 2.1 nm, indicative of a bilayer to pseudo-trilayer arrangement. The exceptionally high basal spacing of 3.4 nm for the S-BEN N-400FP organoclay might be due to interlayer penetration of organosilane hydrolysis products during synthesis. The thermal properties of organoclays are apparently related to the nature of the surfactants and their arrangement in the interlayer space of montmorillonite. [Copyright &y& Elsevier]

Molecular dynamics simulations were performed to provide a systematic study of aqueous uranyl adsorption onto the external surface of 2:1 dioctahedral clays. Our understanding of this key process is critical in predicting the fate of radioactive contaminants in natural groundwaters. These simulations provide atomistic detail to help explain experimental trends in uranyl adsorption onto natural media containing smectite clays. Aqueous uranyl concentrations ranged from 0.027 to 0.162 M. Sodium ions and carbonate ions (0.027–0.243 M) were also present in the aqueous regions to more faithfully model a stream of uranyl-containing groundwater contacting a mineral system comprised of Na-smectite. No adsorption occurred near the pyrophyllite surface, and there was little difference in uranyl adsorption onto the beidellite and montmorillonite, despite the difference in location of clay layer charge between the two. At low uranyl concentration, the pentaaquouranyl complex dominates in solution and readily adsorbs to the clay basal plane. At higher uranyl (and carbonate) concentrations, the mono(carbonato) complex forms in solution, and uranyl adsorption decreases. Sodium adsorption onto beidellite occurred both as inner- and outer-sphere surface complexes, again with little effect on uranyl adsorption. Uranyl surface complexes consisted primarily of the pentaaquo cation (85%) and to a lesser extent the mono-(carbonato) species (15%). Speciation diagrams of the aqueous region indicate that the mono(carbonato)uranyl complex is abundant at high ionic strength. 01igomeric uranyl complexes are observed at high ionic strength, particularly near the pyrophyllite and montmorillonite surfaces. Atomic density profiles of water oxygen and hydrogen atoms are nearly identical near the beidellite and montmorillonite surfaces. Water structure therefore appears to be governed by the presence of adsorbed ions and not by the location of layer charge associated with the substrate. The water oxygen density near the pyrophyllite surface is similar to the other cases, but the hydrogen density profile indicates reduced hydrogen bonding between adsorbed water molecules and the surface. [ABSTRACT FROM AUTHOR]

The effect of exchanging transition metal ions in the alumina-pillared beidellite/montmorillonite FAZA on the conversion of methanol, ethanol, n-propanol and iso-propanol has been investigated. It is concluded that dehydration/dehydrogenation throughout the series of materials occurs preferentially at the oxidic pillar rather than at the aluminosilicate sheet. The basal structure of the smectite, i.e. whether protons lie inside (montmorillonitic) or outside (beidellitic) the aluminosilicate layers, little influences the reactions. By way of contrast, cation exchange (both single and double) greatly influences both product ratio outcomes and conversions, e.g. Ni-FAZA (FAZA=a Fe2+/Fe3+/alumina-pillared mixed beidellite/montmorillonite) has very acidic centres suppressing MeOH dehydration completely. [Copyright &y& Elsevier]

Molecular mechanics simulations, combined with X-ray powder diffraction and infrared spectroscopy, have been used in structure analysis of montmorillonite and beidellite intercalated with tetramethylammonium cations. A complex structure analysis provided us with the detailed structure model, including characterization of the disorder, the total sublimation energy and a charge distribution in the structure of intercalates. The calculated basal spacings (14.36 Å for TMA-montmorillonite and 14.12 Å for TMA-beidellite) are in good agreement with the experimental values (14.31 Å for TMA-montmorillonite and 14.147 Å for TMA-beidellite). Both intercalated structures exhibit positional and orientational disorder in the arrangement of TMA cations, and consequently disorder in layer-stacking. In the present work we analyse the effect of octahedral and tetrahedral substitutions in a 2:1 silicate layer on the arrangement of tetramethylammonium (TMA) cations in the interlayer space of montmorillonite and beidellite. The most significant difference between TMA-montmorillonite and TMA-beidellite is in the charge distribution on the TMA cations and silicate layer. The TMA-beidellite structure is highly polarized, the total charge on one TMA cation is +0.167 e−, while the total charge on the TMA cation in montmorillonite is +0.050 e−. [ABSTRACT FROM AUTHOR]

Atrazine (A) is one of the most applied herbicides and has a negative impact on the environment and health. Density functional theory (DFT) and experimental methods were used in the study of the immobilization of atrazine in two smectites, montmorillonite (Mt) and beidellite (Bd), as well as in their organically modified structures. Four systems were examined: A-Mt and A-Bd, as well as the structures modified by tetramethylphosphonium cation (TMP), A-TMP-Mt and A-TMP-Bd. The calculations revealed a flat arrangement of the atrazine in the interlayer space of both smectites with higher stability of beidellite structures. The presence of the TMP cation increased the fixation of atrazine in both organically modified smectites. The calculated vibrational spectra allowed a detailed analysis of the overlapping bands observed in the experimental FTIR spectra and their correct assignment. Further, selected FTIR bands unambiguously assigned to atrazine and both smectites served for the estimation of the adsorbed amount of atrazine. It was shown that the adsorption capacity of both TMP-modified smectites did not increase in comparison to the adsorption capacity of unmodified smectite samples. [ABSTRACT FROM AUTHOR]

The mechanochemical adsorption of phenol by laponite, saponite, montmorillonite, beidellite and vermiculite was studied by simultaneous DTA and TG analysis. Fitting of the DTA curves showed four endothermic peaks in the r.t. −250°C temperature range. The first peak represents the melting of the excess crystalline phenol. The second peak represents the evolution of water. The third peak represents the boiling of the free phenol. The fourth peak represents the evolution of adsorbed phenol. In the DTA curves of montmorillonite the fourth peak is not observed as a separate peak but as part of the third peak. Laponite, saponite and montmorillonite show a very small exothermic effect which follows the endothermic events. Iaponite adsorbs the greatest amounts of phenol and shows a strong exothermic peak at 390°C. None of the other clays show this exothermic effect at this temperature. [ABSTRACT FROM AUTHOR]

The swelling properties of Al-pillared clays, obtained from five different smectites, were studied using X-ray diffraction. These clays, the dioctahedral beidellite and montmorillonite and the trioctahedral saponite, hectorite and laponite differ in source of isomorphic substitution and represent a series of decreasing basicity along the siloxane plane. An Al oxyhydroxy cation was inserted between the layers to form the respective pillared clays and these clays were heated incrementally to 600°C. The XRD peaks at each stage of heating were recorded as well as the same samples subsequently wetted. Basal spacings of each clay at each stage of dehydration ↭d rehydration indicated that the swelling of tetrahedrally substituted saponite and beidellite was indeed restricted, compared with the other three clays. This was attributed to greater basicity of the oxygen plane of beidellite and saponite due to tetrahedral substitution of Si by Al, resulting in an increase in the strength of hydrogen bonds between either water or the interlayer polyhydroxy cation and the clay. The data from the XRD analyses helped in addition, to clarify the thermal transformations of the Keggin ion itself. According to the changes in the d-spacings of the pillared clays it was concluded that the Keggin ion lost its structural water at ∼200°C and dehydroxylated in a range beginning at 350°C. Between 500 to 600°C this polymer cation, which is thought to form the Al2O3 oxide, did not rehydrate. [ABSTRACT FROM AUTHOR]

The distinction of montmorillonite proper from beidellite is often based on the Li-text proposed by GREENE-KELLY (1953). However, when saturated with Li, true montmorillonite samples that contain considerable admixtures of SiO[sub2] or Fe[2]O[3]hH[sub2]O behave in the same way as beidellites. An irreversible sorption of Li (after annealing at 250°C) is observed in such samples only after the admixed amorphous substance has been removed by leaching with a NaCO[sub3] or HCI solution. [ABSTRACT FROM AUTHOR]

Smectites affecting water and soil exchangeable cations, especially potassium and ammonium related to plants, are common swelling clay minerals in sediments and paleosols that can record paleoenvironmental and paleoclimatic information. Paleoenvironmental and paleoclimatic interpretations are potentially ambiguous, because smectite minerals may have either detrital or pedogenic origins. This study analyzes smectites in Lower Eocene paleosols of the Qaidam Basin, China, with the goal of determining their origin and paleoclimatic significance. Twelve smectite-rich vertisol samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-mass spectrometry (ICP-MS), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) in the Dahonggou (DHG) section of the Lulehe Formation. The samples from DHG section contained predominant dioctahedral smectites (60–65%) in the <2 μm clay fractions. Minor dioctahedral illites, dioctahedral interstratified illite-smectite (I-Sm), kaolinite, and traces of dioctahedral chlorites and palygorskites were present. The chemical data and FTIR spectra revealed both tetrahedral substitutions (i.e., Al for Si) and octahedral substitutions (i.e., Mg and minor Fe for Al) in the dioctahedral smectites. Montmorillonite comprised the dominance of the fine-clay fractions (< 0.2 μm) with minor contributions of beidellite, as determined by the Greene-Kelly test (GK-test), geochemical and FTIR analyses. SEM observations revealed two morphological types of dioctahedral smectites, the less abundant one with typical 'honeycomb' structure, interpreted as neoformed, and the other occurring as thin, well-defined plates and irregular masses considered to be detrital. DSC revealed two dehydroxylation temperatures (500–550 °C and 600–700 °C), indicating the presence of trans- vacant (t v) and cis- vacant (cv) layers, respectively. The cv layers were attributed to montmorillonite originating as catchment-delivered detritus, as the study Lulehe Formation exhibited no evidence of volcanic ash layers or diagenesis that would have promoted formation of montmorillonite with high dehydroxylation temperatures. The tv layers represented pedogenic beidellite formed via solution crystallization in a floodplain pedogenic environment. The accumulation of abundant inherited montmorillonite and neoformed beidellite in paleosols of the Lulehe Formation was probably promoted by a relatively warm climate with alternating wet-dry seasons during the Early Eocene. Admixture of detrital montmorillonite makes paleoclimatic reconstructions based on bulk-smectites concentrations and isotopic compositions unreliable, showing that integrated clay mineralogical analyses of the type undertaken in this study are necessary to identify the provenance and paleoclimatic significance of smectites in sediments. • Two subtypes of smectite, montmorillonite and beidellite, occur in Lulehe palesosols. • Predominant montmorillonite with high thermal ability has a detrital origin. • Minor beidellite with low dehydroxylation temperatures has a pedogenic origin. • Smectite accumulation/formation increased during EECO due to seasonal wet and dry. • Smectites in soils can be of multiple origin making their paleoenvironmental interpretation complex. [ABSTRACT FROM AUTHOR]

• Swelling pressure trend of smectites with alkaline-earth interlayer cations was studied. • Variables used are interlayer cation, d-spacing, dry density, total charge and location. • Swelling pressure increased as the interlayer cation progressed up the group. • Swelling behavior of Mg-smectite was different from other cation-smectites. Interlayer swelling pressure of smectite minerals was investigated at atomic level using molecular dynamics simulation method. Four different alkaline-earth cations were used as interlayer cations for simulated montmorillonite and beidellite minerals. The swelling pressures of the smectite minerals were obtained as function of interlayer cation, layer charge, charge location, dry density and d-spacing. The swelling pressure of the homoionic smectites follows the trend Mg2+ > Ca2+ > Sr2+ > Ba2+. Layer charge and charge location have also a notable effect on swelling pressure. Octahedrally substituted smectites show higher swelling pressure than tetrahedrally substituted, while lower layer charge lead to higher pressure than higher layer charge. The results have practical implications in the selection and predicted behaviour of backfilling clay materials for geodisposal of high-level radioactive waste. [ABSTRACT FROM AUTHOR]

Chitosan-clay bionanocomposites were prepared from chitosan biopolymer solutions and four different smectites (SAz-1 and SWy-2 montmorillonites, SBId-1 beidellite and NAu-1 nontronite), and the effect of layer charge and charge distribution on the formation of bionanocomposites was examined. The intercalation of the chitosan in the homoionic Na+-smectite samples showed that all samples produced chitosan-smectite nanocomposites even at the lowest chitosan-clay ratios. The adsorption isotherms of chitosan by the smectites were classified as L- type isotherms. The equilibrium data were fitted to Langmuir and Freundlich isotherms and the equilibrium adsorption was best described by the Langmuir isotherm model for SWy-2 and SBId-1 while the Freundlich model achieved a better description for SAz-1 and NAu-1 for the higher chitosan concentrations. The adsorption of chitosan exceeded the CEC in the montmorillonites (SWy-2 and SAz-1) whereas in SBId-1 and NAu-1 with tetrahedral charge it either reached or it was lower than the CEC showing that layer charge and charge localization may hinder chitosan adsorption. Unlabelled Image • Chitosan-smectite bionanocomposites were produced even at the lowest chitosan-clay ratios. • Adsorption isotherms follow the Langmuir and the Freundlich model. • The increase of Δ d L spacing is in full accordance with adsorption isotherms. • Adsorption of chitosan in excess of the CEC is evident in smectites with octahedral charge. • Layer charge and charge localization influence chitosan adsorption and formation of bionanocomposites. [ABSTRACT FROM AUTHOR]

• Swelling pressure trend of smectites with monovalent interlayer cations was studied. • Variables used are interlayer cation, d-spacing, dry density, total charge and location. • Swelling pressure decreased as the interlayer cation progressed down the group. • Swelling behavior of Li-smectite was different from other cation-smectites. Interlayer cations largely determine the swellability of expansive clays. An atomic level study was carried out to determine the effect of monovalent interlayer cations on the swelling behavior of two different smectite minerals to establish their swelling pressure trends. Montmorillonite and beidellite clay minerals were simulated with five different alkaline metals as interlayer cations: Li+, Na+, K+, Rb+, and Cs+. A molecular dynamics simulation method was employed with varying charge balancing cations, layer charges and charge locations. The swelling pressure of homoionic smectite decreased as the interlayer cation progressed down the group from Li+ to Cs+ yielding the following trend: Li+ > Na+ > K+ > Rb+ > Cs+. The effects of layer charge and charge location were also evident. The outcome of this study is useful for the selection of smectite buffer materials for use in high-level radioactive waste disposal repositories. [ABSTRACT FROM AUTHOR]

• Effect of iron on the swelling pressure of sodium smectites has been modeled. • The variables are dry density, total charge, charge location and iron content. • Increase in iron content results in decrease in the swelling pressure of smectites. • Effect is more pronounced in montmorillonite than beidellite at high dry densities. Smectites in bentonites contain variable amounts of iron substituting aluminum and silicon in the crystal structure. The swelling pressure of smectite models with varying Fe3+ content, layer charge and charge location were studied using molecular dynamics method. Swelling pressures decrease with increasing iron content at high dry densities, this is more pronounced in montmorillonites than beidellites with same layer charge. This swelling pressure decrease is slightly faster for smectites with lower layer charge. Results obtained from the study is useful in the screening process of iron-containing bentonites intended for use as sealants in High Level Radioactive Waste (HLRW) repositories. [ABSTRACT FROM AUTHOR]