Smectite Structure Research Articles

Impacts of Impurity Removal Chemical Pretreatment Procedures on the Composition and Adsorption Properties of Bentonites

This work is based on a detailed study of samples from three industrial bentonite deposits in Russia (10th Khutor, Zyryanskoe) and Kazakhstan (Taganskoe). The samples of the listed bentonites were undergoing stage-by-stage purification from carbonates, organic matter, and non-clay iron-bearing phase procedures. The conducted research revealed changes in the composition of bentonites already at the first stage of purification, which, however, were not noted for all bentonites, as well as a change in the forms of Fe in smectite structure. The latter is probably associated with changes in cis- and trans-vacant positions. Sorption characteristics with respect to 137Cs are determined using the localization and charges in the smectite layer and the slight changes during purification, along with the specific surface area indicators.

Atomistic Computer Simulations of Uranyl Adsorption on Hydrated Illite and Smectite Surfaces

A quantitative understanding of the molecular-scale mechanisms of radionuclide sorption on different clay minerals is crucial for the development and safe implementation of geological nuclear waste disposal technologies. We apply classical molecular dynamics (MD) computer simulations to study the adsorption of uranyl on the external basal surfaces of two typical clay models. In the illite model, negative charge is primarily localized in the tetrahedral sheets, while in the lower-charge smectite model, the isomorphic substitutions are introduced in the octahedral sheet. The comparison of atomic density distributions at the clay surfaces and adsorption-free energies profiles as a function of distance from these surfaces demonstrates that overall U behavior at the basal clay surface is quite similar for illite and smectite. Uranyl is sorbed as a mixture of outer-sphere aqua complexes [UO2(H2O)5]2+ and hydrolyzed aqua complexes [UO2(H2O)4–5OH]+ on both surfaces. The structural and compositional differences between the models do not greatly affect the uranyl’s nearest coordination environment and are mainly reflected in the specific localization and orientation of the uranyl ions at both surfaces and in the magnitude of the adsorption-free energies. The observed quantitative characteristics of uranyl interactions with illite and smectite surfaces will help to better understand U behavior during the sorption process on clay minerals for the entire range of mixed-layer illite–smectite structures. A comparison of two versions of the ClayFF force field in the simulations made it possible to more accurately and quantitatively evaluate some subtle features of the uranyl–clay interactions and to obtain a more precise composition of uranyl complex with the modified ClayFF force field (ClayFF-MOH).

Palygorskite in Indian tropical soils: a clay mineral of pedogenic, geogenic, or climate-induced origin – a mechanistic review

Abstract Most clay minerals are characterized by a platy morphology. By contrast, palygorskite has a fibrous morphology and is structurally distinct from the typical 1:1 and 2:1 layer structures. Diverse opinions exist on the origin of palygorskite in soils. Many authors suggest that palygorskite forms after smectite. Others favor its authigenesis during pedogenic processes or its inheritance from the parent material. This review provides a critical synthesis on the origin of palygorskite in the semi-arid-tropical (SAT) Vertisols and arid calcic soils of the Thar Desert of India. It also highlights the specific genetic pathway for the presence of palygorskite in the soils. The ubiquitous association of smectite with palygorskite is inadequate to explain the formation at the expense of smectite, because at pH 8.2 and above the smectite structure is subjected to dissolution to create soluble Si and Al, and the recrystallization of the soluble Si and Al to form palygorskite may not be possible in the Vertisols of the Indian SAT environment. Thus, mildly to moderate alkaline pedochemcial environments of the SAT Vertisols do not favor authigenic precipitation of the palygorskite in such soils. This review shows that the presence of palygorskite in the SAT Vertisols is due to its inheritance from the exhumed inter-trappean beds, infra-trappean beds, and bole beds. This view on the genesis of the palygorskite is also justified by its presence in weakly developed calcic soils of the Thar Desert as detrital flux from the adjoining marine sedimentary rocks.

Oxidation State and Structure of Fe in Nontronite: From Oxidizing to Reducing Conditions.

The redox reaction between natural Fe-containing clay minerals and its sorbates is a fundamental process controlling the cycles of many elements such as carbon, nutrients, redox-sensitive metals, and metalloids (e.g., Co, Mn, As, Se), and inorganic as well as organic pollutants in Earth's critical zone. While the structure of natural clay minerals under oxic conditions is well-known, less is known about their behavior under anoxic and reducing conditions, thereby impeding a full understanding of the mechanisms of clay-driven reduction and oxidation (redox) reactions especially under reducing conditions. Here we investigate the structure of a ferruginous natural clay smectite, nontronite, under different redox conditions, and compare several methods for the determination of iron redox states. Iron in nontronite was gradually reduced chemically with the citrate-bicarbonate-dithionite (CBD) method. 57Fe Mössbauer spectrometry, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) spectroscopy including its pre-edge, extended X-ray absorption fine structure (EXAFS) spectroscopy, and mediated electrochemical oxidation and reduction (MEO/MER) provided consistent Fe(II)/Fe(III) ratios. By combining X-ray diffraction (XRD) and transmission electron microscopy (TEM), we show that the long-range structure of nontronite at the highest obtained reduction degree of 44% Fe(II) is not different from that of fully oxidized nontronite except for a slight basal plane dissolution on the external surfaces. The short-range order probed by EXAFS spectroscopy suggests, however, an increasing structural disorder and Fe clustering with increasing reduction of structural Fe.

About the possibility to liberate naturally present fixed cations from dioctahedral smectites by hydrothermal treatment

The chemical analysis of pure dioctahedral smectites exchanged by one of the typical CEC-index cations (e.g. Cu-trien) often shows significant Na-, Ca-, and K-contents which, in contrast to Mg, are not expected to occur in the octahedral sheet. They are supposed to occur in the interlayer but are not exchangeable - hence termed “fixed cations”. The aim of the study was to gain new evidence for the presence of naturally present fixed cations and find out if at least a part of these can be liberated by hydration after hydrothermal treatment.The methodological approach for cation liberation tests included different treatments with water at 60 °C and 150 °C, partly using an ultrasonic bath, and different reaction times prior to addition of the Cu-trien index cation for CEC analysis.The study proved that at least part of the naturally present fixed cations could be rendered exchangeable (liberated) by the hydrothermal treatment before an index cation is added. The differences were small but significant.Liberation of fixed cations was thought to increase the CEC but the increased amount of exchangeable cations was accompanied by a decrease of the CEC, which can probably be explained by partial smectite dissolution. This hypothesis was supported by measurements conducted at higher solid to liquid ratio in order to suppress smectite dissolution. The CEC decrease was less pronounced but at the same time less cations were liberated. In conclusion, the naturally present fixed cations are strongly bound and, supposedly, can only be liberated if the smectite structure is affected.Future work will be devoted to finding alternative methods which are suitable for quantification of naturally present fixed cations in bentonites/smectites.

Oxidation rates and redox stabilization of ferrous iron in trioctahedral smectites

Iron(II)-bearing trioctahedral smectites (saponites) form during anoxic alteration of basaltic rock. They are predicted to have been widespread on the early Earth and are observed in the oceanic subsurface today. Smectite structures, including the occupancy of sites in the octahedral sheet, affect iron redox behavior but the rates and products of trioctahedral smectite oxidation have been largely unexplored to date. In this study we synthesized two Fe(II)-bearing trioctahedral smectites, one moderate (22 wt% Fe) and one high (27 wt% Fe) in iron content. We then examined the rate, extent, and products of their oxidation by dissolved oxygen, nitrite, and hydrogen peroxide. Dissolved oxygen caused partial oxidation of Fe(II) in the smectites with 14 to 43% of Fe(II) unoxidized after 20 to 30 days of exposure. The rate and extent of oxidation correlated with the dissolved oxygen concentration and the Fe(II) content of the clay. The incomplete oxidation in these experiments is consistent with the mixed-valent trioctahedral smectites observed in oxidized natural samples but contrasts with the complete reoxidation by oxygen shown by chemically- or microbially-reduced dioctahedral smectites. Oxidation of structural Fe(II) by 5 mmol L−1 nitrite was negligible for the moderate-iron smectite and yielded only ∼17% oxidation after 54 days of reaction for the high-iron smectite. Hydrogen peroxide caused rapid and near-complete oxidation of both clays. Powder X-ray diffraction, variable-temperature Mössbauer spectroscopy, and extended X-ray absorption fine structure spectroscopy together detected no crystalline or short-range-ordered secondary phases and show that oxidized iron remained in the trioctahedral smectite structure. The recalcitrant Fe(II) pool in oxidized trioctahedral smectites exists in less distorted sites than Fe(II) in the initial clays. Its unreactive nature at prolonged reaction times indicates an elevated redox potential generated by the local coordination environment. Slower oxidation rates create a larger recalcitrant Fe(II) pool, suggesting kinetic competition between oxidation and a process involved in redox stabilization, such as electron exchange between octahedral iron sites or deprotonation of hydroxyl groups in the structure. The resistance to complete oxidation of trioctahedral ferrous smectites and their full retention of iron demonstrates that transitions from anoxic to oxic conditions generate mixed-valence smectites rather than a mixture of new phases. Identifying the diagenetic products of mixed-valent trioctahedral smectites may provide an indicator in the rock record of past redox cycling. Substantial portions of structural Fe(II) in trioctahedral smectites display slow abiotic oxidation kinetics and represent potential electron donors for both microaerophilic iron oxidizing and nitrate-reducing, iron-oxidizing microorganisms in altered mafic rocks and related settings.

Dehydroxylation of smectites from Nudged Elastic Band and Born-Oppenheimer Molecular Dynamics simulations

High–temperature treatment of smectite minerals leads to the dehydroxylation process. One of the consequences of this process is the loss of sorption ability, which is an interesting, but not fully understood, phenomenon. A combination of static and dynamic quantum–chemical methods was used to analyze structural changes associated with the dehydroxylation of smectite layers. The work aims in the determination of multi–path reaction mechanism. The Nudged Elastic Band (NEB) method allowed for the recognition of intermediate structures, transition states and activation energies in the studied reaction mechanisms. Born–Oppenheimer Molecular Dynamics (BOMD) was used to investigate the interatomic distances time–evolution of atoms involved in the dehydroxylation process at different temperatures. The simulations were carried out in the crystalline phase, for which the computational model was prepared based on the dioctahedral structure of smectites with the chemical formula: Ca2+Al4Si8O22(OH)2. It was found, that the process requires a proton transfer to the oxygen atom of the hydroxyl group located in the octahedral sheet, which results in the release of water molecule from the layer. In addition, it has been shown that the process of a water molecule formation on the surface is also possible. The process occurring inside the layer is more energetically favoured than that on the surface. However, high activation energies were obtained in both cases, what proves that the dehydroxylation reaction takes place only at a relatively high temperatures. It has also been demonstrated that this process can follow several different mechanisms.

The Effects of Hydrochloric Acid Pretreatment on Different Types of Clay Minerals

Clay minerals are common in geological samples and are useful paleoclimate and sediment provenance proxies. Acid pretreatment is the most common method for the separation and purification of clay minerals. Given that hydrochloric acid (HCl) can dissolve chlorite and distinguish it from kaolinite, the HCl digestion method is used to simplify the routine method of clay mineral analysis. However, there have been few studies of the effects of acid digestion on different clay minerals in the context of extracting paleoclimate indicators. In this study, we used illite, chlorite, kaolinite, and two types of smectite to assess the effects of pretreatment with different HCl concentrations at variable temperatures. Our results show that chlorite is the most soluble clay mineral in HCl and can be effectively dissolved in HCl with concentrations of >1 N. The variable crystal structure of smectite affects its solubility in HCl. Ca-rich smectite, which has more cation substitution of octahedral Al, can be dissolved with HCl. However, Na-rich smectite, which has less cation substitution for octahedral Al, is hardly dissolved in HCl of any concentration or at any temperature. Illite can be partly dissolved in HCl, and the threshold beyond which dissolution occurs is 5 N HCl at 70 °C. Kaolinite is relatively difficult to dissolve in HCl. Given that the HCl digestion method uses the peak intensity of the bulk sample X-ray diffraction (XRD) analysis, whereas the routine method uses the peak area of clay particles, we compared the results of clay mineral quantification and the paleoclimate proxy obtained using the two methods for synthetically prepared mixed and natural clay samples. The results obtained with the HCl digestion method are less accurate than those obtained with the routine method because of the dissolution of illite and smectite in HCl. Therefore, the HCl pretreatment method is not suitable for clay mineral analysis in paleoclimate studies. The present results provide reference data for future studies that employ the acid dissolution pretreatment of clay mineral samples to acquire and quantify paleoclimate proxies.

Structural and Surface Modification of Oxalic-Acid-Activated Bentonites in Various Acid Concentrations for Bleaching Earth Synthesis—A Comparative Study

The aim of this study is to investigate the oxalic acid activation of bentonites containing different types of smectites, analyse their surface modification as a function of acid concentration and create good quality bleaching earths. In particular, two different bentonite samples (one containing aluminum and one containing ferruginous smectite), after being characterized through XRD, XRF and FT-IR analysis, are treated with oxalic acid at a concentration of 0.5, 0.7 and 1 M. Their structural modifications after treatment are observed through FT-IR spectra and surface area and porosity measurement (using the BET equation and the BJH method, respectively) combined with the determination of the main structural metals’ extraction from them (using an atomic adsorption spectrometer). The results showed that the ferruginous smectite is more susceptible to oxalic acid activation compared to the aluminum smectite, and all the final products have developed extra porosity in their structure while retaining the structure of smectite (even at 0.5 M acid concentration). The activated samples were used as bleaching earths in soybean oil, and the results proved that Lovibond yellow and red colours as well as the chlorophyll of oil (measured spectrophotometrically) were reduced to the values set by the specifications.

Characterization of Bentonites from the In Situ ABM5 Heater Experiment at Äspö Hard Rock Laboratory, Sweden

The Alternative Buffer Material ABM5 experiment is an in situ medium-scale experiment performed at Äspö Hard Rock Laboratory (HRL) conducted by SKB in Sweden with the aim of analysing the long-term stability of bentonites used as an engineering barrier for a high-level radioactive waste repository (HLWR). In this work, four different ring-shaped Ca- and Na-bentonite blocks, which were piled around a carbon steel cylindrical heater, subjected to a maximum temperature of 250 °C and hydrated with saline Na-Ca-Cl Äspö groundwater (0.91 ionic strength), were characterized after dismantling. This work allowed us to identify the main geochemical processes involved, as well as the modifications in the physico-chemical properties and pore water composition after 4.4 years of treatment. No significant modifications in mineralogy were observed in samples close to the heater contact, except an increase in Fe content due to C-steel corrosion, carbonate dissolution/precipitation (mainly calcite and siderite) and Mg increase. No magnetite and a low amount of Fe(II) inside the clay mineral structure were detected. No modifications were observed in the smectite structure, except a slight increase in total and tetrahedral charge. A decrease in external surface area and cation exchange capacity (CEC) was found in all samples, with lower values being detected at the heater contact. As a consequence of the diffusion of the infiltrating groundwater, a modification of the composition at clay mineral exchange sites occurred. Ca-bentonites increased their Na content at exchange sites, whereas Na-bentonite increased their Ca content. Exchangeable Mg content decreased in all bentonites, except in MX-80 located at the bottom part of the package. A salinity gradient is observed through the bentonite blocks from the granite to the heater contact due to anions are controlled by diffusion and anion exclusion. The pore water chemistry of bentonites evolved as a function of the diffusion transport of the groundwater, the chemical equilibrium of cations at exchange sites and mineral dissolution/precipitation processes. These reactions are in turn dependent on temperature and water vapor fluxes.

Effects of Metal-Polycation Pillaring and Exchangeable Cations on Aflatoxin Adsorption by Smectite

AbstractNatural smectites bind aflatoxins from water effectively, but the complex chemical environment in the guts of mammals and other animals can limit binding of aflatoxins. Many efforts have been made to enhance the adsorption capacity and affinity of smectites for aflatoxins in the presence of biological compounds. The main objective of the present study was to modify smectite structures by pillaring and cation exchange to enhance aflatoxin B1 adsorption capacity and selectivity. Smectite was pillared with Al and Al-Fe polycations or saturated with Ca, Mg, Zn, or Li. Structural changes in smectites with or without heat treatment were determined using X-ray diffraction and Fourier-transform infrared spectroscopy. Equilibrium aflatoxin B1 adsorption to the smectites was measured in aqueous solution and in simulated gastric fluid. Pillaring with the polycations expanded smectites in the z-direction to 18.6 Å and the expansion was stable after heating at 500°C. Changes in the Al–OH–Al infrared bands in the stretching region supported the formation of pillared clays. Migration of Mg, Zn, and Li into the octahedral sites of the smectite was observed as Mg and Zn saturation yielded a d spacing of 15 Å at 200°C which collapsed to 9.6 Å at 400°C. The 14.6 Å peak of the Li-saturated smectite collapsed to 9.6 Å at 200°C while the 15 Å Ca-saturated smectite peak was stable up to 400°C. The unheated Al- and AlFe-pillared smectites adsorbed significantly more aflatoxin B1 from an aqueous suspension than did unpillared clay. In both water and simulated gastric fluid, heat treatment decreased aflatoxin B1 adsorption to pillared smectites, but heat treatment increased aflatoxin B1 adsorption to unpillared smectites. Without heat treatment, smectites saturated with divalent cations (Ca, Mg, Zn) adsorbed more aflatoxin B1 from an aqueous suspension than the smectite saturated with a monovalent cation (Li). Ca-saturated smectite showed the greatest aflatoxin B1 adsorption, 114 g kg–1, from aqueous suspension after 400°C heat treatment. The Zn-, Mg-, and Li-saturated smectites showed maximum aflatoxin adsorption of 107, 93, and 90 g kg–1, respectively, after 200°C heat treatment. From simulated gastric fluid with pepsin, the 200°C heated, Zn-saturated smectite had maximum aflatoxin B1 adsorption of 68 g kg–1. Pillared smectites effectively adsorbed aflatoxin B1 from aqueous suspension, but Ca- and Zn-saturated smectites after heat treatment might improve the selectivity of smectites for aflatoxin B1 over pepsin and enhance the efficacy of smectite as a feed additive.

Synthesis of ferrian and ferro-saponites: Implications for the structure of (Fe,Mg)-smectites formed under reduced conditions

Abstract Clay minerals are widely distributed on the surface of Earth, Mars, and Ceres in the solar systems. Among numerous clay minerals, smectites can record the history of the environment through the exchange of interlayer cations with those in water or through redox reactions with the atmosphere. Therefore, characterization of chemical compositions and crystal structures of smectites is crucial for revealing the paleoenvironment. For instance, the crystal structure within octahedral sheets of iron-bearing smectites changes to trioctahedral sheets under reduced or dioctahedral sheets under oxidizing conditions. Orbital infrared and X-ray diffraction (XRD) analyses by Mars orbiters/rovers revealed the presence of (Fe,Mg)-smectites on the surface of Mars; however, it has been difficult to characterize the properties of these (Fe,Mg)-smectites, which are rare on the surface of Earth. In this study, we synthesized ferrian (ferric ion-rich) and ferrous (ferrous ion-rich) (Fe,Mg)-saponite and revealed the effect of valence states and iron contents on the crystal structures. These saponites were synthesized using a hydrothermal method under reduced conditions. The crystal structures and valence states of iron were analyzed by XRD, Fourier-transform infrared spectroscopy, transmission electron microscopy, Mössbauer spectroscopy, and X-ray absorption near edge measurements. The synthesized clays were trioctahedral swelling clays and were identified as saponites. The valence state of iron in these synthesized saponites is altered by oxygen and a reducing agent in water; however, the trioctahedral structures are maintained under both oxidizing and reduced conditions, following a reversible reaction. This mechanism can be interpreted by the desorption and adsorption of hydrogen in the hydroxyls of the octahedral sheets of the smectite layers. The maximum basal spacing of the (02l) lattice plane in the octahedral sheets was defined by compiling various smectite data. When the basal spacing of (02l) is larger than the maximum in dioctahedral smectites, smectite can be identified as trioctahedral smectite. The redox state of iron in the octahedral sheet cannot be determined from the basal spacing of (02l). We revealed that the iron content in the trioctahedral sheet has a linear relationship with lattice parameter b. This provides a method to estimate the iron content in saponite from XRD data. The XRD profiles of smectites found at the Yellowknife Bay on Mars can be explained only by trioctahedral smectites, and the iron content in the octahedral sheet is roughly estimated to be 0.5–1.7 in a half-unit cell. These results indicate that the presence of (Fe,Mg)-saponite implies a reduced environment during the formation and that this iron-bearing saponite has both oxidation and reduction capabilities depending on the environment.

Mineralogical Study on the Consistency Property of Bentonite Mixed Soil

Bentonite mixed soil, which is a mixture of bentonite with sand and gravel, can be easily compacted by adjusting the blending degree, and it is difficult for water to pass through and exhibits low permeability. It is widely used at construction sites because it can be mixed on-site and has good workability. Examples of its use include impermeable layers at waste disposal sites and measures to contain heavy metals from rocks generated from construction sites. The main component of bentonite is smectite, which is a swelling clay mineral, and the swelling property and consistency characteristics determine the engineering properties. In particular, low permeability is considered to be strongly influenced by the electrochemical properties at the crystal level peculiar to smectite. In this study, the mechanism of expression of engineering properties of bentonite mixed soil was investigated focusing on the mineralogy properties of smectite. Specifically, X-ray and near-infrared analysis were performed to clarify the crystal structure of smectite and the adsorptivity of water molecules. The change in interlayer distance were clarified by XRD. Information on the behavior of water molecules was obtained by NIR. From these analyses, we obtained information on the swelling, consistency and micro-physical properties of smectite, which is the main component of bentonite, related to water permeability.

Leaching behaviour of lithium from a clay sample of Kırka borate deposit in sulfuric acid solutions

In this study, the leaching behaviour of lithium, together with cesium and rubidium, from a clay sample of Kırka borate deposit in sulfuric acid solutions was investigated with chemical, XRD, FTIR, DTA analyses methods and specific surface area measurements. It was observed that the leaching behaviours of lithium, cesium and rubidium were quite similar in character in that their extent of leaching, and their leaching markedly increased with the increase of sulfuric acid concentration from 0.1 to 2 M at 90°C. Further increase in the concentration of sulfuric acid to 4 M appeared to have a limited improvement on their leaching ratios. In 2 M H2SO4 solution and at 90°C, the optimum conditions, the leaching ratio values obtained for lithium, cesium and rubidium were 97.2, 83.7 and 65.2%, respectively. The results of XRD, FTIR and DTA analyses applied to the clay sample and the leaching residue obtained after leaching of the clay sample under the optimum conditions collectively showed that almost complete destruction of crystalline smectite structure(s) in the clay sample caused by acid leaching resulted in the formation of amorphous silica phase in the leaching residue. During this transformation, as expected, the specific surface area of the clay sample increased, from 59 to 406 m2/g. The results obtained in this preliminary study may be used in hydrometallurgical treatment of very high tonnages of clay containing processing waste of the Kırka boron plant as a potential lithium (cesium and/or rubidium) resource in the future.

Geology, mineralogy, and geochemistry of the smectite-bearing Eocene İslambeyli Formation in the Pınarhisar region, NW Thrace, Turkey

The Eocene Islambeyli Formation outcrops at the lowest sedimentary unit of the northern part of the Tertiary Thrace Basin. This formation consists mainly of siliciclastics with increasing carbonate contribution in the upper part in the Pinarhisar region (NW Thrace). Mineralogically, this formation is derived from the quartz-rich sedimentary and felsic igneous provenance in an active continental margin (ACM) tectonic setting. The siliciclastics are composed of mainly dioctahedral Ca–smectite with minor amounts of illite, chlorite, and kaolinite, plus non-clay minerals quartz, K-feldspar, plagioclase, muscovite, biotite, calcite, zeolite (heulandite–clinoptilolite), and gypsum. Primary fragments of the Islambeyli Formation transported from the metamorphics of the Paleozoic Strandja Massif were widely altered according to the alteration indices of the massif and resulted in the enrichment and depletion of some elements (e.g., increase in Th/U ratio, negative Eu anomaly). The feldspar volume and total content of Na2O + K2O values of the formation are clearly lower than those of the massif. Mg and Ca in the smectite structure were probably obtained from seawater in a shore and shallow marine-reef environment during and after the deposition of the Islambeyli Formation.

Reducing Competition of Pepsin in Aflatoxin Adsorption by Modifying a Smectite with Organic Nutrients.

Carcinogenic aflatoxins can be inactivated by smectites (e.g., montmorillonite) through adsorption and degradation. Proteins in gastric fluids can reduce smectite’s adsorption capacity for aflatoxins. The objective of this study was to evaluate the efficiency of smectites modified with organic nutrients in restricting the influence of proteins on aflatoxin adsorption. Arginine, histidine, choline, lysine, and vitamin B1 were selected to occupy part of the interlayer space of montmorillonite to achieve a smectite structure more selective for aflatoxin adsorption, but not for the large protein molecules. The unmodified montmorillonite had a maximum adsorption capacity of 0.2 mol/kg in the presence of pepsin. The vitamin B1-montmorillonite showed significant improvements in the aflatoxin affinity constant from 0.065 to 0.201 M and the aflatoxin adsorption to 0.56 mol/kg. Choline-montmorillonite and histidine-montmorillonite showed a moderate increase in AfB1 adsorption. Arginine-montmorillonite and lysine-montmorillonite showed a slight increase in the adsorption capacity, but did not improve the affinity constant. The XRD results indicated that pepsin could still access the interlayer of nutrient-montmorillonite complexes. The intercalation of organic nutrients into the interlayer space of montmorillonite improved the AfB1 adsorption by restricting the adsorption of pepsin.

Swelling suppression of black cotton soil by means of liquid immersion and surface modification

Although the surface organic modification of smectite has been investigated widely, the swelling behavior of clays has been scarcely studied with consideration of civil engineering applications. In this work a facile strategy of liquid-immersion (dilute H2SO4 aqeuous solution) was proposed, and the 3-aminopropyltrimethoxysilane (APS) was utilized as surface modifier to suppress expansibility of black cotton soil (BCS) which is a type of highly swelling soils in tropical areas. Factors such as the incorporation dosage of APS, surface characters of soil treated by solution of H2SO4 or Na2CO3, and reaction temperatures/time were investigated to get lower swelling ratios. The treatment of BCS by H2SO4 was found more effective in immobilizing APS molecules, and hydronium ions were suppressed after the APS modification. The free swelling index (FSI) of BCS was decreased from 120% to 15% after treatment with H2SO4 and appropriate amount of APS modification. The reaction can be completed within several hours at the room temperature to ~80 °C. The soil samples were characterized by different means including the X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscope, thermogravimetric analysis and Zeta potential measurements. The APS molecules were found to react with –OH groups of the clay, and the hydrophobic groups provide surface hydrophobicity, which prevents hydration of cations within clay minerals. The APS was indicated to re-constructed lamellar structures of smectites after H2SO4 treatment, which suppressed the intra-crystalline and the subsequent osmotic swelling. This research highlights the liquid immersion and surface modification is applicable in diminishing swelling ratios of highly expansive black cotton soil.

Influence of Smectite Structure and Hydration on Supercritical Methane Binding and Dynamics in Smectite Pores

While significant effort has gone into studying H2O–mineral interactions over the past few decades, less attention has been paid to the interactions of hydrophobic fluids with mineral surfaces, par...

Characterization and Application of Catalysts Hard Green Clay and MoO<sub>3</sub>/ Hard Green Clay in Transesterification Reaction of Soybean Oil

The present study describes the preparation of catalyst MoO3 supported on smectite clay by the solution impregnation method and its evaluation as a heterogeneous catalyst in the production of biodiesel from soybean oil. The individual effects of catalyst (hard green clay and MoO3/hard green clay) on kinematic viscosity of produced biodiesel and conversion were investigated. The samples were characterized by X-ray diffraction, X-ray fluorescence spectroscopy and N2 adsorption-desorption. Conditions of soybean oil transesterification were: 5% catalyst by weight, 1:12 oil to methanol molar ratio, at 200 oC for 60 minutes. Patterns of X-ray diffraction showed the characteristic peaks of the structure of smectite. The results of X-ray diffraction suggests that MoO3 species exist as highly dispersed surface species. Molybdenum metal identified as effective catalysts for the transesterification reaction of soybean oil with methanol. A preliminary design assessment show that this catalysts (MoO3/HGC) is sufficiently active achieving conversion in excess of 62,07% at temperature below 200 oC.

Structural iron in smectites with different charge locations

The versatile structure of smectites can exhibit large variations in chemical compositions and cationic substitutions in different crystallographic sites, resulting in various locations of layer charge. Natural smectites can contain various amounts of structural iron, the chemical form of which can influence the reactivity of these minerals. The variety of Fe crystal chemistry in smectite was explored for eight natural smectites of distinct chemical compositions and charge locations, together with two synthetic ferric saponites used as reference compounds for tetrahedral Fe(III). All samples were identified as dioctahedral or trioctahedral smectite by X-ray diffraction and Fourier-transform infrared spectroscopy. The extent of [4]Al for [4]Si substitution was determined by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy. The Fe local chemical environment was probed by polarized X-ray absorption spectroscopy. Only Fe(III) could be detected in all samples, with no evidence of cluster formation. The O shell at 1.86 Å in synthetic saponites suggests Fe insertion in tetrahedral sites, and the absence of detected octahedral Fe implies quantitative substitution of [4]Fe3+ for [4]Si4+. In natural smectites, Fe(III) is bound to six O atoms at ~ 2.00 Å, suggesting insertion in octahedral sites. This inference is also supported by the detection of in-plane Mg/Al/Fe atoms at ~ 3.05 Å and out-of-plane Si/Al atoms at ~ 3.25 Å. In one Fe-rich nontronite, the detection of an O subshell at ~ 1.88 Å suggests a concomitant insertion of Fe(III) in tetrahedral sites. Low numbers of octahedral neighbors were detected in natural saponite and hectorite, presumably because of the presence of vacancies and/or Li(I) in adjacent octahedral sites balancing the local charge excess originating from the substitution of Fe(III) for Mg(II). The substitution of [4]Fe3+ for [4]Si4+ can be readily obtained under defined conditions in the laboratory, but seems more rare in natural samples, or present in amounts below the detection limit of spectroscopic methods used in this study.