Conceptual models of smectite hydration include planar (flat) clay layers that undergo stepwise expansion as successive monolayers of water molecules fill the interlayer regions. However, X-ray diffraction (XRD) studies indicate the presence of interstratified hydration states, suggesting non-uniform interlayer hydration in smectites. Additionally, recent theoretical studies have shown that clay layers can adopt bent configurations over nanometer-scale lateral dimensions with minimal effect on mechanical properties. Therefore, in this study we used molecular simulations to evaluate structural properties and water adsorption isotherms for montmorillonite models composed of bent clay layers in mixed hydration states. Results are compared with models consisting of planar clay layers with interstratified hydration states (e.g. 1W–2W). The small degree of bending in these models (up to 1.5 Å of vertical displacement over a 1.3 nm lateral dimension) had little or no effect on bond lengths and angle distributions within the clay layers. Except for models that included dry states, porosities and simulated water adsorption isotherms were nearly identical for bent or flat clay layers with the same averaged layer spacing. Similar agreement was seen with Na- and Ca-exchanged clays. While the small bent models did not retain their configurations during unconstrained molecular dynamics simulation with flexible clay layers, we show that bent structures are stable at much larger length scales by simulating a 41.6×7.1 nm2 system that included dehydrated and hydrated regions in the same interlayer.

Compacted bentonite, used as an engineering barrier for permanent containment of high-level radioactive waste, is susceptible to mineral evolution resulting in compromise of the expected barrier performance due to alkaline–thermal chemical interaction in the near-field. To elucidate the mineral-evolution mechanisms within bentonite and the transformation of the nuclide adsorption properties during that period, experimental evolution of bentonite was conducted in a NaOH solution with a pH of 14 at temperatures ranging from 60 to 120°C. The results showed that temperature significantly affects the stability of minerals in bentonite under alkali conditions. The dissolution rate of fine-grained cristobalite in bentonite exceeds that of smectite, with the phase-transition products of smectite being temperature-dependent. As the temperature rises, smectite experiences a three-stage transformation: initially, at 60°C, the lattice structure thins due to the collapse of the octahedral sheets; at 80°C, the lattice disintegrates and reorganizes into a loose framework akin to albite; and by 100°C, it further reorganizes into a denser framework resembling analcime. The adsorption properties of bentonite exhibit a peak inflection point at 80°C, where the dissolution of the smectite lattice eliminates interlayer pores and exposes numerous polar or negatively charged sites which results in a decrease in specific surface area and an increase in cation exchange capacity and adsorption capacity of Eu3+. This research provides insights into the intricate evolution of bentonite minerals and the associated changes in radionuclide adsorption capacity, contributing to a better understanding of the stability of bentonite barriers and the effective long-term containment of nuclear waste.

Amino acids have been detected in some meteorites and are readily synthesized in prebiotic experiments. These molecules may have been precursors of oligomers and polymers in the early Earth. These reactions were likely to happen in the protected confined spaces on the porous surface of olivine and in the interlayer nanospace of montmorillonite. This study describes experimental and theoretical research on the sorption of l-alanine onto surfaces of silicate minerals, olivine and montmorillonite. Kinetics of the sorption of this amino acid at different pH media was performed. This sorption has been also studied at atomic scale by means of quantum mechanical calculations finding that this sorption is energetically favourable. These results strongly support the premise that minerals could have actively participated in prebiotic reactions.

The adsorption of acetamide and partly hydrolyzed Polyacrylamides onto montmorillonite has been investigated with emphasis on the valency of the exchangeable cation of the clay and the ionic strength of the medium. A difference in the adsorption of acetamide from 3 mg/g to 0.6 mg/g was observed when a Na+-clay was changed to a Al3+-clay. For Polyacrylamide, an opposite effect was observed, and the fixation increased from 3 mg/g (Na+-clay) to 18 mg/g (Al3+-clay). This difference in behavior may be accounted for by the decrease in adsorption sites due to partial flocculation in the presence of polyvalent cations (tactoid formation). Adsorption in a saline medium is two- to fivefold greater for diverse polymers and substantially less for acetamide (from 3.0 mg/g to 0.77 mg/g). An increase in the degree of hydrolysis of the polymer results in a significant increase in adsorption. In contrast, a change of molecular weight has practically no influence upon the adsorption ratio. The results obtained in saline medium may be explained by a size decrease of the macromolecules, allowing a closer approach to the surface of the mineral substrate, and by a chain lengthening as the degree of hydrolysis increases, induced by the electrostatic repulsions between the COO− groups.

Cation-exchange capacities determined by methylene blue adsorption (CEC MB) and by the amount of K displaced from a K-saturated clay by NH4 (CEC K//NH4) correlate closely in five soil clays from Pennsylvania, but differ greatly in two soil clays with a large content of amorphous material. CEC MB was found to provide a more precise distinction between montmorillonite and vermiculite than CEC K//NH4. Synthetic aluminosilicate gels showed CEC K//NH4 > CEC Ca/Mg > CEC MB, but no relation to the behavior of the two soil clays with a large content of amorphous material was found.

Organic matter present in clayey sediments may act as a cement between the clay particles as well as a blocking agent for the swelling of clay minerals like montmorillonite.

Removal of the organic matter is important when a clay sample is to be characterized by X-ray powder diffraction, or when it is to be separated into the composing minerals on the basis of differences in density.

Eight different methods for the removal of organic matter from a North Sea clay sample have been studied. Of all methods tried bromine oxidation was found to be the most effective.

Hydroxy-Al- and hydroxy-Mg-montmorillonite were prepared by treating dispersed Na-montmorillonite with aluminum and magnesium nitrate solutions and titrating with NaOH solutions so that the OH/Al ratio varied from zero to 3.0 and the OH/Mg ratio from zero to 2.0. External precipitation of Al and Mg hydroxides was observed when the OH/M ratios (M = metal) approached 3 and 2, respectively. From chemical analyses of the initial Na-montmorillonite and the hydroxy-metal montmorillonites, structural formulae were derived by assuming that the silicate layer compositions remained unchanged. Prior to the addition of NaOH, the average interlayer material approximated in composition to [Al(OH)2]+ and [Mg(OH)]+. With additions of NaOH the interlayer compositions moved progressively towards Al(OH)3 and Mg(OH)2. When the hydroxy interlayers approached completion, external precipitation was observed. X-ray powder diffraction data showed that the hydroxy-Mg products have less tendency to swell in ethylene glycol and water, and greater thermal stability than the hydroxy-Al products. Initially, when the average interlayer compositions were near Al(OH)2 and Mg(OH), swelling followed more nearly the normal behavior.

Adsorption of Co2+ and Cd2+ on Wyoming montmorillonite was studied by the batch equilibration technique, as a function of salt concentration (0.01–4 M NaCl and NaNO3), pH (5.0–6.5), adsorbate concentration (trace-10−2 moles/liter), and presence of complexing ions. Comparison was made with the adsorbability of Sr2+, known to follow simple ion-exchange equations. The distribution coefficients for Co and Cd in noncomplexing media varied with salt concentration (from ∼500 liters/kg in 0.01 M Na+ to ∼10 liters/kg in 1 M Na+; pH = 5), but to a lesser extent than that of Sr. Adsorbability varied also with pH (∼1 order of magnitude/pH unit), especially at high ionic strength, compared to a negligible pH effect on Sr. The distribution coefficients of Cd and Co decreased with increasing loading on the clay at a very low percentage (0.2%) of the ion-exchange capacity compared to Sr (20%). These data suggest two classes of sites participating in the adsorption of Cd and Co.

The adsorbability of Cd in highly concentrated chloride solution (>1 M) was less than 1 liter/kg, presumably because of the chloride complex formation. This effect increased with increasing pH. The low adsorbability of Cd on montmorillonite from concentrated NaCl solution is promising with respect to its use as a tracer for monitoring flow through formations containing montmorillonite.

The charge density distribution among different classes of a series of reduced charge mont-morillonites is heterogeneous as in the parent Camp Berteau clay. In addition, charge reduction proceeds inhomogeneously. Up to 20% differences in charge density can be accounted for by aikyl chains extending at the edges of the clay particle. A realistic charge density-cation-exchange capacity relationship for hec-torite, Otay montmorillonite, and a series of reduced charge montmorillonites of Camp Berteau is obtained by accounting for the influence of particle radius and for the extent of alkyl chains lying outside the clay layers in the charge density calculations.

Acidified suspensions of Al-saturated kaolinite, montmorillonite, mica, illite, and biotite in 10−3 M NaNO3 were potentiometrically titrated with 0.1 N NaOH and 0.1 N HNO3 in succession in a CO2-free nitrogen atmosphere. The resulting curves were compared with those for Al(NO3)3 solutions of similar Al concentration in the supernatant solution and corrected for Al in the entrained solution in the clay.

Base titrations of Al ions adsorbed on all the minerals, except montmorillonite, showed two pH inflections separated by a buffering range. With montmorillonite, there were three pH inflections similar to those for Al in solution. The first inflections in the titration of suspensions occurred at lower pHs and were less pronounced than for Al in solution. These represent the titration of H3O+ sorbed during the pretreatment. The buffering by adsorbed Al ions is also less than that by Al in solution.

The OH− used up by adsorbed Al ions between the first and last inflections was equal to, or slightly greater than, the CEC of the minerals, except for mica where it was more than twice the CEC, because new interlayer surfaces were formed during the acid pretreatment. Acid titration curves of Al ions in the adsorbed and solution states showed hysteresis when related to the base titration curves. The use of two titration speeds (3 and 0.3 pH units/hr) only slightly affected the titration curves of the minerals suggesting that the observed effects were not caused by lack of equilibrium with added base or acid.

A stable porous system consisting of montmorillonite cross-linked by Al-hydroxide oligomers was synthesized by reacting at room temperature an aqueous solution of such oligomers with a unit-layer dispersion of montmorillonite. The resulting cross-linked montmorillonite (Al-CLM) is a nonswelling material, showing basal spacings of 14.4 to 18.8 Å after air drying and between 14.2 to 18.0 Å after treatment at 110°C. The basal spacing is found to depend on the age and OH/Al ratio of the Al-hydroxide solution, as well as on the relative amounts of the two reactants. A specific surface area of 160 m2/g and a diffraction pattern with a dominant basal spacing of 17.5 Å is obtained by using Al-hydroxide with OH/Al = 1.85, aged for at least 5 days, and by applying an Al/montmorillonite ratio greater than 1.5 in the cross-linking process. The basal spacing of Al-CLM remains essentially unchanged after heating at 220°C, while the specific surface area is not affected by heat treatment up to 480°C.

Two possible configurations of Al-hydroxide oligomers, homogeneously distributed between parallel montmorillonite unit-layers, were considered in order to account for the basal spacing of 17.5–18.8 Å, viz. (a) stacking of two oligomeric ring units in parallel orientation relative to the clay lamellae and (b) perpendicular orientation of individual oligomeric units.

Mössbauer spectroscopy of dioctahedral phyllosilicates showed that on dehydroxylation iron which originally occupied M(2) and M(l) sites became, respectively, 5- and 6-coordinated. The 6-coordinated sites are very distorted. No migration of cations occurs in the course of heating the specimens for 1–3 hr at 600°–700°C.

By using a combination of several physicochemical methods, different successive stages of the dehydroxylation process could be distinguished: (1) migration of protons; (2) localized dehydroxylation of individual associations without significant change in the overall configuration of the octahedral sheets; and (3) loss of most of the hydroxyl groups with concomitant changes in the cell dimensions. Penetration of Li into the octahedral sheets does not affect the course of the reaction, but reduces the dehydroxylation temperature and the stability of the products.

Dehydroxylation was preceded by or associated with the oxidation of any divalent iron present. Fe3+ derived from Fe2+ was indistinguishable by Mössbauer spectroscopy from iron initially present in the trivalent form. High concentrations of Fe lower the dehydroxylation temperature and reduce the stability of the dehydroxylate to the extent that partial disintegration may precede complete dehydroxylation.

Boron adsorption by Ca forms of montmorillonite, illite, and kaolinite was determined as a function of pH and boron concentration in solution. Data from batch experiments were compared with results computed for each clay according to fitted adsorption coefficients (maximum boron adsorption and affinity constants related to the binding energy). The agreement between calculated values and experimental results indicates that a phenomenological equation can be used to predict boron adsorption on clays as a function of both of these variables. For the solution-to-clay ratios examined, the water content does not affect the boron-surface interaction as expressed by the above adsorption parameters. Because the affinity of clays for B(OH)4− is much stronger than for B(OH)3 , the adsorption maximum was obtained only under alkaline conditions at approximately pH 9.0 to 9.7. It is suggested that the pH of maximum adsorption is a function of the ratios of affinity coefficients of the three species B(OH)3, B(OH)4−, and OH− competing for the same adsorption sites. The adsorption coefficients indicate that in some cases the difference in the amount of adsorbed boron between montmorillonite and kaolinite could be either small or large, depending on the circumstances. The main factor that would affect this difference is the total amount of boron in the suspension. Estimated value of the adsorption maximum was 2.94, 11.8 and 15.1 µmole/g for Ca-kaolinite, Ca-montmorillonite, and Ca-illite, respectively.

The integral thermodynamic quantities of adsorbed water on Na- and Ca-montmorillonite have been calculated from water adsorption isotherms on Na- and Ca-montmorillonite at 298° and 313°K and from one adsorption isotherm and calorimetric measurements at 298°K. The integral entropy values decrease and then increase as the amount of adsorbed water approaches zero. In both systems, the curves approach the entropy value of free liquid water at the high content water. The thermodynamics of adsorbate on a non-inert adsorbent is discussed in some detail. The two-isotherm method gives the energy change of the water phase only, whereas the colorimetric method gives the energy change of the whole system (clay, exchangeable cations, and the adsorbed water). Only when the energy changes in the solid phase are negligible (=inert surface) should the two methods give similar results. An hypothesis was developed to explain the entropy-change data of water adsorbed on clay surfaces, in which the clay surface behaves as a non-inert adsorbent.

Organic diacid (oxalic and succinic) adsorption onto montmorillonite is feasible, but weak (~1 mg/g). The comparison of chemical and radiochemical determinations reveals that 80% of the acid in contact with the smectite is used to attack the clay lattice. The pH is the main parameter involved in adsorption, and fixation passes through a minimum for pH 6 to 7. Polyacrylate adsorption is also weak (~1.5 mg/g). It changes with the nature of the exchangeable cation of smectite. Its pH-dependence displays a pronounced maximum for a value corresponding to the pKa of the acidic functions (pH ~6.8), and a minimum at about pH 8. On the assumption that a polyacrylate macromolecule is 100% hydrolyzed, it follows that the-COOH groups carried by 20% hydrolyzed Polyacrylamide molecules (such as those used in the tertiary recovery of petroleum) contribute at the very most to 10% of the total adsorption onto clay. Fixation, therefore, involves predominantly protonation of the amide functions at the edge surfaces of the clay. The acidic functions play a minor role in the adsorption phenomenon in that they affect the length of the macromolecule. The extent of this contribution, however, is virtually impossible to estimate.

Neutron diffraction measurements for a preferentially oriented aggregate slab sample of deuterated Na-montmorillonite from Upton, Wyoming, are described for a series of clay-water contents ranging from 0 to 500 mg/g. A neutron wavelength of 2.39 Å was used with extended detectors to collect much of the “out of plane” component of the diffraction peak intensities.

The diffraction pattern intensities from the 00ℓ planes of the clay, corresponding to a reflection geometry, are a strong function of sample water content and show a variation in basal spacing from 9.8 to 19.0 Å. The hk reflections from transmission geometry measurements show, however, that the lattice a and b axes are constant within experimental uncertainty (0.02 Å) over the range in water content and their intensities vary only by a few percent. In this geometry, a broad, water-like diffraction pattern was noted as a background under the usual hk peak intensity series. This underlying water-like pattern varies in proportion to the sample water content.

Data reduction steps included consideration of background removal, multiple scattering, flux normalization, and attenuation of scattering due to sample thickness. Analysis of the reduced data revealed that the clay-water has a “liquid-like” ordering, with a density increase of approximately 5% over bulk water. An association between a few interlayer water molecules and the silicate superstructure is indicated by the slight change in the hk band intensities, but this change seems to be complete at water contents below 100 mg/g. Fourier analysis of the basal peak series from the dry clay shows that the hydrogens of the lattice hydroxyl groups lie in the same basal plane as their associated oxygen atoms.

Montmorillonite, kaolinite, illite, and chlorite were found to adsorb bitumen and its pentane-soluble and pentane-insoluble fractions. The formation of clay-bitumen complexes is influenced by the nature of the exchangeable cation on the clay and by the solvent carrier which stabilizes the bituminous compounds. Ca-clays adsorb organic compounds more strongly than sodium forms except in the presence of nitrobenzene. Solvents of high dielectric constant, such as nitrobenzene, promote ionization so that the ion-exchange mechanism of adsorption is favored, whereas solvents of lower dielectric constant, such as chloroform, tend to solvate rather than to dissociate bitumens. The behavior of the montmorillonite-bi- tumen complex in variable relative humidity indicates that organic molecules adsorb primarily on external surfaces and cause the clay to become less hydrophilic than prior to treatment. Clay-organic complexes are sufficiently stable to resist powerful organic solvents. The clay-organic complex separated from the Athabasca oil sand behaves similarly during chemical treatment to complexes formed between bitumen and the four reference clay minerals.

The diffusion of water in the title intercalate has been measured by quasielastic neutron scattering. The diffusion coefficient (6.1 × 10−7 cm2 s−1 at 23.5°C) is one order less than that found previously for a sodium-exchanged montmorillonite which, however, contained 4 times as much water in the interlamellar space. The activation energy for the motion has been deduced to be 18 kJ mol−1. Also it has been demonstrated that upon the time scale of the neutron scattering events (faster than 10−9 s) the hydroxyl groups of the clay lattice are not in motion.