Suspensions were produced by mixing Na-saturated, Upton montmorillonite with aqueous solutions containing different concentrations of 1,4-dioxane. Each suspension was deposited on a porous ceramic filter in an environmental chamber, and the solution was expressed from it by admitting gaseous helium to the chamber at a slightly elevated pressure. The chamber was fitted 1) with beryllium windows so that X-rays could be transmitted into and out of it and 2) with a drain so that the expressed solution could be conducted to the outside atmosphere. Once a filter cake had formed on the filter, the pressure of the gaseous helium was raised in successive increments and, after each increment, the c-axis layer spacing(s) was determined by X-ray diffraction. Increasing the concentration of 1,4-dioxane caused some of the fully expanded layers to collapse to the partially expanded state (c-axis spacing = 15 Å) and appeared to cause the remaining fully expanded layers to move farther apart, especially at the higher pressures. Alternative explanations were given for these apparently contradictory results.

Dioctahedral tosudite, a regular interstratification of dioctahedral chlorite-dioctahedral smectite, occurs associated with kaolin in the hydrothermal area of Delgado, Neutla, Mexico. Its composition corresponds to the formula:

$$\left( {S{I_{13.77}}A{L_{2.23}}} \right)A{L_8}{O_{40}}{\left( {OH} \right)_8} \cdot C{A_{0.39}}N{A_{0.03}}{K_{1.06}} \cdot \left( {A{L_{3.79}}F{E^{ + 3}}_{0.09}F{E^{ + 2}}_{0.06}M{G_{0.26}}} \right){\left( {OH} \right)_{12}}.$$

The electrochemical properties of kaolinite before and after modification with chlorodimethyl-octadecylsilane have been studied by electrophoretic mobility, surface charge titration, and extrapolated yield stress measurements as a function of pH and ionic strength. A heteropolar model of kaolinite, which views the particles as having a pH-independent permanent negative charge on the basal planes and a pH-dependent charge on the edges, has been used to model the data. The zeta potential and surface charge titration experimental data have been used simultaneously to calculate acid and ion complexation equilibrium constants using a surface complex model of the oxide-solution interface. The experimental data were modeled following subtraction of the basal plane constant negative charge, describing only the edge electrical double layer properties. Extrapolated yield stress measurements along with the electrochemical data were used to determine the edge isoelectric points for both the unmodified and modified kaolinite and were found to occur at pH values of 5.25 and 6.75, respectively. Acidity and ion complexation constants were calculated for both sets of data before and after surface modification. The acidity constants, pKa1 = 5.0 and pKa2 = 6.0, calculated for unmodified kaolinite, correlate closely with acidity constants determined by oxide studies for acidic sites on alumina and silica, respectively, and were, therefore, assigned to pH-dependent specific chemical surface hydroxyl groups on the edges of kaolinite. The parameters calculated for the modified kaolinite indicate that the silane has reacted with these pH-dependent hydroxyl groups causing both a change in their acidity and a concomitant decrease in their ionization capacity. Infrared data show that the long chain hydrocarbon silane is held by strong bonding to the kaolinite surface as it remains attached after washing with cyclohexane, heating, and dispersion in an aqueous environment.

Diffuse reflectance spectra of kaolins have been recorded in samples from different environments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding to the 2(6A1) → 2(4T1(4G)) transition. The systematic comparison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments of crystal field transitions to goethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination of the Fe-phases associated to kaolins assists in the interpretation of the formation conditions of these minerals.

The effects of three organic ligands on the adsorption of copper on Ca-montmorillonite were studied. The results indicate that these effects include three different processes:

1. 1) Enhanced uptake of positively charged copper-ligand complexes by ion-exchange.

2. 2) Formation of ternary surface complexes involving surface aluminol groups.

3. 3) Inhibited uptake due to competition between the surface ligands and the dissolved ligands for dissolved copper.

Ethylenediamine promotes copper uptake by ion-exchange at low pH but tends to suppress adsorption at aluminol groups by ligand competition at high pH. The same mechanisms are operative for β-alanine; however, the uptake of Cu(β-ala)+ by ion-exchange is not promoted by the attached ligand. The influence of malonate includes both ligand competition and formation of ternary complexes. A quantitative interpretation based on the surface complexation model using the least-squares programs FITEQL (Westall, 1982) and GRFIT (Ludwig, 1992) is presented. The obtained equilibrium constants are listed in Tables 2b and 3.

The hydrolysis of 2-(2-trichlorosilylethyl)pyridine (TCSEP) or 3-aminopropyltrimethoxysilane (APTMS) yields a complex mixture of oligomers, the composition of which is a function of time. 29Si NMR measurements show that the same oligosilsesquioxanes can be obtained from both compounds after prolonged hydrolysis. Ion exchange by montmorillonite with the pillaring solution obtained from TCSEP results in partial intercalation, with a noticeable fraction of the clay exchange sites occupied by protons not by the Si cationic species. Calcination of this pillared clay results in a mixture of phases showing 1.86 or 1.56 nm basal spacings. Pillaring of the same montmorillonite by the solution obtained by partial hydrolysis of APTMS results in a homogeneous solid with a basal spacing of 2 nm, which reduces to 1.65 nm upon calcination above 773 K. The microporous volume (pores < 1 nm) of these Si-clays, determined after calcination at 773 K, 0.15 ml/g for the PILC obtained from TCSEP, and 0.186 ml/g for the PILC obtained from APTMS is comparable to that measured for Al or Zr pillared clays. A better thermal stability is observed for the clay pillared by APTMS, which retains a large microporosity (0.167 ml/g) up to 973 K in air. These Si-pillared clays show a small number of weakly acidic sites and some strongly acidic sites retaining ammonia up to 723 K, which are most likely localized on the clay layers.

Hydrotalcite (HT) interlayered with hexacyanoferrate(II) ions ([Mg4.62Al1.52(OH)12][(Fe(CN)6)0.30-(CO3)0.29]) has been synthesized from the carbonate-HT ([Mg4.52Al1.50(OH)12][(CO3)0.77]) through double ion exchange reactions, and the products have been studied by chemical and physical methods. Mössbauer spectroscopy shows that hexacyanoferrate(II) is held rigidly in the interlayer by electrostatic forces and is characterized by a singlet at 0.01 mm s−1 at 80 K. X-ray diffraction shows an increase of the basal spacing (d003 from 0.78 to 1.1 nm following exchange. Hexacyanoferrate(II) in the interlayer was oxidized to hexacyanoferrate(III) (up to 20%) by dioxygen on dehydrating the interlayer either by drying at 70°C or by washing with nonaqueous solvents like acetone or ethanol. The CN (ν6) band of hexacyanoferrate(II) and (III) is found at 2036 cm−1 and 2112 cm−1, respectively. The presence of an absorption band at 2080 cm−1, assigned to free cyanide anions in the interlayer, suggests that the hexacyanoferrate(II,III) complexes are not inert in the interlayer, cyanide ligands being substituted with either water or hydroxyls. Oxidation and ligand exchange of the hexacyanoferrate(II) are also indicated by Mössbauer spectroscopy.

In studies on the fate of aluminium in the environment, nontronite and saponite have been obtained by synthesis in reducing alkaline conditions close to those prevailing in poorly drained soils developed from limestones. The two minerals obtained have different structures and organizations corresponding to two different growth and/or maturation mechanisms. High-resolution transmission electron microscopy of ultrathin sections of a synthetic aluminous nontronite embedded in resin showed the presence of crystallites consisting of two to ten co-terminating parallel layers, indicating synchronous growth. Electron diffraction showed that the individual crystallites had hk-ordering, i.e., orientation of layers with respect to the six-fold pseudosymmetry of the unit cell. Deposits of a synthetic saponite included hk-ordered crystallites and crinkled films with turbostratic stacking. The two saponite phases had slightly different b dimensions. Lattice fringe images of sections of saponite embedded in resin showed a high angular disorientation of the layers in the stacking direction, suggesting multiple nucleation and growth of individual layers, subsequently aggregated with imperfect parallelism.

Exploration of the synthetic conditions of the aluminous nontronite indicated that calcium was essential for an hk-ordered product. Syntheses using potassium or sodium hydroxides and carbonates for pH control gave poorly organized nontronites. Hydrazine was not essential for nontronite formation, but better crystallized products—judging by their IR spectra—were obtained in its presence by maintaining reducing conditions in the early stages of synthesis. Attempts to prepare ferruginous beidellites under similar conditions to those in which aluminous nontronites formed were unsuccessful.

A synergistic mechanism for the retention of organic and inorganic pollutants in clays is discussed in this paper. The mechanism of adsorption of cis- or trans- 1,2-dichloroethylene vapor (CDE or TDE, respectively) by hydrated smectite clay (hectorite) exchanged with Pb2+, Hg2+, Cd2+, Ca2+, Ag+, or Na+ has been investigated by simultaneously measuring chlorohydrocarbon uptake and water desorption isotherm and by recording the infrared (IR) spectrum of the adsorbed phase. Hydrated hectorite saturated with divalent cations adsorbs about 55% more CDE or 35% more TDE than those saturated with monovalent cations. The quantity of chlorohydrocarbon adsorbed is also a function of the hydration of the clay interlayer space. When dehydrated, hectorite does not adsorb CDE or TDE. Upon long outgassing at room temperature or even at 100°C, the characteristic IR bands of clays with adsorbed chlorohydrocarbon, although much weakened, are still observable. The ratio of the amount of water desorbed to the amount of chlorohydrocarbon adsorbed varied from about 0.22 to 0.34. A shift of the center of gravity of the hydration water OH stretching frequency towards a higher wavenumber and of the asymmetric CC1 stretching vibration toward a lower frequency suggest that the formation of hydrogen bonds between CDE or TDE and water is the driving force for adsorption and that the cation-dipole interaction does not play a major role.

Commercial spreadsheet programs allow calculation of reasonable reaction paths for weathering of silicate minerals in a dilute acidic aqueous solution. These calculations use reaction coefficients of simplified weathering reactions and ignore speciation in the aqueous solution. The method is illustrated using the example of dissolution of microcline. When plotted on a log-log activity diagram, the obtained reaction paths are clearly curved, unlike the straight lines shown in most recent geochemistry textbooks. The calculated reaction paths are strongly dependent on the initial pH of the solution. At higher pH the speciation in the solution must be included in the calculations. The most important speciation reactions (the dissociation of H2O and H4SiO4) can be easily included using commercial mathematical programs. p ]The presented examples include closed systems with different initial pH values and an open system case describing the evolution of a soil profile. All reaction paths calculated using the simplified methods agree well with those obtained using sophisticated modeling codes.

The binary exchange of cations on clays and soils is generally regarded as a thermodynamically reversible process. The literature on soil chemistry and geochemistry, however, abounds with reports on cation exchange reactions that appear to have only limited reversibility, i.e., that exhibit hysteresis. A satisfactory explanation of this phenomenon is still lacking, even though a number of mechanisms have been advocated, e.g., charge or site heterogeneity at the surface, differential hydration of cations, dehydration of the exchanger, crystalline swelling hysteresis, and inaccessibility of sites caused by domain or quasi-crystal formation. In the present article, the relevant literature is reviewed and analyzed critically. On the basis of available evidence, it is shown that exchangeable cations can be classified into three groups, defined in such a way that hysteresis has, in the literature, generally not been observed when exchange reactions involved cations belonging to the same group, but has often been found when the reactions involved cations from different groups. Furthermore, it is argued that none of the five mechanisms mentioned can, in and of itself, account fully for the observed exchange hysteresis. A conceptual model is proposed that combines elements of these five mechanisms and is able to describe, at least qualitatively, the effects of factors such as clay type, electrolyte concentration, and extent of dehydration.

The decomposition of kaolinite by treatment with trimethyl phosphate (TMP) and the composition of the new crystalline phase formed were studied. On hot treatment with TMP, kaolinite forms a crystalline white compound that is soluble in hot water. The X-ray diffraction pattern of the kaolinite treated shows both the typical reflections of kaolinite and, furthermore, a very strong reflection at 8.84 Å. After 30 days of treatment with TMP, the silicate structure of kaolinite is completely destroyed and a crystalline phase identical with that resulting from treatment of aluminium oxide (Al2O3) with TMP is formed. The results show that the compound in question is formed by hydrolysis of TMP, catalyzed by the hydration water of exchange cations of kaolinite, followed by removal of Al from the silicate structure by incompletely hydrolyzed TMP. The new crystalline phase thus formed is an aluminium alkyl phosphate of formula Al(CH3)6(PO4)3.

The crystallization kinetics of goethite were studied colorimetrically under highly alkaline conditions (pH 10.1–12.2) at temperatures from 40° to 85°C. Color changes during crystallization from fresh precipitates, plotted on a*-b* colorimetric diagrams, were used to discriminate between pure goethite and mixtures of goethite and hematite. Only the b* value increased as goethite crystallization proceeded, and even a minor increase in the a* value revealed the existence of hematite. The rate of goethite crystallization, estimated from the b* value, could be modeled by a pseudo-first-order rate law. This rate depended both on pH and on temperature. Apparent activation energies for the reactions of 56.1 kJ/mol at pH 11.7 and 48.2 kJ/mol at pH 12.2 were estimated from Arrhenius plots.