Smectite may impact the ability of saline aquifer–caprock systems to store CO2 effectively, because of changes in pressure, temperature, and brine concentration induced by the injection of CO2. These changes influence the molar volume of smectite, affecting the short-term structural and stratigraphic trapping, or the dissolution of smectite via the long-term geochemical trapping. This study investigated the d001 value of an interlayer-cation-exchanged smectite, Na-rich SWy-2 (Na-SWy-2), with Ca or Mg (hereafter CaSWy-2 and MgSWy-2). Molar volume experiments used X-ray diffraction and a high-pressure environmental chamber. The extent of smectite dissolution was simulated at experimental conditions by geochemical modeling using a rate equation derived from the transition state theory. CaSWy-2-CaCl2 and MgSWy-2-MgCl2 brine systems showed that increasing the brine concentration from 0.17 M to saturation results in a <18% decrease in d001 values, and increasing the temperature from approximately 33 to 150°C results in <11% decrease. The effect of the interlayer cation shows the d001 values of MgSWy-2 are <0.4 Å higher compared with CaSWy-2. Geochemical modeling shows the extent of dissolution of Na-SWy-2, CaSWy-2, or MgSWy-2 is only <1.1% in acidic conditions. Furthermore, the calculated swelling pressure needed to decrease the H2O sheets in the interlayer, from 3W to 2W, of MgSWy-2 and CaSWy-2 are higher compared with Na-SWy-2. The swelling pressure was approximated from the sum of the osmotic repulsive pressure, the van der Waals attractive pressure, and the hydration pressure. The data suggest that Na-SWy-2, CaSWy-2, and MgSWy-2 may affect saline aquifer–caprock systems to store CO2. The molar volume is affected by changes in pressure, temperature and brine concentration, or swelling pressure from the injection of CO2. An increase in the d001 value of SWy-2 can enhance the sealing capabilities of a caprock by making saline aquifers less porous and less permeable and thus increasing the capability for CO2 storage. In contrast, a decrease in the d001 value can create cracks in a caprock and thus provide conduits for the CO2 to escape. Furthermore, the CO2 injection will cause a decrease in pH, causing smectite to dissolve until it reaches a steady state. However, despite acidic aquifer conditions, SWy-2 has low solubility.

Analytical data from aqueous dissolution studies of minerals, mineral systems, and naturally equilibrated solutions such as surface waters and groundwaters provide the basic ingredients necessary to calculate comparative solubility (or activity) products (CKs) and comparative free energies of formation (CΔGf0) of possible minerals or hypothetical minerals. Using a thermodynamic approach, quasi-thermodynamic values are obtained which can help in understanding the relative stabilities of different but similar materials and changes in reacting systems. Illite equilibrated solutions demonstrated that: 1) there is a 5 kcal spread in comparative free energies of formation of the five illites used, 2) the comparative stabilities remain about the same when highly simplified but similar hypothetical mineral formulas are considered, and 3) some of these illites are probably not the most stable phase in a closed chemical system at standard temperature and pressure.

A “mineral index system” composed of common rock-forming minerals, products of chemical weathering and perhaps hypothetical minerals is proposed, which offers a means of studying naturally equilibrated solutions. Such a system can show changes with respect to CΔGf0 of these minerals at a particular site through time or in relationship to spatial distribution and geologic changes through synchronous sampling at different sites.

Goose Lake, Beavers Bend, and Fithian illites were equilibrated in the presence of goethite (or hematite) at room temperature for as long as 2.6 yr. Kaolinite of known stability was added to some samples. Samples of solution were obtained after centrifuging with, or without, immiscible displacement or column leaching. Equilibrium was indicated by constant values over a long period of time, with kaolinite solubility as an internal indicator, and most importantly, by the approach to equilibrium from both undersaturation and supersaturation. Data plots indicate that the illites contain two or more phases or components in equilibrium with each other. Statistical and graphical techniques were used to analyze hundreds of equilibrations. The constancy of pK values for various expressions indicates that bulk illite composition and pyrophyllite are likely solution-controlling phases or components, but that muscovite, leucophyllite, phlogopite, and talc are not. A muscovite-like phase, of lower K content than muscovite, fit one pK expression well, but failed on two others.

High-Mg chlorites from Vermont and Quebec and high-Fe chlorites from Michigan and New Mexico were equilibrated at room temperature in the near-neutral pH range. Gibbsite, kaolinite, and hematite of known stability were added to the samples to control unmeasurable variables at calculable levels. Equilibrium solution compositions were obtained from undersaturation and from supersaturation. Other indicators of equilibrium were good agreement between successive analyses over a long period of time, between duplicate samples, between independent systems, and between independent measures of equilibrium. All four chlorites were stable relative to brucite and, with a few exceptions, relative to talc under the conditions of study. When in equilibrium with gibbsite, the pH − ½Mg2+ value of the chlorites ranged from 6.3 to 6.5, at a pH4SiO4 value of 4.0. These values are in good agreement with prior estimates of chlorite stability. The calculated standard free energy of formation of the chlorites is dependent upon solution Fe2+ calculated from the sample Eh and assumed equilibrium with hematite, with the assumption that the Fe2+-Fe3+ couple is at the same Eh as the sample.

Methane solubilities at 25°C were measured at 350, 550, and 750 psia in dilute (< 11 wt. %) clay slurries of Na-montmorillonites and argillaceous sediment. Methane solubilities were not significantly affected by the presence of clay. Water hydrated onto the external clay surfaces did not appear to exclude methane. In addition there was no detectable sorption of methane onto the clays. The measured solubilities are consistent with an open structure of the water hydrated onto the clay surface for which the partial molal volume is larger than that of normal water. The results imply that aqueous methane solubilities measured in the laboratory can be used to determine degrees of methane saturation in interstitial solutions in unconsolidated sediments.

Parathion and methyl parathion adsorbed at high loads on clays and similar surfaces exhibited properties of a bulk phase, such as a well-defined solubility and a characteristic X-ray powder diffraction (XRD) pattern, different from those of the free substances. The aqueous solubility of parathion condensed at these surfaces was as low as 1.2 μg/ml as compared to 14 μg/ml for free parathion. The BET isotherm described the behavior of the investigated surface condensates even though these condensates were different from the multilayer adsorption for which the BET isotherm was originally developed. The XRD patterns of parathion and methyl parathion adsorbed on Na-bentonite exhibited reflections which were shifted towards higher angles as compared with the XRD patterns of the free compounds. This is in agreement with the lower solubility of the surface condensates. The properties of these condensates were easily controlled by the choice of adsorbent as well as by the choice of conditions under whch they were prepared. The existence of such surface-condensed phases and the manipulation of their properties can be utilized for the slow-release formulation of organophosphate esters which is important, for example, in pesticides application.

According to Lippmann (1977, 1982) wide compositional variations and excess enthalpies of mixing calculated with electrostatic models imply that clay minerals of variable composition are dis-equilibrium solids. However, recent ATEM analyses of illite samples indicate compositional homogeneity of single illite grains and limited compositional variations in sedimentary basins. Moreover, Lippmann's electrostatic model may be inadequate inasmuch as it neglects polarization energy which is known to be a significant component of lattice energy even in dominantly ionic structures. Contrary to the assumptions of Lippmann, I/S minerals have also been shown to undergo Ostwald ripening.

May et al. (1986) reported that smectites do not reversibly control equilibria and further argued that conceptual and experimental deficiencies inherent in the solubility method prevent the attainment and demonstration of equilibrium in experiments with complex aluminosilicates of variable composition. However, equilibrium may be assumed if: (1) steady states are approached from both under- and over-saturation, (2) the slopes of univariant lines representing mineral-solution equilibria are rational over a wide range of solution compositions and temperature, and (3) results are reproducible in experiments of long duration. Recent solubility studies of smectites, chlorites, and illites meet these criteria indicating that clay minerals of variable composition are true phases capable of attaining equilibrium.

Two-line ferrihydrite is an important adsorbent of many toxics in natural and anthropogenic systems; however, the specific structural sites responsible for the high adsorption capacity are not well understood. A combination of chemical and spectroscopic techniques have been employed in this study to gain further insight into the structural nature of sites at the ferrihydrite surface. The kinetics of iron isotopic exchange demonstrated that there are at least two types of iron sites in ferrihydrite. One population of sites, referred to as labile sites, approached iron isotopic equilibrium within 24 hr in 59Fe-NTA solutions, while the second population of sites, referred to as non-labile, exhibited a much slower rate of isotopic exchange. Adsorbed arsenate reduced the degree of exchange by labile sites, indicating that the anion blocked or greatly inhibited the rate of exchange of these sites. Mössbauer spectra were collected from a variety of samples including 56Fe-ferrihydrite samples with 57Fe in labile sites, samples containing 57Fe throughout the structure, and samples with 57Fe in non-labile sites. The spectra showed characteristic broad doublets signifying poor structural order. Refined fits of the spectra indicated that labile sites have larger quadrupole splitting, hence more local distortion, than non-labile sites. In all cases, the spectra demonstrated some degree of asymmetry, indicating a distribution of Fe environments in ferrihydrite. Overall spectral findings, combined with recent EXAFS results (Waychunas et al., 1993), indicate that labile sites likely are more reactive (with respect to iron isotopic exchange) because they have fewer neighboring Fe octahedra and are therefore bound less strongly to the ferrihydrite structure. The labile population of sites probably is composed of end sites of the dioctahedral chain structure of 2-line ferrihydrite, which is a subset of the entire population of surface sites. Mössbauer spectra of samples containing adsorbed arsenate indicated that the anion may slightly decrease the distortion of labile sites and stabilized the structure as a whole by bidentate bonding.

Submicron-sized (~3–60 nm) powders of Al-substituted magnetite were synthesized in the laboratory by precipitation methods by mixing appropriate molar volumes FeCl2, FeCl3 and AlCl3 solutions and precipitating with 20% NH4OH. Precipitates were dialyzed for 48 hr to remove excess salts and then freeze-dried. The nominal Al mole fractions [Als = Al/(Al + Fe)] in the initial precipitate ranged from 0.001 to 0.42. Portions of the resulting powders were heated sequentially in air at 400° and 500°C. Powders were examined using X-ray diffraction (XRD), transmission electron microscopy (TEM), and visible and near-IR reflectance spectroscopy. Solubilities were determined in ammonium oxalate (pH = 3) and dithionite-citrate-bicarbonate (DCB) solutions. As determined by XRD, the mineralogy of precipitated powder samples was predominantly magnetite. Powders having Als > 0.20 contained minor goethite and a poorly crystalline iron oxide phase (ferrihydrite?), and powders having Als > 0.25 also contained gibbsite. The color of the magnetites was black throughout the range of Al-substitution. Powders heated to 400°C were reddish brown; Munsell colors ranged from 5R 2/2 to 10R 3/4 for Als from 0.1–41.5%, respectively. By XRD, these powders were maghemite, but hematite was also detected by Mössbauer spectroscopy. XRD and Mössbauer data indicate powders heated to 500°C are hematite; their Munsell colors are not noticeably different from the corresponding 400°C samples. Mean crystallite dimensions (MCDs) of the magnetite powders increase with the Al mole fraction from ~10 nm for Als = 0.001% to a maximum value of 35 nm for Als = 0.15 and decrease slightly with further increasing Al substitution. Heating magnetite powders to 400°C did not change the MCDs significantly. Heating to 500°C resulted in hematites having MCDs larger than those for corresponding precursor magnetites for Als < 0.10. The opposite is true for hematites derived from magnetites having Als > 0.10. For hematite powders with Als > 0.05, MCD decreased with increasing Al-substitution. Solubilities of powders in oxalate solutions were independent of Al content and decreased in the order unheated samples (mostly magnetite) >400°C-heated samples (maghemite + hematite) >500°C-heated samples (hematite). All powders dissolved completely in DCB. The low crystallinity of the magnetite powders and the presence of ferrous iron are responsible for their relatively high solubility in oxalate solutions.

New experimental data with respect to the solubility of natural and synthesized magadiite at elevated temperatures and in alkaline solutions are presented. The results show that the solubility of magadiite increases according to the expression ln(Kmag) = −8146·T(K)−1 − 5.71 from 20 to 100°C ($(\Delta G_{\text{R}}^0({\text{K}}_{\text{mag}})=19.6\pm 0.3$ and $\Delta H_{\text{R}}^0({\text{K}}_{\text{mag}})=16.2\pm 0.4\text{kcal}{\text{mol}}^{-1}$). The experimental results and the related data from the literature suggest that the formation of magadiite may be favored by a decrease of temperature and pH (pH >9) as well by large amounts of Na+ ions and low ionic strength. These effects are related to the value of $\Delta H_{\text{R}}^0({\text{K}}_{\text{mag}})$, the distribution of dissolved silica species, the stoichiometry of magadiite, and the occurrence of negatively-charged species, respectively.

Dissolution of amorphous silica or silicate is an important reaction to release silicic acid in natural water and this reaction is affected by several factors. The existence of a natural organic compound, tropolone, because of its abundance, is of particular importance. In the present study, the dissolution of amorphous silica in the presence of tropolone (HL) was investigated in an aqueous solution under acidic conditions. The dissolution is controlled by the reaction between silicic acid and tropolone (Si(OH)4 + 3HL + H+ ⇌ SiL3+ + 4H2O), where the conditional formation constant K = [SiL3+]/[Si(OH)4][HL]3[H+]). The solubility of amorphous silica in the presence of tropolone was significantly greater than that in pure water due to the formation of the Si-tropolonate complex in the solution. The acceleration of silica dissolution by tropolone depends on the reaction pH and tropolone concentration. The solubility of the amorphous silica in the tropolone solution at pH 1 was approximately ten times greater than in the 0.1 mol/dm3 HCl used as a reference. This increase in the solubility correlates to a K value of the Si-tropolone complex at each pH. From the solubility experiments, the conditional formation constants (log K) of the Si-tropolonate complex at each reaction pH 1, 2, and 3 were 6.39, 5.88, and 5.77, respectively. The significant acceleration of the dissolution of amorphous silica by tropolone at pH 1 can be attributed to the large formation constant of the Si-tropolonate complex.

Clays and clay minerals dissolve over a broad pH range, such as during sediment diagenesis and in a variety of applications, including nuclear waste storage, landfills, and geopolymer binders in the construction industry. The solubility depends on process parameters (pH, temperature, pressure, etc.) and material properties (phase content, clay mineral composition, particle size, etc.). Pretreatments such as calcination or severe grinding change the material properties and could enhance solubility, which is called activation. The aim of the current study was to determine the solubility of three different clay minerals after calcination (metakaolinite, metamontmorillonite, and metaillite) in high molar alkaline solutions (NaOH) up to 10.79 mol/L and pH = 14.73. Furthermore, the solubility of an Al(OH)3 powder in alkaline solution (NaOH) was analyzed, as it can be used to adjust the Si:Al ratio of geopolymer precursors. The residues of the clay minerals after the alkaline treatment were investigated to disclose potential alterations in their phase contents. Based on the results of the thermal and alkaline activation, conclusions about the suitability as geopolymer precursors were made. All clay minerals showed an increase in solubility proportional to the concentration of the alkaline solution. The solubility decreased in the order metakaolinite > metamontmorillonite > metaillite. Thereby, dissolution was incomplete for all three clay minerals (<90%) after 7 days and congruent for metakaolinite and metaillite but incongruent for metamontmorillonite.

Syntheses of Fe-, Co-, and Ni-containing Mg-Al-layered double hydroxides (LDHs) are described here because Fe, Co, and Ni represent the major constituents in steel containers used for storing spent nuclear fuel. Much evidence exists for the formation of LDHs during the corrosion of such containers under repository-relevant conditions. Because of their anion-exchange properties, LDHs can be considered as materials with the potential to retain and immobilize anionic radionuclides. Evaluation of the thermodynamic properties of LDHs is essential for reliable prediction of their behavior (solubility, anion exchange properties) in geochemical environments. The impact on the thermodynamic properties of the isostructural incorporation of divalent cations into the LDH was the main focus of the present study.

Mg-Al-Cl-LDH and the Fe-, Co-, and Ni-doped LDHs were synthesized by the co-precipitation method and then characterized (using powder X-ray diffraction (PXRD), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX), and thermogravimetric analyses (TGA)).

The PXRD and EXAFS analyses indicated that all synthesized samples were pure LDHs where Co, Ni, and Fe were incorporated isostructurally. The EXAFS and XANES results demonstrated that Ni and Co were incorporated as divalent cations and Fe as a trivalent cation. Thermodynamic calculations were performed assuming an equilibrium state between aqueous solutions and corresponding precipitates after synthesis. The first estimates of the molar Gibbs free energies for Fe-, Co-, and Ni-containing LDHs at 70ºC were provided. The calculated Gibbs free energy of the pure Mg-Al-LDH (-3629 kJ/mol) was slightly less than those for Fe-, Co-, and Ni-containing compositions (-3612±50, -3604±50, -3593±50kJ/mol).

Hydrotalcite-like layered double hydroxides (LDH), of the formulation M2Al(OH)6(CO3)0.5.H2O, where M = Mg, Zn, Co, Ni, have been prepared, the products characterized and their solubility products measured at ionic strengths of 0.0065 and 0.0128 M and at 25°C. Steady-state solubility was reached after 100 days. The solubility products have been formulated according to the following reaction: $M_2\text{Al}{\left(\text{OH}\right)}_{6}0.5{\text{CO}}_3\cdot {\text{H}}_2\text{O}+6{\text{H}}^{+}\to 2M^{2+}+{\text{Al}}^{3+}+0.5{\text{CO}}_3^{2-}+{\text{H}}_2\text{O}$ where

$K_{\text{SO}}=\frac{{\left[M^{2+}\right]}^2\left[{\text{Al}}^{3+}\right]{\left[{\text{CO}}_3^{2-}\right]}^{0.5}}{{\left[\text{H}\right]}^6}$

Average values of Kso for I = 0, estimated using the Davies equation, are 25.43, 20.80, 22.88 and 20.03 for Mg, Zn, Co and Ni, respectively. Model calculations reveal that the thermodynamic stability of the LDHs is greater than that of the corresponding divalent hydroxides for Zn, Co and Ni below a pH of ∼10, 9 and 8, respectively, and at least up to pH 12 for Mg.

Pyrrhotite Fe1–xS is the main sulfide component of platinum-group element (PGE) ores and commonly contains from a few tenths to a few dozen ppm of disseminated Pt. Here we report an X-ray absorption spectroscopy investigation into the state of Pt in synthetic pyrrhotite in combination with theoretical spectra modelling. The pyrrhotite crystals were obtained by means of the salt flux technique, using a eutectic mixture of alkali metal halides as the transport media. Analysis of the chemical composition of synthesised crystals showed that an increase of the temperature and sulfur fugacity yields higher concentrations of Pt in pyrrhotite. The Pt content reached 0.6 wt.% at the maximum temperature and sulfur fugacity (t = 720°C, log $f_{{\rm S}_ 2}$ = –0.1) achieved in a Pt-saturated system. X-ray absorption near-edge structure (XANES) analysis of Pt L3-edge spectra revealed that Pt is present in pyrrhotite in the 4+ and 2+ ‘formal’ oxidation states. Theoretical modelling of XANES and interpretation of extended X-ray absorption fine structure (EXAFS) spectra showed that Pt4+ substitutes for Fe in the crystal lattice of pyrrhotite, whereas Pt2+ forms PtS-like clusters disseminated in the pyrrhotite matrix. Atoms of isomorphous Pt4+ are surrounded by 6 S atoms at a distance of 2.39 ± 0.02 Å. According to theoretical simulations using the FDMNES program, the second coordination sphere of the solid-solution Pt contains one vacancy in the Fe sublattice within the Fe-layer. The Pt2+S-like clusters can be considered as a quench product. High sulfur fugacity stabilises the solid-solution Pt and prevents the formation of the PtS-like clusters during cooling. The maximum content of the solid-solution Pt in pyrrhotite is ca. 50 times lower than in pyrite and can be approximated by a straight line in the log C(Pt) vs. 1/T plot, it increases from 1 ppm at 350°C to 3 wt.% at 900°C.

In situ liquid cell transmission electron microscopy (LC-TEM) is an innovative method for studying the processes involved in the formation of crystals in liquids. However, it is difficult to capture early stages of crystallization because of the small field of view and the unfavorable changes in sample composition resulting from electron-beam radiolysis. Nevertheless, if the radiolysis required to induce the crystallization of a sample could be controlled in LC-TEM, this would be advantageous for observing the crystallization process. Here, we examined this possibility by using a mixture of sodium chlorate (NaClO3) and acetone in the LC-TEM. The electron beam induced the formation of dendritic crystals in a saturated acetone solution of NaClO3; moreover, these crystals consisted of sodium chloride (NaCl), rather than NaClO3, suggesting that chloride ions (Cl−), which were not present in the initial solution, were generated by radiolysis of chlorate ions ${\rm ( ClO}_3^- )$. As a result, the solution then supersaturated with NaCl because its solubility in acetone is much lower than that of NaClO3. The combination of radiolysis and a solvent in which a solute is much less soluble is potentially useful for establishing crystallization conditions for materials that are difficult to crystallize directly in LC-TEM experiments.

The effect of a Mg- and Al-containing layered double hydroxide (LDH), used as an additive or as a matrix, on the dissolution and solubility of two non-steroidal anti-inflammatory drugs (NSAIDs), namely, mefenamic and meclofenamic acids, is reported. The use of MgAl-LDH enhances the solubility of mefenamic acid at the three pH values tested, and the value is larger when the acid is intercalated as opposed to simply being mixed with the layered material. For pure mefenamic acid, the most unfavourable pH is 1.2, while for the physical mixture and the sample with the intercalated drug, the most unfavourable pH is 4.5, for which the minimum increase in the amount of dissolved acid is recorded. The (dissolution) release-rate for both drugs when they are intercalated between the layers is much slower than for the physical mixture or for the dissolution of the pure drug.