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The conditions under which an exchanger phase will behave as an ideal mixture are established from thermodynamic principles. It is shown that, if a stoichiometric cation-exchange reaction is reversible, the exchanger phase will exhibit ideal behavior if the Vanselow selectivity coefficient is independent of the exchanger composition. This criterion is applied to some recently published data for Na+-trace metal cation exchange on Camp Berteau montmorillonite. An analysis of the data suggests that, so long as the exchange process is reversible, Na+-trace metal cation exchange produces an exchanger phase that behaves as an ideal mixture. ?????? ?? ????????????????? ?????????, ???? ??????????? ???????, ??? ??????? ???????? ???? ????? ???? ??? ????????? ?????. ????????, ??? ???? ????????????????? ??????? ?????????? ?????? ???????? ?????????, ???????? ???? ????????? ???????? ????????? ????? ??? ???????, ??? ??????????? ????????????? ???????? ???????? ??????????? ?? ????????? ???????. ???? ???????? ???????? ? ????????? ??????? ?????????????? ??????, ?????????? ?????????? ?????? ??????????? ???????? ? Na+? ??????????????? ???? ???????. ?????? ?????? ???????, ??? ?? ??? ??? ???? ??????? ???????? ?????????, ? ?????????? ?????????? ?????? Na+? ??????????? ????????? ?????????? ???????? ????, ??????? ????? ???? ??? ????????? ?????. Die Bedingungen, unter denen sich eine Austauschphase wie eine ideale Mischung verhält, werden aus thermodynamischen Prinzipien aufgestellt. Es wird gezeigt, daß, wenn eine stöchiometrische Kationenaustauschreaktion umkehrbar ist, die austauschphase ideales Verhalten zeigt, falls der Vanselow Selektivitätskoeffizient unabhängig von der Zusammensetzung der Austauschers ist. Dies Kriterium wird auf einige, vor kurzem veröffentlichte Daten für Na+-Spurenmetallkationen- austausch mit Camp Berteau Montmorillonit angewendet. Eine Analyse der Daten schlägt vor, daß solange wie der Austauschvorgang umkehrbar ist, Na+-Spurenmetallkationenaustausch eine Austauschphase produziert, die sich wie eine ideale Mischung verhält. Les conditions sous lesquelles une phase d’échange se comportera comme un mélange idéal sont établies à partir de principes thermodynamiques. Il peut être démontré que si une réaction stoichiométrique d’échange de cation est réversible, la phase d’échange montrera un comportement idéal si le coefficient de sélectivité Vanselow est indépendant de la composition de l’élément échangeur. Ce critère est appliqué à des données récemment publiées concernant l’échange de cation du métal traceur-Na+sur de la montmorillonite Camp Berteau. Une analyse des données suggère que, tant que la processus d’échange est réversible, l’échange de cation du métal traceur-Na+produit une phase d’échange se comportant comme un mélange idéal.

The surface microtopography of hematite over the course of dissolution in oxalic and citric acids was examined by in-situand ex-situatomic-force microscopy, In-situimaging of the basal-plane surface of a centimeter-scale natural hematite sample immersed in 2 mM citric acid demonstrated that the basal-plane surface was relatively unreactive; rather, dissolution occurred along step edges and via etch-pit formation. Ex-situimaging of synthetic hematite particles following batch dissolution in 1 mM oxalic acid showed similar dissolution features on basal-plane surfaces; in addition, etching along particle edges was apparent. The presence of etch features is consistent with a surface-controlled dissolution reaction. The results are in agreement with previous investigations suggesting that the basal-plane surface is relatively unreactive with respect to ligand exchange. Both in-situand ex-situimaging of particle surfaces can provide valuable information on the roles of surface structures and microtopographic features in mineral dissolution.

Monte Carlo (MC) simulations of molecular structure in the interlayers of 2:1 Na-saturated clay minerals were performed to address several important simulation methodological issues. Investigation was focused on monolayer hydrates of the clay minerals because these systems provide a severe test of the quality and sensitivity of MC interlayer simulations. Comparisons were made between two leading models of the water-water interaction in condensed phases, and the sensitivity of the simulations to the size or shape of the periodically-repeated simulation cell was determined. The results indicated that model potential functions permitting significant deviations from the molecular environment in bulk liquid water are superior to those calibrated to mimic the bulk water structure closely. Increasing the simulation cell size or altering its shape from a rectangular 21.12 Å × 18.28 Å × 6.54 Å cell (about eight clay mineral unit cells) had no significant effect on the calculated interlayer properties.

Monte Carlo (MC) simulations of interlayer molecular structure in monolayer hydrates of Nasaturated Wyoming-type montmorillonites and vermiculite were performed. Detailed comparison of the stimulation results with experimental diffraction and thermodynamic data for these clay-water systems indicated good semiquantitative to quantitative agreement. The MC simulations revealed that, in the monolayer hydrate, interlayer water molecules tend to increase their occupation of the midplane as layer charge increases. As the percentage of tetrahedral layer charge increases, water molecules are induced to interact with the siloxane surface O atoms through hydrogen bonding and Na+counter-ions are induced to form inner-sphere surface complexes. These results suggest the need for careful diffraction experiments on a series of monolayer hydrates of montmorillonite whose layer charge and tetrahedral isomorphic substitution charge vary systematically.

Specimen samples of Crook County montmorillonite and Silver Hill illite, purified and prepared in the Na-form, were imaged under 80% relative humidity using an atomic force microscope. The direct images showed clearly the hexagonal array of hexagonal rings of oxygen ions expected for the basal planes of 2:1 phyllosilicates. Fourier transformation of the digital information obtained by the microscope scanning tip led to an estimate of 5.1 ± 0.3 Å for the nearest-neighbor separation, in agreement with the ideal nearest-neighbor spacing of 5.4 Å for hexagonal rings as derived from X-ray powder diffraction data. The atomic force microscope should prove to be a useful tool for the molecular-scale resolution of clay mineral surfaces that contain adsorbed macromolecules.

The Nriagu polymer model of 2:1 layer type clay minerals develops from the premise that clay minerals are condensation copolymers of solid hydroxides. In the Mattigod-Sposito formulation of the model, standard state chemical potentials (standard Gibbs energies of formation from the elements) of 2:1 clay minerals are predicted quantitatively with a linear correlation equation relating the standard Gibbs energy of the polymerization reaction (ΔGor) to the half-cell layer charge of the clay mineral and to the valence and ionic radius of the exchangeable cation. It is now shown that this correlation equation can be derived from two basic assumptions: (1) that the standard Gibbs energy change for the transfer of a cation in a pure hydroxide solid to a hydroxide component in the tetrahedral or octahedral sheet of a 2:1 clay mineral is independent of the nature of the cation and (2) that the difference between ΔGorfor the polymerization reaction to form a 2:1 clay mineral and ΔGorfor the same reaction to form the zero layer-charge analog of the clay mineral is proportional to the number of interlayer exchangeable cations per unit cell of the clay mineral and to the radius of its exchangeable cation. Both of these assumptions can be tested experimentally, independent of the polymer model.

Infrared (IR) spectra in the fundamental and near-IR regions were obtained for Na-saturated Wyoming montmorillonite and reduced-charge Na/Li-saturated Wyoming montmorillonites hydrated under water vapor at 50% RH and dehydrated under vacuum. For the Na-montmorillonite, changes in the intensities of the structural OH-bending modes, particularly that of the MgAlOH group, were observed as the clay was dehydrated. This result was interpreted as evidence that exchangeable Na ions lose solvation water and settle into the ditrigonal cavities on the clay surface as it becomes desiccated. For the Na/Li-montmorillonites, the structural OH-bending modes also decreased in IR intensity because of Li+migration into the octahedral sheet. The fundamental IR spectra of D2O adsorbed by the montmorillonites showed characteristic absorptions at 2685, 2510, 2400, and 1205 cm-1that decreased in intensity proportionally to the cation-exchange capacity. This result, along with corroborating data from X-ray powder diffractograms and from near-IR diffuse reflectance spectra of H2O adsorbed by the clays, suggest that the exchangeable cations on Na-montmorillonite dissociated from the clay surface as it hydrated and played a significant role in organizing the structure of adsorbed water at low water contents. Infrarot (IR)-Spektren im Bereich der Grundschwingungen und im nahen Infrarot wurden für Na-gesättigten Montmorillonit von Wyoming und für Na/Li-gesättigte Montmorillonite von Wyoming mit reduzierter Ladung, die bei einem Wasserdampfdruck von 50% relativer Feuchtigkeit hydratisiert und im Vakuum dehydratisiert wurden, gefahren. Bei dem Na-Montmorillonit wurden Veränderungen bei den Intensitäten der strukturellen OH-Knickmoden, vor allem der MgAlOH-Gruppe, beobachtet, wenn der Ton dehydratisiert wurde. Dieses Ergebnis wurde als Beweis dafür interpretiert, daß austauschbare Na-Ionen ihr Solvatationswasser verlieren und sich in die ditrigonalen Hohlräume auf der Tonoberfläche anlagern, wenn sie getrocknet wird. Bei den Na/Li-Montmorilloniten nahmen die strukturellen OH-Knick-moden bei der IR-Intensität ebenfalls ab, da das Li+in die Oktaederschicht wandert. Die Spektren im Bereich der Grundschwingungen im IR von an den Montmorillonit absorbiertem D2O zeigen charakteristische Adsorptionsbanden bei 2685, 2510, 2400, und 1205 cm-1, deren Intensitäten proportional mit der Kationenaustauschkapazität abnehmen. Dieses Ergebnis zusammen mit den Ergebnissen aus den Rönt-genpulverdiffraktogrammen und den Ergebnissen aus den diffusen Reflexionsspektren im nahen IR von an Tonen adsorbiertem H2O deuten darauf hin, daß sich die austauschbaren Kationen am Na-Montmorillonit von der Tonoberfläche dissoziieren, wenn diese hydratisiert wird, und somit eine wichtige Rolle dabei spielen, welche Struktur des absorbierten Wassers sich bei niedrigen H2O-Gehalten bildet. [U.W.] La montmorillonite-Na du Wyoming et les montmorillonites-Na/Li de charge réduite ont été étudiées par spectroscopic infrarouge (IR) et proche infrarouge (PIR) à des teneurs en eau correspondant à des pressions relatives de vapeur d’eau de 0 et 50%. On observe pour la montmorillonite-Na des modifications dans les intensités des modes de vibration de déformation des OH de structure en particulier des groupes OH liés au couple Mg-Al lorsqu’on déshydrate l’argile. On interprète ce résultat comme la preuve que les ions Na+échangeables perdent leur eau d’hydratation et se logent dans les cavités ditrigonales à la surface de l’argile lorsqu’on déshydrate celle-ci. L’intensité des bandes de déformation des groupes OH diminue aussi dans le cas des montmorillonites-Na/Li parce que les ions Li+migrent dans les cavités oc-taédriques libres. Le spectre infrarouge de D2O adsorbée sur les montmorillonites est formé des bandes à 2685, 2510, 2400, et 1205 cm-1dont l’intensité diminue avec la capacité d’échange. Ce résultat, conforté par les mesures faites par diffraction des RX et par les spectres de réflexion diffuse dans le domaine proche infrarouge de l’eau adsorbée par les argiles, indique que les cations échangeables de la montmorillonite-Na sont dissociés de la surface de l’argile lorsqu’elle s’hydrate et qu’ils jouent un rôle déterminant dans l’organisation de l’eau adsorbée des sytèmes peu hydratés. Инфракрасные спектры (ИК) были получены в основной и ближней инфракрасной области для Na-насыщенного Уаеминского монтмориллонита и Nа/Li-насыщенных Уаеминских монтмориллонитов с уменьшенным зарядом, уводненных в присутствии водяного пара о 50% относительной влажности и отводненных в состоянии вакуума. Изменения интенсивностей изгибающих колебаний структурных групп ОН, особенно группы MgАlOН, наблюдались для Nа-монтмориллонита после дегидратации глины. Этот результат рассматривался как доказательство того, что обменные ионы Na теряют сольватационную воду и осаждаются в двухтригональных кавитациях на поверхности глины, когда она осушается. Для Nа/Li-монт-мориллонитов, интенсивность изгибающих колебаний структурных ОН также уменьшалась из-за миграции Li+ в октаэдрический слой. Основные ИК спектры D2О, адсорбированний монтмориллонитом, показали характеристическую абсорбцию при 2685, 2510, 2400, и 1205 см-1, интенсивность которой уменьшалась пропорционально катионообменной способности. Этот результат, вместе с подтверждающими данными рентгеновских порошковых диффрактограмм и данными спектров (диффузионного отражения ближней ИК области) воды адсорбированной глинами указывают на то, что обменные катионы из Na-монтмориллонита были диссоцированны с поверхности глины в то время, как она уводнялась. Эти катионы играли значительную роль в организовании структуры адсорбированной воды при низких ее содержаниях. [Е.С.]

A combined application of electron paramagnetic resonance (EPR) and Fourier-transform infrared (FTIR) spectroscopies with thermal methods was used to determine the chemical form of V(IV) in a Georgia kaolinite (KGa-I). Precise values of the EPR spectroscopic g-values and hyperfine coupling constants were obtained for an untreated sample (g∥= 1.940, A∦= 18.71 mT; g⊥= 1.966, A⊥= 7.63 mT). Heating the sample to 1000°C in steps while monitoring phase changes with EPR and FTIR spectra led to the following structural interpretations: 1) Vanadium (IV) occurs almost entirely as an isomorphically substituted species in the octahedral sheet of KGa-1 kaolinite; 2) during the dehydroxylation of kaolinite at about 500°C and the subsequent formation of metakaolinite, V(IV)-substituted octahedral sites are readily converted into truncated octahedra exhibiting fourfold coordination; and 3) in these highly distorted four-coordinated sites, V(IV) is metastable, being completely oxidized at about 800°C.

The adsorption of citrate, at 10 4−10−3M initial concentration, by pseudoboehmite suspended in 0.02 M NaC1O4was investigated at varying pH. Citrate shows a strong affinity for the pseudoboehmite surface, as seen in the adsorption isotherm at pH 6. Adsorption envelopes of adsorbed citrate vs. pH for a given initial citrate concentration are characteristic for the adsorption of a polyprotic acid by a variable-charge mineral. The envelope data were fit well by the Constant Capacitance Model assuming a ligand-exchange adsorption mechanism, three monodentate surface species of citrate, and a reactive surface OH density of 0.4 mol kg−1. Aqueous speciation calculations suggest that solubility equilibrium with pseudoboehmite was attained at pH > 9 and that particulate or polymeric Al may have existed at 6 < pH < 9. Dissolved Al appeared to reduce the adsorption of citrate at pH < 5.5 via solution complexation reactions.

Hydroxy-Al-montmorillonite was prepared by mixing Na-Wyoming montmorillonite with Al(OH)2.5polycations and aging the mixture for 10 days. The reactions of this complex with citric acid at millimolar concentrations were investigated at different pHs for a 4-hr reaction time. The citrate sorption data, X-ray powder diffraction patterns of the montmorillonite adsorbent, and solubility products for Al(OH)3, estimated after computer speciation of the aqueous solution phase, suggested two reaction mechanisms, depending on pH. At 4 < pH < 5.5, the complex was stable, and only external hydroxy-Al polycations could react with citrate, yielding a sorption envelope characteristic of an acid having a low pKa. At higher pH, and particularly at pH > 7, the interlayer Al-polymers became accessible to the ligand and desorbed quickly from the montmorillonite basal planes to form a new, external, X-ray-amorphous solid phase with citrate. This reaction produced a large second peak in the sorption envelopes near pH 8, which was controlled by the increasingly negative surface charge at high pH and by the initial concentration of citric acid. This ligand accelerated, i.e., favored kinetically, the dissociation of adsorbed hydroxy-Al polymers from montmorillonite at pH > 7. On prépare une smectite hydroxyalumineuse en mélangeant et en laissant en suspension 10 jours une montmorillonite du Wyoming Na et des polycations Al(OH)2.5. Les réactions de ce complexe avec l’acide citrique en concentration millimolaire et à différent pH sont analysées à partir des quantités de citrate adsorbé, de la diffraction X appliquée à des dépôts orientés de l’argile ayant réagi, du calcul du produit de solubilité Al(OH)3et des concentrations des diverses espèces en solution. Deux mécanismes sont invoqués selon les domaines de pH. A 4 < pH < 5.5, le complexe est stable et seuls les polymères d’Al des surfaces externes peuvent réagir avec le citrate, conduisant à une fixation de citrate faible, caractérisée par des courbes en fonction du pH avec un maximum apparent. A pH plus élevé et surtout à pH >7, tous les polymères d’Al deviennent accessibles au ligand. et sont rapidement détachés des surfaces basales pour former avec le citrate une nouvelle phase amorphe en dehors des feuillets de l’argile. Cette dernière réaction se traduit par un deuxième maximum plus important dans les courbes enveloppes, vers pH 8, dont l’amplitude est limitée par l’effet de l’augmentation des charges négatives à pH élevé, et par la quantité initiale de citrate. Cet anion accélère la dissociation des polymères d’Al et de la montmorillonite à pH > 7.

The vibrational modes of clay minerals in aqueous suspension are uniquely accessible to Raman spectroscopy, but this potentially powerful approach has not been applied heretofore to study clays in aqueous samples. In this paper, Raman spectra in the 100- to 4000-cm−1region were obtained for kaolinite in aqueous suspension and in air-dry samples. Contact with water perturbed the low-wavenumber Raman spectrum (100 to 1000 cm−1) significantly with respect to relative band intensities and resulted in a pH dependence of the integrated intensity in the OH-stretching region. Comparison of the Raman and infrared (IR) spectra of air-dry kaolinite samples confirmed five Raman-active OH-stretching modes at 3621, 3652, 3668, 3688, and 3696 cm−1, in contrast to four IR-active modes at 3621, 3652, 3668, and 3695 cm−1. The Raman spectra of two kaolinites of different origin showed differences in band positions and intensities. These results suggest that Raman spectroscopy may provide a useful method to study clay mineral-water interactions, colloidal behavior in clay suspensions, and variations in clay mineral structure.