Structure and dynamics of water on muscovite mica surfaces     

Hiroshi Sakuma  Katsuyuki Kawamura 《Geochimica et cosmochimica acta》2009,73(14):4100-16115

The structure and dynamics of water on muscovite mica (0 0 1) surfaces have been investigated by molecular dynamics simulations. X-ray reflectivity profiles highly reflecting the interfacial structure are directly calculated and compared with those of experiments. The direct comparison has validated the usefulness of MD simulations to understand the real interfacial structure of the mica−water system. We observed five distinguished peaks in the density profile of oxygen present in water, and these peaks are attributable to the water molecules directly adsorbed on mica, hydrated to the K⁺ ions on the mica surface, and ordered due to hydrogen bonds between hydrated K⁺ ions. The hydrated K⁺ ions make an inner-sphere complex and have an explicit first hydration shell with a radius of 3.6 Å and a hydration number of 2.9. The change of the viscosity of water located above 1 nm apart from the mica surface was not observed. This feature is in good agreement with a recent experimental study in which the shear measurement was conducted using a surface forces apparatus. The increase of the viscosity by a factor of ca. 2-3 relative to that of the bulk water was observed at water located within 1 nm from the isolated mica surface.  相似文献   

Animated molecular dynamics simulations of hydrated caesium-smectite interlayers     

Rebecca Sutton  Garrison Sposito 《Geochemical transactions》2002,3(1):73

Computer animation of center of mass coordinates obtained from 800 ps molecular dynamics simulations of Cs-smectite hydrates (1/3 and 2/3 water monolayers) provided information concerning the structure and dynamics of the interlayer region that could not be obtained through traditional simulation analysis methods. Cs⁺ formed inner sphere complexes with the mineral surface, and could be seen to jump from one attracting location near a layer charge site to the next, while water molecules were observed to migrate from the hydration shell of one ion to that of another. Neighboring ions maintained a partial hydration shell by sharing water molecules, such that a single water molecule hydrated two ions simultaneously for hundreds of picoseconds. Cs-montmorillonite hydrates featured the largest extent of this sharing interaction, because interlayer ions were able to inhabit positions near surface cavities as well as at their edges, close to oxygen triads. The greater positional freedom of Cs⁺ within the montmorillonite interlayer, a result of structural hydroxyl orientation and low tetrahedral charge, promoted the optimization of distances between cations and water molecules required for water sharing. Preference of Cs⁺ for locations near oxygen triads was observed within interlayer beidellite and hectorite. Water molecules also could be seen to interact directly with the mineral surface, entering its surface cavities to approach attracting charge sites and structural hydroxyls. With increasing water content, water molecules exhibited increased frequency and duration of both cavity habitation and water sharing interactions. Competition between Cs⁺ and water molecules for surface sites was evident. These important cooperative and competitive features of interlayer molecular behavior were uniquely revealed by animation of an otherwise highly complex simulation output.  相似文献   

Effects of substrate structure and composition on the structure, dynamics, and energetics of water at mineral surfaces: A molecular dynamics modeling study     

Jianwei Wang  Andrey G. Kalinichev  R. James Kirkpatrick 《Geochimica et cosmochimica acta》2006,70(3):562-582

Molecular dynamics computer simulations of the molecular structure, diffusive dynamics and hydration energetics of water adsorbed on (0 0 1) surfaces of brucite Mg(OH)₂, gibbsite Al(OH)₃, hydrotalcite Mg₂Al(OH)₆Cl · 2H₂O, muscovite KAl₂(Si₃Al)O₁₀(OH)₂, and talc Mg₃Si₄O₁₀(OH)₂ provide new insight into the relationships between the substrate structure and composition and the molecular-scale structure and properties of the interfacial water. For the three hydroxide phases studied here, the differences in the structural charge on the octahedral sheet, cation occupancies and distributions, and the orientations of OH groups all affect the surface water structure. The density profiles of water molecules perpendicular to the surface are very similar, due to the prevalent importance of H-bonding between the surface and the water and to their similar layered crystal structures. However, the predominant orientations of the surface water molecules and the detailed two-dimensional near-surface structure are quite different. The atomic density profiles and other structural characteristics of water at the two sheet silicate surfaces are very different, because the talc (0 0 1) surface is hydrophobic whereas the muscovite (0 0 1) surface is hydrophilic. At the hydrophilic and electrostatically neutral brucite and gibbsite (0 0 1) surfaces, both donating and accepting H-bonds from the H₂O molecules are important for the development of a continuous hydrogen bonding network across the interfacial region. For the hydrophilic but charged hydrotalcite and muscovite (0 0 1) surfaces, only accepting or donating H-bonds from the water molecules contribute to the formation of the H-bonding network at the negatively and positively charged interfaces, respectively. For the hydrophobic talc (0 0 1) surface, H-bonds between water molecules and the surface sites are very weak, and the H-bonds among H₂O molecules dominate the interfacial H-bonding network. For all the systems studied, the orientation of the interfacial water molecules in the first few layers is influenced by both the substrate surface charge and the ability by the surfaces to facilitate H-bond formation. The first layer of water molecules at all surfaces is well ordered in the xy plane (parallel to the surface) and the atomic density distributions reflect the substrate crystal structure. The enhanced ordering of water molecules at the interfaces indicates reduced orientational and translational entropy. In thin films, water molecules are more mobile parallel to the surface than perpendicular to it due to spatial constraints. At neutral, hydrophilic substrates, single-monolayer surface coverage stabilizes the adsorbed water molecules and results in a minimum of the surface hydration energy. In contrast, at the charged and hydrophilic muscovite surface, the hydration energy increases monotonically with increasing water coverage over the range of coverages studied. At the neutral and hydrophobic talc surface, the adsorption of H₂O is unfavorable at all surface coverages, and the hydration energy decreases monotonically with increasing coverage.  相似文献   

Sorption of Cs to micaceous subsurface sediments from the Hanford site, USA   总被引：1，自引：0，他引：1  

John M Zachara  Steven C SmithChongxuan Liu  James P McKinleyR.Jeffrey Serne  Paul L Gassman 《Geochimica et cosmochimica acta》2002,66(2):193-211

The sorption of Cs⁺ was investigated over a large concentration range (10⁻⁹−10⁻² mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO₃ brine) is the carrier. Cs⁺ sorption was measured on homoionic sediments (Na⁺, K⁺, Ca²⁺) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na⁺ electrolyte, concentrations were extended to near saturation with NaNO_3(s) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A four-parameter, two-site (high- and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs⁺ for both high- and low-affinity sites according to the trend K⁺ >> Na⁺ ≥ Ca²⁺. At high salt concentration, Cs⁺ adsorption occurred only on high-affinity sites. Na⁺ was an effective competitor for the high-affinity sites at high salt concentrations. In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs⁺_(aq), and analyzed by electron microprobe to identify phases and features important to Cs⁺ sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na⁺ in HLW and indigenous K⁺ displaced from the sediments may act to expedite the migration of strongly sorbing Cs⁺ in subsurface environments.  相似文献   

Distribution of barium and fulvic acid at the mica-solution interface using in-situ X-ray reflectivity     

Sang Soo Lee  Kathryn L. Nagy 《Geochimica et cosmochimica acta》2007,71(23):5763-5781

The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10⁻³ m BaCl₂, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10⁻³ m BaCl₂ and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl₂ shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba²⁺ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba²⁺ ions, respectively. This pattern indicates that most Ba²⁺ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba²⁺ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba²⁺, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl₂-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl₂ solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba²⁺ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba²⁺ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl₂, more Ba²⁺ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba²⁺ ions adsorbed directly on the mica in an amount similar to that in the BaCl₂ solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba²⁺ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface.  相似文献   

Multicomponent diffusion of a suite of tracers (HTO, Cl, Br, I, Na, Sr, Cs) in a single sample of Opalinus Clay   总被引：1，自引：0，他引：1  

C.A.J. Appelo  L.R. Van Loon 《Geochimica et cosmochimica acta》2010,74(4):1201-57

Diffusion experiments with HTO, ³⁶Cl⁻, Br⁻, I⁻, ²²Na⁺, ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ at trace concentrations in a single sample of Opalinus Clay are modeled with PHREEQC’s multicomponent diffusion module. The model is used first in a classical approach to derive accessible porosities, geometrical factors (the ratio of pore tortuosity and constrictivity) and sorption behavior of the individual tracers assuming that the clay is homogeneous. The accessible porosity for neutral species and cations is obtained from HTO, the anion exclusion volume from ³⁶Cl⁻ and Br⁻, and the cation exchange capacity from ²²Na⁺. The homogeneous model works well for tritium, the anions and ²²Na⁺. However, the ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ experiments show an early arrival of the tracer and a front-form that suggest a dual porosity structure. A model with 10% dead-end pores, containing 19% of the total exchange capacity, can satisfactorily calculate all the experimental data. The Cs⁺ diffusion model builds on a 3-site exchange model, constructed from batch sorption data. The excellent agreement of modeled and measured data contradicts earlier reports that the exchange capacity for Cs⁺ would be smaller in diffusion than in batch experiments.The geometrical factors for the anions are 1.5 times larger than for HTO, and for the cations 2-4 times smaller than for HTO. The different behavior is explained by a tripartite division of the porespace in free porewater, diffuse double layer (DDL) water, and interlayer water in montmorillonite. Differences between estimated and observed geometrical factors for cations are attributed to increased ion-pairing of the divalent cations in DDL water as a result of the low relative dielectric permittivity. Interlayer and/or surface diffusion contributes significantly to the diffusive flux of Cs⁺ but is negligible for the other solutes. The geometrical factors for anions are higher than estimated, because pore constrictions with overlapping double layers force the anions to take longer routes than HTO and the cations. Small differences among the anions can also be attributed to different ion-pairing in DDL water.  相似文献   

Studies on the solid acidity of heated and cation-exchanged montmorillonite using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy     

Hongmei Liu  Dong Liu  Peng Yuan  Daoyong Tan  Jingong Cai  Hongping He  Jianxi Zhu  Zhiguang Song 《Physics and Chemistry of Minerals》2013,40(6):479-489

The effects of heating and cation exchange on the solid acidity of montmorillonite were investigated using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared spectroscopy. The number of total, Brønsted, and Lewis acid sites showed the same modulation tendency with increasing heating temperature, reaching a maximum at 120 °C and subsequently decreasing until it reaches a minimum at 600 °C. The Lewis acid sites result from unsaturated Al³⁺ cations, and their number increased with the heating temperature due to the dehydration and dehydroxylation of montmorillonite. The generation and evolution of Brønsted acidity were mainly related to interlayer-polarized water molecules. Water adsorbed on the unsaturated Al³⁺ ions also acted as a Brønsted acid. The acid strength of the Brønsted acid sites was dependent on the polarization ability of the exchangeable cation, the amount of interlayer water, and the degree of dissociation of the interlayer water coordinated to exchangeable cations. All cation-exchanged montmorillonites exhibited different numbers of acid sites and various distributions of acid strength. Brønsted acidity was predominant in Al³⁺-exchanged montmorillonite, whereas the Na⁺- and K⁺-exchanged montmorillonites showed predominantly Lewis acidity. Moreover, Mg²⁺- and Li⁺-exchanged montmorillonites exhibited approximately equal numbers of Brønsted and Lewis acid sites. The Brønsted acidity of cation-exchanged montmorillonite was positively correlated with the charge-to-radius ratios of the cations, whereas the Lewis acidity was highly dependent on the electronegativity of the cations. The acid strengths of Al³⁺- and Mg²⁺-exchanged montmorillonites were remarkably higher than those of monovalent cation-exchanged montmorillonites, showing the highest acid strength (H ₀ ≤ ?3.0). Li⁺- and Na⁺-exchanged montmorillonites exhibited an acid strength distribution of ?3.0 < H ₀ ≤ 4.8, with the acid strength ranging primarily from 1.5 to 3.3 in Li⁺-exchanged montmorillonite, whereas only weaker-strength acid sites (1.5 < H ₀ ≤ 4.8) were present in K⁺-exchanged montmorillonite. The results of the catalysis experiments indicated that montmorillonite promoted the thermal decomposition of the model organic. The catalytic activity showed a positive correlation with the solid acidity of montmorillonite and was affected by cation exchange, which occurs naturally in geological processes.  相似文献   

Forces and ionic transport between mica surfaces: implications for pressure solution     

Norma Alcantar  Jim Boles 《Geochimica et cosmochimica acta》2003,67(7):1289-1304

Using a surface forces apparatus (SFA), we have studied the interactions between mica surfaces in pure and mixed NaCl and CaCl₂ solutions at relevant geological conditions of pressure and electrolyte composition. Our results show that the short-range (0-50 Å) colloidal forces, including attractive van der Waals and ion-correlation forces, repulsive electrostatic forces, and oscillatory or monotonically repulsive hydration (surface-induced water structure-dependent) forces are involved in different stages of pressure solution. These forces depend on the type (Na⁺, Ca²⁺, and H⁺) and concentration (6-600 mM) of the cations present in the solution. Equilibrium water film thicknesses were measured as a function of the applied (normal) pressure up to 50 MPa (500 atm) and ranged from 30 to 0 Å at pressures above 10 MPa (100 atm). Measurements were also made of the rates of diffusion and exchange of ions into and out of such ultrathin films, and on the nucleation and growth of ionic crystallites on and between the surfaces, which occurred only in the presence of calcium ions. Diffusion of ions into and out of structured water films as thin as one to five water molecules (3 to 15 Å) were found to be surprisingly rapid and never less than two orders of magnitude below the diffusion in bulk water. In contrast, the rates of binding and exchange of ions to the surfaces were found to be the rate-limiting steps to adsorption and crystal formation. These findings imply that, for certain systems or conditions, pressure solution rates could be limited by surface reactions rather than by ion diffusion in thin fluid films.  相似文献   

Water and ions in clays: Unraveling the interlayer/micropore exchange using molecular dynamics     

Benjamin Rotenberg  Virginie Marry  Natalie Malikova  Pierre Turq 《Geochimica et cosmochimica acta》2007,71(21):5089-5101

  相似文献   

Desorption kinetics of radiocesium from subsurface sediments at Hanford Site, USA     

Chongxuan Liu  John M. Zachara  James P. McKinley 《Geochimica et cosmochimica acta》2003,67(16):2893-2912

The desorption of ¹³⁷Cs⁺ was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline ¹³⁷Cs⁺-containing high level nuclear wastes (HLW, 2 × 10⁶ to 6 × 10⁷ pCi ¹³⁷Cs⁺/g) were studied. The desorption of ¹³⁷Cs⁺ was measured in Na⁺, K⁺, Rb⁺, and NH₄⁺electrolytes of variable concentration and pH, and in presence of a strong Cs⁺-specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). ¹³⁷Cs⁺ desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of ¹³⁷Cs⁺ desorption increased with increasing electrolyte concentration and followed a trend of Rb⁺ ≥ K⁺ > Na⁺ at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of ¹³⁷Cs⁺ desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K⁺, Rb⁺, or NH₄⁺. Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs⁺ sorbents. The removal of these precipitates by acidified ammonium oxalate extraction significantly increased the long-term desorption rate and extent. X-ray microprobe analyses of Cs⁺-sorbed micas showed that the ¹³⁷Cs⁺ distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of ¹³⁷Cs⁺. Controlled desorption experiments using Cs⁺-spiked pristine sediment indicated that the ¹³⁷Cs⁺ diffusion rate was fast in Na⁺-electrolyte, but much slower in the presence of K⁺ or Rb⁺, suggesting an effect of edge-interlayer collapse. An intraparticle diffusion model coupled with a two-site cation exchange model was used to interpret the experimental results. Model simulations suggested that about 40% of total sorbed ¹³⁷Cs⁺ was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60-80%. The remainder of the sorbed ¹³⁷Cs⁺ was fixed or desorbed at much slower rate than our experiments could detect.  相似文献   

Theoretical modeling of the Al paramagnetic center and its precursors in stishovite     

Sanda M. Botis  Yuanming Pan 《Physics and Chemistry of Minerals》2010,37(2):119-127

Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stishovite at 77 K detected an Al hole center, which was proposed to be an [O₂ ³⁻–Al³⁺] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six oxygen atoms at an AlO₆ octahedron, while the calculated ²⁷Al hyperfine constants are similar to those determined by EPR experiments. Theoretical results allow us to propose the Al center to represent an [AlO₆]⁰ defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that the diamagnetic precursors [AlO₆/H⁺]⁰, [AlO₆/Li⁺]⁰, and [AlO₆/Na⁺]⁰ are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO₆/Li⁺]⁰ and [AlO₆/Na⁺]⁰ defects suggest that the monovalent cations such as Li⁺ and Na⁺ are potentially important in accommodating Al in stishovite in the lower mantle.  相似文献   

Geochemistry and quality of groundwater of Gummanampadu sub-basin, Guntur District, Andhra Pradesh, India   总被引：3，自引：3，他引：0  

N. Subba Rao  A. Subrahmanyam  S. Ravi Kumar  N. Srinivasulu  G. Babu Rao  P. Surya Rao  G. Venkatram Reddy 《Environmental Earth Sciences》2012,67(5):1451-1471

Groundwater survey has been carried out in the area of Gummanampadu sub-basin located in Guntur District, Andhra Pradesh, India for assessing the factors that are responsible for changing of groundwater chemistry and consequent deterioration of groundwater quality, where the groundwater is a prime source for drinking and irrigation due to non-availability of surface water in time. The area is underlain by the Archaean Gneissic Complex, over which the Proterozoic Cumbhum rocks occur. The results of the plotting of Ca²⁺ + Mg²⁺ versus HCO₃ ^? + CO₃ ^2?, Ca²⁺ + Mg²⁺ versus total cations, Na⁺ + K⁺ versus total cations, Cl^? + SO₄ ^2? versus Na⁺ + K⁺, Na⁺ versus Cl^?, Na⁺ versus HCO₃ ^? + CO₃ ^2?, Na⁺ versus Ca²⁺ and Na⁺: Cl^? versus EC indicate that the rock–water interaction under alkaline condition is the main mechanism in activating mineral dissociation and dissolution, causing the release of Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃ ^?, CO₃ ^2?, SO₄ ^2? and F^? ions into the groundwater. The ionic relations also suggest that the higher concentrations of Na⁺ and Cl^? ions are the results of ion exchange and evaporation. The influences of anthropogenic sources are the other cause for increasing of Mg²⁺, Na⁺, Cl^?, SO₄ ^2? and NO₃ ^? ions. Further, the excess alkaline condition in water accelerates more effective dissolution of F^?-bearing minerals. Moreover, the chemical data plotted in the Piper’s, Gibbs’s and Langelier–Ludwig’s diagrams, computed for the chloro-alkaline and saturation indices, and analyzed in the principal component analysis, support the above hypothesis. The groundwater quality is, thus, characterized by Na⁺ > Ca²⁺ > Mg²⁺ > K⁺: HCO₃ ^? + CO₃ ^2? > Cl^? > SO₄ ^2? > NO₃ ^? > F^? facies. On the other hand, majority of groundwater samples are not suitable for drinking with reference to the concentrations of TDS, TH, Mg²⁺ and F^?, while those are not good for irrigation with respect to USSL’s and Wilcox’s diagrams, residual sodium carbonate, and magnesium hazard, but they are safe for irrigation with respect to permeability index. Thus, the study recommends suitable management measures to improve health conditions as well as to increase agricultural output.  相似文献   

The coprecipitation of strontium,magnesium, sodium,potassium and chloride ions with gypsum. An experimental study     

Jacob Kushnir 《Geochimica et cosmochimica acta》1980,44(10):1471-1482

The coprecipitation of Sr²⁺, Mg²⁺, Na⁺, K⁺ and Cl^? into gypsum was studied as a function of temperature, brine concentration and growth rate. The concentrations of the studied cations in the gypsum increase with growth rate (kinetic effect), with a tendency to reach a limiting value at high growth rates. The partition coefficients of Sr tend to increase with brine concentration and decrease with temperature. The partition coefficients of the other cations also decrease with temperature but depend only very slightly on brine concentration. The concentrations of coprecipitated chloride are negligibly small.The coprecipitation behavior is explained in terms of the relation between the rate of desorption of the coprecipitating ions from the surface of the growing crystal, and the rate of growth. The studied cations may substitute for Ca²⁺ in its normal lattice sites and/or reside in interstitial positions among the structural water molecules. The relative amount of foreign cations occupying interstitial positions increases with increasing growth rate.The elucidation of the behavior of coprecipitated ions in gypsum given here forms a basis for the utilization of these ions as geochemical indicators for the environment of deposition of gypsum. These indicators may help in reconstructing important parameters such as temperature, brine concentration and growth rate.  相似文献   

Adsorption of organic matter at mineral/water interfaces: I. ATR-FTIR spectroscopic and quantum chemical study of oxalate adsorbed at boehmite/water and corundum/water interfaces     

Tae Hyun Yoon  Stephen B. Johnson  Gordon E. Brown Jr. 《Geochimica et cosmochimica acta》2004,68(22):4505-4518

The types and structures of adsorption complexes formed by oxalate at boehmite (γ-AlOOH)/water and corundum (α-Al₂O₃)/water interfaces were determined using in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy and quantum chemical simulation methods. At pH 5.1, at least four different oxalate species were found at or near the boehmite/water interface for oxalate surface coverages (Γ_ox) ranging from 0.25 to 16.44 μmol/m². At relatively low coverages (Γ_ox < 2.47), strongly adsorbed inner-sphere oxalate species (IR peaks at 1286, 1418, 1700, and 1720 cm⁻¹) replace weakly adsorbed carbonate species, and a small proportion of oxalate anions are adsorbed in an outer-sphere mode (IR peaks at 1314 and 1591 cm⁻¹). IR peaks indicative of inner-sphere adsorbed oxalate are also observed for oxalate at the corundum/water interface at Γ_ox = 1.4 μmol/m². With increasing oxalate concentration (Γ_ox > 2.47 μmol/m²), the boehmite surface binding sites for inner-sphere adsorbed oxalate become saturated, and excess oxalate ions are present dominantly as aqueous species (IR peaks at 1309 and 1571 cm⁻¹). In addition to these adsorption processes, oxalate-promoted dissolution of boehmite following inner-sphere oxalate adsorption becomes increasingly pronounced with increasing Γ_ox and results in an aqueous Al(III)-oxalate species, as indicated by shifted IR peaks (1286 → 1297 cm⁻¹ and 1418 → 1408 cm⁻¹). At pH 2.5, no outer-sphere adsorbed oxalate or aqueous oxalate species were observed. The similarity of adsorbed oxalate spectral features at pH 2.5 and 5.1 implies that the adsorption mechanism of aqueous HOx⁻ species involves loss of protons from this species during the ligand-exchange reaction. As a consequence, adsorbed inner-sphere oxalate and aqueous Al(III)-oxalate complexes formed at pH 2.5 have coordination geometries very similar to those formed at pH 5.1.The coordination geometry of inner-sphere adsorbed oxalate species was also predicted using quantum chemical geometry optimization and IR vibrational frequency calculations. Geometry-optimized Al₈O₁₂ and Al₁₄O₂₂ clusters with the reactive surface Al site coordinated by three oxygens were used as model substrates for corundum and boehmite surfaces. Among the models considered, calculated IR frequencies based on a bidentate side-on structure with a 5-membered ring agree best with the observed frequencies for boehmite/oxalate/water samples at Γ_ox = 0.25 to 16.44 μmol/m² and pH 2.5 and 5.1, and for a corundum/oxalate/water sample at Γ_ox = 1.4 μmol/m² and pH 5.1. Based on these results, we suggest that oxalate bonding on boehmite and corundum surfaces results in 5-coordinated rather than 4- or 6-coordinated Al surface sites.  相似文献   

Prediction of surface charge on oxides in salt solutions: Revisions for 1:1 (ML) electrolytes     

Dimitri A. Sverjensky 《Geochimica et cosmochimica acta》2005,69(2):225-257

Quantitative characterization of the development of proton surface charge on the surfaces of minerals is necessary for a fundamental understanding of reactions between minerals and aqueous electrolyte solutions. Despite many experimental studies of charge development, few attempts have been made to integrate the results of such studies with a theoretical framework that permits prediction. The present study builds on a theoretical framework to analyze a total of 55 sets of proton surface charge data referring to wide ranges of ionic strengths, and types of electrolyte and oxide. The resulting parameters were interpreted with the aid of crystal chemical, electrostatic, and thermodynamic theory, which enable a number of generalizations. Prediction of values of the pHZPC and ΔpKnθ reduces the number of triple-layer parameters to be estimated. New standard states for the equilibrium constants for electrolyte adsorption (KM+θ and KL−θ) permit direct comparison of samples with a range of surface areas or site densities. Predicted cation binding on high dielectric constant solids (e.g., rutile) shows KM+θ, increasing in the sequence Cs+, Rb+, K+, Na+, Li+. In contrast, on low dielectric constant solids (e.g., amorphous silica), the predicted sequence is Li+, Na+, K+, Rb+, Cs+. The opposite sequences are attributable to the large solvation energy contribution opposing adsorption on low-dielectric constant solids. Cation and anion binding constants are in general different, which enables direct prediction of the point-of-zero-salt effect (pHPZSE) relative to the pristine point-of-zero charge. The inner and outer capacitances in the triple-layer model (C1 and C2) are predictable parameters consistent with physically reasonable distances and interfacial dielectric constants for water. In summary, all the parameters in the triple-layer model can be estimated with the revised equations of this study, which enables prediction of proton surface charge for any oxide in 1:1 electrolyte solutions independent of experiments. Such predictions can serve as a complement to the experimental study of new oxide/electrolyte systems, or more complex systems, where additional mechanisms of charge development are likely.  相似文献   

Effects of dry density and exchangeable cations on the diffusion process of sodium ions in compacted montmorillonite     

Tamotsu Kozaki  Seichi Sato 《Engineering Geology》2005,81(3):246-254

As a part of the safety assessment of the geological disposal of high-level radioactive waste, the effects of dry density and exchangeable cations on the diffusion process of Na⁺ ions in compacted bentonite were studied from the viewpoint of the activation energy for diffusion. The apparent self-diffusion coefficients of Na⁺ ions in compacted Na-montmorillonite and in a Na- and Ca-montmorillonite mixture were determined by one-dimensional, non-steady diffusion experiments at different temperatures and dry densities. A unique change in activation energy as a function of dry density was found for the Na⁺ ions in compacted Na-montmorillonite. The activation energy suddenly decreased from 18.1 to 14.1 kJ mol^− 1 as the dry density increased from 0.9 to 1.0 Mg m^− 3, whereas it increased to 24.7 kJ mol^− 1 as the dry density increased to 1.8 Mg m^− 3. Examination of the effect of exchangeable cations on the activation energies determined that the activation energies were almost constant, approximately 25 kJ mol^− 1, for the montmorillonite specimens at a dry density of 1.8 Mg m^− 3. However, three different activation energy values were obtained at a dry density of 1.0 Mg m^− 3. These findings cannot be explained by the conventional diffusion model (the pore water diffusion model), which suggests that the predominant diffusion process alternates among pore water diffusion, interlayer diffusion, and external surface diffusion.  相似文献   

Reach-Scale Cation Exchange Controls on Major Ion Chemistry of an Antarctic Glacial Meltwater Stream     

Michael N. Gooseff  Diane M. Mcknight  Robert L. Runkel 《Aquatic Geochemistry》2004,10(3-4):221-238

McMurdo dry valleys of Antarctica represent the largest of the ice-free areas on the Antarctic continent, containing glaciers, meltwater streams, and closed basin lakes. Previous geochemical studies of dry valley streams and lakes have addressed chemical weathering reactions of hyporheic substrate and geochemical evolution of dry valley surface waters. We examine cation transport and exchange reactions during a stream tracer experiment in a dry valley glacial meltwater stream. The injection solution was composed of dissolved Li⁺, Na⁺, K⁺, and Cl^-. Chloride behaved conservatively in this stream, but Li⁺, Na⁺, and K⁺ were reactive to varying degrees. Mass balance analysis indicates that relative to Cl^-, Li⁺ and K⁺ were taken up in downstream transport and Na⁺ was released. Simulations of conservative and reactive (first-order uptake or generation) solute transport were made with the OTIS (one-dimensional solute transport with inflow and storage) model. Among the four experimental reaches of Green Creek, solute transport simulations reveal that Li⁺ was removed from stream water in all four reaches, K⁺ was released in two reaches, taken up in one reach, and Na⁺ was released in all four reaches. Hyporheic sediments appear to be variable with uptake of Li⁺ in two reaches, uptake of K⁺ in one reach, release of K⁺ in two reaches, and uptake of Na⁺ in one reach. Mass balances of the conservative and reactive simulations show that from 1.05 to 2.19 moles of Li⁺ was adsorbed per reach, but less than 0.3 moles of K⁺ and less than 0.9 moles of Na⁺ were released per reach. This suggests that either (1) exchange of another ion which was not analyzed in this experiment or (2) that both ion exchange and sorption control inorganic solute transport. The elevated cation concentrations introduced during the experiment are typical of initial flows in each flow season, which flush accumulated dry salts from the streambed. We propose that the bed sediments (which compose the hyporheic zone) modulate the flushing of these salts during initial flows each season, due to ion exchange and sorption reactions.  相似文献   

Chemical characteristics of coastal rainwater from Puducherry to Neithavasal,Southeastern coast of India     

S. Chidambaram  P. Paramaguru  M. V. Prasanna  U. Karmegam  S. Manikandan 《Environmental Earth Sciences》2014,72(2):557-567

The chemistry of the rainwater indirectly reflects the composition of the ions in the atmosphere. The study of the rainwater gains its own importance as it forms the basis for the agricultural, domestic and drinking water. Twelve rainwater samples were collected along the southeastern coast of India during southwest monsoon. The samples were analyzed for the major anions (Cl^?, SO₄ ^2?, PO₄ ^3? and HCO₃ ^?) and cations (Na⁺, K⁺, Ca²⁺ and Mg²⁺). The majority of the samples reflect acidic pH. The general dominance of the cations is in the order of Na⁺ > Ca²⁺ > K⁺ > Mg²⁺ and that of anions is HCO₃ ^? > Cl^? > SO₄ ^2? > PO₄ ^3?. The water is classified as calcium bicarbonate to sodium bicarbonate type. The decrease of pH value also increases the pCO₂. In order to study the impact of acidic and alkaline species on rainwater, correlation coefficients were determined for establishing the relationship between different ions. Good correlation was established between cations, and sulfate has no correlation with other ions and pH. Factor analysis reveals that land use pattern, marine source and methanogenesis from the tidal influenced mangroves play a major role in determining the rainwater chemistry of the region.  相似文献   

Solute distribution in porous rhyolite as evaluated by sequential centrifugation     

Tadashi Yokoyama  Satoru NakashimaTakashi Murakami  Lionel MercuryYusuke Kirino 《Applied Geochemistry》2011,26(8):1524-1534

Pore water in a porous rhyolite, having a porosity of 27% and pore radii ranging from >25 μm to 0.008 μm, was centrifugally extracted stepwise with increasing centrifugal speed to examine the potential variations of the compositions of pore water and their relationships to reaction and transport occurring in the rock. The rock was soaked for from 1 h to 7 days in an aqueous solution prior to centrifugation. To evaluate the effect of adsorption under minimum effect of dissolution, Li⁺ and Br⁻ were added to the solution as tracer ions. As centrifugal speed increased, water was extracted in order of large to small pores and the thickness of residual water film became thinner. The concentrations of ions dissolving from the rock (Na⁺, K⁺, Ca²⁺, etc.) after 7 days of immersion were relatively constant in pores of 1-10 μm radii and exponentially increased by 3-100 fold with decreasing pore radius to 0.1 μm. These ions are dissolved from the rock and transported toward the exterior of the rock by diffusion. The calculation using a reactive-transport equation showed that the observed concentration changes reflect the change in solute distribution profile with pore size. The concentration of Si after 7 days of immersion was approximately constant or slightly decreased with increasing centrifugal speed, which appears to be controlled by the solubility. The concentration of Li⁺ decreased with increasing centrifugal speed after 1 h of immersion but the trend changed after 7 days of reaction. Initial behavior of Li⁺ is explained by adsorption on pore walls, and the change of trend is explained by desorption of that previously adsorbed, slight amounts of dissolution, and inflow from the outside of the rock. The change in concentration of Br⁻ with increasing centrifugal speed was small, probably because Br⁻ was not adsorbed on the surfaces. The sequential centrifugation thus provides information on the solute distribution associated with reaction and transport occurring in rock pores.  相似文献   

Reversible ion-exchange fixation of cesium-137 leading to mobilization from reservoir sediments     

David W. Evans  James J. Alberts  Roy A. Clark 《Geochimica et cosmochimica acta》1983,47(6):1041-1049

The radioactive fission product, ¹³⁷Cs, has been observed to mobilize from bottom sediments of two South Carolina reservoirs during summer thermal stratification and hypolimnetic anoxia. Mobilization is attributed to ion-exchange displacement of ¹³⁷Cs from sediments by cations such as NH⁺₄, Fe⁺² and Mn⁺² released under anaerobic conditions.Three types of ¹³⁷Cs binding sites to sediment clay minerals are identified: 1) surface and planar sites from which ¹³⁷Cs is generally exchangeable by all cations studied (Na⁺, NH⁺₄, H⁺, Cs⁺, Ca⁺², Mg⁺², Fe⁺², and Mn⁺²); 2) wedge sites where ¹³⁷Cs exchange is sterically limited to cations of similar size and charge (NH⁺₄, Cs⁺, K⁺, and perhaps H₃O⁺); 3) interlayer sites from which ¹³⁷Cs is not readily exchanged. More than 15 years after final ¹³⁷Cs inputs, the reservoir sediments we studied showed the following percentage distribution of sites: 2 to 9% surface sites, 6 to 13% wedge sites, and 78 to 85% interlayer sites. In contrast, lake and stream sediments near Oak Ridge, Tennessee receiving ¹³⁷Cs inputs more than 20 years earlier had greater than 99% of their ¹³⁷Cs associated with non-exchangeable interlayer sites. The difference is attributed to the paucity in the South Carolina sediments of weathered micaceous clay minerals with their abundant interlayer sites. Such interlayer deficient clays are dominant in the Atlantic and Gulf coastal plains of the United States and elsewhere. This suggests that ¹³⁷Cs will be physically and chemically more mobile in such areas as well as more biologically available. Mobility will be enhanced in regimes where cation inputs favoring ¹³⁷Cs exchange occur. Subsurface waste disposal sites where anaerobic conditions develop with NH⁺₄ production and Fe⁺² and Mn⁺² release might be such a regime.  相似文献