Adsorption of Rb and Sr at the orthoclase (0 0 1)-solution interface

Adsorption of Rb⁺ and Sr²⁺ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb⁺ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K⁺ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å²). These results are consistent with an ion-exchange reaction in which Rb⁺ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO₃)₂ solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr²⁺ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr²⁺ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb⁺, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr²⁺ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr²⁺ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr²⁺ on orthoclase. Differences in the partitioning of Sr²⁺ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.     

Molecular dynamics simulations of the orthoclase (0 0 1)- and (0 1 0)-water interfaces

Molecular dynamics simulations of water in contact with the (0 0 1) and (0 1 0) surfaces of orthoclase (KAlSi₃O₈) were carried out to investigate the structure and dynamics of the feldspar-water interface, contrast the intrinsic structural properties of the two surfaces, and provide a basis for future work on the diffusion of ions and molecules in microscopic mineral fractures. Electron density profiles were computed from the molecular dynamics trajectories and compared with those derived experimentally from high-resolution X-ray reflectivity measurements by Fenter and co-workers [Fenter P., Cheng L., Park C., Zhang H. and Sturchio N. C. (2003a) Structure of the orthoclase (0 0 1)- and (0 1 0)-water interfaces by high-resolution X-ray reflectivity. Geochim. Cosmochim. Acta67, 4267-4275]. For each surface, three scenarios were considered whereby the interfacial species is potassium, water, or a hydronium ion. Excellent agreement was obtained for the (0 0 1) surface when potassium is the predominant interfacial species; however, some discrepancies in the position of the interfacial peaks were obtained for the (0 1 0) surface. The two surfaces showed similarities in the extent of water ordering at the interface, the activation energies for water and potassium desorption, and the adsorption localization of interfacial species. However, there are also important differences between the two surfaces in the coordination of a given adsorbed species, adsorption site densities, and the propensity for water molecules in surface cavities and those in the first hydration layer to coordinate to surface bridging oxygen atoms. These differences may have implications for the extent of dissolution in the low-pH regime since hydrolysis of Si(Al)OSi(Al) bonds is a major dissolution mechanism.     

Structures of quartz (100)- and (101)-water interfaces determined by x-ray reflectivity and atomic force microscopy of natural growth surfaces

The structures of prismatic (100) and pyramidal (101) growth faces of natural quartz crystals, and their modification upon annealing at T ≤ 400°C were investigated ex situ by atomic force microscopy (AFM) and in water by high-resolution X-ray reflectivity. AFM images revealed the presence of ∼ 0.1 to 1 μm-wide flat terraces delimited by steps of one to several unit cells in height. These steps follow approximately directions given by the intersection of growth faces. Modeling of X-ray reflectivity data indicates that surface silica groups on flat terraces have only one free Si-O bond each (presumably hydroxylated), except for some having two free Si-O bonds observed on a single (100) surface. Vertical relaxation of atomic positions (< 0.4 Å for terminal oxygens and < 0.2 Å for silicon and oxygen atoms fully coordinated to structural tetrahedra) is limited to a depth of 14 Å. Electron density profiles for all measured interfaces are consistent with a single layer of adsorbed water, with no evidence for additional organization of water molecules into distinct layers extending into the bulk solution. Similar interfacial structures were observed for natural and annealed surfaces of identical crystallographic orientation, indicating that extensive reconstruction of the silica network at the quartz surface did not occur under the annealing conditions.     

Structure and dynamics of water on muscovite mica surfaces

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.     

Water ordering and surface relaxations at the hematite (1 1 0)-water interface

Structural characterization of iron oxide-water interfaces provides insight into the mechanisms through which these minerals control contaminant fate and element cycling in soil, sedimentary, and groundwater systems. Ordering of interfacial water and structural relaxations at the hematite (1 1 0) surface have been investigated in situ using high-resolution specular X-ray reflectivity. These measurements demonstrate that relaxations are constrained to primarily the top ∼5 Å of the surface. Near-surface iron atoms do not relax substantially, although the uppermost layer displays an increased distribution width, while the undercoordinated oxygens on the surface uniformly relaxed outward. Two sites of adsorbed water and additional layering of water farther from the surface were observed. Water fully covers the (1 1 0) surface and appears to form a continuous network extending into bulk solution, with positional order decreasing to that of a disordered bulk fluid within 1 nm. The arrangement of water is similar to that on the hematite (0 1 2) surface, which has a similar surface topography, although these surfaces display different vibrational amplitudes or positional disorder of adsorbed water molecules and average spacings of near-surface layered water. Comparison between these surfaces suggests that interfacial water ordering on hematite is controlled primarily by surface structure and steric constraints and that highly ordered water is likely common to most hematite-water interfaces.     

Interfacial water structure on the (0 1 2) surface of hematite: Ordering and reactivity in comparison with corundum

Many geochemical reactions that control the composition of natural waters, contaminant fate and transport, and biogeochemical element cycling take place at the interface between minerals and aqueous solutions. A fundamental understanding of these important processes requires knowledge of the structure of mineral-water interfaces. High-resolution specular X-ray reflectivity was used to determine the structure of the hematite (0 1 2)-water interface. Relaxation of the surface was observed to be minor, and water was found to order near the hematite surface. Two sites of adsorbed water are inferred to be ordered laterally, one bridging between triply coordinated functional groups and the other bridging between the singly coordinated functional groups on the surface, as steric constraints limit the possible arrangements of water molecules occurring at the observed heights above the hematite surface. Relaxations of the hematite and corundum (0 1 2) surfaces, which are isostructural, are similar and limited primarily to the top most layer of the structures. No significant changes to the interfacial stoichiometry (i.e., partial occupancy of surface species) are observed in either case. The structure of interfacial water is similar on the hematite and corundum (0 1 2) surfaces as well, although water appeared to be less well ordered on the hematite surface. This may be due to expected differences in the oxygen exchange rates from surface functional groups or the apparent better matching of the corundum oxygen lattice to the natural structural ordering in water, and suggests that the dielectric constant gradients of interfacial water may differ on the two surfaces. Similar charging behavior is expected for these surfaces as similar types of surface functional groups are exposed. Although generally similar, subtle differences in the reactivity of hematite and corundum (0 1 2) surfaces to arsenate adsorption, and possibly the adsorption of other species, may be related to the difference in ordering of interfacial water observed in this study.     

Orthoclase dissolution kinetics probed by in situ X-ray reflectivity: effects of temperature, pH, and crystal orientation

Initial dissolution kinetics at orthoclase (001) and (010) cleavage surfaces were measured for ∼2 to 7 monolayers as a function of temperature using in situ X-ray reflectivity. The sensitivity of X-ray reflectivity to probe mineral dissolution is discussed, including the applicability of this approach for different dissolution processes and the range of dissolution rates (∼10⁻¹² to 10⁻⁶ mol/m²/sec) that can be measured. Measurements were performed at pH 12.9 for the (001) surface and at pH 1.1 for the (001) and (010) surfaces at temperatures between 46 and 83°C. Dissolution at pH 12.9 showed a temperature-invariant process with an apparent activation energy of 65 ± 7 kJ/mol for the (001) cleavage surface consistent with previous powder dissolution results. Dissolution at pH 1.1 of the (001) and (010) surfaces revealed a similar process for both surfaces, with apparent activation energies of 87 ± 7 and 41 ± 7 kJ/mol, respectively, but with systematic differences in the dissolution process as a function of temperature. Longer-term measurements (five monolayers) show that the initial rates reported here at acidic pH are greater than steady-state rates by a factor of 2. Apparent activation energies at acidic pH differ substantially from powder dissolution results for K-feldspar; the present results bracket the value derived from powder dissolution measurements. The difference in apparent activation energies for the (001) and (010) faces at pH 1.1 reveals an anisotropy in dissolution kinetics that depends strongly on temperature. Our results imply a projected ∼25-fold change in the ratio of dissolution rates for the (001) and (010) surfaces between 25 and 90°C. The dissolution rate of the (001) surface is higher than that of the (010) surface above 51°C and is projected to be lower below this temperature. These results indicate clearly that the kinetics and energetics of orthoclase dissolution at acidic pH depend on crystal orientation. This dependence may reflect the different manifestation of the Al-Si ordering between the T1 and T2 tetrahedral sites at these two crystal faces and can be rationalized in terms of recent theoretical models of mineral dissolution.     

Interaction of muscovite (0 0 1) with Pu bearing solutions at pH 3 through ex-situ observations

The interaction of Pu³⁺ bearing solutions with the muscovite (0 0 1) basal plane is explored using a combination of ex-situ approaches including alpha-counting, to determine the Pu³⁺ adsorption isotherm, and X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR), to probe the interfacial structure and Pu-specific distribution, respectively. Pu uptake to the muscovite (0 0 1) surface from Pu³⁺ solutions in a 0.1 M NaClO₄ background electrolyte at pH 3 follows an approximate Langmuir isotherm with an apparent adsorption constant, K_app = 5 × 10⁴ M⁻¹, and with a maximum coverage that is consistent with the amount needed to fully compensate the surface charge by trivalent Pu. The XR results show that the muscovite surface reacted with a 10⁻³ M Pu³⁺ solution (at pH 3 with 0.1 M NaClO₄) and dried in the ambient environment, maintains a 30-40 Å thick layer, indicating the presence of a residual hydration layer (possibly including adventitious carbon). The RAXR results indicate that Pu sorbs on the muscovite surface with an intrinsically broad distribution with an average height of 18 Å, substantially larger than heights expected for any specifically adsorbed inner- or outer-sphere complexes. These results are discussed in the context of recent studies of cation adsorption trends on muscovite and the possible roles of Pu hydrolysis species in controlling the Pu-muscovite interactions.     

Weak interfacial water ordering on isostructural hematite and corundum (0 0 1) surfaces

Ordering of interfacial water at the hematite and corundum (0 0 1)-water interfaces has been characterized using in situ high resolution specular X-ray reflectivity measurements. The hematite (0 0 1) surface was prepared through an annealing process to produce a surface isostructural with corundum (0 0 1), facilitating direct comparison. Interfacial water was found to display a similar structure on this pair of isostructural surfaces. A single layer of adsorbed water having a large vibrational amplitude was present on each surface and additional ordering of water extended at least 1 nm into the bulk fluid, with the degree of ordering decreasing with increasing distance from the surfaces. Consistent with prior studies of the (0 1 2) and (1 1 0) surfaces of hematite and corundum, the configuration of water above the (0 0 1) surfaces is primarily controlled by the surface structure, specifically the arrangement of surface functional groups. However, interfacial water at the (0 0 1) surfaces displayed significantly larger vibrational amplitudes throughout the interfacial region than at other isostructural sets of hematite and corundum surfaces, indicating weaker ordering. Comparison of the vibrational amplitudes of adsorbed water on a series of oxide, silicate, and phosphate mineral surfaces suggests that the presence or absence of a substantial interfacial electrostatic field is the primary control on water ordering and not the surface structure itself. On surfaces for which charge originates dominantly through protonation-deprotonation reactions the controlling factor appears to be whether conditions exist where most functional groups are uncharged as opposed to the net surface charge. The doubly coordinated functional groups on hematite and corundum (0 0 1) surfaces are largely uncharged under slightly acidic to circumneutral pH conditions, leading to weak ordering, whereas singly coordinated groups on (0 1 2) and (1 1 0) surfaces of these phases are always charged, even when the net surface charge is zero, and induce strong water ordering. Surfaces lacking structural charge can thus be divided into two distinct classes that induce either strong or weak ordering of interfacial water. Surface functional group coordination is the ultimate control on this division as it determines the charge state of such groups under different protonation configurations. Ion adsorption and electron transfer processes may differ between these classes of surfaces because of the effect of water ordering strength on interfacial capacitances and hydrogen bonding.     

Molecular modeling of water structure in nano-pores between brucite (001) surfaces

Molecular dynamics (MD) computer simulations of liquid water held in one-dimensional nano-confinement by two parallel, electrostatically neutral but hydrophilic surfaces of brucite, Mg(OH)₂, provide greatly increased, atomistically detailed understanding of surface-related effects on the spatial variation in the structural ordering, hydrogen bond (H-bond) organization, and local density of H₂O molecules at this important model hydroxide surface. NVT-ensemble MD simulations (i.e., at constant number of atoms, volume and temperature) were performed for a series of model systems consisting of 3 to 30 Å-thick water layers (containing 35 to 360 H₂O molecules) confined between two 19 Å-thick brucite substrate layers. The results show that the hydrophilic substrate significantly influences the near-surface water structure, with both H-bond donation to the surface oxygen atoms and H-bond acceptance from the surface hydrogen atoms in the first surface layer of H₂O molecules playing key roles. Profiles of oxygen and hydrogen atomic density and H₂O dipole orientation show significant deviation from the corresponding structural properties of bulk water to distances as large as 15 Å (∼5 molecular water layers) from the surface, with the local structural environment varying significantly with the distance from the surface. The water molecules in the first layer at about 2.45 Å from the surface have a two-dimensional hexagonal arrangement parallel to brucite layers, reflecting the brucite surface structure, have total nearest neighbor coordinations of 5 or 6, and are significantly limited in their position and orientation. The greatest degree of the tetrahedral (ice-like) ordering occurs at about 4 Å from the surface. The translational and orientational ordering of H₂O molecules in layers further from the surface become progressively more similar to those of bulk liquid water. A quantitative statistical analysis of the MD-generated instantaneous molecular configurations in terms of local density, molecular orientation, nearest neighbor coordination, and the structural details of the H-bonding network shows that the local structure of interfacial water at the brucite surface results from a combination of “hard wall” (geometric and confinement) effects, highly directional H-bonding, and thermal motion. This structure does not resemble that of bulk water at ambient conditions or at elevated or reduced temperature, but shares some similarities with that of water under higher pressure.     

Thickness and structure of the water film deposited from vapour on calcite surfaces

Synchrotron X-ray reflectivity (SXR) was used to measure the thickness of the water film that adsorbs on a {} cleavage surface of calcite (CaCO₃) in a sample chamber where relative humidity could be controlled within the range from <4% to 90%. Gases used to carry water vapour were initially either 100% N₂ or 100% CO₂. The product water film was remarkably constant in thickness at 15.5 Å (±1 Å) and independent of humidity. When N₂ was used as the carrier gas, this film displayed a gap in its electron density at between 0.6 and 2 Å distance from the calcite surface, depending on humidity. This implies that a change in the arrangement of water molecules occurs in direct proximity to the surface. This electron density discontinuity was measurably further from the calcite surface, at 3.4 Å, when CO₂ was used as the carrier gas. Except for this thin low density region proximate to the calcite surface, the density of the adsorbed water layer was 0.9 g cm⁻³, therefore suggesting a significant degree of ordering. Atomic force microscopy (AFM) images were completed in conjunction with the SXR measurements on similarly prepared calcite cleavage surfaces. AFM showed that terraces may be atomically flat over 1 μm or more. SXR corroborated this observation, with results showing that carefully cleaved surfaces had a starting root mean square (r.m.s.) roughness of ∼1.2 Å. Diffuse scatter measurements constrained the correlation lengths of these surfaces to be at least 18,000 Å. For comparison with the cleaved samples, a surface was also prepared by chemo-mechanical Syton polishing. This surface gave an r.m.s roughness by SXR that was an order of magnitude higher, equal to 12.1 Å. In this case, diffuse scatter resolved a correlation length of 950 Å, and revealed a fractal dimension that was higher than for the cleaved surface. On Syton polished samples, the water film determined by SXR was about twice as thick as for freshly cleaved surfaces, with a density of 1.0 g cm⁻³, equal to that of bulk water. However, surface roughness was too large to allow resolution of any gap in the electron density within the water layer proximate to the solid surface. Our AFM observations also confirm previous reports of calcite surface recrystallization. The electron density of the solid surface determined by SXR is indistinguishable from that of calcite, indicating that any material recrystallized within the adsorbed water film is compositionally indistinguishable from the calcite substrate.     

Structure and dynamics of water on Li-, Na-, K-, Cs-, H3O-exchanged muscovite surfaces: A molecular dynamics study

The distribution and dynamics of water molecules and monovalent cations (Li⁺, Na⁺, K⁺, Cs⁺, and H₃O⁺) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs⁺ and Cl⁻ ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li⁺ ions near the muscovite surface. For Na⁺, K⁺, Cs⁺, and H₃O⁺ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li⁺, Na⁺, K⁺, and H₃O⁺ ions adsorbed on the muscovite surfaces. The water molecules around Cs⁺ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs⁺ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li⁺ ions and decreased with an increase in the ionic radius.     

Cation sorption on the muscovite (0 0 1) surface in chloride solutions using high-resolution X-ray reflectivity

The structure and mechanism of cation sorption at the (0 0 1) muscovite-water interface were investigated in 0.01 and 0.5 m KCl, CsCl, and CaCl₂ and 0.01 m BaCl₂ solutions at slightly acidic pH by high-resolution X-ray reflectivity. Structural relaxations of atom positions in the 2M₁ muscovite were small (?0.07 Å) and occurred over a distance of 30 to 40 Å perpendicular to the interface. Cations in all solutions were sorbed dominantly in the first and second solution layers adjacent to the mineral surface. The derived heights of the first solution layer in KCl and CsCl solutions, 1.67(6)-1.77(7) and 2.15(9)-2.16(2) Å, respectively, differ in magnitude by the approximate difference in crystallographic radii between K and Cs, and correspond closely to the interlayer cation positions in bulk K- and Cs-mica structures. The first solution layer heights in CaCl₂ and BaCl₂ solutions, 2.46(5)-2.56(11) and 2.02(5) Å, respectively, differ in a sense opposite to that expected based on crystallographic or hydrated radii of the divalent cations. The derived ion heights in all solutions imply that there is no intercalated water layer between the first solution layer and the muscovite surface. Molecular compositions were assigned to the first two solution layers in the electron density profiles using models that constrain the number density of sorbed cations, water molecules, and anions by considering the permanent negative charge of the muscovite and average solution density. The models result in partial charge balance (at least 50%) by cations sorbed in the first two layers in the 0.01 m solutions and approximately full charge balance in the 0.5 m solutions. Damped oscillations of model water density away from the first two solution layers agree with previous X-ray reflectivity results on the muscovite (0 0 1) surface in pure water.     

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

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.     

Water structure at hematite-water interfaces

The atomic-level structure of water at mineral surfaces is an important controlling factor in interfacial reactions such as foreign ion incorporation, crystal growth and dissolution, and redox reactions. Molecular dynamics simulations with four different models based on interatomic potentials have been carried out to determine the atomic-level structure of three hematite-water interfaces. In addition, for each of the three surfaces, different terminations or protonation schemes were considered. The availability of surface X-ray scattering data for the surfaces considered here allowed for an extensive comparison with experimental data. Qualitatively, with the exception of one termination with one model, all models predict the correct arrangement of water molecules at the interface. Quantitatively, the agreement with experimental positions, distances, and layer occupancies is good to excellent, especially given the range of values reported in published experimental studies. Therefore, this study provides further evidence that interatomic potential models can be used to reliably predict the structure of mineral-water interfaces. In addition, molecular simulations are a valuable source of information to complement surface X-ray scattering experiments owing to their ability to directly determine the position of hydrogen atoms and to yield three-dimensional predicted structures at no added cost, as demonstrated in this work. Indeed, the molecular dynamics trajectories were analyzed to determine the surface structural controls on the interfacial water structure. Each of the three surface functional groups present at the surfaces considered in this work, namely, triply-coordinated oxo, doubly-coordinated hydroxo, and singly-coordinated aquo groups, was found to form similar hydrogen bond configurations with adsorbed water molecules at all surfaces. Oxo groups accept long-lasting and linear hydrogen bonds from adsorbed water molecules; hydroxo groups can form hydrogen bonds with other surface functional groups as well as with adsorbed water molecules; and aquo groups normally only donate hydrogen bonds to other surface groups or adsorbed water molecules. Additionally, the majority of adsorbed water molecules were found to adopt multiple configurations and orientations. This information was used to evaluate three-dimensional structural models of the interfaces, which were previously derived experimentally from one-dimensional electron density profiles and steric considerations.     

Probing interfacial reactions with X-ray reflectivity and X-ray reflection interface microscopy: Influence of NaCl on the dissolution of orthoclase at pOH 2 and 85 °C

The role of electrolyte ions in the dissolution of orthoclase (0 0 1) in 0.01 m NaOH (pOH ∼ 2) at 84 ± 1 °C is studied using a combination of in-situ X-ray reflectivity (XR) and ex-situ X-ray reflection interface microscopy (XRIM). The real-time XR measurements show characteristic intensity oscillations as a function of time indicative of the successive removal of individual layers. The dissolution rate in 0.01 m NaOH increases approximately linearly with increasing NaCl concentration up to 2 m NaCl. XRIM measurements of the lateral interfacial topography/structure were made for unreacted surfaces and those reacted in 0.01 m NaOH/1.0 m NaCl solution for 15, 30 and 58 min. The XRIM images reveal that the dissolution reaction leads to the formation of micron-scale regions that are characterized by intrinsically lower reflectivity than the unreacted regions, and appears to be nucleated at steps and defect sites. The reflectivity signal from these reacted regions in the presence of NaCl in solution is significantly lower than that calculated from an idealized layer-by-layer dissolution process, as observed previously in 0.1 m NaOH in the absence of added electrolyte. This difference suggests that dissolved NaCl results in a higher terrace reactivity leading to a more three-dimensional process, consistent with the real-time XR measurements. These observations demonstrate the feasibility of XRIM to gain new insights into processes that control interfacial reactivity, specifically the role of electrolytes in feldspar dissolution at alkaline conditions.     

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

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.     

Structure and oxidation state of hematite surfaces reacted with aqueous Fe(II) at acidic and neutral pH

Structural changes and surface oxidation state were examined following the reaction of hematite (0 0 1), (0 1 2), and (1 1 0) with aqueous Fe(II). X-ray reflectivity measurements indicated that Fe(II) induces changes in the structure of all three surfaces under both acidic (pH 3) and neutral (pH 7) conditions. The structural changes were generally independent of pH although the extent of surface transformation varied slightly between acidic and neutral conditions; no systematic trends with pH were observed. Induced changes on the (1 1 0) and (0 1 2) surfaces include the addition or removal of partial surface layers consistent with either growth or dissolution. In contrast, a <1 nm thick, discontinuous film formed on the (0 0 1) surface that appears to be epitaxial yet is not a perfect extension of the underlying hematite lattice, being either structurally defective, compositionally distinct, or nanoscale in size and highly relaxed. Resonant anomalous X-ray reflectivity measurements determined that the surface concentration of Fe(II) present after reaction at pH 7 was below the detection limit of approximately 0.5-1 μmol/m² on all surfaces. These observations are consistent with Fe(II) oxidative adsorption, whereby adsorbed Fe(II) is oxidized by structural Fe(III) in the hematite lattice, with the extent of this reaction controlled by surface structure at the atomic scale. The observed surface transformations at pH 3 show that Fe(II) oxidatively adsorbs on hematite surfaces at pH values where little net adsorption occurs, based on historical macroscopic Fe(II) adsorption behavior on fine-grained hematite powders. This suggests that Fe(II) plays a catalytic role, in which an electron from an adsorbed Fe(II) migrates to and reduces a lattice Fe(III) cation elsewhere, which subsequently desorbs in a scenario with zero net reduction and zero net adsorption. Given the general pH-independence and substantial mass transfer involved, this electron and atom exchange process appears to be a significant subsystem within macroscopic pH-dependent Fe(II) adsorption.     

X-ray absorption fine structure study of As(V) and Se(IV) sorption complexes on hydrous Mn oxides

X-ray absorption fine structure (XAFS) spectroscopic analysis at the As, Se, and Mn K-edges was used to study arsenate [As(V)O₄³⁻] and selenite [Se(IV)O₃²⁻] sorption complexes on the synthetic hydrous manganese oxides (HMOs) vernadite (δ-MnO₂) and K-birnessite (nominal composition: K₄Mn₁₄O₂₇ · 9H₂O). No significant changes were observed in sorption complex structure as a function of sorbent, pH (5 to 8), surface coverage (0.04 to 0.73 μmol/m²), or reaction time (5 to 22 h) in the arsenate or selenite systems. In the arsenate/HMO system, extended XAFS parameters indicate an average second-neighbor As(V) coordination of 2.0 ± 0.4 Mn at an average distance of 3.16 ± 0.01 Å, which is consistent with formation of As(V)O₄ sorption complexes sharing corners with two adjacent Mn(IV)O₆ surface species (i.e., bidentate, binuclear). In the selenite/HMO system, selenite surface complexes are surrounded by two shells of Mn atoms, which could represent two different adsorption complexes or a precipitate. The first shell consists of 1.6 ± 0.4 Mn at 3.07 ± 0.01 Å, which is consistent with the selenite anion forming bidentate (mononuclear) edge-sharing complexes with Mn(II)O₆ or Mn(III)O₆ octahedra. The second shell consists of 1.4 ± 0.4 Mn at 3.49 ± 0.03 Å, consistent with selenite forming monodentate, corner-sharing complexes with Mn(II)O₆ or Mn(III)O₆ octahedra. Pauling bond valence analysis that uses the extended XAFS-derived bond lengths for As(V)-O, Se(IV)-O, and Mn-O bonds indicates that the proposed surface complexes of selenite and arsenate on HMOs should be stable. Although a nearly identical Se(IV) coordination environment is found in a crystalline Mn(II)-Se(IV) precipitate (which has a structure similar to that of MnSeO₃ · H₂O), there are significant differences in the X-ray absorption near-edge structure and extended XAFS spectra of this precipitate and the selenite/HMO sorption samples. These differences coupled with transmission electron microscopy results suggest that if a precipitate is present it lacks long-range order characteristic of crystalline MnSeO₃ · H₂O.     

Simultaneous inner- and outer-sphere arsenate adsorption on corundum and hematite

The ability to predict the fate and transport of arsenic in aquatic environments, its impact on water quality and human health, and the performance and cost-effectiveness of water treatment systems relies on understanding how it interacts with solid surfaces. In situ resonant surface X-ray scattering measurements of arsenate adsorption at pH 5 in 0.01 M NaCl on corundum and hematite (012) surfaces demonstrate that arsenate surface complexation is unexpectedly bimodal, adsorbing simultaneously as inner- and outer-sphere species. In addition, this bimodal behavior is found to be independent of the total arsenate solution concentration, and thus surface coverage, over the range of 10⁻⁶ to 10⁻³ M. Alternative mechanisms to produce the observed As distributions, such as arsenate dimerization or surface precipitation of an aluminum or ferric arsenate, are inconsistent with the experimentally-determined total and As-specific density profiles. Based on the location of the outer-sphere arsenate in relation to the surfaces studied, possible binding mechanisms include electrostatic attraction, hydrogen bonding to surface oxygen functional group, and configurational stabilization by interfacial water. Although the observation of outer-sphere arsenate surface complexes on a metal oxide surface is unprecedented, it is unclear if such species were absent in previous molecular-scale studies, as it is difficult for methods commonly used to investigate the mechanisms of arsenate adsorption to conclusively identify or rule out the presence of outer-sphere species when inner-sphere species are also present.