Octahedral Surface Research Articles

Computational Study on the Octahedral Surfaces of Magnetite Nanoparticles and Their Solvent Interaction.

Magnetite nanoparticles (MNPs) play an important role in geological and environmental systems because of their redox reactivity and ability to sequester a wide range of metals and metalloids. X-ray absorption spectroscopy conducted at metal and metalloid edges has suggested that the magnetite {111} faces of octahedrally shaped nanoparticles play a dominant role in the redox and sorption processes of these elements. However, studies directly probing the magnetite surfaces, especially in their fully solvated state, are scarce. Therefore, we investigated the speciation and stability over a wide Eh/pH range of octahedrally shaped MNPs of 2 nm size by means of Kohn-Sham density functional theory with Hubbard correction (DFT+U). By altering the protonation state of the crystals, a redox-sensitive response of the octahedrally coordinated Fe could be achieved. Furthermore, the preferential H distribution could be identified, highlighting the difference between the edges, vertices, and facets of the nanocrystals. Subsequently, the interactions of the MNPs with a solvent of pure water or a 0.5 M NaCl solution were studied by classical molecular dynamics (MD) simulations. Finally, a comparison of the corresponding macroscopic magnetite (111) surface to the investigated MNPs was conducted.

Structures Associated with the Borromean Rings’ Complement in the Poincaré Ball

Guided by physical needs, we deal with the rotationally isotropic Poincar´e ball, when considering the complement of Borromean rings embedded in it. We consistently describe the geometry of the complement and realize the fundamental group as isometry subgroup in three dimensions. Applying this realization, we reveal normal stochastization and multifractal behavior within the examined model of directed random walks on the rooted Cayley tree, whose sixbranch graphs are associated with dendritic polymers. According to Penner, we construct the Teichm¨uller space of the decorated ideal octahedral surface related to the quotient space of the fundamental group action. Using the conformality of decoration, we define six moduli and the mapping class group generated by cyclic permutations of the ideal vertices. Intending to quantize the geometric area, we state the connection between the induced geometry and the sine-Gordon model. Due to such a correspondence we obtain the differential two-form in the cotangent bundle of the moduli space.

Calculation of contact angle via Young-Dupré equation with molecular dynamic simulation: Kaolinite as an example

To explore the surface wettability, the contact angle was usually experimentally measured by sessile drop method, which is very sensible to the experimental conditions. Molecular Dynamics (MD) simulation method has been applied to study nanodroplets, but the contact angles were usually obtained via curve fittings of the droplets, which is subjective and the errors inevitable. In this work, Young-Dupré equation was applied to calculate the contact angle with the surface tension coefficient of water and the adhesion work of water/surface, which were achieved via MD simulations results. The wettability of layered kaolinite was studied as an example. The computed contact angle of water on the Al-O octahedral surface of kaolinite was 0°, and those on the Si-O tetrahedral surface were roughly 92° ∼ 104°, which agreed well with the results of curve fitting method, about 0° for the Al-O octahedral surface and 106° ± 9° on the Si-O tetrahedral surface. The dipole moment distribution, dipole angle distribution of water molecules, as well as the evolution of hydrogen bond number during the whole dripping process of water droplet on kaolinite surfaces were analyzed to reveal the mechanism of hydrophobic/hydrophilic of different surfaces of kaolinite.

Understanding the adsorption of calcium sulfate on tetracalcium aluminoferrite and tricalcium aluminate surfaces

AbstractCalcium sulfate (CaSO4), an essential retarder in cement, retards the hydration of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO4f on C4AF and C3A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO4 on C4AF was stronger than that on C3A due to the slower CaSO4 consumption rate, lower driving force for CaSO4 adsorption, and surface coverage of Fe(OH)3 gel. The adsorption of CaSO4 hindered Ca dissolution more markedly on C4AF than C3A, which was pronounced on Fe‐free C4AF surfaces. The adsorption of CaSO4 weakened the affinity of water on C4AF and C3A surfaces, lowering the driving force for H2O adsorption. The adsorption of H2O and CaSO4 promoted the dissolution of Al on the [AlO6] octahedral surface of C4AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO4 on initial C4AF and C3A hydration was explained.

Effects of the surface-dependent properties on the morphological modeling of SrMoO4 nanoparticles

The nanoparticle morphology can significantly affect its physical, chemical, and optical properties. In the scheelite group, strontium molybdate (SMO) is an inorganic material widely investigated due to thermal and chemical stability under higher pressure, and their synthesis has been reported via different routes. This work reported a detailed description of the bulk and (001), (100), (101), (103), (111), (112) and (211) surfaces of SMO and a general mapping of morphological transformations, which can be applied to any material that exhibits the scheelite symmetry. The stability order observed for SMO surfaces was: (001) > (112) > (101) > (110) > (111) = (103) > (211) > (100). Surfaces with the same coordination for outermost Sr-centered polyhedra, the higher distortion, the most unstable. In general, it was observed that the most stable surfaces have a higher Egap compared to the unstable. The highest Egap is associated with (101) surface (4.48 eV), while the lowest is with (100) (3.25 eV). Disk-like morphologies are associated with (001) surface stabilization, while (112), (101), (111), (103), and (221) minimizations favor the generation of octahedral morphologies, and finally, (110) and (100) surfaces growth induces rod-like morphologies. The relationship between the morphology and surface-dependent properties can be useful for obtaining multifunctional SMO-related systems and optimizing its application in several areas, such as catalysis, photocatalysis, electronics and optics.

Occurrence Characteristics of Water in Nano-Slit Pores under Different Solution Conditions: A Case Study on Kaolinite.

The presence of water in narrow pore spaces affects the occurrence and flow of methane, which in turn affects shale gas production. Therefore, studying the occurrence and distribution characteristics of water is of great significance to predict gas production. Based on molecular dynamics simulations, this study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores in situ. Owing to its widespread distribution, kaolinite is the most prevalent clay mineral with two surfaces with different characteristics. Three systems of pure water, a CaCl2 solution, and a H2O/CH4 mixed phase were created at varied temperatures (80-120 °C) and pressures (70-120 MPa). The presence of gas and water in the nanopores was investigated thoroughly. The results showed that the adsorption of water on the Al-O octahedral surface of kaolinite was not affected by external conditions under in situ conditions, whereas the adsorption of water on the Si-O tetrahedral surface decreased with increasing temperature, but the change was small. When ions were present in the system, the water capacity decreased. Based on the aforementioned results, external conditions, such as temperature and pressure do not affect the basic state of water. However, if there are more than two fluid types in the system, the adsorption of water on the mineral surface is reduced owing to competitive adsorption. In addition, a CH4-H2O mixed system was simulated, in which methane molecules were distributed in clusters. There are two types of adsorptions in pores: gas-solid interactions and solid-liquid-gas interactions. CH4 molecules are thought to be clustered in water molecules because of the strong hydrogen bonding interactions among the water.

In situ/operando XAFS investigation of the sorption/precipitation of Zn(II) on palygorskite surface at the molecular scale: Implications for Zn stable isotope fractionation

In aqueous geological environments, the fate and transport of Zn are controlled by geochemical reactions (e.g., sorption) occurring at the mineral/water interface. However, many of the underlying Zn fixation mechanisms, especially the kinetics of surface-induced precipitation, are still unclear. Additionally, Zn stable isotope fractionation during sorption has recently been shown to be an important process, but the relationship between Zn isotope fractionation and sorption mechanisms (e.g., surface complexation, precipitation) is also not well understood. To address these issues, we employed X-ray absorption fine structure (XAFS) spectroscopy, high-resolution transmission electron microscopy (HRTEM), and in situ/operando quick-scanning XAS (QXAS) spectroscopy to elucidate the sorption/precipitation mechanisms of Zn at palygorskite/solution interfaces. We also measured Zn isotope ratios during some sorption experiments to illustrate how Zn isotope fractionation behavior is affected by the evolution of coordination environments at different pH values, initial concentrations, and reaction times. Our results demonstrate that Zn sorption mechanisms vary as a function of pH, ionic strength, initial concentration, and reaction time. At low pH (pH < 7.0) and low ionic strength (I ≤ 0.01 M), Zn(II) predominantly forms an outer-sphere surface complex in octahedral coordination. At low pH (pH < 7.0) and high ionic strength (I = 0.1 M), Zn(II) is predominantly sorbed as an inner-sphere octahedral surface complex with insignificant isotopic fractionation (Δ66Znsorbed-aqueous = −0.02 ± 0.05‰). At high pH and high initial Zn concentration (pH 7.5, C0 ≥ 0.2 mM), the formation of octahedral Zn phyllosilicate precipitates is observed, yielding a small fractionation with an average Δ66Znsorbed-aqueous of 0.10 ± 0.07‰. In addition, in situ/operando QXAS of Zn sorption at pH 7.5 in a flow cell revealed that the predominant sorbed Zn species shifted from inner-sphere tetrahedral complexes during the initial stage (e.g., within minutes) to inner-sphere octahedral complexes during later stages (e.g., within hours), followed by the formation of Zn-phyllosilicate precipitates (e.g., within days). This molecular evidence coincides with the evolution of Zn isotope fractionation from a large Δ66Znsorbed-aqueous value (0.53 ± 0.05‰) to a small value of 0.05 ± 0.04‰ during the sorption process. The combination of QXAS and isotope fractionation results reveals a change in the Zn local environment from tetrahedral coordination to octahedral coordination. The findings in this study not only provide new insight into surface adsorption/precipitation mechanisms but also demonstrate that stable isotope fractionation is linked with the local molecular/bonding environments at mineral–water interfaces.

P-Induced Permeation of Nickel into WO3 Octahedra to Form a Synergistic Catalyst for Urea Oxidation.

Small-molecule induction can lead to the oriented migration of metal elements, which affords functional materials with synergistic components. In this study, phosphating nickel foam (NF)-supported octahedral WO3 with phosphine affords P-WO3 /NF electrocatalyst. Ni is found to form Ni-P bonds that migrate from NF to WO3 under the induction of P, resulting in the complex oxides W1.3 Ni0.24 O4 and Ni2 P2 O7 in the particle interior and nickel phosphide on the octahedral grain surface. The catalytic activity of P-WO3 /NF in the urea oxidation reaction (UOR) is improved by synergistic action of the components in the synthesized hybrid particles. A current density of 10 mA cm-2 can be reached at a potential of 1.305 V, the double layer capacitance of the catalyst is significantly increased, and the electron transfer impedance in catalytic UOR is reduced. This work demonstrates that small-molecule induction is suitable for constructing co-catalysts with complex components in a simple protocol, which provides a new route for the design of highly efficient urea oxidation electrocatalysts.

Effect of the Anticaking Agent FeCN on the Creeping Properties of Alkali Halide Crystals

Fe(CN)64– (abbreviated as FeCN) is a well-known anticaking agent for NaCl that has a nearly perfect geometrical fit to the (001) surface. In this work, the investigation of the action of FeCN on nucleation and creeping is extended to a wide range of alkali halides of both the NaCl and CsCl structures. It was found that FeCN is a powerful nucleation inhibitor and enhances the creeping of alkali halides when FeCN geometrically fits well in the octahedral surface site of hosts with the NaCl crystal structure. The tested alkali halides that do not fall in this group or that have the CsCl structure show some reduction in creeping upon the addition of FeCN, while NaF and LiCl did not show a difference. For this category of crystals, there is a poor fit of the FeCN ion in the crystal surface. This study shows that a strong adsorption strength of FeCN enhances creeping, whereas a weak interaction reduces the creeping of alkali halide crystals.

Adsorption of Naphthalene on Clay Minerals: A Molecular Dynamics Simulation Study.

Naphthalene, as one of the representative polycyclic aromatic hydrocarbons, widely exists in contaminated sites and is a potential threat to human health due to its high mobility in soil. The interaction between naphthalene and clay minerals is of great significance to the environmental behavior of naphthalene and the design of remediation technology. In this study, montmorillonite and kaolinite were selected as representative clay minerals. Naphthalene adsorption behavior on mineral surfaces and water-wet kaolinite surfaces was investigated using molecular dynamics (MD) simulation. The interaction energy was calculated to represent the interaction between naphthalene and soil fractions, and the relative concentration and density distribution of naphthalene was analyzed to describe the distribution of naphthalene on the clay surfaces. The self-diffusion coefficient of naphthalene was obtained to represent its mobility under different water content. The electron density calculation was performed to reveal the different adsorption behavior of naphthalene on different surfaces of kaolinite. The simulation results show that montmorillonite had a stronger interaction with naphthalene due to larger electrostatic interaction energy compared to kaolinite, and naphthalene distributed more intensively on the montmorillonite surface. With regards to kaolinite, naphthalene tended to be absorbed on the alumina octahedral surface rather than the silicon tetrahedral surface due to the weak hydron bond interaction. The results indicate that water impeded the adsorption of naphthalene, and the optimal initial thickness of water film, which was 10 Å, was put forward for the application of thermal remediation technology. Furthermore, the average interaction energies between water and mineral surfaces largely depended on the water content, and the competitive adsorption between water and naphthalene only occurred under absorbed and bound water conditions. Overall, the knowledge of naphthalene–soil fractions interaction gained in this study is critical to the understanding of the environmental behavior of naphthalene and the reference for remediation technology.

Tunnel cation and water structures of todorokite: Insights into metal partitioning and isotopic fractionation

Tectomanganate todorokite and phyllomanganate vernadite are the two major manganese minerals in marine ferromanganese sediments that contain large quantities of trace metals. Todorokite has a less clear metal partitioning mechanism than vernadite, mainly because the metal cation and water structures in the tunnel cavities remain poorly understood. Here, we report the composition-dependent tunnel-cation site disorder of todorokite obtained from molecular dynamics (MD) simulations. Our simulations of homocationic Ni2+-, Zn2+-, Mg2+-, Ca2+-, or Na+-todorokite revealed that tunnel cations form inner-sphere (IS) complexes with Mn octahedral surface at low hydration states, but at higher hydration states, outer-sphere (OS) complexes are the dominant cation species located at the center of the tunnel. However, K+ and Cs+ form only IS complexes regardless of the tunnel hydration state. Optimum water contents were determined based on the free energy changes associated with water intercalation into the tunnel cavities calculated as a function of water content for two heterocationic todorokite models: Mg0.5Na0.4Ca0.1Mn6O12·nH2O (Mg/Mn = 0.08, average Mn oxidation state = +3.73) and Mg1.1K0.1Ca0.1Mn6O12·nH2O (Mg/Mn = 0.18, average Mn oxidation state = +3.58). These models emulate the chemical compositions of well-characterized terrestrial and marine todorokite samples, respectively. At each optimum water content, MD simulations revealed that OS complexes are the major Mg2+ species in the former model. However, IS complexes are the major Mg2+ species in the latter model, indicating that the tunnel cation speciation of diagenetic todorokite can be influenced by the sediment pore-fluid chemistry. The atomic tunnel cation and water structures of todorokite provide a molecular basis for understanding of metal partitioning to Mn oxides and interpretation of metal stable isotopes recorded in ferromanganese nodules.

Adsorption mechanism of hydrated Lu(OH)2+ and Al(OH)2+ ions on the surface of kaolinite

Density functional theory was used to study the adsorption mechanism of hydrated Lu(OH)2+ and Al(OH)2+ ions on (001) kaolinite surface. Calculation results showed that the respective most stable hydration configurations in the water system were Lu(OH)(H2O)72+ and Al(OH)(H2O)52+. Both hydrated ions were adsorbed on the outer layer of the (001) kaolinite aluminium octahedral and silica tetrahedral surfaces through hydrogen bonding. The two hydrated ions were adsorbed on the deprotonated (001) aluminium octahedral kaolinite surface via coordination bonds, with electrons transferred from the water ligand O 2p to the Al 2 s, Lu 5d, and the surface O 2p orbitals. Owing to its smaller radius and fewer water ligands, hydrated Al(OH)2+ was more easily adsorbed on the (001) kaolinite surface than hydrated Lu(OH)2+. The experimental adsorption and microcalorimetry results supported this conclusion, and it is hypothesised that hydrated ions mainly act on the kaolinite surface via inner-layer adsorption.

A comparative study of biopolymer adsorption on model anisotropic clay surfaces using quartz crystal microbalance with dissipation (QCM-D)

Mitigation of colloid clay particles is critical during flotation and flocculation processes in mineral processing. Most organic and inorganic mitigation reagents have negative impacts on the environment and human health; therefore, biologically derived substances have been attracting attention as alternative reagents. Given the anisotropic nature of clay surfaces, it is imperative to understand reagent adsorption on the individual edge and basal plane surfaces of clays. Quartz crystal microbalance with dissipation (QCM-D) was used in this study to determine the adsorption characteristics of three biopolymers (the protein-based biopolymer, lysozyme, and protein and polysaccharide-based oligomers; protamine and pectin) on model surfaces of the anisotropic edge and basal planes of the clays (e.g., kaolinite and serpentine). SiO2 sensor representing the tetrahedral basal plane, Al2O3 and Mg(OH)2 representing the octahedral basal planes, AlSiO and MgSiO representing the edge faces of clays were used as model surfaces of clay minerals. For kaolinite, protamine adsorbed preferentially on the silica (SiO2) tetrahedral surface at pH 7 and on the alumina (Al2O3) surface at pH 10. Protamine adsorbed primarily on magnesium hydroxide (Mg(OH)2), representative of serpentine, at pH 7 and 10. Lysozyme adsorbed preferentially and irreversibly on the edge basal plane surfaces of both clays at pH 10, while it showed a higher affinity for octahedral surfaces (alumina and magnesium hydroxide) at pH 7. In contrast, pectin adsorbed strongly on the magnesium hydroxide, representative of the basal plane surface of serpentine. An adsorption study revealed that electrostatic attraction and/or hydrogen bonding mechanisms contributed to the adsorption of biopolymers on clay surfaces. This investigation provides a fundamental and practical understanding of biopolymer interactions with clay surfaces during selective flotation and flocculation.

Interlayer water structure of phyllomanganates: Insights from MD simulations of chalcophanite-group oxide dehydration

Phyllomanganates transform and concentrate a variety of metals in marine and terrestrial environments through sorption and redox reactions. Water molecules present in the interlayer region of phyllomanganates are assumed to play an important role in the stability, cation exchange, and redox reactions of manganese oxide minerals. The molecular structure of interlayer water remains elusive due to insufficient analytical and computational capabilities for investigating these confined water molecules. This study explores the interlayer water structure of phyllomanganates based on classical molecular dynamics (MD) simulations performed for crystalline chalcophanite group oxides (Me2+Mn3O7·3H2O, Me2+ = Zn2+, Ni2+, Mg2+, Mn2+, or Ca2+) as a function of the interlayer water content. As the oxides were dehydrated, MD simulations revealed collapse of the layer spacing in Zn- and Ni-chalcophanite and gradual decrease of the layer spacing in Mg-, Mn-, and Ca-chalcophanite. MD simulations also revealed that the cation-specific reduction in layer spacing with dehydration is controlled by the reorganization of interlayer water molecules, the extent of which strongly depends on the interlayer cations. Water molecules in chalcophanite group oxides occupied the midplane of the interlayer prior to dehydration, with the exception of Ca-chalcophanite which contained two split atomic planes of water molecules near the oxide surface. When Zn- and Ni-chalcophanite were monohydrated per formula unit, water molecules moved directly above or below the interlayer cations, leading to two split atomic planes of water molecules and a minor increase in the c-axis. In monohydrated Mg-, Mn-, and Ca-chalcophanite, water molecules occupied the midplane in the interlayer, with the c-axis reduced. Dehydration of Zn- and Ni-chalcophanite induced the dipole-moment vectors of water molecules to rotate simultaneously and become arranged perpendicular to the (001) basal plane, forming H-bonds between a water molecule and a single Mn octahedral sheet surface. However, in dehydrated Mg-, Mn-, and Ca-chalcophanite, water molecules were induced to rotate in the opposite direction; in this case, the dipole-moment vectors were nearly parallel to the basal plane, and H-bonds formed between a water molecule and two adjacent sheet surfaces. MD simulations with and without Mn4+-vacancy site disorder demonstrated that vacancy site-disorder causes significant disorder in interlayer cation and water structures. The current study provides atomistic insight into the effects on interlayer water structure of chemical and structural disorders in birnessite-like phyllomanganates.

High electrochemical stability of octahedral LiMn2O4 cathode material in aqueous and organic lithium-ion batteries

In this work, octahedral LiMn2O4 (O-LMO) with exposed {111} crystal facets was prepared using MnCO3 as a precursor via solid phase reactions. O-LMO was revealed to have the superior rate performance and good cycling stability in both a three-electrode system and full aqueous batteries with Li2SO4 solution.The capacity retentions of 87% for 200 cycles at 0.1 A g−1 and 87% for 500 cycles at 1 A g−1 were achieved, in sharp contrast with 56% and 70% for the commercial LMO (C-LMO). Also, O-LMO showed the excellent performance in organic systems as well. The superior performance for both aqueous and organic systems may be related to the maintaining of octahedral structures and surface morphologies for O-LMO cycled 200 times at 0.1 A g−1 in aqueous system and the suppression of Mn dissolution and side reactions in both aqueous and organic systems, which originates from the stable {111} facets.

Mechanical and Electrical Interaction of Biological Membranes with Nanoparticles and Nanostructured Surfaces.

In this review paper, we theoretically explain the origin of electrostatic interactions between lipid bilayers and charged solid surfaces using a statistical mechanics approach, where the orientational degree of freedom of lipid head groups and the orientational ordering of the water dipoles are considered. Within the modified Langevin Poisson–Boltzmann model of an electric double layer, we derived an analytical expression for the osmotic pressure between the planar zwitterionic lipid bilayer and charged solid planar surface. We also show that the electrostatic interaction between the zwitterionic lipid head groups of the proximal leaflet and the negatively charged solid surface is accompanied with a more perpendicular average orientation of the lipid head-groups. We further highlight the important role of the surfaces’ nanostructured topography in their interactions with biological material. As an example of nanostructured surfaces, we describe the synthesis of TiO2 nanotubular and octahedral surfaces by using the electrochemical anodization method and hydrothermal method, respectively. The physical and chemical properties of these nanostructured surfaces are described in order to elucidate the influence of the surface topography and other physical properties on the behavior of human cells adhered to TiO2 nanostructured surfaces. In the last part of the paper, we theoretically explain the interplay of elastic and adhesive contributions to the adsorption of lipid vesicles on the solid surfaces. We show the numerically predicted shapes of adhered lipid vesicles corresponding to the minimum of the membrane free energy to describe the influence of the vesicle size, bending modulus, and adhesion strength on the adhesion of lipid vesicles on solid charged surfaces.

Atomic scale investigation of aluminum incorporation, defects, and phase stability in β-(AlxGa1−x)2O3 films

The development of novel ultra-wide bandgap (UWBG) materials requires precise understanding of the atomic level structural origins that give rise to their important properties. We study the aluminum atom incorporation, defect formation, and their relationships with phase stability in β-(AlxGa1−x)2O3 films, a promising candidate for UWBG applications, to explain atomic scale structural characteristics and properties using a combination of quantitative scanning transmission electron microscopy (STEM) and density functional theory (DFT). Our STEM analysis indicates that ∼54% of the incorporated Al substitutes on the octahedrally coordinated Ga2 site in a series of films grown with different techniques and alloy concentrations. DFT calculations show that, while Al energetically prefers the octahedral site, surface reconstructions and kinetic limitations during the epitaxial growth are responsible for Al occupying both octahedral and tetrahedral sites in (AlxGa1−x)2O3, ultimately limiting the stability of the β-phase at x &amp;lt; ∼50%. Local heterogeneity of composition results in the formation of a planar defect, affecting the stability of the β-phase. The similarity of such inclusions to the metastable γ-phase is discussed.

Influence of Muscovite (001) Surface Nanotopography on Radionuclide Adsorption Studied by Kinetic Monte Carlo Simulations

Mechanistic understanding and prediction of solute adsorption from fluids onto mineral surfaces is relevant for many natural and technical processes. Mineral surfaces in natural systems are often exposed to fluids at non-equilibrium conditions resulting in surface dissolution reactions. Such reactions cause the formation of surface nanotopography and, consequently, the exposure of different types of surface atoms. The quantitative effect of nanotopography on the efficiency of adsorption reactions at crystal surfaces is not known. Using kinetic Monte Carlo simulations, we combined a model of muscovite (001) face dissolution with a consequent model of radionuclide adsorption on the rough mineral surface. The model considers three different adsorption sites based on the muscovite surface cations: silicon, tetrahedral, and octahedral aluminum. Two different nanotopography configurations are investigated, both showing similar adsorption behavior. Octahedral aluminum surface atoms defined by having the highest reactivity toward adsorption are exposed solely on steps and pits on the muscovite (001) face. Thus, their availability directly depends on the surface nanotopography. The model results show the need for a more precise parameterization of surface site-specific adsorption, taking into account the coordination of the involved surface cation such as kink, step, or terrace sites.

Microscopic understanding about adsorption and transport of different Cr(VI) species at mineral interfaces

Cr(VI) ranks as one of the most toxic heavy metals and herein, microscopic mechanisms for adsorption and transport of different Cr(VI) oxyanions (Cr2O72- and CrO42-) at kaolinite interfaces are addressed by dispersion-corrected periodic density functional theory calculations. Cr(VI) oxyanions adsorb favorably at both tetrahedral and octahedral surfaces, and K+ ions serve as bridge for Cr(VI) oxyanions and tetrahedral surfaces while Cr(VI) oxyanions serve as bridge for K+ ions and octahedral surfaces. Adsorption structures are altered significantly by pH variation, and stability trends at different pH ranges are deciphered by the dominant interaction force with clay surfaces: Electrostatic interaction with K+ ions at tetrahedral surfaces whereas combined action of electrostatic and H-bonding interactions with Cr(VI) oxyanions at octahedral surfaces. Electron transfers are strongly pH-dependent, and clay surfaces serve as electron reservoirs. CrO42- rather than Cr2O72- is dominant at clay interfaces, and HCrO4- can co-exist under acidic conditions. Cr2O72- transformation to CrO42- is kinetically blocked at pH ≈ PZC while preferred at pH < PZC. Cr(VI) removal and reclamation should proceed at pH > 7.0 and pH < PZC, respectively. Results greatly promote the understanding about Cr(VI) bioavailability and fate in surficial environments and are also useful for Cr(VI) removal and reclamation.

Molecular Dynamics Simulations of Oil-Water Wetting Models of Organic Matter and Minerals in Shale at the Nanometer Scale.

Wettability is an important physical property of shale. This parameter is related to the shale material composition and the fluid properties in the shale pores and plays an important role in the exploration and development of shale oil. Wettability is affected by the scale and roughness. The contact angle at the nanoscale on a smooth surface can better reflect the wettability of shale than the contact angle at higher scales. Molecular dynamics simulations can be used to measure the contact angle on a smooth surface at the nanoscale. This paper focuses on the effects of organic matter and minerals in shale and different components of shale oil on shale wettability. Wetting models of "organic matter-oil component-water," "quartz-oil component-water" and "kaolinite-oil component-water" at the nanoscale were constructed. Molecular dynamics simulation was used to study the morphological changes of different oil components and water on different surfaces. Studies have shown that organic matter is strongly oleophilic and hydrophobic. Polar components in shale oil can make organic matter slightly hydrophilic. It was recognized by quartz wettability experiments and simulation methods at the nanoscale that the cohesive energy of a liquid has a significant influence on the degree of spreading of the liquid on the surface. The "liquid-liquid-solid" wettability experiment is an effective method for determining mineral oleophilic or hydrophilic properties. The nanoquartz in the shale is strongly hydrophilic. The water wetting angle is related to the crude oil component. Nanokaolinite can have a tetrahedral or an octahedral surface; the tetrahedral surface is oleophilic and hydrophobic, and the octahedral surface exhibits strong hydrophilicity. The wettabilities of both surfaces are related to the crude oil component.