Abstract:
In order to investigate the efficiency of sulfate green rust (GR2) to remove Ni from solution, GR2 samples were synthesized under controlled laboratory conditions. Some GR2 samples were synthesized from Fe(II) and Fe(III) sulfate salts by precipitation. Other samples were prepared by coprecipitation, of Ni(II), Fe(II) and Fe(III) sulfate salts, i.e., in the presence of Ni. In another sample, Ni(II) sulfate salt was added to pre-formed GR2. After an initial X-ray diffraction (XRD) characterization all samples were exposed to ambient air in order to understand the role of Ni in the transformation of the GR2 samples. XRD was repeated after 45 days. The results showed that Nious GR2 prepared by coprecipitation is isomorphous to Ni-free GR2, i.e. Ni is incorporated into the crystalline structure. Fe(II) was not replaced by Ni(II) in the crystalline structure of GR2 formed prior to exposure to solution-phase Ni. This suggests Ni was adsorbed to the GR2 surface. Sulfate green rust is more efficient in removing Ni from the environment by coprecipitation.
Abstract:
The grand canonical Monte Carlo method is used to study a binary mixture of Lennard-Jones atoms confined to a corrugated slit micropore which is in thermodynamic equilibrium with its bulk phase counterpart. The micropore has atomically structured walls; one of the which possesses nanoscale structure in the form of rectilinear grooves (corrugation). The grooved surface divides the confined fluid film into two strip shaped regions, that inside and that outside the grooves. Transverse solidlike order in the film gives rise to shear stress. Transverse order coupled with packing restrictions give rise to a difference between the pore and bulk fluid mixture compositions. Solidlike order may appear within the grooves only, outside the grooves only, or in both regions simultaneously. As the relative alignment of the walls is shifted the pore fluid undergoes freeze-thaw cycles in one or both regions with associated changes in the shear stress and pore fluid composition. The degree of transverse order in the film is less than would be expected in a pure Lennard-Jones film and fluid-solid phase transitions are gradual as opposed to sudden as seen in pure Lennard-Jones films. The magnitude of the shear stress is greatest when a fluid-solid phase transition occurs in both regions of the pore. © 1995 American Institute of Physics.
Abstract:
Ni(II)-Fe(II)-Fe(III) layered double hydroxides (LDH) or Ni-containing sulfate green rust (GR2) samples were prepared from Ni(II), Fe(II) and Fe(III) sulfate salts and analyzed with X ray diffraction. Nickel is readily incorporated in the GR2 structure and forms a solid solution between GR2 and a Ni(II)-Fe(III) LDH. There is a correlation between the unit cell a-value and the fraction of Ni(II) incorporated into the Ni(II)-GR2 structure. Since there is strong evidence that the divalent/trivalent cation ratio in GR2 is fixed at 2, it is possible in principle to determine the extent of divalent cation substitution for Fe(II) in GR2 from the unit cell a-value. Oxidation forms a mixture of minerals but the LDH structure is retained if at least 20 % of the divalent cations in the initial solution are Ni(II). It appears that Ni(II) is incorporated in a stable LDH structure. This may be important for two reasons, first for understanding the formation of LDHs, which are anion exchangers, in the natural environment. Secondly, this is important for understanding the fate of transition metals in the environment, particularly in the presence of reduced Fe compounds.
Abstract:
Molecular dynamics and grand canonical Monte Carlo simulations were conducted in order to understand better the relationship between the diffusion of octamethylcyclotetrasiloxane (OMCTS) and cyclohexane monolayers and the atomic structure of confining mica surfaces. It was found that diffusion increases with reduced fluid density. With the surface separation just large enough to accommodate a monolayer, lateral diffusion was direction dependent due to the influence of the atomically structured surfaces.
Abstract:
Grand-canonical Monte Carlo and microcanonical molecular dynamics methods have been used to simulate an ultrathin monatomic film confined to a slit-pore [i.e., between solid surfaces (walls)]. Both walls comprise atoms rigidly fixed in the face centered cubic (100) configuration; one wall is smooth on a nanoscale and the other is corrugated (i.e., scored with regularly spaced rectilinear grooves one to several nanometers wide). Properties of the film have been computed as a function of the lateral alignment (registry), with the temperature, chemical potential, and distance between the walls kept constant. Changing the registry carries the film through a succession of equilibrium states, ranging from all solid at one extreme to all fluid at the other. Over a range of intermediate registries the film consists of fluid and solid portions in equilibrium, that is fluid-filled nanocapillaries separated by solid strips. The range of registries over which such fluid-solid equilibria exist depends upon the width of the grooves and the frequency of the corrugation. For grooves of width comparable to the range of the interatomic potential, fluid and solid phases cease to coexist. In the limit of very wide grooves the character of the film is similar to that of the film confined by strictly smooth walls. The rich phase behavior of the confined film due to the coupling between molecular (registry) and nano (corrugation) scales has obvious implications for boundary lubrication. © 1994 American Institute of Physics.
