Biomedical subjects

Eugene S Ilton

Publications and source records attributed to Eugene S Ilton.

4 recordsLinked to original sources

PubMed2006

Molecular dynamics simulations of electrolyte solutions at the (100) goethite surface.

Molecular dynamics simulations of electrolyte solutions in contact with a neutral (100) goethite (alpha-FeOOH) surface were used to probe the structure of the mineral-water interface and gain insight into the adsorption properties of monovalent ions. Three electrolyte solutions were considered: NaCl, CsCl, and CsF. The electrolyte ions were chosen to cover a range of ionic sizes and affinities for the aqueous phase. The molecular dynamics simulations indicate the presence of a structured interfacial region resulting from the strong interaction of water with the mineral surface. The specific arrangement and preferred orientation of water that arise from this interaction create adsorption sites in the interfacial region, i.e., as far as 15 A away from the surface, and hence give rise to a strong correlation between the water and ion distributions. The structure of the hydrated ion, its effect on the water arrangement at the interface, and the strength of the ion-water bond are found to be key factors that determine the location and extent of ion adsorption at the interface. Additionally, in all simulations, we find a build up of positive charges near the surface due to cation adsorption, which is compensated by an accumulation of anions in the next few angströms. This creates an excess of negative charges, which is in turn compensated by an excess of positive charges, and so on. As we modeled a neutral surface, the structure of the electrolyte distribution arises from the complex interplay of the interactions between the surface, water, and the electrolyte ions rather than from the need to neutralize a surface charge. In addition, our simulations indicate that the electrolyte distribution does not resemble that of a classical electrical double layer. Indeed, our calculations predict the presence of several condensed layers and oscillations in the net charge away from the surface.

Journal Article↗

PubMed2006

Reduction of uranyl in the interlayer region of low iron micas under anoxic and aerobic conditions.

The role of the interlayer region of three different FeII-poor micas (2.8, approximately 0.02, and 0.01 FeII atom %) in the reduction of UVI was investigated at pH approximately 4.3, low ionic strength, and 23 degrees C under both anoxic and aerated conditions in batch reactors for 1 to 10 days. U diffusion profiles were mapped with X-ray microscopy, and the oxidation state of U was tracked with XANES as a function of distance from the crystal edge. Peak U concentrations only reached about 0.006-0.0002 atom percent, where the full-width-at-half-maximums of the diffusion fronts were roughly 25-50 microm. Results for anoxic conditions indicated appreciable reduction of uranyl in all three micas with no measurable dependency on distance from the edge. Under aerobic (open to the atmosphere) conditions, U was still partially reduced, although to a lesser degree than for anoxic conditions, and UVI/Utot increased with time. Interestingly, reoxidation reached a plateau after about 3-10 days, leaving an appreciable proportion of U reduced. It is likely that measurable reduction occurred, despite low FeII, because U concentrations were even lower, where FeII/U ratios ranged from about 10(4)-10(2). A coupled diffusion-reduction model was developed that successfully simulated the proximal portion of the diffusion front. The results show that reduction of UVI in the interlayer of low FeII micas is, in principle, viable and that under aerobic conditions reduction is initially faster than reoxidation. This extends the range of possible environments where heterogeneous reduction of UVI by micas might occur.

Absorptiometry, Photon↗

PubMed2005

Effects of compositional defects on small polaron hopping in micas.

Hartree-Fock calculations and electron transfer (ET) theory were used to model the effects of compositional defects on ET in the brucite-like octahedral sheet of mica. ET was modeled as an Fe(IIIII) valence interchange reaction across shared octahedral edges of the M2-M2 iron sublattice. The model entails the hopping of localized electrons and small polaron behavior. Hartree-Fock calculations indicate that substitution of F for structural OH bridges increases the reorganization energy lambda, decreases the electronic coupling matrix element V(AB), and thereby substantially decreases the hopping rate. The lambda increase arises from modification of the metal-ligand bond force constants, and the V(AB) decrease arises from reduction of superexchange interaction through anion bridges. Deprotonation of an OH bridge, consistent with a possible mechanism of maintaining charge neutrality during net oxidation, yields a net increase in the ET rate. Although substitution of Al or Mg for Fe in M1 sites distorts the structure of adjacent Fe-occupied M2 sites, the distortion has little net impact on ET rates through these M2 sites. Hence the main effect of Al or Mg substitution for Fe, should it occur in the M2 sublattice, is to block ET pathways. Collectively, these findings pave the way for larger-scale oxidation/reduction models to be constructed for realistic, compositionally diverse micas.

Journal Article↗

PubMed2005

Mica surfaces stabilize pentavalent uranium.

High-resolution X-ray photoelectron spectroscopy was used to demonstrate that reduction of aqueous U6+ at ferrous mica surfaces at 25 degrees C preserves U5+ as the dominant sorbed species over a broad range of solution compositions. Polymerization of sorbed U5+ with sorbed U6+ and U4+ is identified as a possible mechanism for how mineral surfaces circumvent the rapid disproportionation of aqueous U5+. The general nature of this mechanism suggests that U5+ could play an important, but previously unidentified, role in the low-temperature chemistry of uranium in reducing, heterogeneous aqueous systems.

Journal Article↗