Mechanistic insight into biopolymer induced iron oxide mineralization through quantification of molecular bonding

Microbial production of iron (oxyhydr)oxides on polysaccharide rich biopolymers occurs on such a vast scale that it impacts the global iron cycle and has been responsible for major biogeochemical events. Yet the physiochemical controls these biopolymers exert on iron (oxyhydr)oxide formation are poorly understood. Here we used dynamic force spectroscopy to directly probe binding between complex, model and natural microbial polysaccharides and common iron (oxyhydr)oxides. Applying nucleation theory to our results demonstrates that if there is a strong attractive interaction between biopolymers and iron (oxyhydr)oxides, the biopolymers decrease the nucleation barriers, thus promoting mineral nucleation. These results are also supported by nucleation studies and density functional theory. Spectroscopic and thermogravimetric data provide insight into the subsequent growth dynamics and show that the degree and strength of water association with the polymers can explain the influence on iron (oxyhydr)oxide transformation rates. Combined, our results provide a mechanistic basis for understanding how polymer-mineral-water interactions alter iron (oxyhydr)oxides nucleation and growth dynamics and pave the way for an improved understanding of the consequences of polymer induced mineralization in natural systems

Incorporation of uranium into hematite during crystallization from ferrihydrite

Ferrihydrite was exposed to U(VI)-containing cement leachate (pH 10.5) and aged to induce crystallization of hematite. A combination of chemical extractions, TEM, and XAS techniques provided the first evidence that adsorbed U(VI) (≈3000 ppm) was incorporated into hematite during ferrihydrite aggregation and the early stages of crystallization, with continued uptake occurring during hematite ripening. Analysis of EXAFS and XANES data indicated that the U(VI) was incorporated into a distorted, octahedrally coordinated site replacing Fe(III). Fitting of the EXAFS showed the uranyl bonds lengthened from 1.81 to 1.87 Å, in contrast to previous studies that have suggested that the uranyl bond is lost altogether upon incorporation into hematite the results of this study both provide a new mechanistic understanding of uranium incorporation into hematite and define the nature of the bonding environment of uranium within the mineral structure. Immobilization of U(VI) by incorporation into hematite has clear and important implications for limiting uranium migration in natural and engineered environments. © 2014 American Chemical Society

Electrochemical and Photoelectrochemical Investigation of Water Oxidation with Hematite Electrodes

Atomic layer deposition (ALD) was utilized to deposit uniform thin films of hematite (α-Fe2O3) on transparent conductive substrates for photocatalytic water oxidation studies. Comparison of the oxidation of water to the oxidation of a fast redox shuttle allowed for new insight in determining the rate limiting processes of water oxidation at hematite electrodes. It was found that an additional overpotential is needed to initiate water oxidation compared to the fast redox shuttle. A combination of electrochemical impedance spectroscopy, photoelectrochemical and electrochemical measurements were employed to determine the cause of the additional overpotential. It was found that photogenerated holes initially oxidize the electrode surface under water oxidation conditions, which is attributed to the first step in water oxidation. A critical number of these surface intermediates need to be generated in order for the subsequent hole-transfer steps to proceed. At higher applied potentials, the behavior of the electrode is virtually identical while oxidizing either water or the fast redox shuttle; the slight discrepancy is attributed to a shift in potential associated with Fermi level pinning by the surface states in the absence of a redox shuttle. A water oxidation mechanism is proposed to interpret these results

Molecular Dynamics Simulations of the Interfacial Region between Boehmite and Gibbsite Basal Surfaces and High Ionic Strength Aqueous Solutions

Classical molecular dynamics (MD) simulations were used to study the interactions of up to 2 M NaCl and NaNO₃ aqueous solutions with the presumed inert boehmite (010) and gibbsite (001) surfaces. The force field parameters used in these simulations were validated against density functional theory calculations of Na⁺ and Cl^– hydrated complexes adsorbed at the boehmite (010) surface. In all the classical MD simulations and regardless of the ionic strength or the nature of the anion, Na⁺ ions were found to preferably form inner-sphere complexes over outer-sphere complexes at the aluminum (oxy)­hydroxide surfaces, adsorbing closer to the surface than both water molecules and anions. In contrast, Cl^– ions were predicted to distribute preferably in outer-sphere positions. The resulting asymmetry in adsorption strengths offers molecular-scale evidence for the observed isoelectric point (IEP) shift to higher pH at high ionic strength for aluminum (oxy)­hydroxides. As such, the MD simulations also provided clear evidence against the assumption that the basal surfaces of boehmite and gibbsite are inert to background electrolytes. Finally, the MD simulations indicated that the different affinities of NO₃^– and Cl^– for the surfaces might have macroscopic consequences, such as difference in the sensitivity of the IEP to the electrolyte concentration

Is the calcite–water interface understood?: Direct comparisons of molecular dynamics simulations with specular x-ray reflectivity data

New insights into the understanding of calcite-water interface structure are obtained through direct comparisons of multiple classical molecular dynamics (MD) simulations with high-resolution specular X-ray reflectivity (XR) data. This set of comparisons, with four different state of-the-art force fields (including two non-polarizable, one polarizable, and one reactive force field), reveal new insights into the absolute accuracy of the simulated structures and the uniqueness of the XR-derived structural results. These four simulations, though qualitatively similar, have visibly distinct interfacial structures and are distinguished through a quantitative comparison of the XR signals calculated from these simulations with experimental XR data. The results demonstrate that the simulated calcite-water interface structures, taken as a whole, are not consistent with the XR data (i.e., within the precision and accuracy of the XR data). This disagreement is largely due to the simulated calcite interfacial structure. The simulated interfacial water profiles show a higher level of consistency with the XR data, but with substantially different levels of agreement, with the rigid-ion model (RIM) simulations showing semi-quantitative agreement. Further comparisons of the structural parameters that describe the interfacial structure (derived from both the MD simulations and the XR data) provide further insight into the sources of differences between these two approaches. Using the new insights from the RIM simulations, new structures of the calcite-water interface consistent with both the experimental data and the simulation are identified and compared to recent results

Interaction of Ethanol and Water with the {104} Surface of Calcite

Molecular dynamics simulations have been used to model the interaction between ethanol, water, and the {104} surface of calcite. Our results demonstrate that a single ethanol molecule is able to form two interactions with the mineral surface (both Ca−O and O−H), resulting in a highly ordered, stable adsorption layer. In contrast, a single water molecule can only form one or other of these interactions and is thus less well bound, resulting in a more unstable adsorption layer. Consequently, when competitive adsorption is considered, ethanol dominates the adsorption layer that forms even when the starting configuration consists of a complete monolayer of water at the surface. The computational results are in good agreement with the results from atomic force microscopy experiments where it is observed that a layer of ethanol remains attached to the calcite surface, decreasing its ability to interact with water and for growth at the {104} surface to occur. This observation, and its corresponding molecular explanation, may give some insight into the ability to control crystal form using mixtures of different organic solvents