Diffusion Kinetics of Adsorbed Species on Pyrite Surfaces

Surface diffusion can bring redox pairs closer together, which is necessary for electron transfer to happen, thereby strongly influencing the overall kinetics. However, little is known about the diffusion kinetics of redox-active species on pyrite and other sulfides, which aid in catalyzing many redox reactions. Here, we calculate the diffusion of oxidant (UO22+) and reductant (Fe2+, HS–) species on pyrite {100} surfaces using quantum-mechanical calculations. Energy curves along different diffusion paths are derived for both inner- and outer-sphere complexes by moving the species in small increments (0.05–0.25 Å). The diffusion path along the molecular ridges formed by disulfide groups on the uppermost pyrite surface has the lowest energy barrier for the diffusion of all species tested. Single-particle diffusion coefficients along their optimal diffusion pathways are derived from diffusion energy barriers and attempt frequencies. Calculations are performed on flat defect-free pyrite surfaces, while on actual surfaces, diffusion is affected by defects, steps, and impurities. Calculated mobilities of the outer-sphere complexed uranyl and ferrous iron are about 4–5 times faster than their inner-sphere ones. Although the results here focus on single-particle diffusion, UO22+-HS– was used as an example for interdependent multiple-particle diffusion on the pyrite surface. Interactions between diffusing species (uranyl vs HS–), and to a limited degree jump correlations, were derived quantum-mechanically. Interactions are a combination of electronic interactions underneath the mineral surface and through the aqueous near-surface region; their interdependent diffusion can be approximated by apparent Coulomb interactions (with a dielectric constant of ∼7.7) for processing in subsequent Monte Carlo simulations

Actinyl Adsorption and Reduction on Pyrite Surfaces: Insights from DFT Calculations

Interactions of actinides with pyrite surfaces are highly important in catalyzing their reductive immobilization, thereby controlling the movement of these species in the environment. Here, surface adsorption and subsequent reduction of aqueous actinyl­(VI) on pyrite surfaces were explored using density functional hybrid theory (DFT-B3LYP) combined with a first hydration sphere of water molecules and a dielectric continuum for solvation effects. Adsorption of cationic (AnO2(H2O)5)2+(An = U, Np, Pu) and neutral AnO2(OH)2(H2O)3 actinyl onto a small pyrite cluster (Fe4S8) and the effect of coadsorption on the energetics and electron transfer are evaluated by adding either hydroquinone, H2Q (reduced), or quinone, Q (oxidized). The pyrite surface instantaneously transfers an electron to the adsorbed cationic actinyl. Unpaired electron atomic spin densities confirm the electron transfer from the pyrite surface to An atoms. For the neutral actinyl adsorption, electron transfer is confirmed for neptunyl and plutonyl but not for uranyl. Several factors control the overall adsorption energetics and kinetics, such as the nature of the coadsorbate (H2Q/Q), pyrite surface, actinyl, and charge or protonation state (cationic or neutral). The surface-mediated reduction of adsorbed actinyl occurs by receiving electrons either directly from the sulfide or from the coadsorbed H2Q through the sulfide. In the direct reduction case, an H+ ion is added to the surface-bound cationic actinyl, and the mineral surface acts as an electron donor. In contrast, in the proton-coupled electron transfer (PCET) reduction, the surface mediates the electrons through the surface by synergistically aligning relevant orbitals in line. This results in the less soluble and stable An­(IV). Our results indicate that the pyrite surface promotes a faster PCET reaction for the actinyl reduction under circumneutral (pH 4–7) conditions

Effects of Hydroxyl and Carboxyl Functional Groups on Calcite Surface Wettability Using Atomic Force Microscopy and Density Functional Theory

Surface-active compounds, primarily in asphaltene fractions of crude oil, are responsible for binding the nonpolar oil components to mineral surfaces and, therefore, control wettability changes on reservoir rock/mineral surfaces. Surface wettability changes occur mainly through polar functional groups in these compounds, such as hydroxyl, carboxyl, or carbonyl. By using crude oil with its asphaltene fraction removed, so-called maltenes, we investigate the effect of hydroxyl and carboxyl functional groups on wettability changes of calcite surfaces. Atomic force microscopy (AFM) images show significantly increased adsorption of maltenes on calcite samples treated with two asphaltene surrogates (phenol with a hydroxyl group and benzoic acid with a carboxyl one) than that on water-treated samples. However, the adsorbate patterns are different between those two asphaltene surrogates, suggesting different aggregation mechanisms. In addition, we observed the formation of larger surface-adsorbed droplets on the phenol or benzoic acid-treated calcite samples even for relatively short exposure times (<30 min) to maltenes. Quantum-mechanical calculations show more favorable adsorption for benzoic acid onto the calcite surface both on terraces and step edges. However, when a model oil molecule adsorbs onto those two preadsorbed asphaltene surrogates, nonpolar oil molecules preferentially adsorb onto phenol on terrace sites and benzoic acid on step edges. Overall, benzoic acid changes the calcite surface wettability more significantly than phenol

Tuning Electronic Properties of Functionalized Polyhedral Oligomeric Silsesquioxanes: A DFT and TDDFT Study

The structure and electronic properties of polyhedral oligomeric silsesquioxane (POSS) cages functionalized with different organic groups have been studied using density functional theory and time-dependent density functional theory calculations. Accordingly, the POSS-T8 cage is quite rigid upon functionalization and thus provides a means for spatially separating conjugated organic fragments, which is useful for the realization of novel organic molecular architectures for light-emitting diodes. Moreover, electronic properties can be tuned through the choice of functional groups and their positioning on or within the POSS cage. Attaching an electron-donating group, such as 4-carbazolephenyl, to the silicon atom at the corner of the cage raises the HOMO level, while attaching an electron-withdrawing group, such as 4-cyanophenyl, or inserting an inert molecule, such as N2, into the POSS cage lowers the LUMO level. Frontier orbital analysis indicates that the POSS cage is partially conjugated and serves a role as electron acceptor. Carrier transport rates are discussed in the frame of Marcus’ electron hopping theory. On the basis of the calculated reorganization energies, these POSS compounds can be used as carrier transporting or blocking materials, depending on the functionalization. Exciton binding energies strongly depend on the spatial arrangement of frontier orbitals rather than on molecular sizes

Mechanistic Study of Wettability Changes on Calcite by Molecules Containing a Polar Hydroxyl Functional Group and Nonpolar Benzene Rings

Oil extraction efficiency strongly depends on the wettability status (oil- vs water-wet) of reservoir rocks during oil recovery. Aromatic compounds with polar functional groups in crude oil have a significant influence on binding hydrophobic molecules to mineral surfaces. Most of these compounds are in the asphaltene fraction of crude oil. This study focuses on the hydroxyl functional group, an identified functional group in asphaltenes, to understand how the interactions between hydroxyl groups in asphaltenes and mineral surfaces begin. Phenol and 1-naphthol are used as asphaltene surrogates to model the simplest version of asphaltenes. Adsorption of oil molecules on the calcite {101̅4} surface is described using static quantum-mechanical density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations. DFT calculations indicate that adsorption of phenol and 1-naphthol occurs preferentially between their hydroxyl group and calcite step edges. 1-Naphthol adsorbs more strongly than phenol, with different adsorption geometries due to the larger hydrophobic part of 1-naphthol. MD simulations show that phenol can behave as an agent to separate oil from the water phase and to bind the oil phase to the calcite surface in the water/oil mixture. In the presence of phenol, partial separation of water/oil with an incomplete lining of phenol at the water/oil boundary is observed after 0.2 ns. After 1 ns, perfect separation of water/oil with a complete lining of phenol at the water/oil boundary is observed, and the calcite surface becomes oil-wet. Phenol molecules enclose decane molecules at the water–decane boundary preventing water from repelling decane molecules from the calcite surface and facilitate further accumulation of hydrocarbons near the surface, rendering the surface oil-wet. This study indicates phenol and 1-naphthol to be good proxies for polar components in oil, and they can be used in designing further experiments to test pH, salinity, and temperature dependence to improve oil recovery

Strain-Induced Segmentation of Magnesian Calcite Thin Films Growing on a Calcite Substrate

In crystal growth of mineral species or different compositional members of a solid solution on one another, the degree of lattice mismatch at their interface affects the growth pattern of the precipitating mineral phase. Fast layer-by-layer growth of magnesian calcite on pure calcite (101̅4) substrates has been observed at Mg2+/Ca2+ ratios of 2−7 using in situ atomic force microscopy. Under solution conditions of calcite saturation states starting from Ω ≈ 33, depending on Mg2+/Ca2+ ratios and carbonate content, bulging in the epitaxial magnesian calcite thin film led to the formation of networks of ridges along the [4̅41], [481̅], and [421̅] directions. Eventually, spreading of monolayers stopped at the ridges and formed stationary multilayer steps, resulting in separate and individually growing crystal segments. Molecular dynamics computational modeling suggests that relaxation of strain energy, caused by the interfacial lattice mismatch between pure calcite and the isostructural magnesium-containing phase with smaller lattice constants, leads to a semicoherent interface and disordered linear zones cutting through the thin film. As a consequence, the surface bulges up in a way similar to our laboratory observations. This strain-induced segmentation produces aggregates of aligned microcrystals and increase knowledge of the behavior of strained thin films in general

Density Functional Theory Studies of Chloroethene Adsorption on Zerovalent Iron

Adsorption of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on zerovalent iron is investigated using density functional theory (DFT) to evaluate hypotheses concerning the relative reactivity of these compounds on zerovalent iron. Four different chloroethene adsorption modes on the Fe(110) surface were studied using periodic DFT and the generalized gradient approximation (GGA). Of the adsorption sites examined, the atop site, where the chloroethene CC bond straddles a surface iron atom, was the most energetically favorable site for the adsorption of all three chloroethenes. Electronic structure and property analyses provide an indication of the extent of sp2-sp3 hybridization. The strong hybridization of the π-bonding orbital between the chloroethene CC bond and the iron surface suggests that adsorbed chloroethenes are strongly activated on Fe(110) and are likely precursors for subsequent chloroethene dissociation on the Fe surface. When the effect of solvation is indirectly taken into account in the DFT simulations by considering the hydration energies of chloroethenes in bulk water, the ordering of the adsorption energies of chloroethenes from the aqueous phase onto Fe(110) is in agreement with experimental observation (PCE > TCE > cis-DCE). Electronic properties of the adsorbed configurations of chloroethenes are also presented

Effect of EDTA Complexation on the Kinetics and Thermodynamics of Uranium Redox Reactions Catalyzed by Pyrite: A Combined Electrochemical and Quantum-Mechanical Approach

Ethylenediaminetetraacetic acid (EDTA), a strong complexing agent, is a common constituent of liquid nuclear waste streams. The interaction of EDTA or other organic or inorganic ligands with uranyl enhances the mobilization of uranium and therefore significantly influences the disposal and remediation of uranium waste. Although numerous studies have focused on the effect of EDTA on the mobilization of environmental uranium, including the adsorption and redox reactions of uranium on mineral surfaces, few studies have differentiated the reaction-controlling steps (diffusion, adsorption, and chemical reaction/electron transfer) occurring at heterogeneous interfaces to assess the effect of EDTA complexation on each of these processes. Here, a combination of in situ electrochemical methods and quantum-mechanical calculations was used to study the effect of EDTA complexation on the kinetics and thermodynamics of redox reactions of uranium at the pyrite-solution interface. Experiments indicate a potential shift of U­(VI)/U­(V) on the pyrite surface of ∼0.04 V due to surface adsorption and −0.21 V due to EDTA complexation, with similar results from the calculations. The oxidation of the U­(IV) reaction is more sensitive to EDTA complexation in solution than that of U­(V). Disproportionation, affecting 2/3 of the intermediate pentavalent state, is the rate-limiting step of the overall redox cycle of uranyl. The charge transfer was controlled by diffusion and adsorption processes. EDTA can promote the delivery of free uranyl from the bulk solution to the pyrite–water interface where the reaction takes place, making the electron-transfer process less diffusion-limited. Adsorption of uranyl onto pyrite surfaces in the gradient electric field is multimolecular layer adsorption, through which electron transfer can occur

Pentavalent Uranium Enriched Mineral Surface under Electrochemically Controlled Reducing Environments

Redox reactions of uranium (U) in aqueous environments have important impacts on the mobility and isotopic fractionation of U in the geosphere. Pentavalent U as the cationic uranyl ion, UO2+, is rarely observed in naturally occurring samples because of its limited lifetime, but it may be an important intermediate state controlling the redox kinetics between hexavalent and tetravalent U. Increasing evidence has indicated that U­(V) can be stabilized under laboratory conditions. Here, we showed that U­(V) is the dominant species on the magnetite (Fe3O4) surface under reducing conditions controlled by electrochemical methods. Cyclic voltammetry reveals coupled redox peaks corresponding to the U­(VI)­O22+/U­(V)­O2+ one-electron redox reaction. Magnetite electrodes polarized at a series of potentials to reduce U­(VI)­O22+ were characterized by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and Auger electron mapping. The results showed that up to twice the amount of U­(V) to U­(VI) was present on the magnetite surface. U­(V) adopted a typical uranyl-type structure, and the U coverage on the magnetite surface increased with decreasing potentials. The formation of mixed-valence U­(V)/U­(VI) species on the surface of magnetite may hinder the U­(V) disproportionation reaction, thereby eliminating the presence of tetravalent U. These results show that U­(V) can exist over short time scales as the dominant U species on mineral surfaces under selected reducing conditions by the controlled polarization of a mineral electrode

Electrochemical and Spectroscopic Evidence on the One-Electron Reduction of U(VI) to U(V) on Magnetite

Reduction of U­(VI) to U­(IV) on mineral surfaces is often considered a one-step two-electron process. However, stabilized U­(V), with no evidence of U­(IV), found in recent studies indicates U­(VI) can undergo a one-electron reduction to U­(V) without further progression to U­(IV). We investigated reduction pathways of uranium by reducing U­(VI) electrochemically on a magnetite electrode at pH 3.4. Cyclic voltammetry confirms the one-electron reduction of U­(VI) to U­(V). Formation of nanosize uranium precipitates on the magnetite surface at reducing potentials and dissolution of the solids at oxidizing potentials are observed by in situ electrochemical atomic force microscopy. XPS analysis of the magnetite electrodes polarized in uranium solutions at voltages from −0.1 to −0.9 V (E⁰_U(VI)/U(V)= −0.135 V vs Ag/AgCl) show the presence of only U­(V) and U­(VI). The sample with the highest U­(V)/U­(VI) ratio was prepared at −0.7 V, where the longest average U–O_axial distance of 2.05 ± 0.01 Å was evident in the same sample revealed by extended X-ray absorption fine structure analysis. The results demonstrate that the electrochemical reduction of U­(VI) on magnetite only yields U­(V), even at a potential of −0.9 V, which favors the one-electron reduction mechanism. U­(V) does not disproportionate but stabilizes on magnetite through precipitation of mixed-valence state U­(V)/U­(VI) solids