Molecular simulation of the diffusion of uranyl carbonate species in aqueous solution

Potential-based molecular dynamics simulations of aqueous uranyl carbonate species (M_xUO₂(CO₃)_y^2+2x−2y with M = Mg, Ca, or Sr) were carried out to gain molecular-level insight into the hydration properties of these species. The simulation results were used to estimate the self-diffusion coefficients of these uranyl carbonate species, which often dominate uranyl speciation in groundwater systems. The diffusion coefficients obtained for the monoatomic alkaline-earth cations and polyatomic ions (uranyl, carbonate, and uranyl tri-carbonate) were compared with those calculated from the Stokes-Einstein (SE) equation and its variant formulation by Impey et al. (1983). Our results show that the equation of Impey et al. (1983), originally formulated for monovalent monoatomic ions, can be extended to divalent monoatomic ions, with some success in reproducing the absolute values and the overall trend determined from the molecular dynamics simulations, but not to polyatomic ions, for which the hydration shell is not spherically symmetrical. Despite the quantitative failure of both SE formulations, a plot of the diffusion coefficients of the uranyl carbonate complexes as a function of the inverse of the equivalent spherical radius showed that a general linear dependence is observed for these complexes as expected from the SE equation. The nature of the alkaline-earth cation in the uranyl carbonate complexes was not found to have a significant effect on the ion’s diffusion coefficient, which suggests that the use of a single diffusion coefficient for different alkaline-earth uranyl carbonate complexes in microscopic diffusion models is appropriate.The potential model reproduced well published quantum mechanical and experimental data of and of the individual constituent ions, and therefore is expected to offer reliable predictions of the structure of magnesium and strontium uranyl carbonate aqueous species, for which there is no structural data available to date. In addition, the interatomic distances reported for could help with the refinement of the interpretation of EXAFS data of these species, which is made difficult by the similar uranium-distant carbonate oxygen and uranium-calcium distances.An analysis of the dynamics of water exchange around the alkaline-earth cations revealed that the presence of the uranyl tri-carbonate molecule has a strong influence on the geometry of the cation’s first hydration shell, which, in turn, can considerably affect the water exchange kinetics depending on whether the imposed geometry matches that around the isolated alkaline-earth cation. This result shows that the alkaline-earth uranyl carbonate complexes have distinct water exchange dynamics, which may lead to different reactivities. Finally, significant changes in water residence time were also predicted when replacing carbonate for water ligands in the uranyl coordination shell.     

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

The adsorption of uranyl (UO₂²⁺) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO₂ pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO₂)(CO₃)₃⁴⁻, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO₂-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.     

Sorption and catalytic oxidation of Fe(II) at the surface of calcite

The effect of sorption and coprecipitation of Fe(II) with calcite on the kinetics of Fe(II) oxidation was investigated. The interaction of Fe(II) with calcite was studied experimentally in the absence and presence of oxygen. The sorption of Fe(II) on calcite occurred in two distinguishable steps: (a) a rapid adsorption step (seconds-minutes) was followed by (b) a slower incorporation (hours-weeks). The incorporated Fe(II) could not be remobilized by a strong complexing agent (phenanthroline or ferrozine) but the dissolution of the outmost calcite layers with carbonic acid allowed its recovery. Based on results of the latter dissolution experiments, a stoichiometry of 0.4 mol% Fe:Ca and a mixed carbonate layer thickness of 25 nm (after 168 h equilibration) were estimated. Fe(II) sorption on calcite could be successfully described by a surface adsorption and precipitation model (Comans & Middelburg, GCA51 (1987), 2587) and surface complexation modeling (Van Cappellen et al., GCA57 (1993), 3505; Pokrovsky et al., Langmuir16 (2000), 2677). The surface complex model required the consideration of two adsorbed Fe(II) surface species, >CO₃Fe⁺ and >CO₃FeCO₃H⁰. For the formation of the latter species, a stability constant is being suggested. The oxidation kinetics of Fe(II) in the presence of calcite depended on the equilibration time of aqueous Fe(II) with the mineral prior to the introduction of oxygen. If pre-equilibrated for >15 h, the oxidation kinetics was comparable to a calcite-free system (t_1/2 = 145 ± 15 min). Conversely, if Fe(II) was added to an aerated calcite suspension, the rate of oxidation was higher than in the absence of calcite (t_1/2 = 41 ± 1 min and t_1/2 = 100 ± 15 min, respectively). This catalysis was due to the greater reactivity of the adsorbed Fe(II) species, >CO₃FeCO₃H⁰, for which the species specific rate constant was estimated.     

Friction characteristics of Cd-rich carbonate films on calcite surfaces: implications for compositional differentiation at the nanometer scale

Lateral Force Microscopy (LFM) studies were carried out on cleaved calcite sections in contact with solutions supersaturated with respect to otavite (CdCO₃) or calcite-otavite solid solutions (SS) as a means to examine the potential for future application of LFM as a nanometer-scale mineral surface composition mapping technique. Layer-by-layer growth of surface films took place either by step advancement or by a surface nucleation and step advancement mechanisms. Friction vs. applied load data acquired on the films and the calcite substrate were successfully fitted to the Johnson Kendall Roberts (JKR) model for single asperity contacts. Following this model, friction differences between film and substrate at low loads were dictated by differences in adhesion, whereas at higher load they reflect differences in contact shear strength. In most experiments at fixed load, the film showed higher friction than the calcite surface, but the friction-load dependence for the different surfaces revealed that at low loads (0–40 nN), a calcian otavite film has lower friction than calcite; a result that is contrary to earlier LFM reports of the same system. Multilayer films of calcian-otavite displayed increasing friction with film thickness, consistent with the expectation that the film surface composition will become increasingly Cd-rich with increasing thickness. Both load- and thickness-dependence trends support the hypothesis that the contact shear strength correlates with the hydration enthalpy of the surface ions, thereby imparting friction sensitivity in the LFM to mineral-water interface composition.     

Modelling the adsorption of uranyl on the surface of goethite

Atomistic simulations have been carried out to study the adsorption of the hydrated uranyl ion on a variety of faces of goethite (α-FeOOH). The relative stabilities of these faces have been assessed by calculation of the corresponding surface energies. We find that adsorption onto dry stable surfaces yields structures of variance with extended X-ray absorption fine structure data, whilst adsorption onto hydrated (110) and (001) surfaces yields structures compatible with these data.     

Magnetic anisotropy of calcite at room-temperature

The intrinsic room temperature magnetic properties of pure calcite were determined from a series of natural crystals, and they were found to be highly dependent on the chemical composition. In general, dia-, para-, and ferromagnetic components contribute to the magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS). With a combination of magnetic measurements and chemical analysis these three contributions were determined and related to their mineralogical sources. The intrinsic diamagnetic susceptibility of pure calcite is − 4.46 ± 0.16 × 10^− 9 m³/kg (− 12.09 ± 0.5 × 10^− 6 SI) and the susceptibility difference is 4.06 ± 0.03 × 10^− 10 m³/kg (1.10 ± 0.01 × 10^− 6 SI). These diamagnetic properties are easily dominated by other components. The paramagnetic contribution is due to paramagnetic ions in the crystal lattice that substitute for calcium; these are mainly iron and manganese. The measured paramagnetic susceptibility agrees with the values calculated from the known concentration of paramagnetic ions in the crystals according to the Curie law of paramagnetic susceptibility. Substituted iron leads to an increase in the AMS. The paramagnetic susceptibility difference was found to correlate linearly with the iron content for concentrations between 500 and 10,000 ppm. An empirical relation was determined: (k₁ − k₃)^para (kg/m³) = Fe-content (ppm) × (1 ± 0.1) × 10^− 12 (kg/m³/ppm). The maximum susceptibility difference (Δk = k₁ − k₃) was found to be unaffected by iron contents below 100 ppm. Ferromagnetic contributions due to inclusions of ferromagnetic minerals can dominate the susceptibility. They were detected by acquisition of isothermal remanent magnetization (IRM) and their contribution to the AMS was separated by high-field measurements.     

Molecular models of a hydrated calcite mineral surface

Hydrated mineral surfaces play an important role in many processes in biological, geological, and industrial applications. An energy force field was developed for molecular mechanics and molecular dynamics simulations of hydrated carbonate minerals and was applied to investigate the behavior of water on the calcite surface. The force field is a significant development for large-scale molecular simulations of these systems, and provides good agreement with experimental and previous modeling results. Simulations indicate that water molecules are significantly ordered near the calcite surface. The predominant surface configuration (75-80%) results from coordination of a water molecule with a single calcium cation-carbonate anion pair, while the less common situation involves water coordination with two ion pairs. Surface restructuring and variation in coordination in the water layers results in distinct distances for water oxygens above the calcite surface—a two-component first monolayer (2.3 and 3.0 Å) and a secondary monolayer (5.0 Å). The different coordinations also alter lateral displacement, hydrogen bonding, and surface-normal orientation of the water molecules. The ordering of water molecules and the formation of a unique hydrogen bonding network at the calcite surface influence the physical properties of the interfacial water. Surface exchange of water molecules is observed by molecular dynamics simulation to occur at a rate of one exchange per 10 ps. Diffusion coefficients derived from mean square displacement analysis of atomic trajectories indicate a dependence of water transport based on the distance of the water molecules from the calcite surface.     

X-ray absorption spectroscopy study of Cu and Zn adsorption complexes at the calcite surface: Implications for site-specific metal incorporation preferences during calcite crystal growth

We report results from in situ extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Cu(II) and Zn(II) complexes forming at the calcite surface following adsorption from preequilibrated calcite-saturated solutions. Both Cu(II) and Zn(II) coordinate at Ca sites on the calcite surface, forming mononuclear inner-sphere adsorption complexes. The Zn adsorption complexes are in tetrahedral coordination with first-shell O neighbors with R_Zn-O = 1.95 Å, and the Cu complexes are Jahn-Teller distorted, with equatorial R_Cu-O = 1.95 Å. Results from EXAFS data of dilute Cu- and Zn-calcite solid solutions confirm substitution of these metals in the Ca site of the calcite structure as octahedral complexes during coprecipitation. X-ray fluorescence microanalyses of calcite (101?4) hillocks grown in coprecipitation experiments show that divalent Cu and Zn, which have ionic radii smaller than Ca, are preferentially incorporated into the parallel arrays of <4?41>₊ steps that define one pair of symmetrically equivalent vicinal faces on polygonized growth spirals. In contrast, other divalent metals with sixfold ionic radii smaller than Ca (Co, Cd, Mn, Mg) have been shown to be preferentially incorporated into <4?41>₋ growth steps, which define the second pair of vicinal faces on the growth spirals, but which are symmetrically nonequivalent to the steps on the first pair. The distortion from octahedral symmetry observed for the Cu and Zn adsorption complexes likely plays a key role in the observed preference of Cu and Zn for incorporation into the <4?41>₊ steps.     

Arsenate uptake by calcite: Macroscopic and spectroscopic characterization of adsorption and incorporation mechanisms

Batch uptake experiments and X-ray element mapping and spectroscopic techniques were used to investigate As(V) (arsenate) uptake mechanisms by calcite, including adsorption and coprecipitation. Batch sorption experiments in calcite-equilibrated suspensions (pH 8.3; PCO₂ = 10^−3.5 atm) reveal rapid initial sorption to calcite, with sorption rate gradually decreasing with time as available sorption sites decrease. An As(V)-calcite sorption isotherm determined after 24 h equilibration exhibits Langmuir-like behavior up to As concentrations of 300 μM. Maximum distribution coefficient values (K_d), derived from a best fit to a Langmuir model, are ∼190 L kg⁻¹.Calcite single crystals grown in the presence of As(V) show well-developed rhombohedral morphology with characteristic growth hillocks on surfaces at low As(V) concentrations (?5 μM), but habit modification is evident at As(V) concentrations ?30 μM in the form of macrostep development preferentially on the − vicinal surfaces of growth hillocks. Micro-X-ray fluorescence element mapping of surfaces shows preferential incorporation of As in the − vicinal faces relative to + vicinals. EXAFS fit results for both adsorption and coprecipitation samples confirm that As occurs in the 5+ oxidation state in tetrahedral coordination with oxygen, i.e., as arsenate. For adsorption samples, As(V) forms inner-sphere surface complexes via corner-sharing with Ca octahedra. As(V) coprecipitated with calcite substitutes in carbonate sites but with As off-centered, as indicated by two Ca shells, and with likely disruption of local structure. The results indicate that As(V) interacts strongly with the calcite surface, similar to often-cited analog phosphate, and uptake can occur via both adsorption and coprecipitation reactions. Therefore, calcite may be effective for partial removal of dissolved arsenate from aquatic and soil systems.     

The influence of temperature and seawater composition on calcite crystal growth mechanisms and kinetics: Implications for Mg incorporation in calcite lattice

The composition of carbonate minerals formed in past and present oceans is assumed to be significantly controlled by temperature and seawater composition. To determine if and how temperature is kinetically responsible for the amount of Mg incorporated in calcite, we quantified the influence of temperature and specific dissolved components on the complex mechanism of calcite precipitation in seawater. A kinetic study was carried out in artificial seawater and NaCl-CaCl₂ solutions, each having a total ionic strength of 0.7 M. The constant addition technique was used to maintain [Ca²⁺] at 10.5 mmol kg⁻¹ while [] was varied to isolate the role of this variable on the precipitation rate of calcite.Our results show that the overall reaction of calcite precipitation in both seawater and NaCl-CaCl₂ solutions is dominated by the following reaction:

     

Molecular-scale surface processes during the growth of calcite in the presence of manganese

This paper deals with the growth behaviour of the Mn-Ca-CO₃-H₂O solid solution-aqueous solution system on calcite {104} surfaces. This system represents a model example, which allows us to study the effect of a number of controlling factors on the crystallisation: (1) the supersaturation function, β(x), and nucleation rate function, J(x), for the Mn-Ca-CO₃-H₂O system, (2) the relationship of such functions to the molecular scale growth mechanisms operating on growing surfaces, and (3) the surface structure of the calcite {104} faces. In situ atomic force microscopy (AFM) growth experiments revealed a wide variety of surface phenomena, such as the transition between growth mechanisms, anisotropic changes in the step rates, and the influence of the Mn-bearing newly formed surface on subsequent growth (step stoppage followed by the formation of two-dimensional nuclei and the reproduction of the original calcite {104} surface microtopography). These phenomena result from the interplay between the controlling parameters and are explained in those terms.     

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T _2c–T)Q ²+1/6CQ ⁶ with a=24 J·K·mol^-1, C = 30 kJ·mol^– T _c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.     

Toward a conceptual model of the calcite surface: hydration,hydrolysis, and surface potential

Because of a recent increase in interest in the properties of the calcite surface, there has also been an increase in activity toward development of mathematical models to describe calcite’s surface behaviour, particularly with respect to adsorption and precipitation. For a mathematical model to be realistic, it must be based on a sound conceptual model of atomic structure at the interface. New observations from high resolution techniques have been combined with previously published data to resolve the apparent conflict with results from electrokinetic studies and to present a picture of what the calcite surface probably looks like at the atomic scale.In ultra-high vacuum (10⁻¹⁰ mbar), a cleaved surface remains unreacted for at least an hour, but the unreacted surface does not remain as a termination of the bulk structure. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) show that the outer-most atomic layer relaxes and the surface slightly restructures. In air, dangling bonds are satisfied by hydrolysed water. XPS and time-of-flight secondary ion mass spectrometry (TOF-SIMS) reveal the presence of adsorbed OH and H. In AFM images, the features so typical of calcite, namely, alternate-row offset, pairing and height difference, as well as the consistent dependence of these features on the force and direction of tip scanning, are best explained by OH filling of the vacant O sites created during cleavage on the Ca octahedra. Thus there is solid evidence to indicate the presence of OH and H chemi-sorbed at the termination of the bulk calcite structure.Wet chemical studies, however, show that calcite’s pH_pzc (zero point of charge) varies with sample history and solution composition. Electrophoretic mobility measurements indicate that the potential-determining ions are not H⁺ and OH⁻, but rather Ca²⁺ and CO₃²⁻ (or HCO₃⁻ or H₂CO₃⁰). This apparent conflict is resolved by a slight modification of the electrical double layer (EDL) model. At the bulk termination, hydrolysis species are chemi-bonded. At the Stern layer, adsorption attaches Ca²⁺ and CO₃²⁻ (or other carbonate species), but the hydrolysis layer keeps them in outer-sphere coordination to the surface. With dehydration, loss of the hydrolysis species results in direct contact between adsorbed ions and the bulk termination, therefore, inner-sphere sorption is equivalent to extension of the three dimensional bulk network, which is precipitation. Attachment of ions with size and charge compatible with Ca and CO₃ likewise results in coprecipitation and solid–solution formation.     

Sorption of trace metals on calcite: Applicability of the surface precipitation model

Published Sorption isotherm data of Cd²⁺, Mn²⁺, Zn²⁺, and Co²⁺ on calcite are adequately described by the surface precipitation model which was originally developed by FArley et al. (1985) for the sorption of cations on metal oxides. In addition to monolayer adsorption, the model accounts for the formation of a surface phase with a composition that is described by a solid solution having as end members the sorbent calcium carbonate mineral and a pure carbonate precipitate of the sorbing trace metal. The model thus specifies a continuum between adsorption and precipitation. This feature is supported in the literature by observations on the reaction kinetics and the amount of surface coverage during trace metal sorption on calcite. The apparent adsorption constants of these trace metals, as derived from the model, can be ranked according to the degree to which their ionic radii match the ionic radius of Ca²⁺.     

Composition of carbonate overgrowths produced on Iceland spar calcite crystals buried in bahamian carbonate-rich sediments

High-magnesium calcite overgrowths were precipitated on Iceland spar calcite crystals buried in Bahamian sediments. Their composition was within the range of carbonate cements in this area. The precipitation rate of the overgrowths was slower and the magnesium content higher than predicted from laboratory experiments, possibly due to adsorption of organic compounds.     

Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model

Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10^?5 M, (dissolved uranyl 10^?5 to 10^?8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO₃ solutions containing up to 0.01 M NaHCO₃. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10^?3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO₂OH⁺ and (UO₂)₃(OH)⁺₅, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO₂OH⁺ surface complex, and a monodentate, bidentate or tridentate (UO₂)₃(OH)⁺₅surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10^?2 and 10^?3 M.     

松辽盆地宝龙山铀矿床中铀酰碳酸盐矿物的发现与砂岩型铀矿成矿的新途径

1 研究目的(Objective) 国内中新生代沉积盆地(伊犁、吐哈、鄂尔多斯、二连、松辽)中砂岩型铀矿中铀矿物类型以沥青铀矿或铀石为主,也可见少量的次生铀矿物,如水硅铀矿,与铀石可以相互转化.笔者在研究松辽盆地宝龙山铀矿床铀赋存状态过程中发现少量的铀酰碳酸盐矿物,本文运用扫描电镜、能谱与红外吸收光谱分析等方法对铀酰碳...