Diffusion and Adsorption of Uranyl Carbonate ... - ACS Publications

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Diffusion and Adsorption of Uranyl Carbonate Species in Nanosized Mineral Fractures Sebastien Kerisit* and Chongxuan Liu Pacific Northwest National Laboratory, Chemical and Materials Sciences Division, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Atomistic simulations were performed to study the diffusion and adsorption of Ca2UO2(CO3)3 and of some of its constituent species, i.e., UO22+, CO32−, and UO2CO3, in feldspar nanosized fractures. Feldspar is important to uranium remediation efforts at the U.S. Department of Energy Hanford site as it has been found in recent studies to host contaminants within its intragrain fractures. In addition, uranyl carbonate species are known to dominate U(VI) speciation in conditions relevant to the Hanford site. Molecular dynamics (MD) simulations showed that the presence of the feldspar surface diminishes the diffusion coefficients of all of the species considered in this work and that the diffusion coefficients do not reach their bulk aqueous solution values in the center of a 2.5 nm fracture. Moreover, the MD simulations showed that the rate of decrease in the diffusion coefficients with decreasing distance from the surface is greater for larger adsorbing species. Free energy profiles of the same species adsorbing on the feldspar surface revealed a large favorable free energy of adsorption for UO22+ and UO2CO3, which are able to adsorb to the surface with their uranium atom directly bonded to a surface hydroxyl oxygen, whereas adsorption of CO32− and Ca2UO2(CO3)3, which attach to the surface via hydrogen bonding from a surface hydroxyl group to a carbonate oxygen, was calculated to be either only slightly favorable or unfavorable.

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However, several research groups 9−17 have shown, using MD simulations, that the diffusion of aqueous species can be significantly influenced by the presence of a mineral surface. In a previous publication,9 we investigated the effects of confinement and of the presence of the mineral surface on the diffusion of water and electrolyte ions in nanosized mineral fractures and found the presence of a 2.0−2.5 nm interfacial region within which the diffusion coefficients of water and of the electrolyte ions differed from their bulk values and decreased significantly as the species approached the mineral surface. The simulations also indicated that the contribution of the surface effects only become negligible for fractures that are several tens of nanometers wide. We thus expect these microscopic surface effects to play a significant role in the U(VI) diffusion in subsurface materials with a significant fraction of nano- and microsized pores as those found in some of the Hanford Site sediments.3,5 Therefore, the goal of this research is to provide atomic-level insights into the contribution of microscopic surface effects to the diffusion process of uranyl species in porous media with nano- to microsized pores using atomistic simulation techniques. The experimental determination of self-diffusion coefficients with techniques such as the nuclear magnetic resonance spin echoes in the presence of pulsed magnetic field gradients 5,18 or the diaphragm cell technique19 is particularly

INTRODUCTION Uranium is a major radionuclide contaminant in groundwater systems at sites where nuclear materials were processed and stored.1 Consequently, one of the main challenges for uranium remediation at nuclear facilities is to develop a comprehensive understanding of the reactivity and stability of uranium in order to predict its fate in subsurface environments. Uranium has been found predominantly in microscopic intragrain domains in contaminated sediments such as from the US Department of Energy (DOE) Hanford Site.2,3 In addition, ion diffusion has been shown to be a major process contributing to the preferential uranium concentration in these sediments and is predicted to control the future release of uranium from the sediments.2,4,5 One important limitation in studies of uranium diffusion is the lack of self-diffusion coefficients for uranyl species in groundwater,6 which has led to the development of diffusion models, referred to here as apparent diffusion models, whereby the total dissolved U(VI) concentration is considered as a single chemical species. Recently, the self-diffusion coefficients of alkaline-earth uranyl carbonate species were determined using molecular dynamics (MD) simulations.7 This allowed for the development of a species-based diffusion model6 that explicitly treats the variations in mass, charge, and diffusion coefficient of aqueous uranyl species, which proportions can be determined from speciation reactions. In test cases, the species-based diffusion model was shown to give a good description of the uranyl release from and diffusion into a saprolite sediment from the DOE Oak Ridge Site6 and of the mass transfer limited uranyl adsorption in Hanford sediments.8 © 2011 American Chemical Society

Received: Revised: Accepted: Published: 1632

August 8, 2011 December 17, 2011 December 29, 2011 December 29, 2011 dx.doi.org/10.1021/es2027696 | Environ. Sci. Technol. 2012, 46, 1632−1640

Environmental Science & Technology

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the uranyl ion,37 and of Pavese et al. for the carbonate ion and the calcium−carbonate interactions.38 The interactions between calcium and water were described with the parameters of de Leeuw and Parker39 and the parameters of Wander et al. were employed to model the interactions between uranyl and carbonate.40 Some modifications were made to the CLAYFF force field, as previously reported.26 The model parameters used in this work are the same as those employed in our previous publications on the orthoclase-water interface 9,26 and the hydration of uranyl carbonate species.7 All of these parameters are provided in the Supporting Information, SI. The accuracy of molecular dynamics simulations is dependent on the potential parameters chosen to carry out the simulations. Although not all aspects of the results presented in this work can be validated against experimental data, the set of potential parameters used in this work was shown previously to give good agreement with experimental data on several properties of the system of interest including the orthoclase bulk structure,26 the structure of orthoclase−water interfaces,26 the structure and dynamics of water around the aqueous species of interest,7 and the diffusion coefficient of several aqueous species studied.7 All of the simulations were performed in the NVT ensemble (constant number of particles, constant volume, and constant temperature) at 300 K and zero-applied pressure. The temperature was kept constant by use of the Nosé-Hoover41 thermostat with a thermostat relaxation time of 0.5 ps. The electrostatic interactions were calculated by means of the Ewald summation method.42 A 9 Å cutoff was used for the short-range interactions and the real part of the Ewald sum. The Ewald sum parameters were chosen to achieve a relative error on the electrostatic energy of at most 10−7. The Verlet leapfrog integration algorithm was used to integrate the equations of motion with a time step of 1 fs. The geometry of the water molecules was held fixed using the SHAKE algorithm.43 As mentioned above, the (001) orthoclase surface was considered in this work. Orthoclase is the potassium endmember of alkali feldspar, a framework silicate. Corner sharing AlO4 and SiO4 tetrahedra form a three-dimensional lattice within which potassium ions fill cavities to charge compensate for the Al substitution for Si. The (001) orthoclase slab was generated as previously and was terminated by a full layer of hydroxyl groups. The surface area was 25.69 × 26.93 Å2. The plane of the surface was used as the zero distance and was defined as the plane that passes through the topmost Si/Al tetrahedral sites. The direction normal to the surface was extended to create a fracture 25-Å wide. The fracture was filled with 556 water molecules. In this work, we focus on the diffusion and adsorption of four species, namely uranyl (UO22+), carbonate (CO32−), uranyl carbonate (UO2CO3), and calcium uranyl tricarbonate (Ca2UO2(CO3)3). These four species will be referred to, as a group, as the uranyl carbonate species. 1, 1, 2, and 6 water molecules were removed when inserting UO22+, CO32−, UO2CO3, and Ca2UO2(CO3)3 in the mineral fracture, respectively. To construct the diffusion coefficient profiles, approximately 20 constrained MD simulations were carried out for each species along the normal to the surface at 0.5 Å intervals. In each simulation, the center of mass of the species of interest was constrained at a height z above the mineral surface using the SHAKE algorithm.43 The species were allowed to move in the plane parallel to the surface. The atomic velocities were initially scaled for 100 ps to the target temperature followed by

challenging for uranyl species because of the coexistence of various U(VI) aqueous species under most experimental conditions.20 Therefore, atomistic simulation is a powerful tool that can be used to determine the diffusion coefficients of individual uranyl species and that can isolate and quantify the microscopic surface effects. U(VI) is known to form complexes with a range of inorganic ligands. For example, carbonate is a common ligand in groundwater environments and previous atomistic modeling work on uranium in natural systems has thus often concentrated on uranyl carbonate species.7,21−24 In particular, uranyl carbonate species such as Ca2UO2(CO3)3 will dominate U(VI) aqueous speciation25 in the pH conditions relevant to those observed at the DOE Hanford Site. Consequently, one of the specific objectives of this study is to determine the diffusion profile, in the vicinity of a model orthoclase (potassium feldspar, KAlSi3O8) surface, of an alkaline-earth uranyl carbonate species, namely Ca2UO2(CO3)3, and of some of its constituent species, i.e., UO22+, CO32−, and UO2CO3. Orthoclase was selected as a model surface, in this work and in previous work,9,26 as feldspar has been shown to be an important type of minerals that preferentially sequestered U(VI) within its nanoand microsized fractures at the DOE Hanford Site. Moreover, the (001) surface of orthoclase was chosen as its calculated interfacial electron density profile was previously found26 to be in excellent agreement with that derived from high-resolution X-ray reflectivity measurements by Fenter and co-workers.27 In addition, we calculated the free energy profile of these species as they adsorb on the orthoclase surface to determine the number and nature of adsorbed surface species, their affinity for the orthoclase surface, and the kinetics of adsorption/desorption. Finally, information on the atomiclevel structure of uranyl adsorbed on mineral surfaces can be obtained using extended X-ray absorption fine structure spectroscopy (EXAFS); indeed, data is available for uranyl adsorption on sodium feldspar28 and other silicate minerals29 and EXAFS studies of uranyl carbonate species adsorbed on mineral surfaces have also been reported,30,31 although not on feldspar surfaces to the best of our knowledge. Atomic-level information on the adsorption configuration of uranium species on mineral surfaces is critical to the development of surface complexation models aimed at predicting uranium transport in contaminated groundwater systems. However, interpretation of EXAFS spectra can be complicated, in particular if the atom probed with X-ray is present in a mixture of different species.32 Consequently, analysis and interpretation of EXAFS spectra can greatly benefit from input from atomistic simulations.33 Therefore, mean interatomic distances of the adsorbed species calculated in this work are also presented to help interpretation of future EXAFS measurements and to contrast the calculated distances with published experimental data.

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COMPUTATIONAL METHODS The MD simulations were carried out with the computer program DL_POLY.34 In these simulations, atoms are represented as point-charge particles that interact via longrange Coulombic forces and short-range interactions. The latter are described by parametrized functions and represent the repulsion between electron-charge clouds, van der Waals attraction forces, and many-body terms such as bond bending. The short-range potential parameters and ionic charges used in this study are those of the SPC/E model for water,35 the CLAYFF force field for orthoclase,36 of Guilbaud and Wipff for 1633

dx.doi.org/10.1021/es2027696 | Environ. Sci. Technol. 2012, 46, 1632−1640


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data collection for 5 ns. For each simulation, the diffusion coefficient in the direction parallel to the surface, D, was computed from the mean square displacement (MSD):

Dt =

1 ⟨|ri(t ) − ri(0)|2 ⟩ 4

(1)

where ri(t) is the position of atom i at time t. Only the atomic coordinates in the direction parallel to the surface were considered. A configuration was recorded every 0.2 ps for calculating D. The uncertainty in D at each height was obtained by calculating the standard deviation of the mean when each trajectory was divided into five subtrajectories of equal length. From the same set of constrained MD simulations, the free energy difference between a species at a height z and the same species in the middle of the fracture (i.e., at height z0) can be obtained by integrating the average force in the direction perpendicular to the surface, fz, acting on the center of mass of this species from z0 to z:44

ΔA(z) = A(z) − A(z 0) =

∫z

z 0

⟨fz (z)⟩dz

(2)

In addition to the simulations mentioned in the previous paragraph, additional simulations were performed at 0.125 Å intervals in the close vicinity of the surface to better describe the free energy of the adsorbed surface species. The additional simulations were identical to those described above with the exception of the data collection time, which was reduced to 1 ns. Indeed, longer simulations were not necessary as the force on the center of mass was found to converge much more rapidly than the value of the diffusion coefficient.

Figure 1. (Top) Diffusion coefficients of the uranyl carbonate species as a function of distance from the orthoclase (001) surface. Also shown are the corresponding bulk diffusion coefficients (horizontal lines) and the water density (hashed line). (Bottom) Diffusion coefficients normalized to the corresponding bulk diffusion coefficients. For H2O, (1) and (2) refer to the unconstrained and constrained approaches, respectively.

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RESULTS AND DISCUSSION Diffusion of Uranyl Carbonate Species in Orthoclase Fracture. The diffusion coefficients of the four species of interest were calculated as a function of distance and are shown in Figure 1. The water density profile is also displayed in Figure 1 and it shows the presence of an adsorbed layer at 2.89 Å above the topmost tetrahedral site, which is consistent with the experimental value of 2.75 ± 0.1 Å obtained by Fenter et al.27 from X-ray reflectivity measurements. A detailed description of the orthoclase−water interface including an extensive comparison with experimental data was presented in a previous publication.26 As seen previously for water and sodium and chloride ions,9 the diffusivity of uranyl carbonate species decreases as they approach the surface. Figure 1 also shows the corresponding bulk diffusion coefficients. The bulk diffusion properties of the uranyl carbonate species was extensively discussed in a previous publication.7 For all four species, the diffusion coefficient in the middle of the fracture does not reach the bulk value. This result is consistent with the results obtained for water in a previous publication.9 Indeed, this work showed that a pore size of at least 5 nm is required for the water diffusion coefficient to reach its bulk value. UO22+ and CO32−, which have similar bulk diffusion coefficients, also show similar diffusion profiles. In both profiles, the diffusion coefficient plateaus and becomes negligible within approximately 5.5 Å from the surface. As discussed in detail later, this distance corresponds to the height at which the two ions become adsorbed on the surface and thus stop diffusing in the plane parallel to the surface. Figure 1 also shows that, due to its much larger size, Ca2UO2(CO3)3 shows a diffusion profile that plateaus further away from the surface. To

evaluate whether the presence of the mineral surface affects the aqueous species differently, the diffusion coefficient profiles were normalized to their respective bulk value, as shown in Figure 1, together with the equivalent profile for water. Interestingly, the diffusion profiles of the uranyl carbonate species diminish much more rapidly than that of water. Among the uranyl carbonate species, the largest species, Ca2UO2(CO3)3, shows a profile that decreases more quickly than that of the other species; therefore, the MD simulations suggest that the relative effect of the mineral surface on the diffusion of aqueous species increases with the species size and that, consequently, the water diffusion profile cannot be used to predict that of other aqueous species. Also shown in Figure 1 is the normalized diffusion coefficient of water obtained in ref 9 with a different approach whereby a MD trajectory is divided into 10-ps subtrajectories and the geometric center of each subtrajectory is used to assign the calculated MSD for that subtrajectory to a particular height above the surface. This approach is applicable to water due to its high concentration, which also leads to smaller error bars.9 The constrained and unconstrained diffusion profiles are in good agreement in the middle of the water slab. The constrained profile then diminishes slightly more rapidly, although remains within error of the unconstrained profile. Closer to the surface, the constrained profile is lower than the unconstrained one, albeit with a similar gradient. It should be noted that the unconstrained approach can potentially lead to 1634



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away from the surface (Figure 3). This is illustrated in Figure 2, which shows the mean and variance of the distribution of the angle between the uranium−carbon vector and the normal to the surface, whereby an acute angle corresponds to the carbonate ion pointing away from the surface. At each height, the angle distribution is fitted with a Gaussian curve to extract the mean angle and its variance. Below approximately 5.5 Å, the angle is low indicating that the U−C vector points mostly away from the surface. In addition, the variance is very low, which shows that the orientation of UO2CO3 is significantly constrained when it is adsorbed on the surface. Above 5.5 Å, the orientation of UO2CO3 appears random, as evidenced by the increase in the average angle, its fluctuations around 90°, and the large increase in the variance. The profiles show either a very shallow minimum or no minimum at all in the region corresponding to outersphere complex formation. Both profiles also show essentially no free energy barrier for adsorption. It should be noted that this approach assumes that the reaction coordinate is the normal to the mineral surface and that, therefore, if the true reaction coordinate has components in the other two directions, the free energy barrier for adsorption could be miscalculated. However, a MD simulation of UO22+ initially placed in an outersphere position (i.e., at a height of 4.625 Å with five waters of hydration) adsorbed in the first innersphere position after only approximately 200 ps, confirming that the free energy barrier for adsorption can easily be overcome at room temperature. Interestingly, this suggests the feldspar surface has a catalytic effect on water exchange since the calculated residence time of water in the uranyl first hydration shell is approximately 1200 ps in aqueous solution for the model used in this work.7 The free energy profiles show two minima close to the surface. The first minimum has a large adsorption energy of approximately 50 kJ·mol−1. The second minimum is narrow and higher in energy than the first minimum by about 10 kJ·mol−1. It should be noted that, for the simulations where UO22+ and UO2CO3 are within the free energy well, a surface potassium ion originally positioned in the vicinity of the adsorbing species dissolves from the surface into the aqueous solution. This is to be expected when UO22+ is adsorbing on the surface due to its net positive charge and the fact that the potassium ions positioned in the surface cavities can exchange fairly rapidly with water molecules.26 However, UO2CO3 has no net charge. Therefore, we suggest that because UO2CO3 adsorbs with its uranyl moiety exposed to the surface, the positive charge of the UO22+ moiety is not screened by the carbonate ion and, consequently, UO2CO3 also has the ability to expel a potassium ion from the surface. In other words, the geometrical constraint on UO2CO3 due to adsorption on the surface creates a net dipole, which can interact with surface species. Figure 2 also shows the coordination number of uranium with water, surface hydroxyl, and surface bridging oxygens in the UO22+ simulations. The coordination numbers were obtained by integrating the radial distribution function (RDF) of the corresponding ion pair up to its first minimum. A bin size of 0.01 Å was used in all RDF analyses. The coordination number profiles indicate that, when adsorbing in the first minimum, the overall 5-fold equatorial coordination is maintained but uranyl exchanges a water molecule from its first hydration shell for a surface hydroxyl (Figure 2 and Figure 3). In the second minimum, uranyl looses an additional water molecule, which is now replaced by a surface bridging oxygen (Figure 2 and Figure 3). The coordination number of hydrogen

overestimations in the first hydration layer as short excursions of water molecules out of the hydration layer during a 10-ps subtrajectory would affect the calculated MSD more significantly than the geometric center due to the large gradient in diffusion coefficient in the vicinity of the interface. Adsorption of Uranyl Carbonate Species at the (001) Orthoclase Surface. The free energy profiles obtained for UO22+ and UO2CO3 are shown in Figure 2. The two profiles are

Figure 2. (Top) Free energy profiles of UO22+ and UO2CO3 adsorbing on the orthoclase (001) surface. (Middle) Coordination number of UO22+ and of the hydrogen atoms of its first hydration shell (with the latter shown as negative values for clarity) with water (OW), surface hydroxyl (OH), and surface bridging (OB) oxygen atoms. (Bottom) Mean and variance of the angle between the U−C vector and the normal to the surface.

essentially identical with the UO2CO3 profile slightly shifted to longer distances because the center of mass of the adsorbing species is used to determine its position. The two profiles are almost identical because UO2CO3 coordinates to the surface via its uranium atom and with its carbonate ion mostly pointing 1635



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Figure 3. Snapshots of adsorbed species. (a) UO22+ innersphere complex 1, (b) UO22+ innersphere complex 2, (c) UO2CO3, (d) CO32−, and (e) Ca2UO2(CO3)3. Uranium is shown in blue, carbon in light gray, calcium in orange, oxygen in red, hydrogen in white, potassium in green, silicon in yellow, and aluminum in gray. For clarity, only the water molecules directly coordinated to the adsorbed complexes are shown.

where kTST is the transition state rate constant,

atoms in the uranyl first hydration shell was also calculated to determine the region of the interface where the uranyl ion forms an outersphere complex and to evaluate whether the first shell water molecules contribute to the surface bonding in the innersphere position. Figure 2 shows that the outersphere complex is bound via up to two hydrogen bonds from first shell waters to surface hydroxyls and up to one hydrogen bond with surface bridging oxygens. In addition, hydrogen bonds are also formed with the surface when UO22+ is in innersphere coordination. The coordination number profile of UO2CO3 shows the same features as that of UO22+, and is thus not displayed, with the exception that the initial number of water molecules in the first hydration shell is three instead of five. The rate of desorption of UO22+ from the first innersphere minimum into the aqueous solution was calculated from the free energy profile to estimate the residence time of UO22+ at the surface. This calculation serves two purposes: first, it provides information on the extent by which the feldspar surface can retard UO22+ diffusion on a microscopic level and, second, it allows for a comparison with rates obtained at higher scales, such as those obtained from batch or stirred-flow systems.45 From transition state theory, the rate of desorption can be written as follows:

k = κkTST

kTST =

kBT 2πμ

exp( − WPMF(r *)/kBT ) r*

∫0 exp( − WPMF(r )/kBT )dr

(4)

where μ simplifies to the mass of the adsorbing molecule, kB is the Boltzmann constant, T is the temperature, WPMF is the free energy determined from the potential of mean force calculations, and r* is the transition state distance, i.e., the position of the free energy maximum between the adsorbed and desorbed states. The transition state rate constant was calculated to be 5.8 × 104 s−1. The transmission coefficient, κ, was determined from the plateau value of the normalized reactive flux, which can be computed as follows:

k (t ) =

⟨r(0) ̇ θ[r(t ) − r*]⟩c ⟨r(0) ⟩c ̇ θ[r(0)] ̇

(5)

where θ[x] is the Heaviside function, which is 1 if x is larger is the initial velocity of the than 0 and 0 otherwise, and r(0) ̇ uranyl ion along the reaction coordinate. The subscript c means that the initial configurations have been generated in the constrained reaction coordinate ensemble. To compute κ, a pool of initial configurations was produced by running a 5-ns MD simulation where the uranyl ion is constrained at the transition state distance (4.5 Å) and collecting a configuration

(3) 1636



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every 2 ps for a total of 2500 configurations. Each configuration was run both backward and forward in time for 2.5 ps and a value of κ of 0.015 was determined from averaging k(t) over the last 1.0 ps (SI Figure S1). For comparison, previous work46 on the adsorption of divalent alkaline-earth cations at the (10.4) calcite surface resulted in values of κ ranging from 0.006 to 0.029. Therefore, the uranyl desorption rate constant was calculated to be 8.8 × 102 s−1, which translates into a residence time at the surface of approximately 1 ms. In comparison, previous work on the kinetics of water exchange at this surface showed that the residence time of water in the adsorbed layer was approximately 290 ps, i.e., several orders of magnitude lower. Therefore, the MD simulations predict that UO22+ can be significantly retarded by adsorption at the surface relative to water molecules. The same conclusion is expected for UO2CO3 as it shows essentially the same free energy profile. The free energy profiles of CO32− and Ca2UO2(CO3)3 adsorbing on the (001) orthoclase surface are shown in Figure 4. The free energy profile of CO32− also shows two innersphere minima; however, the free energy of adsorption (approximately 5 kJ·mol−1) is much less than that calculated for UO22+ and UO2CO3. The coordination number profile is also shown in Figure 4. Similarly to what was predicted for UO22+, CO32− exchanges water molecules in its first hydration shell for surface hydroxyls (one and two for the first and second minima, respectively). Therefore, the lower adsorption energy can be explained by the fact that CO32− only attaches to the surface via hydrogen bonds. The free energy profile of Ca2UO2(CO3)3 is significantly different from that of the other species. Notably, the free energy barrier for adsorption is large, only one innersphere minimum is found, and the MD simulations predict the free energy of adsorption to be unfavorable. As for CO32−, Ca2UO2(CO3)3 exchanges water molecules for surface hydroxyls as it approaches the surface; however, unlike CO32−, exchanging water molecules for the first surface hydroxyl is not energetically favored. Figure 4 indicates that Ca2UO2(CO3)3 exchanges approximately two water molecules for a single surface hydroxyl, whereas CO32− shows one-to-one exchange ratio, which can explain the difference in energetics. This difference in water to hydroxyl exchange ratio is likely due to the extended structure of Ca2UO2(CO3)3, which, when adsorbed on the surface, leads to an exclusion region that cannot be accessed by waters of solvation. Comparison with Experimental Data. In this section, we compare the results of the MD simulations with available experimental data and in particular with EXAFS of uranium published in the literature for adsorption of uranyl or uranyl carbonate species at different mineral surfaces. We also consider the structure of the uranyl carbonate species in aqueous solution as previously calculated7 with the same potential model to evaluate the effects of adsorption on the structure of these species. For the purposes of this section, additional 5-ns MD simulations were carried out, in which the species of interest were not constrained and were initially placed in the innersphere positions determined in the PMF calculations. All calculated distances are reported in Table 1 and snapshots of the adsorbed species are shown in Figure 3. Walter et al.28 studied uranium(VI) adsorption to pristine and leached albite (sodium feldspar: NaAlSi3O8) surfaces using EXAFS and time-resolved laser-induced fluorescence spectroscopy (TRLFS). Walter et al. characterized three pristine samples and two leached samples whereby the different samples

Figure 4. (Top) Free energy profiles of CO32− and Ca2UO2(CO3)3 adsorbing on the orthoclase (001) surface. (Middle) Coordination number of carbonate oxygens (OC) with water (HW) and surface hydroxyl (H) hydrogens. (Bottom) Same as middle plot but for Ca2UO2(CO3)3, also shown is the coordination number of calcium with water oxygens (OW).

were reacted with uranium solutions of different concentrations. The TRLFS results indicated the presence of two adsorbed species in two of the three pristine samples, which is consistent with our results of two free energy minima at the orthoclase−water interface. For the third sample, Walter et al. derived a uranium-surface cation bond length of 3.09 Å, but could not differentiate between silicon and aluminum; they concluded that this distance was indicative of a mononuclear, bidendate innersphere complex. This is consistent with our distance of 3.17 Å to a surface aluminum when UO22+ is adsorbed in the second innersphere minimum (Table 1) and is coordinated to a surface hydroxyl oxygen and a surface bridging oxygen. We note that, based on electrostatics, we expect UO22+ to be more likely to coordinate to an aluminol site than a silanol site. 1637



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Table 1. Mean Inter-Atomic Distances between Uranium and Other Atom Types When Ca2UO2(CO3)3 and Some of Its Constituent Species Are Adsorbed As Innersphere Complexes onto the Orthoclase (001) Surfacea UO22+ 1

species

UO22+ 2

UO2CO3 1

UO2CO3 2

Ca2UO2(CO3)

shell

N

R

N

R

N

R

N

R

N

R

OU OW OH OB Si Al C OC OC Ca

2.0 4.0 1.0

1.82 2.51 2.42

1.83 2.51 2.40

3.65 2.76 2.36 3.93

1.83 2.50 2.41 2.50 3.78 3.15 2.75 2.35 3.92

1.83

1.0 1.0 2.0 1.0

2.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 1.0

2.0

3.68

1.83 2.49 2.41 2.54 3.74 3.17

2.0 2.0 1.0

1.0

2.0 3.0 1.0 1.0 1.0 1.0

3.0 6.0 3.0 2.0

2.79 2.39 3.97 4.00

a

The labels 1 and 2 refer to the innersphere complexes furthest and closest to the surface, respectively. OU, OW, OH, OB, and OC stand for uranyl, water, hydroxyl, bridging, and carbonate oxygens, respectively,.

2.79 Å, respectively. However, these distances are essentially the same as those calculated for the same species in aqueous solution (2.74 and 2.79 Å for UO2CO3 and Ca2UO2(CO3)3, respectively).7 EXAFS measurements of aqueous uranyl carbonate species showed distances varying from 2.87 to 2.95 Å.30,31,50−54 Therefore, both the calculated and observed distances indicate that the uranium−carbonate coordination distances are not affected upon adsorption. A common feature to all the reported bidendate adsorbed configurations 30,31,47−49 is the presence of a split equatorial oxygen shell with a first set of approximately 2 to 3 oxygens between 2.25 and 2.35 Å and a second set of 2 to 4 oxygens between 2.41 and 2.48 Å. This is consistent with the configurations calculated in this work for adsorption on feldspar, which have asymmetrical equatorial oxygen shells with uranium−water oxygen distances remaining around 2.49 to 2.51 Å, distances to surface hydroxyl oxygens ranging from 2.40 to 2.42 Å, and distances to carbonate oxygens from 2.35 to 2.39 Å. It should also be noted that Rossberg et al.32 and Sherman et 49 al. found evidence for a monodendate uranyl carbonate complex adsorbed on ferrihydrite and goethite, respectively, whereby the oxygen of a carbonate ligand is directly bonded to a surface iron atom. Similar features to those discussed above were observed by Catalano and Brown29 from EXAFS measurements of uranyl carbonate complexes adsorbed onto montmorillonite, namely, U−C distances ranging from 2.84 to 2.91 Å with a maximum uncertainty of ±0.09 Å and split equatorial oxygen shells with shorter distances assigned to coordination to the mineral surface with distances ranging from 2.26 to 2.40 Å and longer distances to water molecules (2.41 to 2.54 Å). To conclude, the simulations provided insights into the microscopic surface effects on diffusion of uranyl carbonate species in nanosized mineral fractures. Interestingly, the simulations suggest that Ca2UO2(CO3)3 has a low affinity for the orthoclase surface, whereas UO2CO3 and UO22+ attach strongly. Therefore, the MD simulations indicate that it might be energetically favored for Ca2UO2(CO3)3 to exchange some of its carbonate ligands or calcium ions when adsorbing on the orthoclase surface. This implies that, although Ca2UO2(CO3)3 often dominates aqueous speciation as observed in the Hanford groundwater25 and at other sites,55 the dominant surface complex species may be different.

For all three pristine samples, the equatorial oxygen shell was found between 2.34 and 2.36 Å with uncertainties up to ±0.02 Å. Walter et al. noted that the equatorial oxygen shell was not split or that the extent of splitting was less than the resolution of 0.19 Å. However, for both leached samples, a clear split of the equatorial oxygen shell was observed with one set of distances (U−Oeq 1) at 2.22−2.23 Å and a second set (U− Oeq 2) at 2.43−2.44 Å. The coordination of UO22+ adsorbed on the leached samples could not be unambiguously resolved but analysis of the EXAFS spectrum of one sample suggested the presence of one Si atom at 3.87 Å and, hence, the two most likely models were a mononuclear monodendate complex and a binuclear bidendate complex. The MD simulations do predict the formation of a strongly bound mononuclear monodendate innersphere complex; however, this complex is adsorbed to an aluminum tetrahedron rather than to a silicon tetrahedron. This could be due to the fact that the simulated orthoclase surface is not leached and is therefore not enriched in silicon. The shorter distance of 3.68 Å to Al, compared to the EXAFS distance of 3.87 Å to Si, is consistent with the reduced electrostatic repulsion between Al3+ and UO22+ compared to between Si4+ and UO22+. The short set of oxygen distances (U−Oeq 1) is not observed in the MD simulations; however, deprotonation of surface hydroxyl groups could lead to shorter U−Oeq distances and the type of model used in our study does not allow for the dissociation of surface hydroxyl groups. Therefore, it was not possible to determine whether these groups deprotonated upon UO22+ adsorption. To the best of our knowledge, no EXAFS data have been published on adsorption of uranyl carbonate species on feldspar surfaces. However, several studies have reported EXAFS data on uranyl carbonate species adsorbed on other minerals with the largest body of work on adsorption onto iron (hydr)oxides. Evidence for a mononuclear bidendate uranyl carbonate adsorbed species has been found on hematite,30,31,47 ferrihydrite,48 and goethite.49 Bargar et al. 30,31 reported an adsorbed species on hematite consisting of 0.8 ± 0.1 to 1.7 ± 0.2 carbonate ligands, depending on the pH in the pH range 4.5 to 8.5, with U−C distances ranging from 2.87 to 2.94 Å. Catalano et al.47 obtained similar results at pH 7 with two carbonate ligands with U−C distances of 2.91 Å. Ulrich et al.48 and Sherman et al.49 reported, at pH 8, 1.8 ± 0.8 carbon atom at 2.92 Å and 1 carbon atom between 2.91 and 2.93 Å, respectively. The calculated U−C distances for adsorbed UO2CO3 and Ca2UO2(CO3)3 are shorter: 2.75−2.76 and 1638



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ASSOCIATED CONTENT

S Supporting Information *

Potential parameters and ionic charges; reactive flux for uranyl desorption. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy (DOE) Biological and Environmental Research (BER) Division through the Subsurface Biogeochemistry Research (SBR) Program of the Science Focus Area (SFA) program at Pacific Northwest National Laboratory (PNNL). The computer simulations were performed in part using the Molecular Science Computing (MSC) capabilities in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research (OBER) and located at PNNL. PNNL is operated for the DOE by Battelle Memorial Institute under Contract DE-AC05-76RL01830.

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