A Comparison of Adsorption, Reduction, and Polymerization of the

Sep 28, 2016 - In contrast, uranyl shows only negligible, if any, adsorption .... X-ray Reflectivity Measurements ... solution (adsorbed water layers,...
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A Comparison of Adsorption, Reduction, and Polymerization of the Plutonyl(VI) and Uranyl(VI) Ions from Solution onto the Muscovite Basal Plane Stefan Hellebrandt,† Sang Soo Lee,‡ Karah E. Knope,‡,⊥ Aaron J. Lussier,§ Joanne E. Stubbs,∥ Peter J. Eng,∥ L. Soderholm,*,‡ Paul Fenter,‡ and Moritz Schmidt*,† †

Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01314, Germany Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ∥ Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States ‡

S Supporting Information *

ABSTRACT: X-ray scattering techniques [in situ resonant anomalous X-ray reflectivity (RAXR) and specular crystal truncation rods (CTR)] were used to compare muscovite (001) surfaces in contact with solutions containing either 0.1 mM plutonyl(VI) or 1 mM uranyl(VI) at pH = 3.2 ± 0.2, I(NaCl) = 0.1 M, as well as in situ grazing-incidence X-ray absorption near-edge structure (GI XANES) spectroscopy and ex situ alpha spectrometry. Details of the surface coverage are found to be very different. In the case of Pu, alpha spectrometry finds a surface coverage of 8.3 Pu/AUC (AUC = 46.72 Å2, the unit cell area), far in excess of the 0.5 Pu/AUC expected for ionic adsorption of PuO22+. GI XANES results show that Pu is predominantly tetravalent on the surface, and the CTR/RAXR results show that the adsorbed Pu is broadly distributed. Taken together with previous findings, the results are consistent with adsorption of Pu in the form of Pu(IV)-oxo-nanoparticles. In contrast, uranyl shows only negligible, if any, adsorption according to all methods applied. These results are discussed and compared within the context of known Pu and U redox chemistry.

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INTRODUCTION

solubility, either as intrinsic colloids or as adsorbates on dissolved mineral nanoparticles.3,5−7 On the basis in part of studies revealing unexpectedly facile transport under environmental conditions,8−10 interest has focused on the role played by mineral interfaces in controlling Pu speciation and solubility. Because of early suggestions that PuIV may be the dominant surface species,9 many of these studies have focused on redox active mineral surfaces, notably those of reducing iron and manganese bearing phases. Assumed to be dependent on relative ion-surface redox couples, both oxidized11−14 and reduced11,15−24 Pu have been identified as surface complexes. Of particular interest are studies involving redox-inactive minerals and related surfaces in which Pu is also found as an oxidized or reduced surface complex relative to its dominant solution speciation,21,25−28 raising questions about the complementary roles of solution and interfacial chemistry in sorbed Pu speciation. From a broader perspective, redox behavior has also been observed in other actinide-surface complexes.7 Specifically, reduction behavior has been observed with the UO22+ ion, although in this case the reduction to a UIV

Understanding the chemical reactivity and speciation of hazardous heavy metals in aqueous solutions is a first step toward modeling their behavior, both in the laboratory and under natural conditions. Such modeling efforts are particularly important for the transuranic actinide elements (Np, Pu, and Am), all of which are currently man-made and only recently reintroduced into the geosphere.1 As a result, there is no geologic or mineralogic precedent upon which to judge the potential behavior of these radioactive, heavy-metal poisons. Pu can be particularly difficult to model in this regard because of its redox characteristics, with the trivalent through hexavalent states stable, and sometimes in equilibria, in aqueous solution.2,3 The relevance of this behavior centers on their differences in solubility, and hence their potential mode of transport, with PuIV insoluble and PuV and PuVI, which exist as the plutonyl ions PuO2+ and PuO22+, highly soluble under most groundwater conditions. The pentavalent cation is expected to be the dominant species in acidic solutions, although its solubility even at low concentrations is limited by the formation of PuIV, a small, highly charged, hard cation that is readily hydrolyzed.4 Despite its low solubility, studies have shown that the hydrolyzed PuIV aggregates can significantly increase © 2016 American Chemical Society

Received: July 8, 2016 Revised: August 30, 2016 Published: September 28, 2016 10473

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equilibrium concentration of Pu(IV) is significantly higher (∼25×) than the concentration of U(IV) in bulk solution under the conditions used in the experiment (pH ≈ 3.2, c(NaCl) = 0.1 M, in air). Hydrolysis is expected to be very weak for UO22+ or PuO22+ at this pH44 and should not significantly influence the actinides’ solution chemistry and interfacial reactivity. A more detailed discussion of Pu and U redox chemistry, including Pourbaix diagrams, can be found in the Supporting Information. Ex situ alpha-spectrometry measurements were performed as a direct measure of the U/Pu coverage on the (001) basal plane of muscovite. Freshly cleaved single crystals were reacted with U/Pu solution, with the same concentrations used for the X-ray reflectivity experiments, overnight. These were subsequently rinsed with a background electrolyte solution (c(NaCl) = 0.1 M), to remove surplus, nonsorbed uranium/plutonium, and deionized water (DIW), to prevent any precipitation of the background electrolyte on the crystal surface during drying. This procedure may also remove weakly adsorbed ion species.51 Furthermore, a stainless steel mask was used to define the specific area exposed to the detector and exclude contributions from Pu/U adsorbed to the edges to the measured activity. This method is described in more detail elsewhere.52 Surface X-ray Diffraction. X-ray Reflectivity Measurements. Surface X-ray diffraction comprises a versatile group of techniques to investigate the mineral/solution interface on an atomic level (resolution < 1 Å). Two techniques are used here, crystal truncation rod (CTR) and resonant anomalous X-ray reflectivity (RAXR). Briefly, in surface X-ray diffraction experiments, synchrotron-generated X-rays are reflected by the surface of the muscovite crystal, in contact with the solution. The scattered intensity is measured in the vicinity of the specular reflection conditions. The measured reflectivity data can be understood from a molecular perspective based on the interfacial structure (i.e., as seen in an interfacial electron density profile) to reveal structure- and element-specific results (see the Supporting Information for fitting procedure). The X-ray reflectivity measurements were performed at the GeoSoilEnviroCARS (GSECARS) undulator beamline 13-ID-C at the Advanced Photon Source (APS), using a Newport 6-circle kappa diffractometer. X-rays of the desired wavelength are selected by a liquid nitrogen cooled Si (111) double crystal monochromator, and the beam is either focused or collimated both vertically and horizontally using a pair of 1-m long Rh-coated silicon Kirkpatrick− Baez mirrors. The typical flux of the beam is ∼1012 photons/s, with a 0.05−0.25 mm horizontal and 0.5−1.0 mm vertical dimension. To minimize beam damage, some X-ray experiments were performed with a partly defocused beam, different incident beam sizes, and the beam position on the sample was changed repeatedly. No beam damage was observed, as judged by performing multiple CTR measurements at different spots on the crystal surface, which showed no changes during the course of the experiments. Sample stability during RAXR experiments was monitored by fiducial measurements at q = 0.20 Å−1, where q is the vertical momentum transfer. Because of the longer duration of RAXR experiments (∼4 h), it is not possible to differentiate beam-induced damages from ongoing interfacial reactions (see below). The scattered X-ray data were collected using a Dectris PILATUS 100 K 2D pixel array detector.53 All CTR measurements were performed under specular reflection conditions, where the reflected intensity is measured as a function of q, oriented along the surface-normal direction, defined as

surface adsorbate has been limited to either a redox active mineral surface29−34 or in the presence of a solution reductant, notably an organic species such as humic acids.35,36 Our interest in understanding the role of a redox-inactive surface on the speciation of adsorbed Pu has focused on muscovite mica,37 a phyllosilicate whose primary, basal-plane surface is ideal for studies using X-ray surface-scattering probes.38 Pu has been shown to adsorb to the mica surface as the tetravalent ion, with an estimated intrinsic adsorption constant K° of about 7 × 107 M−1,37 a value similar to that seen for natural clay minerals.39 We have found very similar surface scattering features when exposing muscovite surfaces to acidic solutions containing PuIII,26 ThIV,40 or PuIV nanoparticles.41 The adsorbed species are qualitatively different from those seen for redox-stable lanthanide ions,42 suggesting the important role of hydrolysis and condensation reactions for the higher-valent actinides.43,44 In the work presented herein, we extend our studies to the adsorbed species present on muscovite in contact with mildly acidic pH (3.2 ± 0.2) aqueous solutions containing actinyl(VI) ions, specifically PuO22+ and UO22+. Our specific goals were to understand whether muscovite will adsorb actinyl species and whether the same redox-inactive surface that saw the oxidation of PuIII solution to PuIV complexes will stabilize the hexavalent actinyl(VI) complexes. Our results reveal very different behaviors for the plutonyl and uranyl ions, with PuIV surface complexes comparable to those characterized from other Pu solutions. In contrast, U does not appear to form any surface complexes under the conditions of our experiments. These results are discussed in terms of what can be learned about Pu surface complexation reactions from a comparison of the known chemistry of these two actinyl ions.

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MATERIALS AND METHODS

Uptake Quantification. Caution! 242Pu and 238U are radionuclides with half-lives of 3.75 × 105 and 4.47 × 109 years, respectively. Their use requires the appropriate infrastructure and personnel trained in the handling of alpha-emitting isotopes. Muscovite Substrates. Muscovite is a dioctahedral, monoclinic phyllosilicate formed of interconnected TOT (tetrahedral−octahedral−tetrahedral) layers. These layers have a net negative charge due to partial substitution of Al3+ for Si4+.45 The discrete layers are bridged by charge compensating K+ in the interlayer. Muscovite exhibits large, high-quality, defect-free crystals that are ideally suited to investigations using the CTR/RAXR techniques. Thus, muscovite also serves as an effective analogue for many clay minerals that have similar structure, although it has a higher layer charge. Cleavage of the (001) basal plane results in atomically flat surfaces. Muscovite has a unit cell area of AUC = 46.72 Å2 with a (001) spacing of ∼19.96 Å. All crystals used in the experiments were obtained from the Asheville-Schoonmaker Mica Co. (VA) and were of V1 quality (clear, hard, of uniform color, nearly flat, free of all stains, foreign inclusions, cracks, and other similar defects). By cleaving parallel to the (001) plane, the exposed surface has a fixed lattice charge of 1e− per unit cell, when the surface K is removed by plunging the cleaved surface in water.46−48 This charge is independent of solution pH, and no surface protonation is expected on muscovite (001). Cations adsorb under very acidic conditions (pH = 1), indicating that full surface protonation did not occur, even under such harsh conditions.49 Therefore, cation sorption on the (001) plane of muscovite is primarily an electrostatically driven process. Speciation and Uptake Quantification. The concentrations of Utot and Putot are 1 mM (M denotes mol L−1) and 0.1 mM, respectively. Speciation calculations (not shown) were performed using the PHREEQC solution modeling suite (version 2.18) with data of the NAGRA/PSI chemical thermodynamic database.50 While there are just minor differences in the speciation of U(VI) and Pu(VI), the

q=

⎛ 2θ ⎞ 4π × sin⎜ ⎟ ⎝2 ⎠ λ

(1)

where λ is the wavelength of the X-ray beam and 2θ is the diffraction angle. CTR data directly probe the structure of the near-surface crystal layers and the surface associated solution (adsorbed water layers, background electrolytes, metal ions, etc.). CTR measurements were performed at a fixed incident photon energy (16 keV), approximately 1 and 2 keV away from the LIII absorption edges of U and Pu, respectively, which is sufficiently far from the absorption edges to minimize resonant contributions to these measurements. 10474

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Figure 1. LIII-edge GI-XANES spectra at the muscovite (001)−(a) Pu(VI) and −(b) U(VI) solution interfaces. Measured spectra are shown with reference spectra of Pu(IV)59,60 and Pu(VI)60 as well as U(IV) and U(VI).61 Measured spectra are shifted by the indicated energies, and all spectra were normalized to a unit peak intensity for easier comparison. RAXR measurements probe the reflectivity signal, R, as a function of photon energy, E (at a fixed q) by scanning photon energy over a 600 eV range centered at the LIII absorption edge of U (17.18 keV) and Pu (18.06 keV), respectively. Element-specific electron density profiles of the U- and Pu-adsorbed ion distribution can be derived by combining these RAXR data with CTR data. X-ray absorption near-edge structure (XANES) spectroscopy was performed in fluorescence mode under grazing-incidence conditions to ascertain the oxidation state of the element of interest, under the experimental setup conditions. Fluorescence was measured using an SII Vortex ME4 X-ray fluorescence detector. XANES spectra are also used to extract the elements’ anomalous dispersion terms by applying a difference Kramers−Kronig transformation.54 X-ray Reflectivity Sample Preparation. For the X-ray reflectivity experiments, freshly cleaved muscovite crystals were immersed overnight in the reaction solution. The crystal was then removed from the solution and immediately placed in the sample holder. After 20 μL of the reaction solution was pipetted on top of the mounted crystal, the whole sample holder40 (Figure S2) was covered with a first layer of Kapton film, and then sealed with two additional domes of Kapton foil as secondary containment layers. The sorption reaction was performed with a concentration of 1 and 0.1 mM 238U or 242Pu, respectively, in 0.1 M NaCl as background electrolyte at pH ≈ 3.2. No pH adjustments were done to avoid local changes in uranium/plutonium chemistry.

used to determine the oxidation state of the resonant metal, albeit with some limitations.55,56 XANES spectra of the pentaand hexavalent actinides with actinyl AnO22+ structure have characteristic shoulders on their LIII edges that have been attributed to strong backscattering of the tightly bound -yl oxygens.57 This difference in edge shape is used to distinguish between low oxidation-state ions (III, IV) and their more oxidized (V, VI) dioxo moieties, MO2n+ (n = 1,2). The distinction between the III and IV oxidation states is complicated by the fact that no significant shape change is observed, and shifts in the edge position with changes in oxidation state are of magnitude similar to those arising from changes in chemical environment.55 The same problem exists when comparing XANES spectra from V and VI oxidation state species. Measurements were performed at grazing-incidence (GI) conditions, that is, with an angle of incidence α slightly smaller than the critical angle αc of the mica substrate (α = 0.1° as compared to αc = 0.11° at 18 keV). Under these incident beam conditions, the surface signals are enhanced by the X-ray standing wave effect,58 while contributions from the bulk substrate are suppressed.37 However, GI XANES is not inherently surface specific and may contain contributions from absorption of the X-ray beam by ions of the bulk solution. The Pu and U GI XANES spectra are shown in Figure 1 together with reference spectra of the tetravalent59−61 and hexavalent60,61 oxidation states. Reference spectra were taken from solutions with different compositions than used in the current study; hence chemical shifts in the edge positions can be expected. Comparison of the surface Pu XANES spectrum with the solution references (Figure 1a) shows no evidence of the shoulder feature expected for the plutonyl(VI) ion. Instead, the strong, broad white line is an excellent match to the Pu(IV) reference spectrum, indicating a reduced Pu species on the mica surface. It is noteworthy that UV/visible spectroscopy of the same reaction solution, but not in contact with muscovite (not shown), provides no indication for the presence of Pu(IV) even over significantly longer periods of time. In contrast to the Pu GI XANES for mica immersed in a plutonyl(VI) solution, the U GI XANES measured for mica immersed in a uranyl(VI) solution had a lower signal-to-noise ratio. This poorer data quality was mainly attributed to an extremely low intensity of the U Lα fluorescence signal (Figure 1b). This weak signal at a GI condition indicates only minimal uranium sorption. However, the XANES clearly shows the presence of a prominent “uranyl-shoulder”, with almost no

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RESULTS Alpha Spectrometry. The Pu coverage measured by alpha spectrometry52 under the same solution conditions as used in the surface diffraction experiment ([Pu] = 0.1 mM, pH = 3.2, 0.1 M NaCl), θPu(alpha), was 8.3 ± 0.2 Pu/AUC, equivalent to 0.71 μg/cm2. This coverage is similar to, although somewhat less than, the coverages found upon sorption of Pu(IV)-oxonanoparticles formed from a Pu(III) solution (9.0 ± 0.5 Pu/ AUC),26 and upon sorption of preformed Pu(IV)-oxo-nanoparticles (10.8 ± 0.6 Pu/AUC).41 All of these coverages are far in excess of that expected for simple ion adsorption whose coverage would be determined by charge compensation of the muscovite charge (1 e−/AUC). In contrast, no U(VI) was detected on the muscovite (001) basal plane, under the same solution conditions as used in the surface diffraction experiment ([UO22+] = 1 mM, pH = 3.2, 0.1 M NaCl). The limit of detection for 238U measured with alpha spectrometry is θU(alpha) = 0.04 U/AUC, equivalent to 3.52 ng/cm2. These results show that the surface loading of U was more than 200-fold less than that of Pu despite the 10-fold higher concentration of U in these studies. X-ray Absorption Near-Edge Structure (XANES). X-ray absorption near-edge structure (XANES) spectroscopy was 10475

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Figure 2. (a) Specular X-ray reflectivity of the muscovite (001) equilibrated with a solution of 0.1 mM Pu(VI) at pH 3.2 with 0.1 M NaCl background electrolyte as a function of momentum transfer, q, measured with a photon energy E = 16.0 keV. Measured reflectivity data are shown as red symbols. The best fit is shown as a blue line. Also shown are data measured after reaction of muscovite with 30 mM NaCl (gray symbols).62 (b) Total electron density profile derived from CTR (black line). The total electron density profile observed on muscovite equilibrated with 30 mM NaCl62 is also plotted (dashed red line) for comparison. The electron density is normalized to that of bulk water, ρw = 0.33 e−/Å3. The average height of muscovite surface oxygens is set to be z = 0 Å.

Figure 3. (a) Specular X-ray reflectivity of the muscovite crystal (001) equilibrated with a solution of 1 mM U(VI) at pH 3.2 with 0.1 M NaCl background electrolyte as a function of momentum transfer, q, measured with a photon energy E = 16.0 keV. Measured reflectivity data are shown as red symbols. The best fit is shown as a thick blue line. Also shown are data measured after reaction of muscovite with 30 mM NaCl (gray symbols),62 which serve as a reference. (b) Total electron density profile derived from CTR (black line). The total electron density profile observed on muscovite equilibrated with 30 mM NaCl is also plotted (dashed red line) for comparison. The electron density is normalized to that of bulk water, ρw = 0.33 e−/Å3. The average height of muscovite surface oxygens is set to be z = 0 Å.

process, as shown by the comparison with the mica−NaCl system62 (Figure 2b, dashed red line). The total electron density profile for the Pu sample consists of a broad, structured peak within the first 20 Å of the solution near the interface, followed by a shallow broad peak that reaches bulk water electron density at around 45 Å. In contrast, such features are absent in the profile for the NaCl sample, suggesting that the significant electron density enhancements mainly originate from sorption of Pu at the interface. The integrated excess electron density corresponds to a total coverage of ∼4 Pu/AUC, if we make the simplifying assumption that it is due solely to Pu. The results are therefore consistent with the unexpectedly high Pu coverages observed by alpha counting. Uranium Adsorption Structure (CTR). Specular X-ray reflectivity data measured from muscovite equilibrated with 1 mM UO22+ in 0.1 M NaCl as background electrolyte are shown in Figure 3a. For reference, data from Lee et al. (2012)62 measured in NaCl solution of a similar concentration (30 mM) are also shown. The data sets show minor differences at higher q values (e.g., q ≈ 4 or 4.7 Å−1), consistent with similar

discernible white line feature (Figure 1b), suggesting that any adsorbed U was present as the uranyl ion, the same species as present in solution. Interfacial Structures. Plutonium Adsorption Structure (CTR). The specular CTR data measured in 0.1 mM PuO22+ solution are shown in Figure 2. The data show typical strong variations, over several orders of magnitude, with the highest reflectivities corresponding to the tails of the bulk Bragg peaks. For comparison, CTR data for mica in contact with a 30 mM NaCl solution are also shown to represent data measured without Pu (i.e., muscovite in the background electrolyte) (Figure 2a). Substantial differences between the two data sets are observed, indicating sizable changes in the interfacial structure. Such large changes in interfacial electron density are attributed to Pu sorption because of its large Z (atomic number) and hence enhanced scattering power. The derived interfacial electron density profile (Figure 2b), including contributions from all species within the interfacial structure, specifically the adsorbed Pu species, all other solute ions, and water, is also strongly modified by the Pu adsorption 10476

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Figure 4. Selected resonant anomalous X-ray reflectivity (RAXR) data (○) measured from the muscovite (001) basal plane at different fixed momentum transfers, q (Å−1). RAXR measurements were obtained by scanning the incident photon energy near the LIII absorption edges of (a) Pu (18.06 keV) and (b) U (17.18 keV), respectively. The deviation of the RAXR signal from the Pu/U free baseline (dashed red lines) is a result of adsorbed resonant ions (as exemplarily shown by the gray arrow for the q = 0.14 Å−1 data of Pu). Spectra are normalized using the resonance amplitude normalization [(|Ftot(q,E)|2 − |FNR(q)|2)/(2|FNR(q)|)],63 where Ftot and FNR are total and nonresonant structure factors, respectively, and offset vertically for clarity (offset given in parentheses for each spectrum). The blue lines are a result of the model-independent analysis and are intended as a “guide-to-the-eye”.

interfacial structures in the two systems. The total electron density profile obtained from the mica−solution interface in the presence of UO22+ (Figure 3b) is also very similar to the profile in the absence of UO22+. Both profiles include a double peak feature close to the interface (z < 4 Å), followed by weaker oscillatory features at distances further from the interface. The most notable difference in the profiles is that the first two peaks are shifted away from the surface by ∼0.4 and 0.2 Å, respectively, in the system containing UO22+. Within the context of the potential for large scale nanoparticle adsorption, we do not see any differences in interfacial structure that can be attributed to uranium nanoparticle adsorption. We consider the difference in these electron density profiles with respect to the 30 mM NaCl profiles62 to be small, possibly attributable to a variety of factors, including differences in solution pH and the larger concentration of NaCl in our experiment. Resonant Anomalous X-ray Reflectivity (RAXR) of Puand U-Reacted Surfaces. The contributions from the actinides adsorbed to the interfacial structures were elucidated using RAXR measurements performed at the LIII edge for Pu or U (Figures 4 and S1). The Pu RAXR data show the largest RAXR modulation at the lowest q (=0.14 Å−1) whose magnitude quickly decreases with increasing q. Considering that the magnitudes of RAXR modulations are proportional to the amount of adsorbed species, it is evident that there was measurable Pu adsorbed at the muscovite−solution interface. The abrupt decrease in magnitude in the low q region indicates that the sorbed Pu was distributed broadly (i.e., with a wide variation in adsorption height) at the interface. In contrast, the U RAXR data show almost no resonant signal near the edge energy of the element, indicating that little or no U adsorbed. This clear difference between the Pu and U RAXR data

provides direct confirmation of the alpha spectrometry measurements and CTR results. Although the interfacial distribution of an adsorbate (including its total coverage and vertical distribution) can be determined from RAXR data, it was not possible to obtain an accurate and unique quantification of the Pu distribution from the current data set. The difficulty arose because the sample was changing during the time of data collection of ∼12 h. Although the muscovite sample was reacted with the Pu(VI) solution overnight, the reduction of Pu(VI) to Pu(IV) is generally known to be slow and may not have reached completion within the time frame of the experiment.2,11,23,25 In addition, although the experiment was designed to minimize the impact of the X-ray beam on the interfacial chemistry (e.g., by using a defocused beam and with frequent sample translations), Pu is known to be sensitive to X-ray beam perturbations under similar conditions.2 The importance of the sample changing in time lies in the requirement in RAXR analysis for a nonresonant structure factor as input as well as the need for all RAXR spectra to represent the same adsorbate structure.64 The nonresonant information is normally acquired from CTR data obtained from the same sample, but in this case was not available because of the sample evolution during RAXR data acquisition. Further complicating the analysis, the RAXR spectra were only observed over a narrow q range, and consequently the intrinsic aspects of the structure were not fully probed by these measurements (especially if the adsorbed structure has a complexity comparable to that previously observed for Pu(III), with an extended distribution associated with the formation of adsorbed PuO2 aggregates). Despite these issues, the model-independent RAXR analyses, which allow us to estimate the overall distribution of Pu,64 10477

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adsorption of the more complex UO22+ ion with respect to that of the simple divalent ions. Recent MD simulations68 also indicate relatively weak adsorption strength for uranyl ions to mica, showing that adsorption of divalent uranyl ions to the mica surface was significantly reduced by monovalent K ions when the same concentrations of uranyl and K ions were allowed to compete in one system. Whereas changes in hardness or charge distribution may be used to distinguish the adsorption of UO22+ on muscovite from other formally divalent ions such as Sr2+ and Ba2+, these arguments are not sufficient to explain the large excess in PuO22+ uptake. Although Pu is slightly smaller than U, both of their actinyl ions have similar local overall charge distribution. From this point of view, the competition of PuO22+ and UO22+ with the counterion Na+ (and hydronium in an acidic solution) would be expected to be very similar. Instead, the significantly enhanced adsorption from the PuO22+ versus the UO22+ solutions in contact with the muscovite surface, together with the GI XANES data, suggests that the adsorbed layer is comprised of reduced Pu, predominantly PuIV. The redox chemistries of both Pu and U in aqueous solutions are well-known and provide some insight into the different behaviors observed for plutonyl and uranyl in our surfaceadsorption experiments.75 A comparison of the standard potentials of Pu and U for the two-electron reduction of the hexavalent actinyl to the tetravalent ion in acidic solution reveals Pu has the more favorable half-cell reaction by 0.727 V (+1.00 vs 0.273 V, respectively, vs SHE). Important in this comparison is the relative stability of the pentavalent Pu species PuO2+ with respect to its uranyl(V) counterpart. In fact, Pu(V)O2+ is expected to be the stable Pu aqueous species under the conditions of our experiment,4 in contrast to the known stability of U(VI)O22+ ion over a wide range of aqueous solution conditions. Thus, the initial step in the reduction of the actinyl(VI) is expected to be kinetically favored for the plutonyl ion. Once formed, the reduced PuIV ion is known to hydrolyze and subsequently condense to form oxidic aggregates or nanoclusters.5,6,76 The known half-cell reduction potential of PuO2+ to PuO2 solid is 1.585 V (vs SHE), significantly higher than the 0.58 V for the UO22+ to UO2 reaction. This comparison reflects the widespread observation of hydrolyzed Pu species5−7,44 and the known stability of the Pu-dioxide phase.77 Whether the aggregated PuIV oxide phase forms in solution and/or directly on the mineral surface, the latter has been shown to facilitate the process.15,26−28 The total electron density profile obtained from the CTR data (shown in Figure 2a) is compared in Figure 5 with similar profiles previously obtained from muscovite surfaces contacted with either a PuIII solution78 or a Pu-nanoparticle colloidal solution.41 Specifically, the broad intense feature extending out to about 20 Å above the mineral surface, combined with the additional enhanced density even further out from the surface, agrees well with the profile seen when the contacting solution contains PuIII or hydrolyzed Pu-oxide nanoparticles. CTRs and normalized CTRs for each sample are included in Figure S3. More generally, the formation of PuIV surface layers with features similar to reported PuO2 nanoparticle structures measuring 1−2 nm in size has been previously characterized by high-resolution transmission electron microscopy (HRTEM).79 Details of the surface structure seem to vary with the mineral phase. Notably, the colloid structure was shown to have undergone a phase change from the expected

confirm the enhanced adsorption layer is comprised of Pu and average height above the surface of the Pu distribution ⟨z⟩ = limq→0[ϕ(q)/q] of >30 Å where ϕ(q) is the phase of the resonant structure factor. These observations are fully consistent with the CTR, alpha spectrometry, and GI-XANES measurements, which indicate a vertically extended distribution and a high Pu coverage.

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DISCUSSION Beginning from a mildly acidic solution containing dissolved PuO22+ in contact with a muscovite surface, our results are all consistent with the formation of an adsorbed Pu surface layer. Alpha counting experiments yield a coverage of about 8 Pu/ AUC, significantly larger than the 0.5 Pu/AUC expected for full surface coverage by a divalent ion, such as PuO22+, on a muscovite surface. Results from GI XANES of the Pu LIII-edge reveal no significant plutonyl adsorption, as judged by the absence of the multiple-scattering feature above the edge indicative of the linear [OPuO]n+ moiety.65 Instead the edge is consistent with the presence of either trivalent or tetravalent Pu, a result that decreases the number of ions required to provide surface charge neutrality to 0.33 or 0.25 Pu/AUC, respectively, more than an order of magnitude lower than the adsorbed Pu coverage observed experimentally. Similar experiments involving a UO22+ solution in contact with a muscovite produced very different results. Consistent with precedent, we observed that uranyl weakly adsorbs to the muscovite surface.66−68 Alpha-counting measurements, together with the synchrotron-based X-ray studies, reveal substantially lower adsorbed coverages when compared to Pu. The U coverage under our experimental conditions was sufficiently low that the measurements were only able to provide an upper limit for the coverage (≤0.04 U/AUC). More generally, the surface structure bears a strong resemblance to that formed upon reaction of muscovite with NaCl,62 with only a slight shift of all features to larger distances from the interface. Assuming a simple electrostatic adsorption process, the uranyl(VI) ion would be expected to behave similarly to Sr2+ or Ba2+, whose adsorption properties have been previously studied.63,69,70 For example, surface coverage under similar conditions for divalent Sr and Ba is consistent with values expected for single-ion, electrostatically dominated adsorption processes. Both ions were found to compensate the surface charge of muscovite, with only minor deviations (occupancy of Sr2+ or Ba 2+ is 0.57 ± 0.03 35 and ≥0.44 ions/A UC , respectively).63,70 There may be multiple reasons for the starkly different behavior of UO22+ with respect to other ions. From a simple electrostatic perspective, solute and electrolyte ions such as monovalent Na or divalent Sr compete with the hydronium ion, the latter of which has an effective adsorption strength significantly higher than the other monovalent cations.49 For the conditions of this study, we estimate that the Sr coverage would be only ∼50% of the saturation coverage expected to compensate the muscovite surface charge.63,69,70 From a more atomic/molecular structural perspective, both Sr2+ and Ba2+ can be roughly approximated as point charges, whereas the uranyl ion is linear, with a charge distribution that retains some anionic behavior with the terminal oxo groups.42,71−73 The larger size of UO22+ in solution (the U−O distance is ∼1.7 Å, to which one-half the ionic radii of the oxygen atom must be added) as compared to that of Sr2+ or Ba2+(∼1.2−1.5 Å),74 especially when hydrated, may further destabilize the 10478

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The results obtained here, specifically demonstrating that a PuO2-nanoparticle-like layer forms on the muscovite surface in contact with a solution containing dissolved plutonyl(VI), are directly comparable with previous results when the same mineral is contacted with solutions containing either PuIII26 or a monodisperse solution containing PuIV nanoparticles41 with diameters on the order of 1−2 nm.80 Taken together, these results suggest that a driving force for adsorption is the broadranging stability of the hydrolyzed Pu aggregates.5,83 The higher stability of Pu(V) relative to U(V) under the experimental conditions may facilitate the reduction process through Pu(V)’s capability to disproportionate to form Pu(IV) and Pu(VI). Our results show that this is not a directly coupled redox process involving muscovite, because it is a redox-inactive mineral and both oxidation and reduction of Pu occur to result in PuIV-oxide aggregates at its surface. We can only speculate what the other half of the redox couple is. As the substrate itself is redoxinactive, one could hypothesize that water serves as the reductant, but our experiments do not allow a direct determination of the oxidized species. That said, the reaction kinetics at the muscovite surface appears to be significantly enhanced, pointing to its potential role in facilitating PuO2-like phase formation. This behavior has been studied previously, where it has been suggested that a montmorillonite surface enables plutonyl(V) reduction through a proton-mediated process.25 The importance of a plutonyl(V) species in the reduction process would explain in part the significant difference observed between plutonyl(VI) and uranyl(VI), because PuV is more stable than its UV counterpart.2

Figure 5. A comparison of total electron-density profiles from muscovite contacted with solutions containing PuIII,26 PuVI, or PuIV nanoparticles41 in comparison with that from muscovite with a 30 mM NaCl solution62 (without Pu).

fcc-type packing seen in solution80,81 to a bcc-like, Pu4O7 phase on the surface of goethite.79 A similar phase change was not observed for the adsorbed layer on a quartz surface. Both surfaces were prepared by contact with a dilute PuIV solution. Given these differences, the results suggest that the templating of the Pu(IV) nanoparticles through an epitaxial coincidence with the interfacial unit cell is not necessary for their formation at an interface, even though the presence of such a template may well alter the reaction products as observed on goethite. The development of PuIV on a mineral surface in contact with dissolved plutonium, from solutions containing trivalent through pentavalent Pu, has been previously observed. Many of the reported studies have involved iron or manganese bearing surfaces, such that the full redox couple has been assumed to involve the mineral phase itself.11,15−24 However, similar behavior has also been observed with solutions in contact with redox-inactive mineral surfaces.21,25−28 Related studies of Pu(V) to Pu(IV) reduction on montmorillonite, a redoxinactive mineral surface, show similar characteristics. The reduction was slow, and, on the basis of pH and ionic-strength dependence of the rate, it was hypothesized to involve a proton-mediated surface process.25 On the basis of known thermodynamic stabilities,2 this reaction would be expected to be significantly disfavored for the uranyl(VI) ion, whose adsorption properties onto a montmorillonite surface have been studied in detail.82 The data obtained from the plutonyl(VI) solutions in contact with a muscovite surface are very similar to those previously observed in the presence of both trivalent Pu and Pu-oxide nanoparticles. The analysis of the X-ray reflectivity data suggests that the adsorbed layer obtained from plutonylcontaining solutions is somewhat thinner and has a smaller Pu coverage (deduced by the smaller degree of enhancement in electron density at the surface) than that obtained from PuIII solutions, as depicted in Figure 5. These differences may originate from the more complex reaction kinetics involved in the reduction of PuO22+ to PuIV, which is known to be kinetically hindered by the fragmentation of the PuO22+ moiety,2 and is consistent with our observation that the sample evolved during the measurement. Consequently, the structure observed here may represent an intermediate stage in the adsorption process.

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CONCLUSION Muscovite (001) surface structures in contact with aqueous solutions containing 0.1 mM PuO22+ and 1 mM UO22+ were evaluated and found to be very different, although in neither case was the -yl(VI) species identified as the surface-complexed species. For the case of Pu, ex situ α-counting measurements, coupled with results from GI XANES and CTR/RAXR scattering studies, were consistent with the formation of a surface layer of reduced PuIV . The enhanced surface concentration, about 8 Pu/AUC as deduced from the alpha counting, and its distribution profile perpendicular to the planar muscovite surface, are similar to results obtained for the same surface in contact with a solution containing either PuIII or PuO2-like nanoparticles. In contrast, for the case of U solutions, there was no evidence of any adsorption. The significant difference in the surface coverage in the presence of aqueous actinyl(VI) solutions is consistent with both the more facile standard reduction potential for the plutonyl(VI) to PuIV reaction and the subsequent hydrolysis and condensation to PuO2-related nanoparticles. A similar reaction scheme does not occur for U under our solution conditions and in the presence of a muscovite (001) surface. Although a reaction scheme for UO22+ similar to that observed for PuO22+, that is reduction, hydrolysis, and subsequent condensation, is thermodynamically available to uranyl(VI) as well, it is less energetically favored. The different behavior of these two ions under similar conditions provides insight into the relative role of the redox window afforded by the aqueous solutions in contact with the redox-inactive muscovite surface. Assuming the major difference in the two systems can be attributed to differences in their formal reduction potentials, as opposed to the sorption potentials of the hexavalent cations, it can be concluded by comparison that the approximately 1 V increased relative 10479

DOI: 10.1021/acs.langmuir.6b02513 Langmuir 2016, 32, 10473−10482

Article

Langmuir potential required for the UO22+ (aq) to UO2(c) half-cell reaction over that for the plutonyl reduction is outside of the range provided by the muscovite surface. This information can be used to provide estimates for the role of a muscovite surface in contact with other dissolved redox-active ions.

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(7) Geckeis, H.; Lützenkirchen, J.; Polly, R.; Rabung, T.; Schmidt, M. Mineral−water interface reactions of actinides. Chem. Rev. 2013, 113, 1016−1062. (8) Kersting, A. B.; Efurd, D. W.; Finnegan, D. L.; Rokop, D. J.; Smith, D. K.; Thompson, J. L. Migration of plutonium in ground water at the Nevada Test Site. Nature 1999, 397, 56−59. (9) Novikov, A. P.; Kalmykov, S. N.; Utsunomiya, S.; Ewing, R. C.; Horreard, F.; Merkulov, A.; Clark, S. B.; Tkachev, V. V.; Myasoedov, B. F. Colloid transport of plutonium in the far-field of the Mayak Production Association, Russia. Science 2006, 314 (5799), 638−641. (10) Kersting, A. B. Plutonium transport in the environment. Inorg. Chem. 2013, 52, 3533−3546. (11) Keeney-Kennicutt, W. L.; Morse, J. W. The redox chemistry of Pu(V)O2+ interaction with common mineral surfaces in dilute solutions and seawater. Geochim. Cosmochim. Acta 1985, 49, 2577− 2588. (12) Duff, M. C.; Hunter, D. B.; Triay, I. R.; Bertsch, P. M.; Reed, D. T.; Sutton, S. R.; Shea-McCarthy, G.; Kitten, J.; Eng, P.; Chipera, S. J.; Vaniman, D. T. Mineral associations and average oxidation states of sorbed Pu on Tuff. Environ. Sci. Technol. 1999, 33, 2163−2169. (13) Stout, S. A.; Reilly, S. D.; Smith, D. M.; Neu, M. P. In PuV Removal from Solution by Oxidation and Adsorption onto Manganese Dioxide; 11th International Symposium on Water-Rock Interaction; Wanty, R. B., Seal, R. R. I., Eds.; Saratoga Springs: New York, 2004; pp 709−713. (14) Malin, J. N.; Geiger, F. M. Uranyl adsorption and speciation at the fused silica/water inferface studied by resonantly enhanced second harmonic generation and the χ(3) method. J. Phys. Chem. A 2010, 114, 1797−1805. (15) Sanchez, A. L.; Murray, J. W.; Sibley, T. H. The adsorption of plutonium(IV) and -(V) on goethite. Geochim. Cosmochim. Acta 1985, 49 (11), 2297−307. (16) Shaughnessy, D. A.; Nitsche, H.; Booth, C. H.; Shuh, D. K.; Waychunas, G. A.; Wilson, R. E.; Gill, H.; Cantrell, K. J.; Serne, R. J. Molecular interfacial reactions between Pu(VI) and manganese oxide minerals Manganite and hausmannite. Environ. Sci. Technol. 2003, 37, 3367−3374. (17) Powell, B. A.; Fjeld, R. A.; Kaplan, D. I.; Coates, J. T.; Serkiz, S. M. Pu(V)O2+ Adsorption and Reduction by Synthetic Magnetite (Fe3O4). Environ. Sci. Technol. 2004, 38 (22), 6016−6024. (18) Powell, B. A.; Fjeld, R. A.; Kaplan, D. I.; Coates, J. T.; Serkiz, S. M. Pu(V)O2+ Adsorption and Reduction by Synthetic Hematite and Goethite. Environ. Sci. Technol. 2005, 39 (7), 2107−2114. (19) Hu, Y.-J.; Schwaiger, L. K.; Booth, C. H.; Kukkadapu, R. K.; Cristiano, E.; Kaplan, D.; Nitsche, H. Molecular interactions of plutonium(VI) with synthetic manganese-substituted goethite. Radiochim. Acta 2010, 98, 655−663. (20) Kirsch, R.; Fellhauer, D.; Altmaier, M.; Neck, V.; Rossberg, A.; Fanghänel, T.; Charlet, L.; Scheinost, A. C. Oxidation state and local structure of plutonium reacted with magnetite, mackinawite, and chukanovite. Environ. Sci. Technol. 2011, 45, 7267−7274. (21) Begg, J. D.; Zavarin, M.; Zhao, P.; Tumey, S. J.; Powell, B.; Kersting, A. B. Pu(V) and Pu(IV) sorption to montmorillonite. Environ. Sci. Technol. 2013, 47 (10), 5146−5153. (22) Romanchuk, A. Y.; Kalmykov, S. N.; Egorov, A. V.; Zubavichus, Y. V.; Shiryaev, A. A.; Batuk, O. N.; Conradson, S. D.; Pankratov, D. A.; Presnyakov, I. A. Formation of crystalline PuO2+xnH2O nanoparticles upon sorption of Pu(V,VI) onto hematite. Geochim. Cosmochim. Acta 2013, 121, 29−40. (23) Hixon, A. E.; Powell, B. A. Observed changes in the mechanism and rates of Pu(V) reduction on hematite as a function of total plutonium concentration. Environ. Sci. Technol. 2014, 48 (16), 9255− 9262. (24) Emerson, H. P.; Powell, B. A. Observations of surface-mediated reduction of Pu(VI) to Pu(IV) on hematite nanoparticles by ATR FTIR. Radiochim. Acta 2015, 103 (8), 553−563. (25) Zavarin, M.; Powell, B. A.; Bourbin, M.; Zhao, P.; Kersting, A. B. Np(V) and Pu(V) ion exchange and surface-mediated reduction

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02513. Analysis of crystal truncation rod (CTR) data; and description of the sample cell (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: (630) 252-4364. E-mail: [email protected]. *Phone: +49 351 260 3136. E-mail: [email protected]. Present Address ⊥

Department of Chemistry, Georgetown University, Washington, DC 20057, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. S. Skanthakumar for his assistance in the preparation of XR samples. This work was cofinanced (M.S. and S.H.) by the Helmholtz Gemeinschaft Deutscher Forschungszentren by supporting the Helmholtz-Nachwuchsgruppe “Structures and Reactivity at the Water/Mineral Interface” (VH-NG-942). Work conducted at Argonne National Laboratory, operated by UChicago Argonne, LLC, was funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under contract number DEAC02-06CH11357, by the Geoscience (S.S.L. and P.F.) and Chemical Sciences (K.E.K. and L.S.) research programs. The Xray data were collected at the GeoSoilEnviroCARS beamline 13-ID-C at the Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviro-CARS is supported by the National Science Foundation-Earth Sciences (EAR-1128799) and Department of Energy-Geosciences (DE-FG0294ER14466) (J.E.S. and P.J.E.).

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