Surface-Mediated Formation of Pu(IV) Nanoparticles at the Muscovite

Article pubs.acs.org/est

Surface-Mediated Formation of Pu(IV) Nanoparticles at the Muscovite-Electrolyte Interface Moritz Schmidt,†,§ Sang Soo Lee,† Richard E. Wilson,† Karah E. Knope,† Francesco Bellucci,† Peter J. Eng,‡ Joanne E. Stubbs,‡ L. Soderholm,† and P. Fenter*,† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States

‡

S Supporting Information *

ABSTRACT: The formation of Pu(IV)-oxo-nanoparticles from Pu(III) solutions by a surface-enhanced redox/polymerization reaction at the muscovite (001) basal plane is reported, with a continuous increase in plutonium coverage observed in situ over several hours. The sorbed Pu extends >70 Å from the surface with a maximum concentration at 10.5 Å and a total coverage of >9 Pu atoms per unit cell area of muscovite (0.77 μg Pu/cm2) (determined independently by in situ resonant anomalous X-ray reflectivity and by ex-situ alpha-spectrometry). The presence of discrete nanoparticles is confirmed by high resolution atomic force microscopy. We propose that the formation of these Pu(IV) nanoparticles from an otherwise stable Pu(III) solution can be explained by the combination of a highly concentrated interfacial Pu-ion species, the Pu(III)−Pu(IV) redox equilibrium, and the strong proclivity of tetravalent Pu to hydrolyze and form polymeric species. These results are the first direct observation of such behavior of plutonium on a naturally occurring mineral, providing insights into understanding the environmental transport of plutonium and other contaminants capable of similar redox/polymerization reactions.

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INTRODUCTION Plutonium, a synthetic element first produced by Seaborg and co-workers only about 70 years ago,1 is an environmental contaminant both as a heavy metal and as an alpha-emitting radionuclide.2 The absence of natural mineral analogues upon which to base an understanding of its chemistry and its lack of a geochemical history adversely impact efforts to predict plutonium fate and transport in the environment. Laboratory studies3,4 reveal complex aqueous behavior and facile redox transformations. Pu is often present with oxidation states ranging from trivalent through hexavalent, each with dramatically different properties. Pu(V) forms highly soluble plutonyl species PuO2+ and is relatively stable under dilute conditions,5 whereas Pu(IV) is generally considered to be insoluble and undergoes hydrolysis, forming plutonium “polymers.”6 These polymers, or colloids,3,7−10 enhance overall Pu(IV) solubility and have been shown to have a sizable role in environmental transport.11−15 Pu(III) exhibits a solution chemistry very similar to that of the trivalent lanthanides and the predominantly trivalent heavier actinides (Am, Cm, ...). Polymerization reactions as observed for Pu(IV) have not been reported for trivalent plutonium. The interaction of dissolved radionuclides with mineral and colloid surfaces is widely considered to play an important role in their environmental transport.15,16 These interactions are often described through simple electrostatics, in the form of Gouy−Chapman−Stern models,17,18 but the actual behavior is generally more complex and not uniquely described by such models. A further complication is that the solution conditions © 2013 American Chemical Society

(e.g., ion concentration) near an interface can vary significantly from the bulk.19 Consequently, an understanding of the molecular-scale interfacial reactivity requires direct observations of the actual near-interface composition and structure. The potential complexity of Pu−mineral interactions was highlighted in ex situ studies of Pu(III) sorption to the basal surface of muscovite mica, using similar solution conditions as were applied for this study (c(Pu3+) = 1 mM, pH = 3, 0.1 M NaClO4).20,21 Crystal truncation rod (CTR) measurements and resonant anomalous X-ray reflectivity (RAXR),22−24 which enable a detailed atomic-level and surface-specific characterization of near-interface structures, showed that Pu sorbed in a broad vertical distribution with an average sorption height of 18 Å, a height considerably larger than expected for either inner or outer sphere sorption. The measured Pu coverage was also larger (0.48 ± 0.06 NPu/AUC) than expected for compensation of the fixed muscovite surface charge by Pu(III) (0.33 NPu/ AUC). This suggested the formation of Pu(IV)-oxo-nanoparticles on the surface, even though muscovite is not redoxactive.25 A recent in situ study26 revealed that presynthesized monodisperse Pu(IV) nanoparticles, with composition [Pu38O56]40+,27 readily adsorbed to the muscovite (001) basal plane. These observations are particularly interesting in light of recent results from ex situ transmission electron microscopy Received: Revised: Accepted: Published: 14178

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Surface X-Ray Scattering and Spectroscopy. CTR, RAXR, and X-ray absorption near-edge spectroscopy (XANES) experiments for the f resh sample were performed at GeoSoilEnviroCARS, beamline 13-ID-C at the Advanced Photon Source, APS, at Argonne National Laboratory. The incident Xray beam with a typical flux of ∼1012 photons/s was collimated by Kirkpatrick-Baez mirrors and slits to a profile of 0.05 × 0.5 mm2 (H × V) and was reflected from the sample and subsequently collected using a PILATUS area detector33 in the scattering plane defined by the incident and reflected X-ray beam. CTR data were measured in specular geometry as a function of vertical momentum transfer q (0.2−5.2 Å−1) with a fixed incident X-ray energy (E = 16.0 keV) that was well separated from any Pu X-ray absorption lines. RAXR data were measured at fixed q by scanning the incident energy through the Pu LIII absorption edge (18.061 keV) determined from Xray fluorescence on the same sample as described previously.20 RAXR spectra were measured at 25 selected values of q. For both experiments, CTR and RAXR, system stability was confirmed by repeated reference measurements at selected values of q at regular time intervals. Measurements for the aged sample were performed at the APS, beamline 6-ID-B (XOR, formerly MU-CAT) with slight differences in the experimental details: the beam was collimated by slits and then focused vertically by a Kirkpatrick-Baez mirror to a profile of 0.05 × 1.00 mm2 (V × H) at the sample position. Data were collected using an X-ray CCD detector. The q range for the CTR experiment was 0.2−5.3 Å−1, and 18 RAXR spectra were recorded.

(TEM) that revealed only weak association of Pu(IV) clusters with goethite, when the nanoparticles are preformed in solution but a strong epitaxial interaction when they are grown on the interface by incrementally adding Pu(IV).28 The same study also finds adsorption of Pu(IV) nanoparticles onto quartz surfaces. The experiments that suggested the growth of Pu nanoparticles at a surface (i.e., by CTR/RAXR and TEM)20,21,28 were performed ex situ, leaving open the potential for chemical changes through intermediate washing steps and subsequent removal of the liquid phase. Here, we present new in situ findings on the interfacial reactivity of plutonium at the muscovite (001)−aqueous solution interface, providing evidence for an interface-mediated pathway to the formation of Pu(IV) nanoparticles. The interfacial structure was elucidated by in situ CTR and RAXR; the time-dependent Pu uptake was monitored by in situ grazing-incidence X-ray absorption near-edge spectroscopy (GI-XANES) and ex situ alpha-spectrometry. The morphology of sorbed Pu(IV) nanoparticles was imaged using ex situ atomic force microscopy (AFM).

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MATERIALS AND METHODS Three samples will be discussed here. A freshly cleaved muscovite crystal was mounted in a recently developed cell for X-ray experiments with radiological samples29 and was exposed to a solution of a 0.1 mM Pu(III) at pH = 2.6 without added background electrolyte (f resh sample). A second sample was reacted with a solution of 2 mM Pu(III) in 100 mM NaCl at pH = 2.6 for 24 h (aged sample). An aliquot of this solution was monitored by UV/vis spectroscopy and was found to be stable over more than one week without measurable quantities (detection limit 160 μM for a 1 mm cell30) of Pu(IV) or polymeric phases.31 A third sample was prepared similarly to the aged sample for parallel AFM measurements. In order to prevent precipitation of the background electrolyte as well as dissolved plutonium onto the mica surface, the sample was washed in deionized water at pH = 3 and blown dry with dry N2 after exposure to the Pu(III) solution. Solution Preparation. All Pu solutions were prepared from acidic stock solutions of 242Pu (t1/2 = 375 000 a), electrochemically reduced to the trivalent state,32 by dilution with deionized water (DIW) or a solution of the appropriate background electrolyte. No pH adjustments were performed so as to prevent accidental formation of Pu-oxo-nanoparticles. The prepared solutions were monitored by UV/vis spectroscopy31 in a 1 mm cell over extended periods of time (∼10 days) to confirm the stability of the initial oxidation state of Pu(III) and the absence of the nanoparticles. Within detection limits (160 μM for Pu(IV)30), no oxidation to Pu(IV) and no formation of nanoparticles was observed in the reference solution without muscovite over the whole time period. Sorption Experiments. Samples were reacted under atmospheric conditions. The f resh sample was reacted in a radiological “thin-film” cell described previously.29 The solution was flowed over the crystal surface and allowed to react for the desired time period using an automated flow system. For X-ray measurements, the solution layer thickness was reduced to a thin film (10 nm. The images are in excellent agreement with the RAXR results and confirm the formation and adsorption of approximately nanometer-sized units, which aggregate further to form large clusters, reaching sizes of more than 10 nm. We

did not reach a steady state even after 90 min. The slow increase in adsorbed Pu is in contrast to previously observed sorption kinetics of cations on the muscovite (001) basal plane where sorption typically was found to occur almost instantaneously.34 The slow kinetics may be related to the chemistry of the sorbed Pu species, but such information could not be obtained solely from the GI-XANES data. The position of the Pu LIII absorption edge cannot be used to unambiguously determine the oxidation state of plutonium when the exact chemical environment is unknown because the edges for triand tetravalent plutonium are closer than potential edge shifts induced by changes in the speciation of plutonium (e.g., upon sorption).35 To further investigate plutonium sorption behavior, the interfacial structure was explored by CTR and RAXR. Data acquisition started after the XANES measurements and lasted approximately 9 h. A full structural analysis of the f resh sample was complicated by the continuous evolution of the interfacial system with time. Instead, data obtained from this sample were used as the basis for analysis of the aged sample. Specific information about the Pu sorption structure can be obtained directly from the RAXR spectra (i.e., R(E) at various q). From these spectra the q-dependent variation of the Puspecific amplitude A(q) (Figure 2, upper half) and phase Φ(q)/

Figure 2. Fresh sample. Amplitude A(q) and phase Φ(q)/q variation of the element-specific partial structure factor F of Pu adsorbed at the muscovite (001)−aqueous solution interfaces determined by modelindependent RAXR. Black solid lines are guides for the eye representing possible extrapolations of the data to q = 0.

q (Figure 2, lower half) can be derived using a modelindependent analysis.36 In the limit of q → 0, Φ/q and A have the asymptotic values: ⟨z⟩ = limq → 0[ϕ(q)/q] θ = limq → 0[A(q)]

where ⟨z⟩ is the average height of the resonant atom (Pu) distribution and θ is its surface loading (in units of ions per muscovite unit cell, AUC = 46.72 Å2). For the f resh sample, we obtain a lower limit for both quantities at q = 0.15 Å−1, revealing an average Pu height >40 Å and a coverage of >1.1 Pu/AUC. The data directly reveal an extended plutonium 14180

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Table 1. Details for the Experimental Conditions for the Fresh and Aged Samples sample

[Pu]sol [mM]

fresh

0.1

aged

2

AFM

2

background DIW pH 2.6 0.1 M NaCl pH 2.6 0.1 M NaCl pH 2.6d

θa (α, RAXR) [NPu/AUC] 1.4 ± 0.1 (α) > 1.1 (RAXR) 9.0 ± 0.5 (α) 9.9 (RAXR) n.d.

exptl.b

time [h]c

29

2

26

24

24

a Surface loading, determined by alpha-spectrometry (α) and resonant anomalous X-ray reflectivity (RAXR)21 in units of number of Pu atoms (NPu) per area of the muscovite unit cell (AUC). bExperimental conditions correspond to those described in the references; for details, refer to the Supporting Information (SI). cReaction time before the start of the in situ CTR/RAXR or ex situ AFM experiments, respectively. dSolution removed from the surface by blow-drying with N2 before the experiment.

Figure 4. (a) AFM micrograph of the muscovite (001) basal plane after overnight exposure to 2 mM Pu(III) in 100 mM NaCl at pH = 2.6. (b) Three-dimensional representation of the same micrograph. (c) Profile along line “1” shown in a.

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Figure 3. Total electron density profile derived from CTR (black line) and Pu electron density distribution derived from RAXR (blue area, the 1σ uncertainty in the electron density (ED) is shown as a black band39). The electron density profile for muscovite in DIW is shown as a dashed red line,40 and the Pu distribution upon adsorption of Pu nanoparticles is shown as a dashed green line.26 The ED is plotted normalized by that of bulk water ρwater = 0.33 e−/Å3. A schematic representation of species contributing to the observed ED profile is shown in the upper part of the figure. (Key: purple tetrahedra, SiO4; blue octahedra, AlO6; red spheres, O; purple, Pu; green, Cl; white, H.) The representation is not quantitative and serves only for visualization.

DISCUSSION The present data shed light on the unexpected reactivity of trivalent plutonium solutions in the presence of a muscovite (001) surface. The in situ results show that the surface enhances the spontaneous formation of plutonium(IV)-oxo-nanoparticles. Pu(III) is not known to form nanoparticles of any kind; thus the formation of such entities requires the previous oxidation of Pu(III) to Pu(IV). The structure of the Pu particles is indeed qualitatively similar to the one that was found upon sorption of preformed Pu(IV) nanoparticles on the same mineral surface. While similarities in the profiles outweigh the differences, it is not surprising that the total and Pu-specific profiles differ in details. The nanoparticles that form at the interface appear to exhibit a wider size distribution close to the interface than found by adsorption of preformed monodisperse nanoparticles with an edge length of ∼10 Å. All Pu solutions appeared stable in the absence of a muscovite interface, and the total Pu-sorbed coverage appeared to be self-limiting while not significantly depleting the bulk solution concentration. From this behavior, we conclude that the observed formation of nanoparticles is not simply a bulk

can distinguish three different morphologies. The majority of the particles are small with heights of about 1−2 nm and base areas of ∼1 × 1 nm2 up to 10 × 10 nm2. Aggregates of these particles can also be seen, forming larger islands with essentially flat tops (∼1 nm height variation over a 20−50 nm lateral range, Figure 4c). A third morphology (not shown in Figure 4) consists of large stacks of particles with ragged features on the top that grow well beyond 10 nm in height. 14181

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precipitation process but instead is likely driven by interactions of aqueous plutonium species at the muscovite−electrolyte interface. Since Pu condensation to form nanoparticles has only been observed for Pu(IV), there are two chemical changes that must occur for the formation of the observed Pu(IV) nanoclusters from an ostensibly Pu(III) aqueous solution: (1) the oxidation of Pu(III) to Pu(IV) and (2) the condensation of Pu(IV) into clusters. Whereas both reactions are known to occur in Pu solutions without an interface,9 the data presented herein suggest an additional, surface-enhanced, pathway to Puoxo-nanoparticle formation. The solution redox potential was not controlled during either measurement. The redox potential for the Pu(III)/Pu(IV) couple is similar to that of the water couple itself, so direct solvent reduction (i.e., reduction of H+ to H2) is expected to provide the other half-cell reaction for at least some of the Pu oxidation. Due to the very small total amount of adsorbed Pu (1.24 μg, 5.1 nmol), no macroscopic effects, e.g. changes in pH or formation of a gas, could be observed. According to published thermodynamic data,41 for the Pu solution concentrations used in this study there should be a Pu(III)/Pu(IV) equilibrium establishing a small relative fraction of tetravalent Pu ( 0.6 mol/L in this 15-Å-thick layer of solution adjacent to the muscovite surface (whether this layer contains primarily Pu(III) or Pu(IV)), a ∼1000-fold increase in the interfacial Pu concentration with respect to that in the bulk solution. Since condensation reactions are known to be concentration dependent, this enhancement of local Pu concentration would be expected to facilitate polymerization and/or aggregation,7 even at very low pH. (The pH at the interface could not be quantified in this study.) These reactions will not be limited by the depletion of bulk or interfacial Pu(IV) due to condensation, as these species can be replenished through the Pu(III)−Pu(IV) redox equilibrium. The results from the aged sample suggest that the reaction is essentially selflimiting. Although the surface loading exceeds the amount expected for a simple trivalent cation by nearly a factor of 30, only 2.5% of plutonium was removed from the original sorption solution. The factor limiting the reaction’s progress is unclear, but our proposed mechanism suggests one possible explanation: the compensation of the negative surface charge by the positively charged nanoparticles can be expected to limit the polymerization process by inhibiting an enhanced Pu solution concentration in the interfacial region. These combined X-ray scattering and AFM results suggest the surface-mediated formation of nanometer-sized Pu(IV) aggregates on muscovite. In this sense, the surface appears to “catalyze” aggregate formation, accelerating a reaction that, in solution, can take several weeks by enhancing the local interfacial Pu concentration, thereby opening up a distinct heterogeneous interfacial reaction pathway that occurs within hours. This process has many implications for plutonium chemistry in general, but particularly for its geochemistry and geochemical transport behavior. Such a pathway is consistent with previous ex situ observations regarding the preferential adsorption of environmental Pu on the surface of Fe nanoparticles,13 but the present results reveal that this process occurs rapidly and does not necessarily require a redox-active mineral substrate. While the formation of nanoparticles increases the retention capacity of the mineral almost 30-fold, in other systems (e.g., where adsorption to mobile nanoparticulate matter is possible) the same mechanism may lead to increased transport in aquatic systems. This process may also be relevant to Pu sorption on other substrates and might be enhanced by redox-active substrates. More generally, this result highlights the possibility that the nanoparticle−polymer phase may form spontaneously under environmental conditions, even at very low bulk Pu solution concentrations. Moreover, the 14182

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(9) Neck, V.; Altmaier, M.; Fanghänel, T. Solubility of plutonium hydroxides/hydrous oxides under reducing conditions and in the presence of oxygen. C. R. Chim. 2007, 10 (10−11), 959−977. (10) Knope, K. E.; Soderholm, L. Solution and Solid-State Structural Chemistry of Actinide Hydrates and Their Hydrolysis and Condensation Products. Chem. Rev. 2012, 113 (2), 944−994. (11) Ryan, J. N.; Illangasekare, T. H.; Litaor, M. I.; Shannon, R. Particle and plutonium mobilization in macroporous soils during rainfall simulations. Environ. Sci. Technol. 1998, 32, 476−482. (12) 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 (6714), 56−59. (13) 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. (14) Xie, J.; Wang, X.; Lu, J.; Zhou, X.; Lin, J.; Li, M.; Xu, Q.; Du, L.; Liu, Y.; Zhou, G. Colloid-associated plutonium transport in the vadose zone sediments at Lop Nor. J. Environ. Radioactivity 2013, 116, 76−83. (15) Kersting, A. B. Plutonium transport in the environment. Inorg. Chem. 2013, 52, 3533−3546. (16) Geckeis, H.; Lützenkirchen, J.; Polly, R.; Rabung, T.; Schmidt, M. Mineral−Water Interface Reactions of Actinides. Chem. Rev. 2013, 113 (2), 1016−1062. (17) Chapman, D. L. LI. A contribution to the theory of electrocapillarity. Philos. Mag. Ser. 6 1913, 25 (148), 475−481. (18) Stern, O. Zur Theorie der Elektrolytischen Doppelschicht. Z. Elektrochem. 1924, 30 (21/22), 508−516. (19) Schmidt, M.; Lee, S. S.; Wilson, R. E.; Soderholm, L.; Fenter, P. Sorption of tetravalent Thorium on Muscovite. Geochim. Cosmochim. Acta 2012, 88, 66−76. (20) Fenter, P.; Lee, S. S.; Park, C.; Soderholm, L.; Wilson, R. E.; Schwindt, O. Interaction of muscovite (0 0 1) with Pu3+ bearing solutions at pH 3 through ex-situ observations. Geochim. Cosmochim. Acta 2010, 74, 6984−6995. (21) Wilson, R. E.; Schwindt, O.; Fenter, P.; Soderholm, L. Exploitation of the sorptive properties of mica for the preparation of higher-resolution alpha-spectroscopy samples. Radiochim. Acta 2010, 98 (7), 431−436. (22) Fenter, P. X-ray Reflectivity as a Probe of Mineral-Fluid Interfaces: A User Guide. Rev. Mineral. Geochem. 2002, 49, 149−220. (23) Lee, S. S.; Nagy, K. L.; Park, C.; Fenter, P. Heavy Metal Sorption at the Muscovite (001)−Fulvic Acid Interface. Environ. Sci. Technol. 2011, 45 (22), 9574−9581. (24) Park, C.; Fenter, P. A.; Sturchio, N. C.; Regalbuto, J. R. Probing Outer-Sphere Adsorption of Aqueous Metal Complexes at the OxideWater Interface with Resonant Anomalous X-Ray Reflectivity. Phys. Rev. Lett. 2005, 94 (7), 076104. (25) Bailey, S. W. Micas; Mineralogical Society of America: Chantilly, VA, 1984; Vol. 13. (26) Schmidt, M.; Wilson, R. E.; Lee, S. S.; Soderholm, L.; Fenter, P. Adsorption of Plutonium-Oxide Nanoparticles on Muscovite. Langmuir 2012, 28, 2620−2627. (27) Soderholm, L.; Almond, P. M.; Skanthakumar, S.; Wilson, R. E.; Burns, P. C. The structure of the plutonium oxide nanocluster [Pu38O56Cl54(H2O)(8)](14−). Angew. Chem., Int. Ed. 2008, 47 (2), 298− 302. (28) Powell, B. A.; Dai, Z.; Zavarin, M.; Zhao, P.; Kersting, A. B. Stabilization of Plutonium Nano-Colloids by Epitaxial Distortion on Mineral Surfaces. Environ. Sci. Technol. 2011, 45 (7), 2698−2703. (29) Schmidt, M.; Eng, P.; Stubbs, J.; Soderholm, L.; Fenter, P. On a new X-ray reflectivity environmental cell design for in situ studies of radioactive and atmosphere-sensitive samples. Rev. Sci. Instrum. 2011, 82 (7), 075105−1 -075105−10. (30) Wilson, R. E.; Hu, Y.-J.; Nitsche, H. Detection and quantification of Pu(III, IV, V, and VI) using a 1.0-meter liquid core waveguide. Radiochim. Acta 2005, 93 (4−2005), 203−206. (31) Cohen, D. The Absorption Spectra of Plutonium Ions in Perchloric Acid Solutions. J. Inorg. Nucl. Chem. 1961, 18, 211−218.

potential importance of mineral−Pu interactions for understanding the aquatic chemistry of plutonium in the environment, as well as the significance of Pu-oxo-nanoparticles as an intermediate state in precipitation reactions, is emphasized.

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ASSOCIATED CONTENT

S Supporting Information *

The CTR and RAXR data, fitting parameters, details on the fitting procedure, and experimental procedures, electron density difference profiles, preliminary structure for the f resh sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 (630) 252-7053. E-mail: [email protected]. Present Address §

Institute of Resource Ecology, Helmholtzzentrum DresdenRossendorf, P.O. Box 510119, 01314 Dresden, Germany Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work, conducted at Argonne National Laboratory, operated by UChicago Argonne, LLC for the United States Department of Energy under contract number DE-AC0206CH11357, is jointly supported by the United States Department of Energy Office of Science, BER, NSF, and the EPA (MS), and by the DOE/BES Geoscience (S.S.L., F.B., and P.F.) and Chemical Sciences (K.E.K., R.E.W., and L.S.) research programs. The X-ray data were collected at the GeoSoilEnviroCARS beamline 13-ID-C and the X-ray Operations and Research beamline 6-ID-B at the Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation−Earth Sciences (EAR-1128799) and Department of Energy−Geosciences (DE-FG02-94ER14466). Notes

The authors declare no competing financial interest.

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REFERENCES

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