Review pubs.acs.org/CR
Solution and Solid-State Structural Chemistry of Actinide Hydrates and Their Hydrolysis and Condensation Products Karah E. Knope* and L. Soderholm* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States 4.4.2. Mononuclear Hydrolysis Products 4.4.3. Pu(IV) Dimers 4.4.4. Pu Polymer or Colloid 4.5. Berkelium(IV) 5. Pentavalent An 5.1. Pa(V) 5.1.1. Hydrates/Aqua Ion 5.2. Uranyl(V) 5.2.1. Hydrates/Aqua Ion 5.2.2. Oligomers 5.3. Neptunyl(V) 5.3.1. Hydrates/Aqua Ion 5.3.2. Oligomers 5.4. Plutonyl(V) 5.4.1. Hydrates/Aqua Ion 6. Hexavalent An 6.1. Uranyl(VI) 6.1.1. Hydrates/Aqua Ion 6.1.2. Mononuclear Hydrolysis Products 6.1.3. Oligomers 6.2. Neptunyl(VI) 6.2.1. Hydrates/Aqua Ion 6.2.2. Oligomers 6.3. Plutonyl(VI) 6.3.1. Aqua Ion/Hydrates 6.3.2. Oligomers 7. Heptavalent An 7.1. Neptunium(VII) 7.1.1. Mononuclear Species 7.1.2. Oligomers 7.2. Plutonium(VII) 7.2.1. Mononuclear Species 8. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References Note Added after ASAP Publication
CONTENTS 1. Introduction 2. Background: Actinide Aqueous Chemistry 2.1. Actinide Ion Stabilities in Aqueous Environments 2.2. Actinide Ion Hydrolysis and Condensation Reactions 2.3. Structural Probes of Actinide Speciation 3. Trivalent An 3.1. An(III) Solution Species 3.1.1. U(III) 3.1.2. Np(III) 3.1.3. Pu(III) 3.1.4. Am(III) 3.1.5. Cm(III) 3.1.6. Bk(III) 3.1.7. Cf(III) 3.2. An(III) Solid-State Structural Chemistry 3.2.1. Aqua Complexes 3.2.2. An(III) Dimers 4. Tetravalent An 4.1. Thorium(IV) 4.1.1. Hydrates/Aqua Ion 4.1.2. Mononuclear Hydrolysis Products 4.1.3. Oligomers 4.1.4. Colloids 4.2. Uranium(IV) 4.2.1. Hydrates/Aqua Ion 4.2.2. Mononuclear Hydrolysis Products 4.2.3. Oligomers 4.2.4. Colloids 4.3. Neptunium(IV) 4.3.1. Hydrates/Aqua Ion 4.3.2. Oligomers 4.4. Plutonium(IV) 4.4.1. Hydrates/Aqua Ion © 2012 American Chemical Society
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1. INTRODUCTION Metrical knowledge about a metal ion’s speciation in aqueous solution is a critical step in the development of a predictive understanding of its solubility, stability, and reactivity. Gaining Special Issue: 2013 Nuclear Chemistry Received: May 29, 2012 Published: October 29, 2012 944
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Unfortunately for the actinides, particularly the transuranic ions, there are relatively few structural studies that provide metrical information to guide and support chemical species assigned during this modeling process. The so-called “proton ambiguity” complicates even further attempts to provide a cohesive basis for understanding the fundamental thermodynamic properties of aquated ions and their hydrolysis products.8 In standard solution experiments, notably potentiometry, it may not be possible to distinguish equilibrium constants because the same number of protons is released from possibly multiple, different equilibrium equations. In this regard, the structural characterization of hydroxo- or oxo-bridged dimeric or higher order oligomeric precipitates through X-ray diffraction may be useful in support of the solution modeling, that is, the correct constitution and structure of the complexes. In other cases, there is only indirect evidence for oligomer assignment, obtained from chemical techniques such as solvent extraction, or probes such as optical or NMR spectroscopies. From a molecular level, recent advances in theory are providing a new avenue with which to understand actinide speciation, reactivity, solubility, and relative stability of metal complexes in solution.9−22 Potential successes of this approach, particularly when coupled with experiment, are of particular interest to the actinide community. The use of more advanced theory to guide experiments would help to minimize the difficulties associated with obtaining and handling these radionuclides as well as shedding light on their often intractable chemistry, the understanding of which can be vitiated by their complex chemistry, notably the hydrolysis and condensation reactions that are the focus of this Review. Generally, quantum mechanical methods have recently become useful for structural predictions regarding very small molecules. Estimating energy differences between their various isomers is more challenging. Areas in need of continued work, important to the solution chemist, center on the need to include higher order coordination spheres in the calculations, whether they arise from other metals or solvent interactions. Related to this need is the recently recognized importance of the anion on calculated energetics in aqueous solutions. Arguably the most important problem, the ability to probe the effect of pH on ion speciation is still couched by theorists in terms of a “rare event” problem, one that remains largely intractable. This being said, the theorist needs more, and more accurate, metrical information on the metal ion’s coordination environment in solution. There is often little structural information to guide and benchmark theoretical studies. This is particularly true when probing metal ions in solution. Although there are a variety of techniques that provide indirect information about a metal ion’s correlations and coordination complexes in solution, there are a very limited number of techniques that provide direct, metrical information including metal−ligand coordination numbers and distances. For the purpose of this Review, we have chosen to limit our scope to experimental studies that provide this information directly. The focus of this rReview is on providing a detailed metrical description of a hydrated ion’s coordination environment and how it changes as it further reacts with water through hydrolysis. Understanding in detail how to predict and control hydrolysis and condensation reactions is a central goal behind many of the efforts to develop the current thermodynamic databases. Although this is the same goal taken up by the theoreticians and modelers, they approach the problem from a microscopic, molecular level instead of a statistical, macroscopic
such knowledge is particularly challenging for the actinide elements because, unlike most elements across the periodic table, there is no geologic precedent to guide the understanding of general chemical trends across the series. What is clear is the diversity of their chemistries, including their rich redox behavior, which includes disproportionation, their radioactivity, which can lead to radiolysis, and their hydrolysis and condensation reactions, a phenomenon prevalent for metal ions in general.1 The first of the transuranic elements was discovered in 1940,2 with production in macroscopic quantities of the longer lived isotopes following shortly thereafter. What followed was the introduction of these radioactive, toxic elements into the environment.3,4 The potential to continue this trend in the future provides a sense of urgency for a basis upon which to build a well-founded predictive capability for their migration and biological uptake. Examples demonstrating the impact of their aqueous solution behavior extend from the laboratory, where liquid−liquid separations can provide an opportunity to reduce waste volume, to geological fate and transport models, where groundwater transport predictions will impact policy and decision making regarding nuclear energy. Biological uptake and subsequent health issues such as accumulated dose and heavy metal poisoning extend these examples into even more complex solution environments. Much effort and some headway have been made on understanding the solution chemistry of these “new” elements. Central to the work to date has been the development of an understanding of the metal−ion complexes formed in solution, both with the solvent and with other solute ions. Largely application driven to enable separations and processing associated with defense and nuclear energy programs, the enormous opportunity to address fundamental questions of metal−ion behavior in aqueous and nonaqueous solutions remains largely unexplored. Understanding of actinide speciation in solution is a daunting task. To limit and focus the scope of this Review, we have chosen to focus on hydration, hydrolysis, and condensation reactions in aqueous solution because of their fundamental importance across a broad range of disciplines, from biological systems to nuclear waste reprocessing to geological fate and transport. An impressive thermodynamic knowledge-base exists, and theory has focused on hydration and hydrolysis as a place to test their theories. Understanding these simple systems is the first critical step to a predictive understanding of actinide solution chemistry. Perhaps at least in part because of its importance to separations science and to modeling actinide migration from nuclear waste repositories, one area that has received considerable attention has been the development of thermodynamic databases covering a wide range of actinide−ion equilibria in aqueous solution and the stability constants of the associated complexes. As a result, excellent reviews exist on the solution thermodynamic stabilities of hydrated actinide ions and their hydrolyzed moieties.5−7 Included in these reports are descriptions of reaction products introduced during thermodynamic data analysis and interpretation. Oligomeric species are prevalent in much of the modeling. In most cases, it is relatively straightforward during data interpretation to identify oligomer formation, but their detailed constitution is not necessarily unambiguous. Often independent knowledge of chemical behavior is critical in determining a robust description of solution speciation under the specific experimental conditions. 945
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view. It is our goal to facilitate information transfer and comparisons between these two approaches to the same problem. Identified as a missing link, we focus on relating the metal−ligand correlated moieties and aggregates identified from thermodynamics with molecular level structures for which the theorist can assess their results. Metrical information provided by applying modern techniques to solution studies is providing some guidance; however, the extensive and detailed data on atomic correlations provided by single-crystal X-ray diffraction experiments provide guidance and insights on available structural motifs. This is particularly true for actinide structural chemistry, the understanding of which is still in its relative infancy. For solution speciation, we rely almost exclusively on extended X-ray absorption fine structure (EXAFS), a singleion spectroscopic probe that has been widely used to provide first coordination sphere structural data, and X-ray scattering techniques such as wide-angle X-ray scattering (WAXS), largeangle X-ray scattering (LAXS), and high-energy X-ray scattering (HEXS), which are not single-ion probes but do provide pair correlations for all atoms present in the solution.23,24 Scattering techniques work particularly well because X-rays scatter off electron density, so the high atomic numbers of the actinides serve to provide significant contrast to the solvent atoms, thus allowing metal−centric pair correlations to be determined with a high degree of precision. For comparative solid-state structural work, we rely solely on single-crystal X-ray diffraction studies. Highlighted herein are a number of An hydrates, mononuclear hydrolysis products, and polynuclear complexes, the latter of which result from metal−ion hydrolysis and condensation in aqueous and nonaqueous solution. When we set out to write this Review, we thought that we could limit its scope by confining our discussion to those compounds isolated from aqueous solution. However, in doing so, it became obvious that many of the larger clusters, in particular those observed for the tetravalent metal ions and the mixed valent U(IV)/U(V) compounds, would be excluded from this Review, despite their relevance. Hence, we include representative examples of polynuclear An oxide/hydroxide-bridged oligomers irrespective of the conditions under which they were synthesized.
Table 1. Actinide Oxidation States Available, X, in Aqueous Solution and Covered in This Review Th 3 4 5 6 7
X
Pa X X
U
Np
Pu
Am
Cm
Bk
Cf
X X X X X
X
X
X X
X
X X X
X X X X X
2.2. Actinide Ion Hydrolysis and Condensation Reactions
Formation of an actinide−water complex enables hydrolysis, which involves the deprotonation of the bound water according to: [M(OH 2)m ]n + → [M(OH 2)m − h (OH)h ](n − h) + + hH+
(1)
In aqueous solution, H2O can serve both as a solute ligand (Lewis base) and as the solvent. The aquated cation is a Bronsted acid. The degree of cation hydrolysis, h as written in eq 1, is principally determined by the cation charge (n) to radius ratio and is sensitive to solution conditions including the presence or absence of other dissolved species and their concentrations, temperature, and pH. In dilute solutions, there can be an entire series of hydrolysis products spanning the monohydroxo- to the fully hydrolyzed [M(H2O)m(OH)h]0 moiety and can include equilibra that result in the simultaneous presence of two or more such species. Once formed, hydrolyzed metal complexes can condense to form larger oligomers. This condensation can occur via two different reactions.27 (i) The first is olation, which involves one hydrolyzed metal and results in a hydroxo-bridged product. The nucleophilic attack on a metal ion by the OH− group from another metal complex has the form: M − OH + Maq → M − OH − M
(2)
(ii) The second is oxolation, which involves the reaction of two OH− groups attached to two hydrolyzed metal centers and results in the formation of an oxo-bridged species with the elimination of H2O: M − OH + OH − M → M − O − M + H 2O
2. BACKGROUND: ACTINIDE AQUEOUS CHEMISTRY
(3)
Which of reactions 2 or 3 occurs can be considered, in the most elementary sense, in terms of hard and soft acid base theory28,29 and more specifically in terms of the metal−ion hardness30 and electronegativity,31 and their effect on M− (OH) n stability in aqueous solution with respect to deprotonation. These reaction types are based more on stoichometery than on experimental evidence from mechanistic studies. On the basis of their charge and electronegativity, the tri- and tetravalent actinide ions are generally considered to undergo olation reactions, forming oligomeric hydroxo-bridged species with ill-defined structures and chemistry. The formally highvalent actinides do not exist in aqueous solution as the simple aquated cation but instead as the linear dioxo [OAnO]n+ moiety, where n = 1,2 for the penta- and hexavalent ions and the tetraoxo AnO4− species for heptavalent Np and Pu in basic solution. These oxo ions behave as low-valent cations, undergoing primarily olation reactions to form hydroxo-bridged species. This perspective is evident when studying the vast
2.1. Actinide Ion Stabilities in Aqueous Environments
Water is a demanding solvent because it affords to its solutes only a small electropotential window of stability. Derivable from application of the Nernst equation, this window covers the range 0.0−1.23 V (versus the stand hydrogen electrode) in acidic solutions and −0.826 to 0.404 V in alkaline solutions.4,25 Outside of these limits, solute−solvent redox reactions impact the solution chemistry and oxidation-state stability. Complicating the issue is the rich redox chemistry exhibited by the actinide ions themselves.26 Taken together, these factors limit the oxidation states covered in this Review to those listed in Table 1. Oxidation states outside of this range, while perhaps stable in other media, have not been considered as part of this Review. From the perspective of this Review, we assume knowledge of the actinide redox state and for the solution studies will be assumed to be invariant in the hydrolysis and condensation reactions on which we focus. 946
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X-ray energy, it is possible to probe the oxidation state and coordination environment of the absorbing ion. For the data near the absorbing edge, the X-ray absorption near-edge spectroscopy (XANES) is ideal for determining oxidation states. Farther from the edge, the extended X-ray absorption fine structure (EXAFS) provides information about the number, identity, and distance of coordinating ligands about the absorbing ion. It is the results from EXAFS data analyses that are used in this Review. It is a single-ion probe that can provide information about an absorbing ion in the gas, solution (liquid), or solid phase. The coordination numbers are generally considered accurate to ±10% and the distances to about 1%. Two subshells with separations less than 0.1 Å cannot be resolved from solution data. X-ray scattering has been used for a number of years to resolve metal−ion coordination environments in solution,39,40 as exemplified by early studies of dissolved thorium and uranyl ions.41−46 These experiments were conducted using in-house diffractometers and often took months for data aquisition. This technique, often referred to as large-angle X-ray scattering (LAXS) or wide-angle X-ray scattering (WAXS), probes all correlations in solution. Analyses of the data were done by directly taking into account all of the correlations in the pattern.47 The low counting statistics and the restricted momentum (q) range of the data (the maximum of which is defined by the Xray energy) limited the use of this technique. This situation changed with the availability, from third-generation synchrotron sources, of a high flux of high-energy X-rays (>60 keV). High-energy X-ray scattering (HEXS) extends the q-range well into the self-scattering regime,24,48 thus enabling the removal of solvent−solvent and solvent−counterion correlations49 using data from carefully prepared background solutions. Data so treated provide information on metal−ion correlations in solution and are so represented in this Review. HEXS data can provide first-shell coordination numbers to an accuracy of about ±1−2% and distances accurate to about 2%. HEXS is also able to provide relatively detailed information on secondcoordination-sphere interactions. EXAFS and HEXS techniques provide complementary strengths, with EXAFS able to probe metal−ion correlations for submillimolar solution concentrations. HEXS generally requires significantly higher solute concentrations, but because X-rays scatter off electrons, and actinides are high-Z ions, the contrast is optimal and concentrations in the low (2, the details of which are dependent on metal ion concentration, temperature, and the presence of other ions.5,79,92,93 A recent thorough review of the literature pertaining to the determination of stability constants of oligomeric Th hydrolysis products, specific to 1 M NaClO4, identifies mono- and polynuclear Th hydroxide species based solely on potentiometric studies in which they have been identified as major components of the solution.5 The review identifies the mono- and dihydroxo Th monomers Th(OH)3+ and Th(OH)22+ as well as the dimer Th2(OH)26+, tetramer Th4(OH)88+, and hexamer Th6(OH)159+. The tetrameric species Th4(OH)124+, needed to fit data from a 3 M NaClO4 solution, is also included in the list of stable hydrolysis products. These products are used for modeling data in nitrate ionic media as well. In contrast, in chloride media Th2(OH)26+, Th2(OH)35+, and Th6(OH)1410+ are sufficient to model the available data. Together with monomeric Th hydrolyzed species, these oligomers were used to evaluate equilibrium constants at zero ionic strength and to evaluate their ionic strength dependence in perchlorate, chloride, and nitrate solutions.5 It is of note that all species included in the modeling are hydroxo complexes; no mixed oxo-hydroxo or oxo-based moieties were included. As explained in the Introduction, it is impossible to distinguish between oxo- and hydroxo complexes using pH-based methods because of the proton ambiguity. It is interesting to note that to date very few of the model species have been observed in solidstate structures. Nonetheless, over 50 solid-state structures containing polynuclear species have been reported for the tetravalent actinides. Such compounds include sulfates, phosphates, carboxylates, phosphonates, etc. wherein the metal centers are often bridged into polynuclear entities by donor atoms of the complexing ligands. However, for the purposes of this Review, we will limit our discussion of oligomers identified as dimers, trimers, and higher order complexes to those structures containing hydroxo- or oxo-bridged metal cations consistent with the condensation of hydrolyzed species that is believed to proceed via the formation of hydroxo or oxo linkages. 4.1.3.1. Dimers. 4.1.3.1.1. DimersAqueous Solution State. The complex stoichometries and oligomeric complexes identified from solution thermodynamics have relatively little support from structural studies, through either metrical solution speciation or solid-state structural studies. The first direct observation of polynuclear hydrolyzed Th species in solution was provided by X-ray scattering experiments.41 Analyzing LAXS data from aqueous nitrate solutions with Th concentrations of about 1.95 M, a Th−O distance of about 2.50 Å, and a Th−Th distance of about 3.95 Å were determined, the latter of which appeared to have a slight pH dependence. The latter distance was attributed to the presence of a dihydroxo-bridged Th dimer, and the distance of 3.95 Å compares well with Th− Th interatomic distances observed in dihydroxo-bridged Th dimers isolated in the solid state as nitrates (3.998(2) and 3.981(2) Å), and a chloride (4.020(2) Å)94 as well as other Th dimer distances listed in Table 4. A HEXS study of the solution from which the dihydroxo-bridged Th chloride crystals were obtained found a correlation peak at 4.05 Å, confirming the presence of the dimer in solution.94 These results confirm an olation-type condensation for Th dimer formation in moderately acidic, aqueous solution. A detail of interest in
Figure 5. The homoleptic aqua complex isolated for Th; 10 water molecules are found in the inner coordination sphere.86
2.45(1) Å determined by and analysis of HEXS data obtained from solutions made by dissolving the solid in water. 4.1.2. Mononuclear Hydrolysis Products. 4.1.2.1. Aqueous Solution State. The stability constants of mononuclear hydrolyzed Th complexes Th(OH)n(4−n)+, determined using a variety of indirect probes such as liquid−liquid extraction, and potentiometric titration data, have been recently reviewed and the results compiled.5 Anions of choice for these experiments have included perchlorate, chloride, and nitrate because either they are not complexing, as in the former case, or they have some applicable relevance, such as in reprocessing separations. Recent structural work indicates that bromide would also be a suitable ligand, providing only outer-sphere complexes.86 The stability constants reported in these studies rely on a methodical and thoughtful modeling of the data. Often several models can describe the experimental data equally well; the choice is then made using additional chemical information, both quantitative and qualitative. Unfortunately, there are no analytical tools that can be used to directly confirm, or provide metrical information about, the structure of these species in solution. The most commonly used tools, EXAFS, LAXS, or HEXS, lack the accuracy necessary to distinguish a Th−OH2 from a Th−OH bond distance. There is one EXAFS report of a homoleptic Th hydroxide [Th(OH)8]4− stabilized in aqueous CaCl2 solutions at pH = 10−12.90 This is a particularly interesting result because there had been no previous indication for the formation of anionic hydrolyzed Th monomers in aqueous solutions at pH values
