The Chemical Interactions of Actinides in the Environment Wolfgang Runde F rom a chemist’s perspective, the more oxidation states simultaneously in sessing the feasibility of storing nuclear environment is a complex, partic- the same solution. Accordingly, the waste in geologic repositories. The ularly elaborate system. Hundreds light actinides exhibit some of the rich- actinides1 are radioactive, long-lived, of chemically active compounds and est and most involved chemistry in the and highly toxic. Over the last few minerals reside within the earth’s periodic table. decades, vast quantities of transuranic formations, and every patch of rock and As a result, the chemical interactions actinides (those with atomic numbers soil is composed of its own particular of actinides in the environment are in- greater than that of uranium) have been mix. The waters that flow upon and ordinately complex. To predict how an produced inside the fuel rods of com- through these formations harbor a vari- actinide might spread through the envi- mercial nuclear reactors. Currently, ety of organic and inorganic ligands. ronment and how fast that transport most spent fuel rods are stored above- Where the water meets the rock is a might occur, we need to characterize all ground in interim storage facilities, but poorly understood interfacial region that local conditions, including the nature of the plan in the United States and some exhibits its own chemical processes. site-specific minerals, temperature and European countries is to deposit this Likewise, from a chemist’s point of pressure profiles, and the local waters’ nuclear waste in repositories buried view, the actinides are complex pH, redox potential (Eh), and ligand hundreds of meters underground. The elements. Interrupting the 6d transition concentrations. We also need a quanti- Department of Energy (DOE) is study- elements in the last row of the periodic tative knowledge of the competing ing the feasibility of building a reposi- table, the actinides result as electrons geochemical processes that affect the tory for high-level waste in Yucca fill the 5f orbitals. Compared with the actinide’s behavior, most of which are Mountain (Nevada) and has already 4f electrons in the lanthanide series, the illustrated in Figure 1. Precipitation and licensed the Waste Isolation Pilot Plant 5f electrons extend farther from the dissolution of actinide-bearing solids (WIPP) in New Mexico as a repository nucleus and are relatively exposed. limit the upper actinide concentration in for defense-related transuranic waste. Consequently, many actinides exhibit solution, while complexation and redox Given the actinides’ long half-lives, multiple oxidation states and form reactions determine the species’ distrib- these repositories must isolate nuclear dozens of behaviorally distinct molecu- ution and stability. The interaction of a waste for tens of thousands of years. lar species. To confound matters, the dissolved species with mineral and rock light actinides are likely to undergo surfaces and/or colloids determines if 1Although “actinides” refers to the fourteen reduction/oxidation (redox) reactions and how it will migrate through the elements with atomic numbers 90 through 103 and thus may change their oxidation environment. (thorium through lawrencium), in this article we states under even the mildest of condi- Understanding this dynamic inter- limit the term to refer only to uranium, neptunium, plutonium, americium, and curium, unless stated tions. Uranium, neptunium, and espe- play between the actinides and the en- otherwise. These five actinides are the only ones cially plutonium often display two or vironment is critical for accurately as- that pose significant environmental concerns. 392 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment (cid:0) Figure I. Overview of Actinide Behavior in the Environment NpO (CO ) 5–(cid:0) (cid:0) 2 33 The Np(IV) species(cid:0) Np(H O) 4+ 2 8 n Complexation with different ligands (cid:0) o ati can stabilize actinides in solution(cid:0) x and enhance their transport(cid:0) Redox reactions change an actinide's (cid:0) e oxidation state and help to establish (cid:0) mpl through the (cid:0)environment(cid:0) equilibrium between species o R C (cid:0) e d o x (cid:0) Transport Bioavailability Actinides can migrate by (cid:0) water transport or by sorption(cid:0) onto mobile particulates or colloids Microbes can facilitate actinide(cid:0) redox processes, while sorption(cid:0) or uptake by the microbes may(cid:0) Actinides typically form (cid:0) be a potential transport or (cid:0) large molecular complexes(cid:0) immoblilzation mode.(cid:0) in solution. Neptunium assumes (cid:0) (cid:0) the Np(V) species, NpO (H O) +, (cid:0) Solvated actinide species can(cid:0) 2 2 5 in many natural waters. precipitate, forming a solid that in turn(cid:0) PrDis determines the upper concentration(cid:0) es ciol limit for the solvated species. pituti ation(cid:0) n o o n pti or S Np(V) complex sorbed (cid:0) to montmorillonite At low actinide concentrations, sorption onto particulates, (cid:0) clays, or rock surfaces determines the environmental behavior. (cid:0) Actinides can also diffuse into rock, or coprecipitate with (cid:0) (cid:0)(cid:0)(cid:0) natural ligands and become (cid:0) NaNpO CO (s) (cid:0)(cid:0)(cid:0) incorportated into minerals 2 3 Number 26 2000 Los Alamos Science 393 Chemical Interactions of Actinides in the Environment reactions with chloride ions are known PuO CO (aq) 2 3 to stabilize plutonium in the VI oxida- 1.0 PuO F+ PuO2(CO3)22– tion state. Plutonium(VI) species are 2 much more soluble than Pu(IV) species, PuO (CO ) 4– and plutonium mobility would be PuF 2+ 2 33 2 PuO + enhanced at WIPP unless a reducing 2 PuO (OH ) (aq) Rain/streams 2 22 eonr vairroounnmde tnhte c wana sbtee mcoanitnatianienresd. within s) 0.5 volt Ocean PuO2CO3– Unfortunately, very few studies of h ( actinide geochemistry can be conducted ential, E Pu3+ Groundwater PuO OH(aq) isnim suitlua,t es oe nthvairto wnme eanreta flo crocendd ittoions in ot 2 the laboratory. The concentrations of p ox 0 Pu(OH) (aq) the actinides in natural waters are d 4 Re typically on the order of 10–6 molar (M) or lower. While those concentra- Pu(III) tions are high enough to be of environ- Pu(IV) PuOH + mental concern, they are too low to 2 Pu(V) allow direct study with conventional –0.5 Pu(VI) spectroscopies. We have thus had to natural waters adapt advanced spectroscopic tech- niques, such as x-ray absorption fine 0 2 4 6 8 10 12 14 structure (XAFS) and laser-induced pH fluorescence spectroscopies, to study these toxic, radioactive elements. Figure 2. Pourbaix Diagram for Plutonium As an example, recent XAFS investiga- This Eh-vs-pH diagram is calculated for plutonium in water containing hydroxide, tions of Pu(IV) colloids have yielded carbonate, and fluoride ions. (The ligand concentrations are comparable to those found significant insights into the colloids’ in water from well J-13 at Yucca Mountain, Nevada, while the plutonium concentration structure, which for years has compli- is fixed at 10–5M). Specific complexes form within defined Eh/pH regions, while the cated the determination of Pu(IV) stable oxidation states (colors) follow broader trends. For example, more-oxidizing solubility. Scanning electron conditions (higher Eh values) stabilize redox-sensitive actinides like plutonium in the microscopy (SEM) has helped us eluci- higher oxidation states V and VI. The red dots are triple points, where plutonium can date the confusing sorption behavior exist in three different oxidation states. The range of Eh/pH values found in natural of U(VI) onto phosphate mineral waters is bounded by the solid black outline. In ocean water or in groundwater, surfaces. The box on page 412 high- plutonium is likely to be found as Pu(IV), while in rainwater or streams, plutonium can lights a few examples of these assume the V state. Other natural environments will favor Pu(III) or Pu(VI) complexes. spectroscopic studies. The dashed lines define the area of water stability. Above the upper line, water is The scope of actinide interactions in thermodynamicallyunstable and is oxidized to oxygen; below the lower line, water the environment is too broad to cover is reduced to hydrogen. in a single article. We will concentrate therefore on the solubility and specia- Over the course of millennia, however, Our solubility studies, for example, tion of actinides in the presence of it is possible that water seeping into a confirm that neptunium is more than a hydroxide and carbonate ions, which repository will eventually corrode the thousand times more soluble in Yucca are the two most environmentally rele- waste containers. The actinides will Mountain waters than plutonium. That vant ligands. We will then briefly dis- then have access to the environment. is because plutonium in those waters cuss some aspects of actinide sorption Research at Los Alamos has focused favors the IV oxidation state while and actinide interactions with microor- on characterizing the behavior of neptunium favors the far more soluble ganisms. The solubility and sorption of actinides in the environments surround- V state. Thus, for Yucca Mountain, actinides pose two key natural barriers ing those repository sites. (Only small neptunium is the actinide of primary to actinide transport away from a amounts of transuranic elements are concern. WIPP, however, is built within geologic repository, while microorgan- generally found in other environments, in a geologically deep salt formation. In isms pose a less-studied but potential as discussed in the box “Actinides in the extremely salty brines found there, third barrier. While we are fully aware Today’s Environment” on page 396.) the focus shifts to plutonium, since of additional geochemical reactions in 394 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment Table I. Oxidation States of Light Actinidesa The stability of Pu(V) in natural waters containing carbonate is an example. Th Pa U Np Pu Am Cm The plutonium will complex with the carbonate ligands, and if the plutonium III III III III III III III concentration is low (less than about IV IV IV IV IV IV IV 10–6 M), radiolytically induced redox V V V V V reactions are minimized. Consequently, VI VI VI VI the stability of Pu(V) is enhanced and VII VII its disproportionation to Pu(IV) and Pu(VI) is reduced. As discussed later, the solubilities of solids formed from aThe environmentally most important oxidation states are bolded. Pu(V) complexes are orders of magni- tude greater than those of Pu(IV) solids, so the enhanced stability of Pu(V) would serve to increase the total pluto- the environment, such as coprecipita- where species in three different oxida- nium concentration in solution. tion, mineralization, and diffusion tion states are in equilibrium. Another example for the stabilization processes (and have thus included them Because of intrinsic differences in of actinides in solution is the oxidation in Figure 1), we will not discuss their redox potentials, each actinide will of Am(III) and Pu(IV) through radiolytic roles here. exhibit a different set of oxidation formation of oxidizing species, such as states for a given set of solution condi- peroxide (H O ) or hypochlorite 2 2 tions. In contrast to plutonium, U(III) is (ClO–). At high actinide concentrations Oxidation States and Redox unstable under most conditions and and hence under the influence of (its Behavior oxidizes easily to U(IV), while U(V) own) alpha radiation, those normally disproportionates easily to U(IV) and stable oxidation states are oxidized to Water is the dominant transport U(VI). Neptunium(III) and Np(VI) are form Am(V) and Pu(VI), respectively, medium for most elements in the envi- on the edges of the water stability especially in concentrated chloride ronment. Compared with the pH values region and can exist only under strongly brines. and ionic strengths that can be obtained reducing or oxidizing conditions, Despite the complexity of actinide in the laboratory, most natural waters respectively. Americium and curium redox behavior, however, we should are relatively mild. Typically, they are will only be found in the III oxidation emphasize that within a given oxidation nearly neutral (pH 5 to 9), with a wide state under most conditions. Similarly, state, actinides tend to behave similarly. range of redox potentials (from –300 to all actinides beyond curium are domi- For example, we can study a U(IV) +500 millivolts) and low salinities nated by the lanthanide-like trivalent complex and from it infer the behavior (ionic strengths ≤ 1 molal). The water oxidation state. of the analogous Np(IV) and Pu(IV) conditions determine which actinide As a rule of thumb, we expect to complexes. In the following discus- oxidation states predominate and which find U(VI), Np(V), Pu(IV), Am(III), sions, therefore, we often refer to actinide species are stable. and Cm(III) as the prevalent oxidation “generic” actinides—e.g., An(IV) For example, Figure 2 shows the states in most ocean or groundwater complexes—where An is shorthand Pourbaix diagram (Eh vs pH) for pluto- environments. But in other aqueous en- for actinide. nium in water containing the two most vironments, including streams, brines, environmentally relevant ligands, the or bogs, U(IV), Np(IV), and Effective Charge. It has been hydroxide (OH–) and carbonate (CO 2–) Pu(III,V,VI) are also common and like- known for many years that An(IV) 3 ions. Even in this simple aqueous ly to be stable. Table 1 summarizes the forms the strongest, most stable system, plutonium can exist in four oxidation states of the actinides and complexes and An(V) the weakest. This oxidation states: III, IV, V, and VI. highlights the environmentally most behavior follows directly from the (Plutonium is unique in this regard. relevant ones. Some of the states listed effective charges of the ion. The Pourbaix diagrams for the other in the table, such as Pa(III) or Pu(VII), In the III and IV states, the actinides actinides are simpler.) The normal can be synthesized only under extreme form hydrated An3+ and An4+ ions in range of natural waters is outlined in conditions, far from those found in solution, respectively. In contrast, the the figure and overlaps with the stability natural systems. highly charged ions in the V and VI fields of plutonium in the III, IV, and V Additional chemical processes occur- states are unstable in aqueous solution oxidation states. Within this range, ring in solution are likely to influence and hydrolyze instantly to form linear plutonium exhibits two triple points, the actinide’s oxidation state stability. trans-dioxo cations, AnO + and 2 Number 26 2000 Los Alamos Science 395 Chemical Interactions of Actinides in the Environment AnO 2+, with overall charges of +1 and 2 +2, respectively. These cations are Actinides in Today’s Environment often referred to as the actinyl ions. The covalent bonding between the actinide and the two oxygen atoms in In 1942, Enrico Fermi built mankind’s first fission reactor, and nuclear reactions within the actinyl ion (O=An=O)n+, where that nuclear pile created the transuranic elements neptunium, plutonium, americium, n = 1 or 2, enhances the effective and curium. With the subsequent development of nuclear power, the inventory of charge of the central actinide ion to transuranics has increased dramatically. As of 1990, the estimated accumulations in 2.3±0.2 for AnO + and 3.3±0.1 for spent nuclear fuel were approximately 472 tons of plutonium, 28 of tons of neptunium, 2 AnO 2+ (Choppin 1983). A ligand 6 tons americium, and 1.6 tons of curium. 2 approaching the actinyl ion sees this enhanced effective charge and bonds Fortunately, the amount of radioactive material accidentally released from nuclear directly to the actinide in the equatorial power plants has been relatively small. Environmental contamination is severe in only a plane of the linear actinyl ion. Thus, the few locations. Major releases were due to a fire in a reactor at Windscale (now preference for actinides to form com- Sellafield) in the United Kingdom in 1957, an explosion at the Kyshtym nuclear waste plexes generally follows their effective storage facility in the former USSR in 1957, and an explosion and fire at the Chernobyl ion charges, as shown below: reactor in Russia in 1986. In addition, a number of DOE sites in the United States, including the Hanford Site in Washington, Rocky Flats Environmental Technology Site An4+ > AnO 2+ ≥ An3+ > AnO + in Colorado, and Idaho National Engineering and Environmental Laboratory in Idaho, 2 2 +4 +3.3 +3 +2.3 contain high local concentrations of actinides. IV VI III V The main source of transuranics in the environment has been nuclear weapons testing. The second and third lines show the Since the first detonation of a nuclear device in 1945 at Trinity Site in Alamogordo, ion’s effective charge and formal oxida- New Mexico, more than 4 tonnes of plutonium have been released worldwide (account- tion states. ing only for announced nuclear tests). Atmospheric and underground tests have also Recent measurements of bond injected about 95 kilograms of americium into the environment, along with tiny amounts lengths in the actinyl ions of V and VI of the still heavier element curium. complexes reflect the above sequence. A higher effective charge correlates Until they were banned by international treaty in 1963, aboveground nuclear tests with a stronger, hence shorter, bond to propelled the actinides directly into the atmosphere. There they dispersed and migrated the coordinated ligands. Accordingly, around the globe before settling back to earth. Little can be done to isolate or retrieve the bond length of the actinyl bonds in- this material. It is a permanent, albeit negligible, component of the earth’s soils and creases from about 1.75 angstroms (Å) oceans and poses little health risk to the general public. in the VI oxidation state (U=O is 1.76 Å; Pu=O is 1.74 Å) to about 1.83 Å in the Underground nuclear tests injected transuranics into highly localized regions surround- V oxidation state (Np=O is 1.85 Å; ing the detonation sites. These concentrated actinide sources became mineralized and Pu=O is 1.81 Å). integrated into the rock matrix once their areas cooled and solidified. For the most part, Because An(IV) has the highest the nuclear material is fixed in place. However, plutonium from a test conducted at the effective charge, it forms the most Nevada Test Site in 1968 has been detected in a well more than a kilometer away. Its stable complexes in solution and also remarkably fast migration rate (at least 40 meters per year) is likely due to colloidal forms the most stable precipitates with transport through the highly fractured, water-saturated rock surrounding the detonation site. the lowest solubility. Conversely, An(V) complexes are the weakest, and its solubility-controlling solids are the most soluble. An(V) species are there- soluble and easily transported actinide complexes with highly anionic ligands fore the most likely to migrate through and is the actinide of major concern in by electrostatic interactions. Because the environment. Table 1 shows that the assessing the performance of Yucca the actinide-ligand bond is substantially only transuranics that favor the V state Mountain. ionic, the number of bound ligands (the are neptunium and plutonium. But in coordination number) and the relative the environments surrounding repository Coordination Number and Ionic positions of the ligands around the sites, as well as in most groundwaters Radii. Another fundamental aspect cation are determined primarily by and ocean environments, plutonium will about the actinide cations is that they steric and electrostatic principles. most likely assume the IV state. Thus tend to act as hard Pearson “acids,” Consequently, for a given oxidation neptunium is predicted to be the most which means that they form strong state, a range of coordination numbers 396 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment (a) Figure 3. Total An(III) Concentration 10–2 Versus Carbonate Concentration (a) The partial pressure of CO in natural An(OH) (s) 2 M) 10–4 3 waters spans about three orders of n ( AnOHCO3(s) magnitude (gray area). The stability atio An2(CO3)3(s) diagram for An(III) solid phases (at pH 7) entr 10–6 shows that the An(III) hydroxocarbonate nc is favored in most natural waters. Lower o n(III) c 10–8 An3+(aq) pcoanrtciaeln ptrraetsiosnu rtersa nosrl astoel utob lien ccraerabsoenda te A stability of the An(III) hydroxide; higher pressures mean increased stability for 10–10 the normal carbonate solids. (b) The red 10–5 10–4 10–3 10–2 10–1 10 curve is the total concentration of An(III) CO Partial Pressure (atm) 2 in solution when the solubility-controlling (b) solid is An (CO ) •2–3H O(s). Calculated 10–4 2 33 2 concentrations for individual solution M) An (CO ) •2–3 H O(s) 244Cm species are indicated by the gray lines n ( 2 33 2 241Am that run tangent to the solubility curve, atio or by the gray dashed lines. At low entr 10–5 An3+ carbonate concentrations, the An3+ion c dominates the solution species. The total n o c An(III) concentration drops rapidly with An(III) An(CO3)+ An(CO3)33– hoifg thheer scoalridb opnhaatsee c pornecceinptitraatteiosn. sB uast more ated 10–6 carbonate complexation begins to stabi- olv An(CO3)2– lize the amount of actinide in solution. s al By the time the anionic bis- and triscar- ot An(OH)2+ T bonate complexes dominate, the solution concentration increases. 10–7 10–7 10–6 10–5 10–4 10–3 10–2 Carbonate concentration (M) is allowed. Coordination numbers be- ionic radii decrease with atomic favors the formation of a solid of high- tween 6 and 12 have been reported for number. The ionic radii for An(IV) ions er solubility. Formation of the solid sets complexes of An(III) and An(IV) and with coordination numbers of 8 are an upper concentration limit for the between 2 and 8 for the actinyl ions. reported to be 1.00, 0.98, 0.96, 0.95, actinide in solution. For example, the aquo ions of the and 0.95 Å for uranium, neptunium, Figure 3 illustrates these concepts for actinides, which have only water mole- plutonium, americium, and curium, re- An(III), where An is either americium or cules in the first coordination sphere spectively (Shannon 1976). curium. The data points in the lower surrounding the ion, exhibit coordina- graph show the measured total concen- tion numbers that vary with oxidation tration of An(III) in solution. At low state: 8–10 for An3+, 9–12 for An4+, Solubility and Speciation pH with increasing carbonate concen- 4–5 for AnO + and 5–6 for AnO 2+. tration, the hydrated carbonated solid 2 2 These coordination numbers have been The solubility of an actinide is lim- An (CO ) •2–3H O(s), where (s) indi- 2 3 3 2 determined from nuclear magentic reso- ited primarily by two properties: the cates the solid phase, readily precipi- nance (NMR) and XAFS spectroscopy stability of the actinide-bearing solid tates and hence lowers the concentration studies (see the article by Conradson et (the solubility-controlling solid) and the of An3+ in solution. But complexation al. on page 422). stability of the complexed species in of An3+ with carbonate stabilize the Generally, the stabilities of actinide solution. An actinide species will solution species and increases the over- complexes at a given oxidation state precipitate when its dissolved concen- all An(III) solubility. As the carbonate increase with atomic number. This tration is above the thermodynamic concentration increases and the different increase in stability follows the trend of (equilibrium) solubility of its solubility- carbonate species form, the total the actinide contraction, wherein the controlling solid, or if the kinetics actinide concentration in solution first Number 26 2000 Los Alamos Science 397 Chemical Interactions of Actinides in the Environment reaches a minimum and then increases due to the formation of anionic An(III) complexes. The solubility would change Solubility and Speciation Parameters if conditions favored the formation of a new solubility-controlling solid. We obtain solubility data by Solubility products (K ) of actinide-bearing solid phases are indispensable primary data sp performing experiments in the laboratory for predicting the concentration limits of actinides in the environment. The solubility under well-controlled conditions in product describes the dissolution reaction of the solid into its ionic components. For which the actinide concentration is example, the solubility product of AmOHCO (s), where (s) refers to the solid phase, is 3 measured while varying the ligand given by the product of the concentration of the dissolved ions Am3+, OH–, and CO 2– 3 concentration. Such experiments enable according to the following reaction: us to measure the solubility product K sp and the stability constant β. These AmOHCO (s) Am3+ + OH–(cid:0) + CO(cid:0)(cid:0)2– (cid:0) . (1) 3 3(cid:0) thermodynamic parameters are the basis for modeling the solubility boundaries The solubility product is then given by for actinides in natural waters, as discussed in the box on “Solubility and Ksp = [Am3+][OH–(cid:0)][CO32–(cid:0)] , (2) Speciation Parameters.” We should note, however, that where the brackets indicate ion concentration. The formation of an actinide complex in meaningful interpretation of solubility solution, for example, data requires a detailed knowledge of the composition, crystallinity, and solu- Am3++nCO 2– (cid:0) Am(CO ) 3–2n (3) 3 3n bility of the solubility-controlling solid, along with the steady-state concentra- is described by the complex’s formation constant, β . For the forward reaction in n tion and composition of the solution Equation (3), the formation constant is species. Steady state is assumed to be established when the actinide concen-   Am(CO3)n3–2n tration remains stable for several weeks βn = [ ][ ]n , (4) or longer. But the solubility-controlling Am3+ CO32–(cid:0) solids of the actinides generally precipi- tate as an amorphous phase because where n refers to the number of ligands. For the reverse reaction, β changes sign and n radiolysis affects their crystallization. is known as the stability constant. The concentration of an actinide solution complex Actinide solids may become less soluble can be calculated by using the known solubility product of the solubility-limiting solid with time as they transform from their and the formation constant of the species of interest as follows: initially formed, disordered structures into ordered crystalline solids, thereby [ ][ ] [CO 2–](cid:0)n–1 lcohwanegrien,g h tohweierv ferre, em eanye rngoyt. aSpupceha ra for Am(CO3)n3–2n = βn Am3+ CO32–(cid:0) n = βn Ksp [OH3–](cid:0) . (5) several years. Thus, the solids formed in laboratory experiments may not Accordingly, the total actinide concentration in solution is given by the sum of the con- represent the most thermodynamically centrations of species present in solution as follows: stable solids with the lowest solubility. Laboratory-based solubility studies therefore provide an upper limit to [Am(III)] = [Am3+] + Am(CO3)n3–2n + [Am(OH)m3–m(cid:0) ] actinide concentrations in a potential rreelpeoassieto srcye.nario from a nuclear waste = [CO3K2–s(cid:0)]p[OH–(cid:0)] × 1+∑n βn[CO32–(cid:0)]n +∑m βm[OH–(cid:0)]m . (6) Because of their omnipresence, hydroxide ions (OH–) and carbonate Complexation of the dissolved actinide ion with ligands in solution (large β values) n ions (CO 2–) are the most important generally increases the total actinide concentration in solution. 3 ligands that complex to actinides in the environment. While other ligands, such as phosphate, sulfate, and fluoride, can lower the actinide concentration 398 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment (because of the low solubility of the plutonium oxide, PuO . However, the tion of anionic hydroxo complexes: 2 corresponding solid phase), their XAFS data reveal that these particles AnO (OH) – for An(V) and 2 2 concentrations in natural waters are possess multiple functional groups, AnO (OH) 2–n (n = 3 and 4) for 2 n generally low. Consequently, they such as terminal hydroxo and bridging An(VI). The hydrolysis of U(VI) is cannot compete successfully with oxo and hydroxo groups. Each colloid complicated even more by the hydroxide or carbonate complexation. particle can be considered an amor- formation of polymeric hydroxide However, organic biodegradation prod- phous oxyhydroxide compound. The species at high U(VI) concentration ucts, such as humates or fulvates, are functional groups originate from the (>10–5 M), i.e., (UO ) (OH) + or 2 3 5 generally present in natural aquifers and polymerization of monomeric species (UO ) (OH) +. We are currently 2 4 7 can potentially play a role in actinide formed through hydrolysis, and the conducting experiments to see if Pu(VI) solubility and migration. number of functional groups depends undergoes a similar polymerization. on the history of the solution prepara- Hydrolysis. The interaction of tion and aging process. While the Carbonate Complexation. Carbon- actinide and hydroxide ions produces chemical “structure” of every Pu(IV) ate is ubiquitous in nature. In surface monomeric and—particularly for colloid may be similar, each colloid can waters, such as oceans, lakes, or Pu(IV)—polymeric solution species. have a unique distribution of functional streams, its concentration ranges gener- The solid-phase structures are oxides, groups depending on solution prepara- ally between 10–5 and 10–3 M. In oxyhydroxides, or hydroxides of low tion, particle size, and water content. groundwaters, its concentration is solubility. Once they form, however, the colloids enhanced (up to 10–2 M) because of the In the absence of carbonate or are inordinately stable in near-neutral increased CO partial pressure, which is 2 other strong ligands (or with very low solution, presumably because over time, about 10–2 atm deep underground concentrations of them), trivalent the hydroxo groups that join the pluto- compared with the atmospheric partial actinides form the positively charged nium atoms together in the polymer are pressure of about 3 × 10–4 atm. These or neutral hydroxo complexes of converted into chemically resistant oxy- relatively high carbonate concentrations general formula An(OH) 3–m, where gen bridges. influence the environmental chemistry m m = 1, 2, or 3, that is An(OH)2+, The generation of Pu(IV) colloids is of actinides in all oxidation states. An(OH) +, and An(OH) (aq). Similarly, one of the main processes that compli- Carbonate generally bonds to 2 3 An(IV) forms complexes in solution cate determining thermodynamic actinides in a bidentate fashion, that is, of general formula An(OH) 4–n, where constants and accurately predicting two of the three oxygen atoms in the n n = 1, 2, 3, or 4. The solid hydroxide soluble plutonium concentrations in complex bond to the actinide. Thus, An(OH) (s), where n = 3 for An(III) natural waters. Both the colloids and actinide carbonate complexes are very n and 4 for An(IV) or the hydrated other amorphous Pu(IV) hydroxide strong and highly stable both in oxide AnO •nH O for An(IV) are solids span a wide range of structural solution and in the solid state. 2 2 expected to control the solubility of features, crystallinities, and stabilities, We have investigated the structure actinides in both oxidation states. and hence exhibit a wide range of of carbonate complexes involving the Understanding the An(IV) solution solubilities. The presence of Pu(IV) actinyl ions AnO + and AnO 2+. (These 2 2 chemistry is still a formidable chal- colloids will therefore complicate any complexes are more soluble and thus lenge. The high charge allows the An4+ calculation of plutonium solubility, as more easily studied.) As shown in ions to easily hydrolyze and form mul- will be evident when we review Figure 4, three monomeric solution tiple species simultaneously, even at plutonium solubility and speciation in complexes form. Depending on the low pH. Plutonium(IV) is known to Yucca Mountain waters. solution conditions, these complexes hydrolyze even in dilute acidic Solid hydroxides and oxides are also may coexist with each other and with solutions and, at concentrations greater expected to limit the solubility of hydroxo complexes. It is therefore diffi- than 10–6 M, will undergo oligomeriza- An(V) and An(VI) complexes. For cult to obtain clean, unambiguous struc- tion and polymerization to form very example, Np O (s) has been determined tural data about a single species. 2 5 stable intrinsic colloids. to be the solubility-controlling solid in We can make inroads into that prob- Recent x-ray absorption fine-structure waters from Yucca Mountain, while lem by investigating the stability and (XAFS) studies, discussed on page 432 schoepite, UO (OH) •nH O(s) is most structure of the triscarbonato species, 2 2 2 in the article by Conradson, have given stable for U(VI). Interestingly, these AnO (CO ) m-6, shown in Figure 4(c). 2 3 3 more insight into the structural details solids exhibit both acidic and basic This is the limiting (fully coordinated) of Pu(IV) colloids. The sub-micron- characteristics. With increasing pH, An(V,VI) carbonate complex in solu- scale colloid particles are nominally their solubilities tend to decrease in tion. At carbonated concentrations composed of Pu(IV) hydroxo polymers acidic media and increase in alkaline above 10–3 M, this complex forms at and contain structural characteristics of (basic) solutions because of the forma- the exclusion of the others. Using a Number 26 2000 Los Alamos Science 399 Chemical Interactions of Actinides in the Environment battery of spectroscopic techniques, (a) AnO (CO )m–2 (b) AnO (CO ) m–4 (c) AnO (CO ) m–6 2 3 2 32 2 33 including multinuclear NMR spec- troscopy, x-ray diffraction, and XAFS, we have obtained formation constants, bond lengths, and other structural parameters for the An(V) and (VI) car- bonate complexes (Clark et al. 1999). This information has helped us under- stand various aspects of the An(V,VI) carbonate system. (The article by Clark (d) (UO ) (CO ) 6– that begins on page 310 describes struc- 23 36 tural investigations of the Pu(VI) Oxygen tris-carbonate.) Bridging Carbon carbonate Carbonate Solids. In many natural Actinide waters, carbonate effectively competes with hydrolysis reactions, and mixed Terminal hydroxocarbonate solids are likely to carbonate form. These solids have general formu- las An(OH)(CO )(s) for An(III) and 3 An(OH) (CO ) (s) for An(IV), where 2m 3 n An stands for uranium, neptunium, plu- Figure 4. Carbonate Complexes of the Actinyl Ions tonium, or americium. (The equilibria Actinides in the V and VI state form similar carbonate complexes in solution, although in these systems are quite complicated, the charge of the species depends on the oxidation state. Thus, in the three monomer- and neither the structure nor the values ic complexes shown in (a)–(c), m = 1 for An(V) and m = 2 for An(VI). Actinides in the for n has been determined accurately V state have the lowest effective charge and form the weakest complexes. In these for An(IV) compounds.) complexes, Np(V) has the longest actinyl bond (An=O) length (1.86Å). We have also In the laboratory, however, we can determined only slight changes in the equatorial An–O distances to the carbonate study actinide solids of different ligands: they vary between 2.42 and 2.44 Å for An(VI) and between 2.48 and 2.53 Å for compositions by increasing the carbon- Np(V). (d) Uranium(VI) may also polymerize at high carbonate and uranium concentra- ate concentration. This increase reduces tions to form the polynuclear actinyl carbonate complex (UO ) (CO ) 6–. If present, 23 36 the extent of hydrolysis and allows this polynuclear complex stabilizes U(VI) in solution and increases the overall U(VI) the formation, for example, of pure solubility in comparison with the solubility of its monomeric counterpart, AnO (CO ) 2–. 2 32 An(III) carbonate solids, such as Am (CO ) •nH O(s) or 2 3 3 2 NaAm(CO ) •nH O(s), which are 3 2 2 the solubility-controlling solids in solu- incorporate a monovalent cation, such enhances the solubility of the actinide tions containing high concentrations as Na+, K+, or NH +, into their struc- by several orders of magnitude. We 4 of sodium and carbonate. These solids ture (otherwise they would have an exploited that fact when we studied the are analogous to the ones formed overall charge). Thus, we observe only neptunyl “double carbonate” solids by the trivalent lanthanides Nd(III) ternary An(V) solids with general com- M NpO (CO ) . These solids exhibit 3 2 3 2 and Eu(III). positions M AnO (CO ) •mH O(s), such low solubilities in near neutral (2n–1) 2 3 n 2 Similarly, we have observed the where n = 1 or 2, M is the monovalent solutions (10–4 to 10–7 M) that the formation of solid An(VI) and An(V) cation, and An stands for neptunium, solution species cannot be studied by compounds in solutions containing high plutonium, or americium. many spectroscopic methods. To modi- concentrations of alkali metals and As the number of cations incorporated fy the solubility and obtain more- carbonate. These An(V,VI) carbonate into the An(V,VI) carbonate solids in- concentrated neptunyl solutions, we solids are inherently interesting. creases (stoichiometrically from 0 to 1 examined the use of large, bulky Together, they can be viewed as a to 3), the structures become increasing- cations in an attempt to generate an structural series, as evident in Figure 5. ly open, and the solids generally unfavorable fit of the complex into the Binary An(VI) solids have a general become more soluble under conditions stable lattices shown in Figures 5(b) composition AnO CO (s) and are made in which the uncomplexed actinide ion and 5(c). We found that tetrabutyl- 2 3 from highly ordered actinyl carbonate dominates solution speciation. Replace- ammonium provided stable solutions of layers. However, An(V) solids need to ment of the cations with larger ones NpO CO – and NpO (CO ) 3– in 2 3 2 3 2 400 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment UO CO KPuO CO K NpO (CO ) •nH O 2 3 2 3 3 2 3 2 2 Figure 5. Actinide(V,VI) Alkali Metal/Carbonate Solids Actinide(V,VI) carbonate solids form a “structural series” that progresses from dense, highly ordered layered structures to more open structures. A single layer is shown above a perspective drawing of each solid. (a) An(VI) will combine with carbonate to form binary solids, such as UO CO (s) or PuO CO (s). Within a layer, each actinide is bonded to two carbonates in a bidentate fashion 2 3 2 3 and to two others in a monodentate fashion. (This is in contrast to solution species, in which carbonate always bonds to an An(VI) in a bidentate fashion.) (b) Actinide(V) ions cannot form binary carbonate solids, since the structural unit would have an overall charge. Solid An(V) carbonates therefore form ternary complexes in conjunction with monovalent metal cations (M), such as Na+, K+, or NH +. Within a layer, each actinyl ion binds to three carbonate ligands in a bidentate fashion. The metal ions sit between the 4 layers and form the ternary complex MAnO CO (s). This opens the structure, in comparison with the An(VI) binary structure, and 2 3 increases solubility. (c) At higher metal ion concentrations, metal ions will be incorporated into each layer to form very open structures of general formula M AnO (CO ) (s). 3 2 32 millimolar concentrations over a wide K = [Na+]2n-1 [NpO +] [CO 2–]n . Solubility and Speciation in sp 2 3 pH range. The enhanced solubility Yucca Mountain Waters allowed us to study the temperature For the An(VI) solid AnO CO (s), 2 3 dependence of the carbonate complexa- n is equal to 1 and its solubility is As a way of tying together many of tion reactions with near-infrared (NIR) nominally independant from the sodium the concepts presented in the previous absorption spectroscopy and to obtain concentration in solution. At the low sections, we can consider the solubility additional structural information about ionic strengths of most natural waters, and speciation of neptunium and pluto- the Np(V) solution carbonate complex- an unrealistically high NpO + concen- nium in Yucca Mountain waters. (The 2 es with XAFS spectroscopy. tration would be needed to precipitate a article by Eckhardt, starting on page The compositions and stabilities of ternary Np(V) carbonate solid. Thus, 410, discusses more of the research these actinyl carbonate solids depend solid An(V) or An(VI) carbonates are conducted for Yucca Mountain.) As on the ionic strength and carbonate in general not observed in natural wa- mentioned earlier, neptunium tends to concentration and are controlled by the ters. However, because the ternary exist in the V oxidation state in natural actinide, carbonate, and sodium concen- compounds of Np(V) have been waters and because of the lower effec- trations in solution. For example, the observed to form in concentrated tive charge of the AnO + ion, forms 2 stability of the Np(V) solids electrolyte solutions, these solids are weak complexes. Thus, neptunium has Na NpO (CO ) •mH O(s) is given important for solubility predictions in the distinction of being the most soluble (2n–1) 2 3 n 2 by the solubility product Ksp: brines found at WIPP. and transportable actinide and is the Number 26 2000 Los Alamos Science 401