The Chemical Interactions of in the Environment

Wolfgang Runde

rom a ’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 Fularly elaborate system. Hundreds light actinides exhibit some of the rich- actinides1 are radioactive, long-lived, of chemically active compounds and est and most involved in the and highly toxic. Over the last few minerals reside within the ’s . decades, vast quantities of transuranic formations, and every patch of 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 ) 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- might spread through the envi- mercial nuclear reactors. Currently, ety of organic and inorganic . 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, potential (Eh), and 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 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 series, the illustrated in Figure 1. Precipitation and licensed the Waste Isolation Pilot Plant 5f electrons extend farther from the dissolution of actinide-bearing (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 , the dissolved species with and rock light actinides are likely to undergo surfaces and/or 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. ( through ), in this article we states under even the mildest of condi- Understanding this dynamic inter- limit the term to refer only to uranium, , , , and , 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

Figure I. Overview of Actinide Behavior in the Environment

NpO (CO ) 5– 2 3 3

The Np(IV) species 4+ Np(H2O)8 Complexation with different ligands can stabilize actinides in solution and enhance their transport Redox reactions change an actinide's through the environment and help to establish equilibrium between species Redox Complexation


Bioavailability Actinides can migrate by water transport or by sorption onto mobile particulates or colloids Microbes can facilitate actinide redox processes, while sorption or uptake by the microbes may Actinides typically form be a potential transport or large molecular complexes immoblilzation mode. in solution. Neptunium assumes + Solvated actinide species can the Np(V) species, NpO2(H2O)5 , in many natural waters. PrecipitationDissolution precipitate, forming a that in turn determines the upper concentration limit for the solvated species.

Np(V) complex sorbed Sorption to montmorillonite

At low actinide concentrations, sorption onto particulates, clays, or rock surfaces determines the environmental behavior. Actinides can also diffuse into rock, or coprecipitate with natural ligands and become NaNpO CO (s) incorportated into minerals 2 3

Number 26 2000 Los Alamos Science 393 Chemical Interactions of Actinides in the Environment

reactions with are known PuO CO (aq) 2 3 to stabilize plutonium in the VI oxida- 1.0 PuO (CO ) 2Ð PuO F+ 2 3 2 tion state. Plutonium(VI) species are 2 much more soluble than Pu(IV) species, PuO (CO ) 4Ð 2+ 2 3 3 and plutonium mobility would be PuF2 PuO + enhanced at WIPP unless a reducing 2 PuO (OH ) (aq) Rain/streams 2 2 2 environment can be maintained within or around the waste containers. 0.5 Ocean Ð PuO2CO3 Unfortunately, very few studies of actinide can be conducted Groundwater in situ, so that we are forced to Pu3+ PuO OH(aq) simulate environmental conditions in 2 the laboratory. The concentrations of

0 Pu(OH)4(aq) the actinides in natural waters are –6 Redox potential, Eh (volts) typically on the order of 10 molar (M) or lower. While those concentra- Pu(III) tions are high enough to be of environ- Pu(IV) + PuOH2 mental concern, they are too low to Pu(V) Ð0.5 allow direct study with conventional Pu(VI) . We have thus had to natural waters adapt advanced spectroscopic tech- niques, such as x-ray absorption fine 02468101214structure (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 , tions of Pu(IV) colloids have yielded , 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 10–5 M). 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 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 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 thermodynamically unstable and is oxidized to ; below the lower line, water the environment is too broad to cover is reduced to . 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 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 VVV reactions are minimized. Consequently, VI VI VI VI the stability of Pu(V) is enhanced and VII VII its 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 - 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 (H2O2) or 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 – 2– hydroxide (OH ) and carbonate (CO3 ) Pu(III,V,VI) are also common and like- known for many years that An(IV) 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 . 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, 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, AnO2 and

Number 26 2000 Los Alamos Science 395 Chemical Interactions of Actinides in the Environment

2+ AnO2 , with overall charges of +1 and +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 in In 1942, 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 , 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 AnO2 and 3.3±0.1 for spent were approximately 472 tons of plutonium, 28 of tons of neptunium, 2+ AnO2 (Choppin 1983). A ligand 6 tons americium, and 1.6 tons of curium. approaching the actinyl ion sees this enhanced effective charge and bonds Fortunately, the amount of radioactive 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 in 1986. In addition, a number of DOE sites in the United States, including the in Washington, Rocky Flats Environmental Technology Site 4+ 2+ 3+ + An > AnO2 ≥An > AnO2 in Colorado, and Idaho National Engineering and Environmental Laboratory in Idaho, +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 testing. The second and third lines show the Since the first detonation of a nuclear device in 1945 at 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 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 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 ) 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 “,” 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) 10Ð2 Figure 3. Total An(III) Concentration Versus Carbonate Concentration

(a) The partial pressure of CO2 in natural An(OH)3(s) 10Ð4 waters spans about three orders of AnOHCO3(s) magnitude (gray area). The stability

An2(CO3)3(s) diagram for An(III) solid phases (at pH 7) 10Ð6 shows that the An(III) hydroxocarbonate is favored in most natural waters. Lower partial pressures or soluble carbonate Ð8 10 An3+(aq) concentration translate to increased An(III) concentration (M) stability of the An(III) hydroxide; higher pressures mean increased stability for Ð10 10 the normal carbonate solids. (b) The red 10Ð5 10Ð4 10Ð3 10Ð2 10Ð1 10 curve is the total concentration of An(III) CO2 Partial Pressure (atm) in solution when the solubility-controlling (b) solid is An (CO ) •2–3H O(s). Calculated Ð4 2 3 3 2 10 concentrations for individual solution 244Cm species are indicated by the gray lines An (CO ) ¥2Ð3 H O(s) 2 3 3 2 241Am that run tangent to the solubility curve, or by the gray dashed lines. At low 10Ð5 carbonate concentrations, the An3+ ion An3+ dominates the solution species. The total An(III) concentration drops rapidly with

+ 3Ð higher carbonate concentrations as more An(CO3) An(CO3)3 of the solid precipitates. But Ð6 10 carbonate complexation begins to stabi- Ð An(CO3)2 lize the amount of actinide in solution. By the time the anionic bis- and triscar- An(OH)2+ Total solvated An(III) concentration (M) solvated Total bonate complexes dominate, the solution concentration increases. 10Ð7 10Ð7 10Ð6 10Ð5 10Ð4 10Ð3 10Ð2 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 - 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- + 2+ 4–5 for AnO2 and 5–6 for AnO2 . tration, the hydrated carbonated solid These coordination numbers have been The solubility of an actinide is lim- An2(CO3)3•2–3H2O(s), where (s) indi- determined from nuclear magentic reso- ited primarily by two properties: the cates the solid phase, readily precipi- nance (NMR) and XAFS 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 . 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 (Ksp) of actinide-bearing solid phases are indispensable primary data 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 AmOHCO3(s), where (s) refers to the solid phase, is 3+ – 2– measured while varying the ligand given by the product of the concentration of the dissolved ions Am , OH , and CO3 concentration. Such experiments enable according to the following reaction: us to measure the solubility product Ksp 3+– 2– and the stability constant β. These AmOHCO3(s) Am + OH + CO3 . (1) thermodynamic parameters are the basis for modeling the solubility boundaries The solubility product is then given by for actinides in natural waters, as 3+– 2– discussed in the box on “Solubility and Ksp = [Am ][OH ][CO3 ] , (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 3+ 2– 3–2n the composition, crystallinity, and solu- Am + nCO3 Am(CO3)n (3) bility of the solubility-controlling solid, β along with the steady-state concentra- is described by the complex’s formation constant, n. For the forward reaction in tion and composition of the solution Equation (3), the formation constant is species. Steady state is assumed to be established when the actinide concen-  3–2n Am(CO3)n  β = , (4) tration remains stable for several weeks n n 3+ 2– or longer. But the solubility-controlling [][]Am CO3 solids of the actinides generally precipi- β tate as an amorphous phase because where n refers to the number of ligands. For the reverse reaction, n changes sign and 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 2– n–1   n []CO3 lowering their free energy. Such a Am(CO ) 3–2n = β [][]Am 3+ CO 2– = β K . (5)  3 n  n 3 nsp – change, however, may not appear for []OH 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 [] 3+ Am(CO ) 3–2n Am(OH) 3–m Am(III) = []Am +  3 n  + []m actinide concentrations in a potential K  n m sp × ββ2– – release scenario from a nuclear waste = 1+∑∑n []CO3 + m [OH ]  (6) . 2– –   repository. [][]CO3 OH n m Because of their omnipresence, – β hydroxide ions (OH ) and carbonate Complexation of the dissolved actinide ion with ligands in solution (large n values) 2– ions (CO3 ) are the most important generally increases the total actinide concentration in solution. ligands that complex to actinides in the environment. While other ligands, such as phosphate, , 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 , PuO2. However, the tion of anionic hydroxo complexes: – corresponding solid phase), their XAFS data reveal that these particles AnO2(OH)2 for An(V) and 2–n concentrations in natural waters are possess multiple functional groups, AnO2(OH)n (n = 3 and 4) for generally low. Consequently, they such as terminal hydroxo and bridging An(VI). The of U(VI) is cannot compete successfully with oxo and hydroxo groups. Each 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 –5 + ucts, such as humates or fulvates, are functional groups originate from the (>10 M), i.e., (UO2)3(OH)5 or + generally present in natural aquifers and polymerization of monomeric species (UO2)4(OH)7 . We are currently 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. - 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 , groups depending on solution prepara- ally between 10–5 and 10–3 M. In oxyhydroxides, or 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 CO2 partial pressure, which is 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 3–m general formula An(OH)m , where gen bridges. influence the 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)2 , and An(OH)3(aq). Similarly, one of the main processes that compli- Carbonate generally bonds to An(IV) forms complexes in solution cate determining thermodynamic actinides in a bidentate fashion, that is, 4–n of general formula An(OH)n , where constants and accurately predicting two of the three oxygen atoms in the n = 1, 2, 3, or 4. The solid hydroxide soluble plutonium concentrations in complex bond to the actinide. Thus, An(OH)n(s), where n = 3 for An(III) natural waters. Both the colloids and actinide carbonate complexes are very and 4 for An(IV) or the hydrated other amorphous Pu(IV) hydroxide strong and highly stable both in oxide AnO2•nH2O for An(IV) are solids span a wide range of structural solution and in the solid state. 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 + 2+ Understanding the An(IV) solution solubilities. The presence of Pu(IV) actinyl ions AnO2 and AnO2 . (These 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, Np2O5(s) has been determined tural data about a single species. 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, UO2(OH)2•nH2O(s) is most structure of the triscarbonato species, m-6 in the article by Conradson, have given stable for U(VI). Interestingly, these AnO2(CO3)3 , shown in Figure 4(c). 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, mÐ2 mÐ4 mÐ6 (a) AnO2(CO3) (b) AnO2(CO3)2 (c) AnO2(CO3)3 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 ) 6Ð that begins on page 310 describes struc- 2 3 3 6 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)(CO3)(s) for An(III) and An(OH)2m(CO3)n(s) for An(IV), where 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 (1.86 Å). 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- 6– ate concentration. This increase reduces tions to form the polynuclear actinyl carbonate complex (UO2)3(CO3)6 . If present, the extent of hydrolysis and allows this polynuclear complex stabilizes U(VI) in solution and increases the overall U(VI) 2– the formation, for example, of pure solubility in comparison with the solubility of its monomeric counterpart, AnO2(CO3)2 . An(III) carbonate solids, such as Am2(CO3)3•nH2O(s) or NaAm(CO3)2•nH2O(s), which are 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 NH4 , into their struc- by several orders of magnitude. We of 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 Nd(III) ternary An(V) solids with general com- M3NpO2(CO3)2. These solids exhibit and Eu(III). positions M(2n–1)AnO2(CO3)n•mH2O(s), such low solubilities in near neutral 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 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 AnO2CO3(s) and are made in which the uncomplexed actinide ion and 5(c). We found that tetrabutyl- from highly ordered actinyl carbonate dominates solution speciation. Replace- ammonium provided stable solutions of – 3– layers. However, An(V) solids need to ment of the cations with larger ones NpO2CO3 and NpO2(CO3)2 in

400 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment

UO2CO3 KPuO2CO3 K3NpO2(CO3)2¥nH2O

Figure 5. Actinide(V,VI) Alkali /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 of each solid. (a) An(VI) will combine with carbonate to form binary solids, such as UO2CO3(s) or PuO2CO3(s). Within a layer, each actinide is bonded to two in a bidentate fashion 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 NH4 . Within a layer, each actinyl ion binds to three carbonate ligands in a bidentate fashion. The metal ions sit between the layers and form the ternary complex MAnO2CO3(s). This opens the structure, in comparison with the An(VI) binary structure, and increases solubility. (c) At higher metal ion concentrations, metal ions will be incorporated into each layer to form very open structures of general formula M3AnO2(CO3)2(s).

+ 2n-1 + 2– n millimolar concentrations over a wide Ksp = [Na ] [NpO2 ] [CO3 ] . Solubility and Speciation in pH range. The enhanced solubility Yucca Mountain Waters allowed us to study the temperature For the An(VI) solid AnO2CO3(s), 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 NpO2 concen- nium in Yucca Mountain waters. (The 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 AnO2 ion, forms stability of the Np(V) solids electrolyte solutions, these solids are weak complexes. Thus, neptunium has Na(2n–1)NpO2(CO3)n•mH2O(s) is given important for solubility predictions in the distinction of being the most soluble by the solubility product Ksp: brines found at WIPP. and transportable actinide and is the

Number 26 2000 Los Alamos Science 401 Chemical Interactions of Actinides in the Environment

temperatures expected at the repository. Not surprisingly, changing these basic 10−2 parameters greatly affected the actinide NaNpO CO ¥nH O(s) 2 3 2 behavior (Efurd et al. 1998). Figure 6 summarizes the results of our studies. Np (Nitsche et al. 1993) The controlling solids for the −4 Np undersaturated actinides in J-13 water—in all oxidation

) 10 1 − states—were mainly oxides and hydrox- ides. We observed only the formation Np2O5 ¥nH2O(s) Np oversaturated of greenish brown Np(V) crystalline − 10 6 solids of general formula Np2O5•nH2O(s), green Pu(IV) solids of general formula Pu (Nitsche et al. 1993) PuO2•nH2O and/or amorphous Pu(IV) hydroxide/colloids. The solubility of − Pu oversaturated –3 10 8 neptunium was about 10 M at pH 6, but dropped to about 5 × 10–6 M at pH Species concentration (mol kg 8.5. The neptunium species distribution

Pu(OH)4(s) (Lemire and Tremaine 1980) similarly changed with conditions. At 10−10 pH 8.5, about 31 percent of Np(V) + was present as the neptunyl ion NpO2 . The remaining soluble Np(V) was com- plexed with either carbonate or hydrox- −12 – 10 ide, with about 58 percent NpO2CO3 0.01 0.1 1.0 and 11 percent NpO OH(aq). At pH 6, −1 2 J-13 water Ionic strength (mol kg ) however, Np(V) hydrolysis and carbon- ate complexation reactions were mini- Figure 6. Solubility of Neptunium(V) and Pu(IV) J-13 Water mized, and all of the neptunium was + The figure shows our measured total soluble concentrations of neptunium (red circles) present as NpO2 . and plutonium (red square) in J-13 water at a fixed carbonate concentration of 2.8 mM By comparison, at all pH values (indicated by the dashed line), as well as earlier measurements conducted at a higher examined in our study, 99 percent of ionic strength (black circle and square). The lines are theoretical predictions for differ- the soluble plutonium was calculated to ent solubility-controlling solids. The Np(V) concentration is about 100 times greater be in the IV state as Pu(OH)4(aq). than that of Pu(IV). Our measurements confirm that the controlling Np(V) solid is the When using Pu(OH)4(s) as the solubility oxide Np2O5(s), rather than the previously postulated solid -limiting solid in our geochemical NaNpO2CO3(s). The solubility of plutonium in J-13 water presumably is controlled by models, the plutonium concentration in amorphous oxides/hydroxides or colloidal forms of Pu(IV), since the measured Pu(IV) solution was predicted to be about solubilities are nearly an order of magnitude higher than predicted by Lemire and 10–9 M (using the thermodynamic data

Tremaine for the solubility product of Pu(OH)4(s). Our data are consistent with the available in the open literature). As range of Pu(IV) solubilities as calculated from Ksp data of Pu(IV) hydroxide or hydrous seen in Figure 6, experimental data are oxide reported in the open literature (gray area). about one to two orders of magnitude higher. The discrepancy presumably arises because plutonium solubility is one of greatest concern when assessing However, groundwater conditions with- instead controlled by amorphous the environmental safety of the poten- in and around Yucca Mountain vary. oxides/hydroxides or colloidal forms of tial underground storage site at Yucca The carbonate concentration changes Pu(IV). (Note that if crystalline PuO2(s) Mountain. Plutonium is considered the (because the CO2 partial pressure were the solubility-controlling solid, most toxic actinide and is always of changes with depth), as does the pH, Pu(IV) solubility would be about 10–17 environmental concern. Eh, and temperature. We mimicked this M, well below the Pu(IV) hydroxide Our studies were conducted in water variation in the natural waters by con- solubility range.) from a particular borehole in Yucca ducting our studies at various pH val- The J-13 water is almost neutral Mountain (J-13 water). The sodium and ues while maintaining a constant ionic (pH ~7) and has a redox potential of carbonate concentrations are low (both strength. We also conducted our studies about +430 millivolts. But different around 2 to 3 mM), as is the ionic at different temperatures (25˚C, 60˚C, water conditions would dramatically strength of J-13 water (0.003 molal). and 90˚C) to approximate the range of affect neptunium solubility, as illustrated

402 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment

(a) 1 (b) pH 7 400 mV Np O (s) 10−4 2 5 10−2 Np2O5(s) ) ) 1 1 − − 300 mV 10−4 10−6 200 mV

10−6 100 mV 10−8 −8 Np concentration (mol kg Np concentration (mol kg 10 0 mV −100 mV Np(OH)4(s) −200 mV Np(OH)4(s) 10−10 46 810 12 −400 −2000 200 400 600 pH Eh (mV)

Figure 7. Neptunium Solubility in Yucca Mountain J-13 Water Neptunium solubility varies tremendously depending on the Eh and pH of the water. (a) The graph shows the neptunium concentra- tion as a function of pH. Calculated solubility boundaries at various Eh values are superimposed on the graph. The colored region contains mainly Np(V) in solution, while the upper and lower bold lines are the solubility boundaries of the Np(V) solid phase,

Np2O5(s), and the Np(IV) solid phase, Np(OH)4(s), respectively. Solutions with neptunium concentrations above these boundaries are oversaturated and will precipitate the respective solid. The total neptunium concentration in solution may vary by more than six orders of magnitude over the entire pH range under oxidizing conditions. (b) This graph shows the solubility as a function of Eh at a fixed pH of 7, and corresponds to the Np solubility as one moves along the red line in the previous graph. Under reducing condi- tions (below –120 mV), Np(IV) dominates the solution speciation. Its concentration in solution is limited to less than 10–8 molal by the solubility of Np(OH)4(s). As conditions become more oxidizing, Np(V) becomes the redox-stable oxidation state and begins to dominate the speciation. [The Eh determines the Np(IV)/Np(V) ratio in solution, while the overall Np(IV) concentration is maintained –4 by the solubility of Np(OH)4(s).] The neptunium concentration rises with redox potential until it reaches approximately 10 molal at an Eh of ~250 mV. Solutions containing a higher neptunium concentration are oversaturated, and the Np(V) solid phase Np2O5(s) precipitates. The soluble neptunium concentration remains constant from then on. The shaded area spans the region where Eh and solubility of the Np(IV) solid control the neptunium concentration in solution. Outside this area, the solubilities of the Np(IV) and Np(V) solids govern neptunium solution concentration, independent of Eh. in Figure 7 (Kaszuba and Runde 1999). ing and Np(V) becomes more stable in far-field environment. Spent nuclear Under strong reducing conditions solution. Eventually, the Np(V) concen- fuel contains mainly UO2 and actinides (below about –120 millivolts), the total tration is great enough to allow a in low oxidation states (III and IV). If soluble neptunium concentration is Np(V) solid phase, Np2O5(s), to precip- conditions at the disposal site are limited by a Np(IV) solid phase, itate. Formation of this solid sets an reducing—for example, because of Np(OH) (s), and is conservatively upper limit to the soluble neptunium microbial activities or the reduction 4 assumed to be about 10–8 M. Similar to concentration. Within the pH and Eh capacity of corroding from storage plutonium, the presence of a more ranges of most natural aquifers containers—the actinides will be main- ordered solid oxyhydroxide or oxide, (between 4 and 9 and between –200 tained in their lower oxidation states such as NpO2(s), would reduce that and +700 millivolts, respectively), that and soluble actinide concentrations may conservative boundary by several orders concentration may vary by more than be low. If local reducing conditions of magnitude. Under these reducing four orders of magnitude. change because oxidizing waters infil- conditions, Np(IV) also dominates the When modeling actinide release trate from the surroundings, actinides solution speciation in the form of the from a repository, we will enter their more-soluble oxidation neutral complex Np(OH)4(aq). The total must anticipate the conditions at the states and begin to migrate into the neptunium concentration will rise as the source (e.g., close to a spent nuclear environment. Once far from the reposi- water conditions become more oxidiz- fuel container) as well as those in the tory, conditions may change drastically,

Number 26 2000 Los Alamos Science 403 Chemical Interactions of Actinides in the Environment

(a) that are saturated with chloride salts 10 1-M NaCl and exhibit very high ionic strengths (between 4 and 8 molal). With increas- 1-M HClO 4 4-M NaCl ing distance from the repository, the 5 brines’ ionic strength decreases and 15-M LiCl may ultimately reach the lower values of most natural surface and ground- waters. This variation complicates our 0 transport models, since the species that form within the WIPP site will not be [Cl] Pu=O PuÐO PuÐCl eq those that control actinide behavior (M) (Å) (Å) (Å) Ð5 far from the site.

Fourier Transform magnitude Fourier Transform 2.40 Pu=O PuÐO 1 1.74 — Chloride has been shown to affect eq 2.43 4 1.75 2.75 the solubility and speciation of PuÐCl 15 1.75 2.49 2.70 actinide compounds significantly. As Ð10 mentioned earlier, in most natural 01234waters plutonium favors the IV and V R Ð φ (Å) oxidation states, but radiolytic (b) hypochlorite formation in concentrated PuO 2+ 2 chloride solution may result in the formation of Pu(VI). Furthermore, + PuO2Cl complexation by chloride may enhance

PuO2Cl2 stabilization of Pu(VI) with respect to Ð reduction to Pu(V) and Pu(IV). PuO2Cl3 We have studied the complexation PuO Cl 2Ð 2 4 reactions of U(VI) and Pu(VI) with Intensity (arbitrary units) chloride using a variety of spectroscopic techniques, including ultraviolet, 820 830 840 850 860 870 visible, and NIR absorption and XAFS Wavelength (nm) spectroscopies (Runde et al. 1999). We could determine when U(VI) and Figure 8. XAFS Data and NIR Spectra of Pu(VI) in NaCl Solutions Pu(VI) chloride species, such as + (a) By comparing XAFS data for Pu(VI) in 1-M NaCl, 4-M NaCl, and 15-M LiCl, and non- PuO2Cl and PuO2Cl2, formed as a complexing 1-M HClO4, we have obtained condition-specific structural information function of chloride concentrations (see about the solution species. Chloride appears to bond above 1-M chloride in solution. Figure 8). Interestingly, the tris- and Bonding is clearly evident by 4-M chloride. As seen in the table (inset), the plutonyl tetrachloro complexes are not formed in bond length (Pu=O) increases slightly after the bonds and replaces water significant concentrations in NaCl (indicated by the Pu–Oeq bond) in the equatorial plane of the O=Pu=O moiety. solutions (up to the point of NaCl (b) The absorption bands at 838, 843, 849, and about 855 nm, plus the characteristic saturation). However, we have observed 2+ 2– band at 830 nm for the PuO2 ion, indicate the formation of inner-sphere Pu(VI) chlo- spectroscopically UO2Cl4 and β 2– ride complexes. These bands allowed us to determine the formation constant, , of the PuO2Cl4 at very high LiCl + most important Pu(VI) chloride complexes in NaCl solutions, PuO2Cl and PuO2Cl2(aq). concentrations and obtained structural data from single synthesized from concentrated chloride solutions. and the transport behavior of redox- Complexation with Chloride The interaction of actinides with sensitive elements such as plutonium or and Implications for WIPP chloride and other weak anionic ligands neptunium may change. Clearly, evalu- significantly affects solubility predic- ation of actinide mobility in the geo- In most natural waters, actinides are tions and must be considered in risk chemical environment around nuclear usually associated with hydroxide or assessments of WIPP and other nuclear waste repositories must account for carbonate ligands. However, WIPP, waste repositories in salt formations. soluble actinide concentrations that are which is designed to hold defense- With regard to neptunium, chloride controlled by the redox potential of the related transuranic waste, is carved out enhances the Np(V) solubility because transport medium and as well by the of a salt formation. Water at the WIPP of a strong ion interaction between the solubility of the actinides’ solid phases. site thus consists of concentrated brines Np(V) compounds and chloride ions

404 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment

(see Figure 9). Runde et al. (1996) 100 simulated this interaction using the NpO (CO )− 5− 2 3 NpO2(CO3)3 Pitzer ion-interaction model and devel- 80 + oped a Pitzer parameter set to predict NpO2 the Np(V) solubility at different NaCl concentrations and in artificial brine 60 solutions (Runde et al. 1999). Neglect- NpO (CO ) 3− ing the chloride interaction to a 40 2 3 2 predicted Np(V) solubility about an order of magnitude too low at a neutral 20 pH. The change of water activity with ionic strength affects the chemical potential of the solid phase because of 10−3 hydration water molecules in the solid. 5-M NaCl 5-M NaClO Np(V) solubilities were measured 4 from oversaturation in two artificial −4 WIPP brines, AISinR and H-17. The 10

) Spatial distribution (%) NaNpO CO ¥3.5H O(s) solutions reached a steady state after 1 2 3 2 − 241 days. The measured Np(V) solubil- ities reported in the literature are 10−5 2 × 10–6 molal and 1 × 10–6 molal in AISinR and H-17, respectively, while An(V) (mol kg our model predicts Np(V) solubilities of − 1 × 10–5 and 3 × 10–6 molal, respec- 10 6 tively. The prediction is acceptable for Na3NpO2(CO3)2¥nH2O(s) the H-17 brine but lacks accuracy for the AISinR brine. This is because the 10−7 model accounts only for interactions 10−7 10−6 10−5 10−4 10−3 10−2 10−1 between Np(V) and Na+ and Cl– ions, Carbonate concentration (mol kg−1) while the Pitzer parameters for electrolyte ions such as K+, Mg2+, Figure 9. Solubility of Np(V) in Concentrated Electrolyte Solutions 2+ 2– Ca , and SO4 (other constituents in The upper graph shows the distribution of predominant Np(V) solution species as a the brines) are unknown. Additional function of carbonate concentrations, with environmentally relevant concentrations data in various electrolyte solutions are shown in gray. The lower graph shows the soluble Np(V) concentrations in 5-M NaCl required to develop a more accurate solution—conditions that are relevant for WIPP—and noncomplexing 5-M NaClO4 model for the Np(V) solution chemistry solution over the same range. Similar data are obtained for Am(V). The neptunium in complex brines. solubility is about an order of magnitude higher in NaCl because of the stabilizing effect of actinide-chloride complexation reactions. The solubility-controlling solid for both curves at low carbonate is the hydrated ternary Np(V) carbonate,

Sorption NaNpO2CO3•3.5H2O(s). Interestingly, this solid initially forms at high carbonate concentrations. While it is kinetically favored to precipitate, it is thermodynamically The solubility of an actinide species unstable. Over time, it transforms to the thermodynamically stable equilibrium phase provides an upper limit to the actinide Na3NpO2(CO3)2•nH2O(s) (dashed curve). concentration in solution and may be considered as the first barrier to actinide transport. Once an actinide expected to dominate actinide transport tration, the flow rate and chemistry of has dissolved in water, its sorption through the environment. (Simple dilu- the water, and the composition of the onto the surrounding rock or mineral tion will lower the actinide concentra- surrounding rocks. In addition, different surfaces is expected to act as a tion, typically to trace concentrations, mechanisms, such as physical adsorp- secondary barrier. When the actinide once the actinide migrates away from tion, , chemisorption, or concentrations are far below the the contaminant or waste source.) even surface precipitation, can fix the solubility limits, as is likely in most Actinide sorption onto mineral actinide to a mineral/rock surface or in surface and groundwater systems, surfaces depends on many factors, such general inhibit its transport. Further- water/rock interfacial processes are as the actinide’s speciation and concen- more, redox processes complicate the

Number 26 2000 Los Alamos Science 405 Chemical Interactions of Actinides in the Environment

120 high values imply effective immobiliza-

Fe2O3 (syn) tion due to sorption onto rock surfaces. Batch-sorption experiments provide 100 helpful information about the sorption capacity of a mineral or soil. But while Fe O 80 2 3 they are easy to perform, they have SiO2 (syn) some drawbacks. The experiments are fundamentally static and provide only

60 SiO2 partial information about the dynamic, nonequilibrium processes that occur in nature.2 Additionally, the results of Adsorbed Pu(V) (%) 40 batch-sorption experiments are very sample specific and difficult to extrapo- Mont (syn) late to the different conditions found 20 Mont throughout a repository. To apply sorption data accurately, 0 we must obtain sorption coefficients 0 40 80 120 160 200 240 from both individual, well-characterized Time (h) minerals, such as zeolites, oxides, or iron oxyhydroxides, and Figure 10. Uptake of Pu(V) by Colloids from mineral with varying Lu et al. (1998) obtained data for Pu(V) onto colloids, including but known compositions. Sorption

(Fe2O3), silica (SiO2), and montmorillonite (Mont), in natural J-13 water and in a coefficients from these studies can synthentic (syn) J-13 water. The colloid concentration was maintained at 200 mg/L, serve as input data into a transport with the colloids being about 100 nm is size. The sorption clearly depends on the type model that will estimate the actinide of mineral, but the sorption mechanism remains unknown. The time-dependence of retention along water flow paths of the sorption onto montmorillonite suggests a more complicated interaction than known mineral content. The model occurs with oxide minerals. predictions can then be compared with experimental results obtained from Table II. Desorption of Plutonium from Colloids in J-13 dynamic experiments that use complex Water after 150 Days soils and rocks. Studies using minerals (hematite, Colloid Material Amount Desorbed calcite, gibbsite, albite, and ), Pu(IV) Pu(V) clays (montmorillonite), and zeolites (%) (%) (clinoptilolite) have shown that Am(III) and Pu(IV) sorb very rapidly to these α Hematite ( -Fe2O3) 0.0 0.01 solids in the following descending (α-FeOOH) 0.57 0.77 order: hematite, montmorillonite, Montmorillonite 8.23 0.48 clinoptilolite, and calcite, followed to a Silica (SiO2) 19.93 1.04 much lesser extent by gibbsite, albite, and quartz. But other factors, such as the concentration, solution speciation, and oxidation-state distribution of the sorption behavior of neptunium and remains in solution. The difference be- actinide species, significantly affect the plutonium. Unfortunately, we are only tween the remaining and initial actinide extent of surface sorption. As an exam- beginning to understand such actinide concentrations equals the amount that ple, K values for neptunium sorption d sorption and transport mechanisms. sorbed to the sample. This difference onto hematite range from 100 to Generally, batch-sorption experi- allows us to calculate the batch-sorption 2Flow-through, or dynamic, column experiments, ments are performed to investigate how distribution coefficient Kd, defined as actinides will be retained in the envi- the moles of per gram of wherein the actinide solution is allowed to migrate through a short column of packed ronment. An actinide-containing solu- solid phase divided by the moles of ra- geomaterial, provide information about dynamic tion is allowed to come in contact with dionuclide per milliliter of solution and transport properties. Dynamic column experi- ments are difficult to perform and interpret, and the geomaterial of interest, and we reported as milliliters per gram (mL/g). most of the sorption studies on the actinides are measure the amount of actinide that Low Kd values mean low sorption, and still performed as batch experiments.

406 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment

Ca P Ca



O Ca


012345 012345 keV keV

Figure 11. SEM Micrograph of Uranium Precipitate on Apatite A solution containing U(VI) at 10–4 M was kept in contact with apatite for 48 hours. The central SEM micrograph (1100 × magnifica- tion) shows crystalline plates on the smooth apatite mineral surface. Analysis of these plates by energy-dispersive spectroscopy (right graph) indicated the presence of , oxygen, uranium, and , consistent with crystalline uranium phosphate. Similar analysis of the bare regions of apatite (left graph) showed signals corresponding to only calcium, oxygen, and phosphorus. The precipitates were observed at uranium concentrations above 10–5 M. Studies show that at lower concentrations, uranium sorbs, rather than precipitates, to the mineral surface.

2000 mL/g, while plutonium sorption their findings: Surprisingly, Pu(IV) is reduced to Pu(IV) through the onto hematite results in K values exhibits a greater tendency to desorb formation of strong mineral-surface d above 10,000 mL/g. This difference is than Pu(V). Particularly striking is the complexes. Quite surprisingly, Neu mainly caused by the different stabili- relatively high desorption of Pu(IV) et al. (2000) showed that Pu(V) is not ties of the actinide oxidation states and from silica, which indicates a different reduced when sorbed on MnO2. X-ray their complexation strengths. The low sorption mechanism or different absorption near-edge structure indicated + charge of the NpO2 ion results in a binding to the silica functional groups the presence of Pu(V) on MnO2, even low sorption of neptunium, while than to the Fe(III) minerals, including though the starting material of the Pu(IV) dominates the complexation and possible changes in plutonium batch-sorption experiment was colloidal sorption processes of plutonium. Pluto- oxidation states. Pu(IV) hydroxide. Their data also nium(V) is expected to follow the low These investigations indicate the indicated that the plutonium was sorption affinity of Np(V). importance of understanding the oxida- oxidized to Pu(VI). The only time they Lu et al. (1998) determined the tion-state stability of the actinide sorbed did not observe the higher-oxidation- sorption of Pu(V) onto oxide and clay on a mineral surface. Redox-active state surface species was when Pu(IV) colloids (Figure 10). Sorption onto iron minerals, such as manganese or iron was complexed with a strong ligand, oxides, such as hematite, is strong and oxide/hydroxides, are present in subsur- such as nitrilo triacetic (NTA). irreversible, but only 50 percent of the face environments and may alter the These observations strongly support the Pu(V) is sorbed on the montmorillonite redox states of uranium, neptunium, hypothesis that plutonium undergoes a (clay). However, the sorption mecha- and plutonium. At the Rocky Flats redox cycling that is enhanced by nisms are largely unknown. The time- Environmental Technology Site, surface interactions. We need more data dependence of the sorption onto the seasonal variations of plutonium con- about the surface species, however, to clay suggests a more complicated inter- centrations in settling ponds correlate model such interactions. action than occurs with oxide minerals. astonishingly well with soluble Obviously, batch experiments alone Interestingly, the desorption of manganese concentrations, indicating a cannot elucidate sorption mechanisms. Pu(V) does not follow this general pat- potential release of plutonium by way Specifically, we have to distinguish be- tern. Lu et. al. (1998) also discussed the of seasonal redox cycling. tween surface precipitation and interfa- desorption of Pu(IV) and Pu(V) from It has been generally hypothesized cial molecular reactions in order to various colloids. Table II summarizes that the environmentally mobile Pu(V) model sorption behavior. In the event

Number 26 2000 Los Alamos Science 407 Chemical Interactions of Actinides in the Environment

Figure 12. Correlation between An(III) (a) 100 Solution Speciation and Sorption onto Montmorillonite 0.1-M NaCl (a) Even with an order-of-magnitude 80 1.0-M NaCl difference in NaCl concentration, sorption of the trivalent actinides Am(III) and 60 Cm(III) peaks in near-neutral waters. At lower pH, the An3+ ion is stable and does 40

not participate in complexation reactions Sorption (%) with the most common ligands, hydroxide or carbonate, no if the ligands are 20 in solution or on a surface. At higher pH, negatively charged An(III) species are 0 formed that increase the species’ (b) solubility and reduce their sorption + Ð 3Ð An(CO3) An(CO3)2 An(CO3)3 because they are repelled from the An3+ 80 negatively charged functional groups on the mineral surface. (b) Calculations for a weak sodium chloride solution 60

(0.1-M NaCl and CO2 partial pressure of 0.01 atm) suggest that sorption is mainly 40 correlated with the presence of cationic solution species. The gray area denotes 2+ Species distribution (%) 20 An(OH) the pH range of natural waters.

456810 7 9 pH of a surface precipitation, the Kd value ence the actinide sorption processes that begun to address the fundamental may represent the solubility product of also depend strongly on pH (Figure 12). mechanism of actinide adsorption onto the surface precipitate but will not For the trivalent actinides Am(III) and mineral surfaces. Janecky and Sattel- reflect the sorption behavior of an Cm(III), the sorption reaches a peak berger (unpublished results) modeled actinide species. under conditions of near-neutral waters. the adsorption behavior of Np(V) on For example, phosphate-containing There, the cationic An(III) solution calcite by minimizing the free energy minerals, such as apatite, are known to species predominate solution speciation. of the coordination. The structural 3+ – reduce thorium and uranium concentra- At lower pH, the An ion is stable and parameters of NpO2CO3 , which is the tions in contaminated solutions. Sorp- does not participate in bonding with main solution complex in neutral and tion distribution coefficients for U(VI) hydroxide or carbonate ligands. At low-carbonate waters (such as J-13 and Th(IV) have been reported in the higher pH, negatively charged An(III) water), were used to mimic the adsorp- literature, but there are alsoreports of species are formed that exhibit reduced tion. The resulting surface-complex U(VI) and Th(IV) precipitates. Our interactions with chemically active modeling indicates a distance of about SEM studies have identified microcrys- groups at the mineral surface. Mineral 4 Å between the neptunium center and talline precipitates on the apatite when or rock surfaces at high pH are generally the nearest calcium ions in the calcite the initial U(VI) concentrations were negatively charged, and apparently the structure. Data from preliminary XAFS above 10–5 M (see Figure 11). At lower negatively charged actinide species are studies support this distance. Although U(VI) concentrations, no surface repelled from the anionic surface this agreement between experiment and precipitation was observed. Thus, at the groups. As such, anionic actinide theory is encouraging, more detailed higher concentrations, the Kd values species behave as simple anionic lig- studies have to be performed to develop 2– 2– 3– actually reflect a solubility product, ands, such as SO4 , CrO4 or AsO4 . much more accurate transport models, while at the lower concentrations, the In addition to An(III) species, the same including this surface complexation decrease of U(VI) in solution results trend of enhanced stability and low model. from sorption of the actinide onto the surface retention has been observed for We are also trying to better under- 4– mineral surface. the An(VI) complex UO2(CO3)3 . stand the sorption/desorption reactions Complexation reactions can influ- Enhanced modeling efforts have of actinides with colloids and the

408 Los Alamos Science Number 26 2000 Chemical Interactions of Actinides in the Environment

actinides’ resulting transport character- contaminants is still virtually unknown. sp. Within the first week of the experi- istics. This area of environmental Microorganisms, especially bacteria, ment, the bacteria concentrated plutoni- migration received heightened attention are also known to mediate redox um to levels that were nearly 10,000 with the discovery of plutonium in a processes. They may reduce elements times greater than a sterile control or- borehole at the Nevada Test Site from higher oxidation states and use the ganism. Lower uptake was observed for (Kersting et al. 1999). The plutonium enthalpy of the redox reaction to grow contact times of two weeks or longer. had evidently migrated 1.3 kilometers and reproduce. Through metabolic Another strain of this bacteria, in only 30 years. (Each nuclear test is activity, microbes can also establish pseudomonas mendocina, is known to characterized by a unique 239Pu/240Pu reducing conditions by changing the pH remove from Fe(III) hydroxide/ ratio, and that ratio can be used and Eh of the local waters. While direct oxides and was investigated for its to identify the plutonium source.) As experimental evidence is still lacking, it potential to remove from discussed in the article by Maureen is almost certain that some microbes those environmentally abundant miner- McGraw, we now believe that colloidal can catalyze the transformation of als. In fact, Kraemer et al. (2000) found transport was responsible for this uranium, neptunium, and plutonium that 20 percent of the sorbed plutonium remarkably fast movement of plutonium into less-soluble forms. Several bacteria was removed by this aerobically grow- through the water-saturated rock. It is species have been reported in the litera- ing bacteria after about four days. The not clear, however, whether the ture to effectively reduce the highly experimental conditions are presently transport was facilitated by intrinsic mobile U(VI) into the far less soluble optimized to remove the majority of the plutonium colloids or natural (clay or and mobile U(IV). sorbed plutonium and to maximize its zeolite) colloids. What is clear is that Given these characteristics, potential application in soil remediation transport models to date have underesti- microbes could pose a third natural strategies. mated the extent of colloidal transport barrier to actinide transport from Leonard et al. (1999) also studied on plutonium mobility. geologic repositories and could also the inhibitory and toxic effects of mag- help remediate actinide-contaminated nesium oxide, MgO, on these WIPP-de- soil, groundwater, and waste streams. rived bacterial cultures. Microbial Interactions The ultimate goal is to use microorgan- oxide is of specific interest because it isms as “living” backfill to immobilize was chosen as backfill material at the We have discussed the most promi- actinides through processes such as WIPP repository to maintain the pH nent chemical processes involving bioreduction, biosorption, or bioprecip- between 9 and 10, to reduce the amount actinides in the environment: dissolu- itation and mineralization. At this time, of infiltrating water, and to limit the tion and precipitation of solids, com- however, actual applications of bacteria amount of CO2 that can accumulate plexation in solution, and sorption onto in remediation strategies and field through degradation of cellulose and surfaces. But there is growing attention demonstrations are rare. other organic compounds in the wastes. to the importance of actinide interac- Recent WIPP-related studies at Los Initial studies indicated an inhibited cell tions with microorganisms. Great vari- Alamos have focused on the interaction growth as the amount of MgO eties of microorganisms exist in all of urnaium and plutonium with the increased, as well as a decrease in aquifers and soils, and many survive bacteria’s cell walls or byproducts, such microbially generated (N2O) by under the most extreme conditions, as extracellular biopolymers, and the active halophilic bacteria. Ultimately, such as around hydrothermal vents at and viability of microorganisms the toxic effect of MgO inhibits micro- the bottom of the ocean, within deep in the presence of the actinides. Francis bial growth, and consequently microor- subterranean aquifers, or within the et al. (1998) have shown that actinides ganisms may be unable to create a hypersalinity of geologic salt beds. are associated only at low levels with reducing environment at the repository Microorganisms can accumulate on the halophilic bacteria isolated from that would stabilize actinides. solid surfaces often via formation of muck pile salts around the WIPP site. A more detailed study on the micro- biofilms, or they may be suspended in The extent of association varied with bial association mechanism included the aquifer. As such they act as mobile bacterial culture, actinide species, pH, analysis of the uptake of uranium and or even self-propelled colloids. The and the presence of organic or inorganic plutonium by sequestering agents and concern is that the actinides can be complexants. The radiolytic effects on extracellular polymers (exopolymers). associated with microbes, either the microbial viability were found to As an example, some exopolymers pro- through surface binding or through be negligible. duced by bacillus licheniformis consist metabolic uptake, and be transported by However, other microorganisms can of repeating γ-polyglutamic acid (γ- them through the environment. However, concentrate metal ions to a great extent. PGA). Neu et al. (manuscript in prepa- the microorganisms’ role in environ- Hersman et al. (1993) observed a strong ration) observed actinide hydroxide pre- mental transport behavior of radioactive uptake of plutonium by pseudomonas cipitation at metal-to-glutamate ratios

Number 26 2000 Los Alamos Science 409 Chemical Interactions of Actinides in the Environment

higher than 1:20, but both U(VI) and In general, actinide solubilities are at the Nevada Test Site. In these specu- Pu(IV) ions formed soluble complexes low in most natural waters—below lations, however, they failed to include at ratios 1:100 and 1:200, respectively. micromolar concentrations. Actinides influential environmental factors. Presumably at the higher PGA concen- in the V oxidation state will have the While we acknowledge that the trations, the actinide ion is encapsulated highest solubilities; those in the IV potential presence of Pu(VI) in a and isolated from hydroxo-colloid for- oxidation state, the lowest. Conversely, PuO2(s) matrix may increase the mation. The binding strength for U(VI) An(V) species show low sorption onto dissolution kinetics, the chemical condi- and Pu(IV) is lower than for other mineral and rock surfaces, and An(IV) tions of the local environment (water strong ligands and chelators, such as species tend to sorb strongly. Typically, and rock compositions) determine the Tiron or NTA, but on the same order as neptunium is the only actinide that stability of plutonium oxidation states for other exopolymers or humic acids. favors the V oxidation state, and as in the aqueous phase and the overall An upper binding boundary for the such it is the most soluble and most plutonium mobility. Under ambient polyglutamate exopolymer was deter- easily transportable actinide under envi- conditions and in most aqueous envi- mined to be about 0.5 micromoles ronmental conditions. Plutonium favors ronments, Pu(VI) is unlikely to be Pu(IV) per milligram of γ-PGA. Active the IV oxidation state in many natural stable. Once in solution, it is typically cells of bacillus licheniformis bonded to waters, and normally its solubility is reduced to lower oxidation states and, more than 90 percent of the introduced quite low. Once plutonium is in solu- depending on the water redox potential, Pu(IV)—up to 8.4 micromoles of tion, its migration is dominated by a may eventually precipitate as Pu(IV) were added. As expected, other variety of processes, including redox Pu(OH)4(s) or PuO2(s). Consequently, functional groups on the exterior cell reactions in solution or on mineral the fact that the superoxide PuO2+x(s) surface (in addition to the PGA exo- surfaces, sorption, colloidal transport, may include Pu(VI) does not automati- ploymer) increase the number of bind- simple diffusion into rock interfaces, cally imply enhanced plutonium solubil- ing sites and enhance metal binding. coprecipitation, and mineralization. ity or mobility in the natural environ- This research clearly shows that Furthermore, increasing research ment. Furthermore, given the actinides bind with microorganisms and activites indicated that microbes can results on colloidal transport, it is their metabolic byproducts. Depending affect actinide migration, either through improbable that Pu(VI) is responsible on the static or dynamic nature of the direct association or by helping to for the plutonium migration at the microorganism, the interaction of pluto- maintain a reducing environment. Nevada Test Site. nium and other actinides with extra- To understand this complex set of As the inventory of actinides in cellular ligands may affect the interacting processes requires that we interim storage facilities grows and actinides’ mobility both away from have a strong, multidisciplinary founda- storage containers age, increasing the storage containers and within the tion in environmental chemistry, molec- risk of accidental releases, and as the environment. Future studies will focus ular science, and interfacial science and safety of permanently storing nuclear on understanding the mechanisms of the requisite spectroscopies, subsurface waste in geologic repositories must be actinide/microbial interactions and testing, and laboratory experiments. But evaluated, it becomes increasingly exploiting their potential for remedia- we also need to interpret experimental important to understand how actinides tion and immobilization efforts. Uptake results within the context of the multi- will interact with the environment. studies of plutonium by means of component natural system. More sophisticated models are needed sequestering agents are highlighted in A case in point is the recent identifi- to account for all the potential the box on “Siderophore-Mediated cation of a higher oxide of plutonium migration paths away from an actinide Microbial Uptake of Plutonium” on of general formula PuO2+x(s), which is source. Theoretical and experimental page 416. formed by water-catalyzed oxidation of scientists will be challenged for years PuO2(s). As discussed by Haschke in by the demands of developing those the article beginning on page 204, this models. Concluding Remarks Pu(IV) “superoxide” may also contain Pu(VI), which would increase plutoni- From this overview of actinide inter- um solubility. In a recently published actions in the environment, it should paper (Haschke et al. 2000), the authors be clear that such interactions are com- speculated that the Pu(VI) could plex. Each local environment is unique, dissolve and be transported more easily and the site-specific conditions deter- from a nuclear waste storage site than mine which actinide species predomi- previously estimated. Further, they nate as well as each species’ overall conjectured that Pu(VI) might have transport and migration characteristics. caused the rapid migration of plutonium

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Further Reading Kim, J. I. 1986. Chemical Behavior of Wolfgang Runde earned B.S. and Ph.D. Transuranic Elements in Natural Aquatic degrees in chemistry from the Technical Univer- Systems. In Handbook on the Physics and sity of Munich, Germany, in 1990 and 1993, Brown, G. E., V. E. Heinrich, W. H. Casey, Chemistry of the Actinides. Edited by A. J. under the direction of Professor Dr. J. I. Kim. D. L. Clark, C. Eggleston, A. Felmy, D. W. Freeman and G. H. Lander. New York: He then worked for two years as a scientific staff Goodman, et al. 1999. Chem. Rev. 99: North-HollandPublishing Co. member at the 77–174. Institute for Langmuir, D. 1997. Chap. 13. In Aqueous , Clark, D. L., S. D. Conradson, D. W. Keogh, Environmental Geochemistry. Upper Saddle TU Munich. M. P. Neu, P. D. Palmer, W. Runde, et al. River, NJ: Prentice Hall. During this time, 1999. X-Ray Absorption and Diffraction he was invited to Studies of Monomeric Actinide Tetra-, Penta-, Leonard, P. A., B. A. Strietelmeier, work as a guest and Hexavalent Carbonato Complexes. In L. Pansoy-Hjelvik, R. Villarreal. 1999. scientist at San- Speciation, Techniques and Facilities for Microbial Characterization for the Source- dia National Lab- Radioactive at Synchrotron Light Term Waste Test Program (STTP) at Los oratories on tech- Sources, OECD Nucl. Energy Agency. 121. Alamos. Conference on nical aspects of 99, Tucson, AZ. the actinide Choppin, G. R., and E. N. Rizkalla. 1994. source term Solution Chemistry of Actinides and Lemire, R. J. and P. R. Tremaine. 1980. model for WIPP. Wolfgang then spent a year as Lanthanides. In Handbook on the Physics and J. Chem. Eng. Data. 25: 361. postdoctoral fellow at the G. T. Seaborg Institute Chemistry of Rare . Edited by K. A. for Transactinium Science at the Lawrence Gschneidner Jr. and L. Eyring. New York: Lu, N., I. R. Triay, C. R. Cotter, H. D. Kitten, Livermore National Laboratory before joining North-Holland Publishing Co. and J. Bentley. 1998. "Reversibility of the Chemical Science and Technology Division sorption of plutonium-239 onto colloids of at Los Alamos National Laboratory in 1996. Choppin, G. R., J.-O. Liljenzin, and J. Rydberg. iron oxides, clay and silica." Abstract of He is presently a staff member and leader of the 1995. Behavior of Radionuclides in the Agronomy, p.31 ASA, CAA, SSSA Annual Actinide Environmental Chemistry team in the Environment. In Radiochemistry and Nuclear Meetings, Baltimore, MD. Environmental Science and Waste Technology Chemistry. Oxford: Butterworth-Heinemann. Division with a joint appointment in the Nuclear Neu, M. P., W. Runde, D. M. Smith, and Radiochemistry in the Chemical Choppin, G. R., 1983. Radiochim. Acta 32: 43. S. D. Conradson, M. R. Liu, D. R. Janecky, Science and Technology Division. His research and D. W. Efurd. 2000. EMSP Workshop, interests are in the areas of inorganic, environ- Dozol, M., R. Hagemann, D. C. Hoffman, Atlanta, GA. mental, and radiochemistry of f elements, J. P. Adloff, H. R. Vongunten, J. Foos, et al. including the actinides, with the focus on 1993. Pure and Appl. Chem. 65: 1081. Nitsche, H., A. Muller, E. M. Standifer, R. S. nuclear waste isolation and the environment. His Deinhammer, K. Becraft, T. Prussin, R. C. research activities have been closely affiliated Efurd, D. W., W. Runde, J. C. Banar, Gatti. 1993. Radiochima. Acta. 62: 105. with the national nuclear waste programs in the D. R. Janecky, J. P. Kaszuba, P. D. Palmer, US (WIPP and Yucca Mountain) and Germany et al. 1998. Environ. Sci. Tech. 32: 3893. Runde, W., S. D. Reilly, M. P. Neu. 1999. (Gorleben). Geochim. Cosmochim. Acta. : 3443. Francis, A. J., J. B. Gillow, C. J. Dodge, M. Dunn, K. Mantione, B. A. Strietelmeier, Runde, W., M. P. Neu, D. L. Clark. 1996. et al. 1998. Radiochim. Acta 82: 347. Geochim. Cosmochim. Acta 60: 2065.

Hascke, J., T. Allen, and L. Morales. 2000. Shannon, R. D. 1976. Acta Cryst. 32A: 751. Science 287: 285.

Hersman, L.E., P. D. Palmer, and D. E. Hobart. 1993. Mater. Res. Soc. Proc. 294: 765.

Kraemer, S. M., S. F. Cheah, R. Zapf, J. Xu, K. N. Raymond, and G. Sposito. 2000. Geochim. Cosmochim. Acta (accepted).

Kazuba, J. P., and Runde, W. 1999. Environ. Sci. Tech. 33: 4427.

Kersting, A. B., D. W. Efurd, D. L. Finnegan, D. J. Rokop, D. K. Smith, J. L. Thompsom. 1999 Nature 397: 56.

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