The Chemical Interactions of Actinides in the Environment Wolfgang Runde 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 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 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 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 oxidation state and help to establish equilibrium between species Redox Complexation Transport 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 solid 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 chloride ions are known PuO CO (aq) 2 3 to stabilize plutonium in the VI oxida- 1.0 PuO (CO ) 2– PuO F+ 2 3 2 tion state. Plutonium(VI) species are 2 much more soluble than Pu(IV) species, PuO (CO ) 4– 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 – PuO2CO3 Unfortunately, very few studies of actinide geochemistry 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) –0.5 allow direct study with conventional Pu(VI) spectroscopies. 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 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 10–5 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 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 thermodynamically unstable 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 VVV 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.