The Chemical Interactions of Actinides in the Environment

From a chemist’s perspective, the environment is a complex, particularly elaborate system. Hundreds of chemically active compounds and minerals reside within the earth’s formations, and every patch of rock and soil is composed of its own particular mix. The waters that flow upon and through these formations harbor a variety of organic and inorganic ligands. Where the water meets the rock is a poorly understood interfacial region that exhibits its own chemical processes. Likewise, from a chemist’s point of view, the actinides are complex elements. Interrupting the 6d transition elements in the last row of the periodic table, the actinides result as electrons fill the 5f orbitals. Compared with the 4f electrons in the lanthanide series, the 5f electrons extend farther from the nucleus and are relatively exposed. Consequently, many actinides exhibit multiple oxidation states and form dozens of behaviorally distinct molecular species. To confound matters, the light actinides are likely to undergo reduction/oxidation (redox) reactions and thus may change their oxidation states under even the mildest of conditions. Uranium, neptunium, and especially plutonium often display two or more oxidation states simultaneously in the same solution. Accordingly, the light actinides exhibit some of the richest and most involved chemistry in the periodic table. As a result, the chemical interactions of actinides in the environment are inordinately complex. To predict how an actinide might spread through the environment and how fast that transport might occur, we need to characterize all local conditions, including the nature of site-specific minerals, temperature and pressure profiles, and the local waters’ pH, redox potential (Eh), and ligand concentrations. We also need a quantitative knowledge of the competing geochemical processes that affect the actinide’s behavior, most of which are illustrated in Figure 1. Precipitation and dissolution of actinide-bearing solids limit the upper actinide concentration in solution, while complexation and redox reactions determine the species’ distribution and stability. The interaction of a dissolved species with mineral and rock surfaces and/or colloids determines if and how it will migrate through the environment. Understanding this dynamic interplay between the actinides and the environment is critical for accurately assessing the feasibility of storing nuclear waste in geologic repositories. The actinides1 are radioactive, long-lived, and highly toxic. Over the last few decades, vast quantities of transuranic actinides (those with atomic numbers greater than that of uranium) have been produced inside the fuel rods of commercial nuclear reactors. Currently, most spent fuel rods are stored aboveground in interim storage facilities, but the plan in the United States and some European countries is to deposit this nuclear waste in repositories buried hundreds of meters underground. The Department of Energy (DOE) is studying the feasibility of building a repository for high-level waste in Yucca Mountain (Nevada) and has already licensed the Waste Isolation Pilot Plant (WIPP) in New Mexico as a repository for defense-related transuranic waste. Given the actinides’ long half-lives, these repositories must isolate nuclear waste for tens of thousands of years.