Copyright © Amy E. Hixon. All rights reserved.
Research in the Hixon group is built on understanding and quantifying the behavior of actinide elements in natural and engineered systems. The actinide elements are those with atomic numbers 89-103 (i.e., actinium through lawrencium) and carry great societal importance due to their use in medicine, power generation, national security, and nuclear waste management. Due to the complex nature of the actinide elements and the relative difficulty of working with radioactive materials, research in actinide chemistry has lagged far behind that of most other elements on the periodic table.
Actinide Science at Notre Dame
The long-term goal in this field is to improve predictions of the fate and transport of actinide elements. Many factors can influence whether these elements are mobile or immobile in the subsurface environment, such as aqueous complexation, colloid formation, and the relative contributions of precipitation versus dissolution and sorption versus desorption. The Hixon research group is moving the field towards a detailed understanding of more complex, and thus more environmentally-relevant, systems. For example, we can combine the two sub-themes described below by looking at how the presence of a ligand in the aqueous phase affects actinide sorption and testing whether sorption behavior can be predicted from the two independent binary systems (e.g., determining whether Pu-EDTA and Pu-mineral models predict the behavior of plutonium in the Pu-EDTA-mineral system).
Behavior of actinide elements at the mineral-water interface
This sub-theme has received recognition via an NSF CAREER award; other funding comes from the Actinide Center of Excellence (ACE). Work in this sub-theme typically involves fundamental studies of uranium, neptunium, and plutonium reactions with pure mineral phases under controlled laboratory conditions. A particular emphasis is placed on why or how fast reactions occur and on being able to describe the redox cycling of actinide elements at the mineral-water interface. In addition, we have pioneered the macroscopic characterization of uranyl peroxide nanocluster interactions with several different mineral surfaces.
Aqueous-phase chemistry of the actinide elements
This sub-theme is primarily supported by a DOE Early Career award with additional student support through the LANL-Carlsbad Field Office and an international collaboration with Karlsruhe Institute of Technology's Institute for Nuclear Waste Disposal. A large emphasis lies in determining the thermodynamics and kinetics of neptunium, plutonium, and americium interactions with uranyl peroxide nanoclusters, but we are also interested in understanding how ligands such as EDTA control the oxidation state distribution of plutonium.
This second research theme represents the addition of new skills in solid-phase characterization as applied to nuclear forensics and is supported by a collaboration with Oak Ridge National Laboratory (ORNL) and Savannah River National Laboratory (SRNL). The Nuclear Security Advanced Technologies group at ORNL has a broad mission of exploring the chemical and physical properties of materials in the nuclear fuel cycle and is responsible for developing and improving analytical frameworks to identify elemental, isotopic, and chemical information on nuclear materials with known and unknown history. They have established a Fuel Cycle Science Fellowship through the Hixon research group, which will support Ph.D. students in research focused on chemical and physical characterization of fuel-cycle related plutonium-containing compounds.
This final research theme represents the addition of new skills in synthetic solid-state chemistry. While generally a separate line of research, the compounds that we synthesize as part of this third research theme have applications to environmental radiochemistry, the nuclear fuel cycle, and national security. ACE is the major sources of funding for this research thrust. Our major scientific contribution thus far has been the expansion of the family of plutonium oxide nanoclusters and synthesis and characterization of the first plutonium metal-organic framework (MOF).