Metal-organic frameworks (MOFs), porous structures that are formed through metal atoms linked with organic molecules, have become versatile materials for storage, separation, and sequestration of various molecules due to their modularity and porous nature. Early MOF studies were typically associated with gas adsorption and separation due to their incredibly high surface area. Now, immobilizing “hot” radionuclides from nuclear waste can be added to the list of MOF applications. A key advantage of MOFs for actinide integration is their unprecedented versatility that allows for replication of structural motifs related to radionuclide-containing natural minerals. This offers several advantages, such as being applicable to adsorption of radionuclides in aqueous environments. Technetium (Tc) is of particular interest because it is a pervasive groundwater contaminant.
Members of the Center for Hierarchical Waste Form Materials (CHWM)—specifically the Shustova group at the University of South Carolina that leads a collaborative effort with researchers from Savannah River National Laboratory, Pacific Northwest National Laboratory, Alfred University, Clemson University, University of Florida, and Commissariat à l'énergie atomique et aux énergies Alternatives (CEA, France), have been developing MOF chemistry to expand the versatility of MOFs as a carrier of radionuclides. CHWM’s multidisciplinary team integrates several groups from universities and national labs in order to model, synthesize, and characterize incorporation of actinides into MOFs.
Synthesizing a new type of MOF
The highly tunable structure of MOFs allows for actinide integration in multiple ways for radionuclide waste immobilization and storage: inclusion inside the pores, metal substitution, or attachment to organic linkers. Furthermore, MOFs not only efficiently trap radionuclide species in their voids but also allow for the possibility of having their pores sealed through the inclusion of so-called “capping linkers.” All these actinide integration methods have been successfully realized by the Shustova group with the support of CHWM.
Prof. Natalia Shustova and her group at University of South Carolina have produced some of the world’s first actinide-bearing MOFs. Her research team was able to integrate uranium and thorium into the MOF matrix through utilization of multiple synthetic routes. Through collaboration with Dr. Jake Amoroso and Dr. David DiPrete at Savannah River National Laboratory, Prof. Shustova and her group were able to not only prepare a unique plutonium-based MOF but also to characterize radionuclide leaching rates. In order to understand how complexing ligands affect actinide interactions with the MOF structure, CEA studied radionuclide interactions with grafted ligands on mesoporous silica. These studies showed that MOFs have a promising future to immobilize transuranic radionuclides from the environment.
Proving MOF waste form suitability
Creating a radionuclide MOF is only the first step in using radionuclide-incorporating MOFs as a viable nuclear waste form. Understanding leaching of the radionuclide from the MOF to the surroundings is critical because it informs us about the lifetime for absorption. In order to provide insights into factors controlling radionuclide leaching from MOFs, Dr. Shenyang Hu at Pacific Northwest National Laboratory, with the help of Prof. Simon Phillpot at the University of Florida, is developing physics-based mesoscale models of radionuclide leaching to understand structural and environmental conditions to optimize MOF stability. Quantum-mechanic calculations provide atomic-scale insight into leaching mechanisms and thermodynamic and kinetic properties that are not scalable to meso- and macroscopic properties but are important inputs for the development of quantitative mesoscale models of leaching kinetics.
Phillpot and his team systematically studied the atomic energy landscape of various MOFs, determining the preference for incorporation of various radionuclides with quantum-mechanical simulations. Once the atomic energy landscape and diffusion barriers were calculated, Hu and his team used them to determine the thermodynamic and kinetic properties in the mesoscale leaching models that predict MOFs’ performance. This allows computational exploration of how the capping linkers, particle size, and aqueous conditions affect a MOFs’ leaching kinetics.
In order to determine the validity of the atomic-scale models, Prof. Kyle Brinkman and Dr. Nancy Birkner at Clemson University and Prof. Scott Misture at Alfred University perform structural and thermodynamic studies on the various ligand combinations using calorimetric, synchrotron x-ray scattering and spectroscopy tools. Calorimetric studies assess the stability of the MOFs under different environmental conditions, while x-ray studies help to understand the complex and often disordered structures that exist, especially within the MOF pores. Through studies on actinide-bearing MOFs from both modeling and characterization, the Shustova group can better understand MOF parameters that could affect radionuclide leaching kinetics and tailor MOF design to desired applications.
A new era for MOFs
The field of actinide-containing MOFs is currently like an iceberg where only the surface has been exposed. There remains great opportunity to expand the field and to understand how MOFs can be used to advance the fields of radionuclide waste processing, separation, immobilization, and storage. The CHWM Energy Frontier Research Center (EFRC) team has shown that it is up to the challenge to advance the science and technology and is committed to meeting this opportunity for making significant research contributions.
