A research team from the Department of Energy's Oak Ridge and Lawrence Berkeley National Laboratories, the University of California - Berkeley, and the University of South Florida have developed a low-cost polymer adsorbent that can selectively bind to dissolved uranium in water. A report on the new process has just been published in the Nature Communications journal. This new process could significantly lower the cost and raise the efficiency of extracting uranium from the oceans of the world for sustainable nuclear energy production.
Ilja Popov works in the ORNL's Chemical Sciences Division. He is one of the coauthors of the report. He said, “Our approach is a significant leap forward. Our material is tailor-made for selecting uranium over other metals present in seawater and can easily be recycled for reuse, making it much more practical and efficient than previously developed adsorbents.”
Microorganisms that have an affinity for iron were an inspiration for this research. Such organisms use natural compounds called “siderophores” to draw nutrients such as iron from their hosts. Popov said, “We essentially created an artificial siderophore to improve the way materials select and bind uranium.” His team used computer modeling and laboratory experimentation to identify a novel functional group known as “H2BHT” (2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine) that preferentially selects uranyl ions (water-soluable uranium) over other metal ions from other elements such as vanadium which are found in seawater.
The discovery of this new material is supported by the promising behavior of a proof-of-principle H2BHT polymer adsorbent in the lab. Uranyl ions are readily adsorbed to the surface of the polymer fibers because of the unique chemical properties of H2BHT. One of the benefits of the new polymer is that it increases the “storage space” on the polymer fibers for uranium. H2BHT is highly selective for uranium and it is recyclable. It is more efficient than any other such method for recovering uranium. A practical method for recovering uranium from seawater could provide fuel for nuclear power reactors for thousands of years at our current rate of consumption.
There are uranium deposits on the floor of the oceans which are constantly being eroded to supply uranyl ions to seawater. It is estimated that there are about three milligrams of uranium per ton of seawater. Even at a very dilute concentration, the oceans of the world contain an estimated four billion tons of uranium. This is roughly a thousand times more uranium than sources on land.
Researchers have been working on the creation of efficient uranium adsorbents to extract uranium from seawater for over fifty years, but the goal has been elusive. Alexander Ivanov does computational studies of H2BHT at the ORNL. He said, “The goal is to develop efficient adsorbent materials at a low cost that can be processed using mild conditions to recover uranium, and also reused for multiple extraction cycles.”
DoE’s Office of Nuclear Energy's Fuel Cycle Research and Development has been supporting this research. The focus of the team has been to identify the factors that selectively influence and increase the volume of uranium that can be extracted by new materials. Previously reported studies of uranium extraction utilizing amidoxime-based polymers revealed that amidoxime has a greater affinity for vanadium than uranium that has been difficult to overcome. The development of H2BHT which is not based on amidoxime has a better ability to selectively target uranium in seawater. Popov said, “The result is that amidoxime-based materials, the current front-runners for commercially available adsorbents, fill up more quickly with vanadium than uranium, which is difficult and costly to remove.”
The old amidoxime-based polymers require concentrated acidic solutions to wash out the vanadium which increases the cost. The result is caustic waste streams and degradation of the polymer fibers which limits the number of times they can be recycled. Popov said, “To work as a scaled-up concept, ideally, unwanted elements would not be adsorbed or could easily be stripped during processing and the material reused for several cycles to maximize the amount of uranium collected.”
In contrast, the process utilizing H2BHT is carried out with mild basic solutions and the fibers can be recycled many times without degradation. The lower cost over the amidoxime-based process and the lesser environmental impact means that H2BHT may be the key to practical uranium extraction from seawater without the environmental devastation caused by mining uranium on land.