The world’s oceans hold nearly 1,000 times more uranium than all known land-based sources. The total, an estimated 4 billion metric tons, could supply the nuclear power industry’s fuel needs for centuries, even if the industry grows rapidly.
But until now, few studies have addressed the difficulties of extracting low concentrations of the dissolved metal from the sea. And very few substances stand out as good uranium sponges (C&EN, Sept. 3, 2012, page 60). But researchers have just added a new class of materials to that short list—metal-organic framework (MOF) compounds (Chem. Sci., DOI: 10.1039/c3sc50230a).
MOFs are crystalline materials composed of metal ions or clusters that are connected by organic linker groups. The materials are endowed with extremely high surface areas and porosities and can be readily tailored via synthesis. Those properties have led to numerous record-breaking demonstrations and some commercialization in gas separation and storage applications.
For those reasons, Wenbin Lin and coworkers at the University of North Carolina, Chapel Hill, set out to design MOFs that can grab onto uranium. Guided by studies indicating that compounds with phosphorylurea groups exhibit high affinity for actinides, Lin’s team prepared a series of MOFs built from zirconium clusters, phosphorylurea groups, and terphenyldicarboxylic acid bridging ligands.
Lin explains that the group varied the lengths of the bridging ligands to tailor the MOFs’ channel sizes to accommodate uranyl (UO22+) ions, the dominant form of dissolved uranium.
The team reports that under some conditions, one of the new MOFs extracts uranium from water, including artificial seawater, with a capacity of 217 mg U per g of absorbent material. That value, which corresponds to one uranyl ion for every two phosphorylurea groups, exceeds the capacity of amidoxime-functionalized polymers, the most commonly studied material for absorbing uranium, by a factor of four. Now the team is optimizing methods for removing the uranium and recycling the MOFs.
University of California, Berkeley, MOF specialist Jeffrey R. Long notes that the number of MOFs exhibiting the durability required for use in a variety of industrial applications is quickly increasing. “This groundbreaking work provides an initial demonstration that MOFs could potentially even be of utility in the extraction of uranium from seawater.”