The most advanced system today for extracting uranium from seawater employs plastic fibers with uranium-binding chemical groups grafted onto their surface. Now, researchers led by Wenbin Lin, a professor of chemistry at the University of North Carolina at Chapel Hill, have designed a metal-organic framework (MOF) to collect common uranium-containing ions dissolved in seawater. In lab tests, the material was at least four times better than the conventional plastic adsorbent at drawing the potential nuclear fuel from artificial seawater.
Metal-organic frameworks are considered very promising for certain technological applications, including gas storage and chemical separation. Their structure can be tuned for different purposes. This allows them to be made extremely porous, resulting in very high surface areas—an order of magnitude larger than that of zeolites, a porous material already used in many commercial adsorbents. And like organic polymers, metal-organic frameworks have surfaces that can be modified so as to bind to specific molecules.
There is about 4 billion tons of uranium in the oceans. This is about 600 times the total uranium reserves on land.
One reason it’s challenging to draw uranium-containing ions from seawater efficiently is that they occur at an extremely low concentration of three parts per billion. The established method, which has been demonstrated at a fairly large scale, entails dropping large amounts of plastic absorbent into the ocean and leaving it for several weeks before retrieving it and removing the uranium haul. But the ocean contains many other ions that can bind to the adsorbent and block uranium from attaching.
The most advanced materials, which can be reused several times, can draw between three and four milligrams of uranium per gram of plastic each time they’re used, says Costas Tsouris, a researcher at Oak Ridge National Laboratory who is working on that system.
In the lab, with no competition from other ions, Lin’s material collected over 200 milligrams of uranium per gram of adsorbent. This affinity for uranium, says Lin, is due to the precise design of the material’s three-dimensional structure.
The new material is “very promising,” because it performed better than the best available materials have done under similar conditions.
Uranium obtained using the traditional process today would cost between $1,000 and $2,000 per kilogram—about 10 to 20 times the current market price, says Schneider. (The price of uranium did rise to around $300 per kilogram as recently as 2007, however.) The new process could cut that cost significantly.
Lin thinks it may eventually be possible to develop a metal-organic framework that is at least several times better than today’s system. He is confident that his lab can exploit the “tunability” of these hybrid materials to improve their affinity for uranium-containing ions and to address weaknesses that further testing may expose.
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