Uranium from the sea?

New absorber materials can extract dissolved uranium from the seawater

The microtomograph shows the structure of the uranium catcher line © DOE / PNNL
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Cords as uranium catchers: more than four billion tonnes of uranium are dissolved in the waters of the oceans. Now, US researchers have developed a method to capture this valuable resource. It suffices to hang cords coated with a new absorber material into the water and wait until enough uranium has accumulated, as initial tests show.

Even though the nuclear phase-out is in progress in Germany, many other countries still rely on nuclear energy for their power supply. And for that they need uranium. But this raw material is finite: The deposits on land are estimated to last for around 100 years. In addition, the processing of uranium ore to uranium-235 needed in nuclear power plants is enormously complex.

Oceans as a source of uranium

But there is another resource: the sea. An estimated four billion tons of uranium are dissolved in the oceans - that would be enough to cover the energy needs of humanity for 10, 000 years. However, capturing the uranium out of the water is not easy. It was only in the 1990s that Japanese scientists succeeded in producing materials that purposely bind uranium from seawater - albeit in rather minimal quantities.

Since then, as part of a large-scale research program, US researchers have worked to make this method more effective and economically rewarding - with success. For the first time, they are presenting materials that reduce the costs of uranium extraction from seawater by three to four times - and significantly increase the yield.

Some oxidation states of uranium are water-soluble - and can also be found in seawater. © Los Alamos National Laboratory

"Lines just hanging in the sea"

And this is how it works: The new "uranium catcher" consists of long strings of polyethylene fibers containing organic amidoxime molecules. These carbon- and nitrogen-containing compounds act as a kind of attractant for the uranium dissolved in the water, because this binds preferentially with them. display

To capture the uranium, it is sufficient to simply hang these cords in the sea, preferably in areas with good mixing of the water, as the researchers explain. After a few weeks, the uranium-containing lice can be caught again. They are subjected to an acid treatment that causes uranyl ions to be released, which can be recovered from the solution and processed further. The uranium catcher cord easily survives this treatment and can be used directly in the ocean again.

Six grams in 50 days

The amount of uranium that can be extracted from these uranium fertilizers has already been tested in three locations on the US West coast, in Florida, and on the coast of Massachusetts. After 49 days in the ocean, the snails had bound at least six grams of uranium per kilogram of absorber material, as the scientists report.

"Understanding how the absorbers work under natural conditions in seawater is crucial, " says Gary Gill of the Pacific Northwest National Laboratory in Richland. For in addition to the yield of uranium, it must also be ensured that this method has no negative effects on the marine environment. "We've already found that most of these absorber materials are not toxic, " says Gill.

Five years of work

The uranium catcher barrels and their absorber materials still have to be tested further. But the researchers already consider their new development promising even if the way was not straightforward: "Synthesizing a material that effectively absorbs uranium from seawater took five years of work and a multidisciplinary approach "Our team, " says Sheng Dai of Oak Ridge National Laboratory (ORNL).

It started with computer modeling that tested which chemical groups selectively bind to uranium. This was followed by thermodynamic and kinetic studies, which determined how fast the uranium from the water binds to the absorber and where the balance of this reaction lies. Because only when more is bound than it dissolves again does the whole thing work. Only after these factors had been clarified did the production of the uranium-catcher-Schn re begin. (Industrial and Chemical Engineering Research, 2016, special edition)

(DOE / Oak Ridge National Laboratory, 25.04.2016 - NPO)