The International Atomic Energy Agency expects nuclear power to grow significantly in the coming decades, by up to 82 percent by the year 2050. That would create an increase in demand for uranium that reserves on land may not be able to meet.
But the world’s oceans, according to the U.S. Department of Energy, contain at least 500 times more uranium than in all known terrestrial reserves. That equates to more than 4.5 billion metric tons of the element in seawater, albeit present at an extremely dilute concentration of 3.3 parts per billion, and scientists have been trying to find efficient ways to extract it.
Researchers recently reported one of the best-performing materials to do this. The porous membrane soaks up 20 times more uranium from seawater than membranes made so far. When the researchers pumped natural seawater across it, the membrane captured over 9 milligrams of uranium per gram of the material in four weeks, more than other uranium-extracting materials reported previously have been able to collect in double that time period.
The membrane’s high uranium-capturing capacity is appealing and indeed surpasses its predecessors, says Costas Tsouris, a chemical engineer at Oak Ridge National Laboratory. What’s also really important, especially for the marine setting, is the membrane’s kinetics. The high speed with which it captures uranium would require a shorter immersion time in seawater, avoiding the growth of microbial films that foul the material, he says. That should make it easier to separate the extracted uranium from the membrane and to reuse the membrane.
The researchers drew inspiration from the porous structure of blood vessels—leading them toward a hierarchical membrane with pores of multiple sizes.
Tsouris and his colleagues worked on seawater uranium extraction for six years starting in 2011. They used an adsorbent—a material that, unlike an absorbent with a ‘b,’ only uses its surface to attract substances—based on a chemical group called amidoxime that Japanese researchers pioneered in the 1980s. Amidoxime shows a preference for binding chemically with uranium ions that occur naturally in seawater over other competing ions that are also present such as iron, copper, and calcium.
The ORNL scientists coated the adsorbent on plastic fibers, braided those fibers, and placed the braids in seawater. Eight weeks later, the fibers had picked up nearly 8 mg/g of uranium.
In 2018, researchers from Pacific Northwest National Laboratory and LCW Supercritical Technologies reported a similar amidoxime-coated fiber that extracted about 5 mg/g of uranium when seawater was pumped through it in the lab.
Some teams have recently put amidoxime on host materials with high surface areas, such as nanostructured materials or porous membrane, which makes more sites available for adsorption and increases uranium uptake. This is the approach that Liping Wen and colleagues from the Chinese Academy of Sciences took. Their research was recently published in the journal Nature Sustainability.
Inspired by the hierarchical porous structure found in blood vessels and other organs, the researchers made a membrane with pores of multiple sizes. The pores are coated with amidoxime. When seawater flows across the membrane, uranium and other molecules first quickly pass through the larger 20 micrometer-wide pores from where they go into narrower channels and into tinier 300–500 nm pores where the slower speed lets uranium bind with amidoxime. This multi-scale design lets water flow through quickly and maximizes surface area for adsorption.
In the laboratory, the researchers pumped a solution with a uranium concentration of 32 ppm across their new membrane and others with uniform pores. The new membrane extracted 20 times more uranium.
Next, they placed a 10-mg membrane between two pieces of sponge in a tube, and continuously pumped natural seawater through the tube. In one week, the membrane accumulated 6.63 mg/g or uranium, reaching 9.03 mg/g after four weeks. The researchers were able to recover the uranium and reuse the membrane five times without any loss in its extraction capacity.
The membrane’s test will of course be in a real-world marine setting, Tsouris says. That will show how stable the membrane is and how much it resists microbial fouling. Plus, it would have to be simply suspended. Pumping seawater across membranes does not make practical sense. “To recover relevant amounts of uranium from seawater we need to process a lot of water,” he says. “If we start pumping that amount, the energy we spend on the process would be more than the energy we get from uranium.”
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