The ocean holds more than 4 billion metric tons (t) of uranium, enough to provide nuclear power to the world for many decades. A new, easy-to-make membrane can extract that uranium with record efficiency, soaking up twice as much of the element by weight as the best materials reported so far (Chem 2022, DOI: 10.1016/j.chempr.2022.07.009).
Nuclear reactors around the world consume about 63,000 t of uranium oxide each year, according to the World Nuclear Association. Land-based uranium reserves are expected to last the next 35 years as nuclear power capacity increases.
In seawater, uranium is present at an ultralow concentration of 3.3 ppb. Amidoximes are the most commonly studied adsorbents for soaking up uranium, but they are unstable or can be used only once or are difficult to make. Researchers have also looked at metal-organic frameworks, proteins, and other materials.
Ketan Patel and Shilpi Kushwaha of the Central Salt and Marine Chemicals Research Institute and their colleagues used a family of porous crystalline materials called hydrogen-bonded organic frameworks (HOFs). HOFs consist of organic building blocks connected by hydrogen bonds. The materials self-assemble from chemical solutions and are easy and cheap to make on a large scale.
The researchers made HOF films that were 40–500 nm thick and composed of pyridine and phenoxy groups connected by imine links. The material contains a network of pores and channels and shows an affinity for binding uranyl ions, the form of uranium found in the ocean. The high surface area and high density of binding sites in the HOF make it very efficient at picking up uranium, Kushwaha says.
Laboratory tests with seawater showed that 1 g of the material could extract 17.8 mg of uranium in 30 days. By recovering the uranium, the researchers could reuse the film five times, although it became less efficient after the first cycle.
The best amidoxime- and protein-based adsorbents made previously soak up about 17 mg/g in 30 days, comparable to the new material. But in addition to being reusable, the HOF is easier to make and stabler under different chemical conditions.
As the properties of such membranes improve, uranium recovery from seawater becomes more economically favorable than mining uranium from land, says Costas Tsouris, who is a chemical engineer at Oak Ridge National Laboratory and was not involved in the work. Real-world tests will reveal whether the film can withstand being fouled by bacteria and other contaminants in seawater, and challenges like mass production and regeneration need to be resolved, Tsouris says. But, he adds, “an efficient film like this may lead to commercial recovery of uranium from seawater in the near future.”