Extracting Uranium from Seawater for Clean Energy

Nuclear energy is becoming a more sought-after clean energy source to phase out fossil fuels alongside renewables. However, unlike renewables, which use the natural elements to produce power, nuclear reactors require a fuel source. Despite the increased interest in nuclear energy, uranium resources are finite, and the continued development and utilisation of new reactors have consumed a lot of uranium resources. 

It's thought that terrestrial ores could be depleted in less than a century. Another option is to extract uranium from sources other than traditional in-ground ores. There is also a lot of uranium in the sea. It's estimated that there are around 4.5 billion tonnes of uranium in seawater, approximately 1000 times that of uranium resources in the ground. Extracting uranium from seawater is seen as an alternative pathway for elevating uranium resources―especially when land-based resources become scarce―but most of the current uranium extraction methods also extract a lot of impurities as well. For nuclear fuel, the uranium needs to be as pure as possible and not contaminated, so more efficient ways of extracting the uranium from seawater are sought that don't co-deposit impurities. 

Electrochemical Extraction of Uranium 

Electrochemical extraction is an extraction process that uses electricity to reduce uranium (VI) (U6+) to uranium (IV) (U4+). It has gathered a lot of attention because it provides an enhanced extraction capacity due to the final uranium products crystalising. This process also has favourable kinetics that are driven by the electric field and a higher resistance for pushing out unwanted non-reductive ions during the extraction process. During the electrochemical extraction process, the extraction efficiency of the uranium relies a lot on the electrode materials, so a lot of the design and development of these systems has centred around the electrodes. 

Different electrode design approaches have been trialled. One of the latest approaches is to create coordination sites for uranyl ions (UO22+) to bind to, which then improves the extraction capability of uranium from the seawater. These coordination sites have taken many forms, from materials that feature a lot of M-O-H bonds to PO43- ion pair sites that selectively bind to uranyl ions and form strong O-U-O bonds before crystallising the uranium into another compound to amidoxime groups and iron nitride active sites that will deposit uranyl ions in the form of sodium-uranyl complexes. 

However, in most of these designs, the uranium products that are created during the extraction process contain alkali metals or the uranium products are deposited in an unstable state (due to a mixture of valence states―the charged state of an ion based on how many electrons are in the outer electron sub-shell―of the uranium ions in the complex). 

Many approaches to date have focused on creating coordination sites for the uranyl ions on the electrode surface with little consideration for the transport and connection between the electrode itself and the uranyl binding sites. This has often led to weak reduction effects and, in some cases, a reoxidation of the captured uranium materials. Reoxidation can also cause further crystallisation with alkali metals, causing a crystal transformation. New approaches need to ensure that the uranium remains stable once captured and reduced to improve the purity of the uranium products being extracted from the seawater. 

New Chemical Extraction Process from Seawater 

Researchers have now created a new synergistic and highly connected coordination-reduction interface that can extract solid black UO2 products from seawater with low levels of impurities through the reduction of uranium (VI) to uranium (IV). The extraction medium was composed of a Ni3S2 fibre with a polyoxometalate CoMo6-derived amorphous CoMoOS layer. The interface has been coined CMOS@NSF by the researchers. 

The CoMoOS layer is chemically similar to its predecessor but with a similar structure. Other than the internal oxygen atoms, which have been replaced with sulphur atoms, the terminal oxygens at the edges of the molecular structure remain exposed. The internal sulphur atoms are responsible for tailoring the electronic distribution in the material, which leads to an accumulation of electrons at the terminal oxygen sites. This makes the terminal oxygen sites strong binding sites for uranyl ions.  

This approach also yielded a strong interfacial connection between the Ni3S2 fibre and CoMoOS layer that improved electron transport and improved the reduction capabilities of the filter. The nickel in the fibre chemically bonded with the oxygen in the uranyl ions (once the uranium had attached to the terminal oxygen groups) to promote the reduction of uranium and accelerate the transfer of electrons, as well as preserve the uranium (IV) compounds and prevent them from reacting again with alkali metals in the seawater. Therefore, the nickel acts as a direct electron transport channel once the uranium has bound the extraction medium. 

From natural seawater, the CMOS@NSF electrochemical extraction process exhibited an electrochemical extraction capacity of 2.65 mg g-1 d-1 for solid UO2 products. This amount was extracted from 25 L of seawater over a period of 24 hours. This equates to an extraction efficiency of 86.67% over the 24-hour period. There will still be a lot of work to do to make these kinds of extraction devices scalable and capable of extracting enough uranium (volume-wise) that it's worth deploying them as an alternative to land-based extraction methods. However, this approach does pave the way for extracting solid UO2 products without the usual impurity contamination and without the worry of the extracted products re-oxidising again after capture. 

Reference: 

Zhu W. et al., A synergistic coordination-reduction interface for electrochemical reductive extraction of uranium with low impurities from seawater, Nature Communications, 16, (2025), 2012.