Team develops an electrochemical method for extracting uranium from solution

Fifty years ago, scientists hit upon what they thought could be the next rocket fuel.

Carboranes – molecules composed of boron, carbon and hydrogen atoms clustered together in three-dimensional shapes – were seen as the possible basis for next-generation propellants due to their ability to release massive amounts of energy when burned.

It was technology that at the time had the potential to augment or even surpass traditional hydrocarbon rocket fuel, and was the subject of heavy investment in the 1950s and 60s.

But things didn’t pan out as expected.

“It turns out that when you burn these things you actually form a lot of sediment,” said Gabriel Ménard, an assistant professor in UC Santa Barbara’s Department of Chemistry and Biochemistry.

In addition to other problems found when burning this so-called “zip fuel,” its residue also gummed up the works in rocket engines, and so the project was scrapped.

“So they made these huge stockpiles of these compounds, but they actually never used them,” Ménard said.

Fast forward to today, and these compounds have come back into vogue with a wide range of applications, from medicine to nanoscale engineering.

For Ménard and fellow UCSB chemistry professor Trevor Hayton, as well as Tel Aviv University chemistry professor Roman Dobrovetsky, carboranes could hold the key to more efficient uranium ion extraction.

And that, in turn, could enable things like better nuclear waste reprocessing and uranium (and other metal) recovery from seawater.

Their research – the first example of applying electrochemical carborane processes to uranium extraction – is published in a paper (link) that appears in the journal Nature.

Key to this technology is the versatility of the cluster molecule. Depending on their compositions these structures can resemble closed cages, or more open nests, due to control of the compound’s redox activity – its readiness to donate or gain electrons.

This allows for the controlled capture and release of metal ions, which in this study was applied to uranium ions.

“The big advancement here is this ‘catch and release’ strategy where you can switch between two states, where one state binds the metal and another state releases the metal,” Hayton said.

Conventional processes, such as the popular PUREX process that extracts plutonium and uranium, rely heavily on solvents, extractants and extensive processing.

“Basically, you could say it’s wasteful,” Ménard said. “In our case, we can do this electrochemically – we can capture and release the uranium with the flip of a switch.

“What actually happens,” added Ménard, “is that the cage opens up.” Specifically, the formerly closed ortho-carborane becomes an opened nido- (“nest”) carborane capable of capturing the positively-charged uranium ion.

Conventionally, the controlled release of extracted uranium ions, however, is not as straightforward and can be somewhat messy. According to the researchers, such methods are “less established and can be difficult, expensive and or destructive to the initial material.”

But here, the researchers have devised a way to reliably and efficiently flip back and forth between open and closed carboranes, using electricity.

By applying an electrical potential using an electrode dipped in the organic portion of a biphasic system, the carboranes can receive and donate the electrons needed to open and close and capture and release uranium, respectively.

“Basically you can open it up, capture uranium, close it back up and then release uranium,” Ménard said. The molecules can be used multiple times, he added.

This technology could be used for several applications that require the extraction of uranium and by extension, other metal ions.

One area is nuclear reprocessing, in which uranium and other radioactive “trans-uranium” elements are extracted from spent nuclear material for storage and reuse (the PUREX process).

“The problem is that these trans-uranium elements are very radioactive and we need to be able to store these for a very long time because they’re basically very dangerous,” Ménard said. This electrochemical method could allow for the separation of uranium from plutonium, similar to the PUREX process, he explained.

The extracted uranium could then be enriched and put back into the reactor; the other high-level waste can be transmuted to reduce their radioactivity.

Additionally, the electrochemical process could also be applied to uranium extraction from seawater, which would ease pressure on the terrestrial mines where all uranium is currently sourced.

“There’s about a thousand times more dissolved uranium in the oceans than there are in all the land mines,” Ménard said.

Similarly, lithium – another valuable metal that exists in large reserves in seawater – could be extracted this way, and the researchers plan to take this research direction in the near future.

“This gives us another tool in the toolbox for manipulating metal ions and processing nuclear waste or doing metal capture out of oceans,” Hayton said. “It’s a new strategy and new method to achieve these types of transformations.”

Research in this study was conducted also by Megan Keener (lead author), Camden Hunt and Timothy G. Carroll at UCSB; and by Vladimir Kampel at Tel Aviv University.

Uranium is a desirable ingredient with significant implications in many technical applications, for instance, the nuclear energy industry, but the uranium reserve on land (4.85 million tons in total) will be exhausted in the next few decades when taking into consideration of current global consumption.¹ To address this issue, the extraction of uranyl (UO₂²⁺) ions from seawater (4.5 billion tons) has attracted enormous attention in past years.^2,3 A number of adsorbents with reasonable adsorption capability such as porous carbon, mesoporous silica, metal–organic frameworks, and ionic liquids have been developed to cope with the steady increase in uranium demand worldwide.⁴⁻¹¹ However, the uranyl ion coexists with a number of transition metal ions including Co²⁺, Ni²⁺, Fe³⁺, and VO₃^–, among others, commonly referred to as interfering metal ions, in seawater at an exceedingly low concentration, which seriously suppresses the extraction capacity and selectivity of currently available adsorbents toward uranyl ions for practical applications.³ It is a daunting challenge to effectively and selectively capture uranium from seawater.

Porous organic materials have been recently recognized as an emerging class of functional platforms, which enable versatile inclusion of functionalized groups thanks to their modular structure, rich stability, and reticular variety.¹²⁻²⁰ This flexibility of tuning the chemical compositions and spatial structures promotes a myriad of applications, for instance, gas separation and storage, optoelectronics, and catalysis.²¹⁻²⁹ Dai et al. and Ma et al. pioneered porous materials with amidoxime groups, which exhibited excellent capability for uranium extraction.³⁰⁻³²However, traditional adsorbents (including oxime-bore products) with flexible and randomly distributed groups lack the selectivity for binding uranyl ions in the presence of interfering ions.³³

To overcome this limitation, two objectives provide such improvement to satisfy the thought-provoking features of capacity and selectivity: First, an adsorption site with the customized size, shape, and composition is required to create both specific and directional interaction with the uranyl ion. Second, a large contact surface in a robust architecture will provide a huge number of accessible spaces for a reciprocity interaction between adsorption sites and target species. These basic necessities rely on the shape persistent structure and suitable directional anchor to enforce specific bonding geometries for uranyl ions (Figure​Figure11). With that in mind, a molecular coordination template strategy is adopted to prepare the selective sites with assembled and arrayed salicylaldoxime groups. Using the porous aromatic framework (PAF-1) as a scaffold, its pore surface was encoded with the salicylaldoxime-based units into dynamic adsorption sites. As a result of the uranyl-specific interaction of bis-salicylaldoxime entity, the resulting porous material (MISS-PAF-1) exhibited excellent adsorption capacity, outstanding selectivity, and good reusability for extraction of UO₂²⁺ ions from real seawater.

Using a molecular coordination template strategy, salicylaldoxime groups were encoded on the porous skeleton of PAF-1 to produce the uranyl-specific bis-salicylaldoxime adsorption sites. Consequently, one resultant (MISS-PAF-1) exhibits excellent capture capacity and ultrahigh selectivity toward UO₂²⁺ ions over interfering species. The selective mechanism is established with the aid of single-crystal X-ray diffraction and XAFS spectroscopy, unraveling that the assembly of salicylaldoxime fragments creates the special coordination environment and settled bond lengths for the uranyl ion. Our work highlights the sharp increase in ion adsorption capacity with high selectivity and good reusability. These results demonstrate that the novel PAF material has proven to be a promising candidate for effective capture of UO₂²⁺ ions from seawater. This therefore resolves the longstanding challenge in the field of uranium extraction.

More information: Megan Keener et al, Redox-switchable carboranes for uranium capture and release, Nature (2020). DOI: 10.1038/s41586-019-1926-4