The world’s largest reservoir of uranium is seawater: there’s about 4.5 billion tonnes dissolved in the oceans. But it’s in vanishingly low concentrations, and all of the other more common things in seawater – like sodium, chloride and magnesium – make uranium very hard to extract.
But not impossible, according to a new study in Energy Advances. An Australian-based team of researchers has made a powder that can efficiently extract uranium from seawater.
“It’s a material that’s called a layered double hydroxide,” explains senior author Dr Jessica Veliscek Carolan, a nuclear chemist at the Australian Nuclear Science and Technology Organisation (ANSTO).
“The one we made has got magnesium hydroxide and aluminium hydroxide in it. As the name suggests, it forms a layer structure.”
In between the layers of the two hydroxide compounds are “anions”: negatively-charged molecules.
Once it’s in seawater, uranium – or more specifically, a compound that contains uranium called uranyl carbonate – displaces these anions and sits between the layers instead. It also binds to the surfaces of the hydroxide layers.
“So there’s two different mechanisms going on, by which the uranium is extracted,” says Veliscek Carolan.
They tested the powder in “seawater-like” conditions – that is, water they’d mixed up to have a similar chemical composition to seawater, and real seawater taken from Coogee Beach in Sydney, spiked with a small amount of extra uranium to make it measurable.
The researchers’ powder stuck only to uranium and not to other substances.
“That was what was really exciting,” says Veliscek Carolan.
“It showed some really nice selectivity, where it particularly pulled out uranium and left all those other salts behind – things like sodium and calcium, that are present in much higher concentrations than uranium.
“Part of that selectivity is coming from the fact that the uranyl carbonate in seawater is anionic or negatively charged, whereas a lot of other metal species like sodium and calcium are cationic, or positively charged.”
But the real clincher was the addition of another element to the substance: neodymium. The researchers tested a few different metals, and found that doping their substance with neodymium made it best at extracting uranium.
“It made that those metal hydroxide layers more ionic in character, which meant that it showed greater selectivity for the uranium,” says Veliscek Carolan.
So, when can we expect uranium extraction to start in the ocean? Not immediately.
“There are a couple of challenges that would still need to be overcome,” says Veliscek Carolan.
The biggest is the powdery form of the substance.
“You can’t really just throw a powder into the ocean and then somehow collect it back. So we would need to be constructing some kind of physical form that allows it to be deployed in an industrial setting.”
The extraction is also slow, and neodymium is a fairly expensive addition.
“It isn’t necessarily a showstopper if it’s slow, but it would be nice if we could make it faster,” says Veliscek Carolan.
“It would also be interesting to see whether we could find a different element in the periodic table that could do a similar thing to the neodymium.” But given that most of the materials are low-cost, the researchers are optimistic that this substance, or one like it, could be used to extract uranium at scale.
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The Ultramarine project – focussing on research and innovation in our marine environments – is supported by Minderoo Foundation's Flourishing Oceans initiative.