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August 23, 2012

Uranium from seawater idea boosted with shrimp shells

BBC News - A report at the 244th meeting of the American Chemical Society described a new technique using uranium-absorbing mats made from discarded shrimp shells. The oceans hold billions of tonnes of uranium at tiny concentrations, but extracting it remains uneconomical.

Although these trials proved the principle of uranium extraction from seawater, the cost was prohibitively high - perhaps around $260 per pound. This compares badly to today's most economic mines on land, which produce uranium at around $20 per pound, while resources at higher costs up to about $115 per pound have already been identified that would last more than a century. The ACS summarised the session saying that the new techniques might reduce the cost of uranium from seawater to around $135 per pound.

Previously Japanese researchers had a design of a mat of plastic fibres impregnated with molecules that both lock onto the fibres and preferentially absorb uranium. That work culminated in a 2003 field test that netted a kilogram of the metal.

The mats can reach 100m in length, suspended underwater at depths up to 200m. They are withdrawn and rinsed with an acid solution that frees the uranium, and the cycle is repeated.

New research has focussed on improving both the braided fibres of the mat and the "ligand" that captures the uranium, which has most often been a molecule called poly-acrylamidoxime.

Several groups at the conference said they had been working on variations on this molecular theme, or variants of porous "nanoparticles" made of silica (the stuff of sand) or carbon.

Robin Rogers of the University of Alabama, who organised the symposium, outlined an improvement developed in his own group: seafood shells.

He said that in the wake of both Hurricane Katrina and the BP oil spill in the region, "we began working with the Gulf Coast Agricultural and Seafood Co-operative... and with the shrimpers and crabbers there, and found they were paying hundreds of thousands of dollars to get rid of their waste [shells]".

"We discovered an 'ionic liquid' - a molten salt - could extract a very important polymer called chitin directly from shrimp shells," he added.



Chitin is a long-chain molecule that is the principal component in crustaceans' shells, but its toughness and its ability to be "electrospun" into fibres that can be made into mats make it an ideal sustainable and biodegradable choice for uranium harvesting.

While research is continuing, there is still some way to go to reach cost parity with the more mature - but more environmentally damaging - technology of mining uranium ores.

"We have not reached a point where we can 'downselect' to a [single] technology, but we have shown that we can double the capacity of what the Japanese have done," Dr Rogers said.

"But the economic analysis being done at the University of Texas has told us that we're not good enough yet, even in today's economy, so we have to improve."

The work is promising enough, though, to begin to remove a concern about the sustainability of those terrestrial sources and any stumbling block that may present to growth in the nuclear power industry.

Eurekalert - Erich Schneider, Ph.D., from the University of Texas, discussed an economic analysis done for the U.S. Department of Energy (DOE) comparing seawater extraction of uranium to traditional ore mining. It shows that DOE-funded technology now can extract about twice as much uranium from seawater as the first approaches, developed in Japan in the late 1990s.

That improvement reduces production costs down to around $300 per pound of uranium, from a cost of $560 per pound using the Japanese technology. However, extraction from seawater remains about five times more expensive than uranium mined from the ground.

Schneider explained, however, that the current goal is not to make seawater extraction as economical as terrestrial mining. Instead, scientists are trying to establish uranium from the ocean can act as a sort of "economic backstop" that will ensure there will be enough uranium to sustain nuclear power through the 21st century and beyond

uclear power plants, he noted, are built to operate for 60 years or longer and involve a huge investment. In 2008, for instance, one energy company in Florida estimated it would cost more than $14 billion to build a new two-reactor plant. Before making that kind of outlay, energy companies want assurance that reasonably priced uranium fuel will be available on a century-long time frame.

"This uncertainty around whether there's enough terrestrial uranium is impacting the decision-making in the industry, because it's hard to make long-term research and development or deployment decisions in the face of big uncertainties about the resource," said Schneider. "So if we can tap into uranium from seawater, we can remove that uncertainty."

Another advantage of seawater extraction could be avoiding some of the environmental costs of extracting uranium ore. Like other kinds of mining, recovering uranium can produce contaminated wastewater, impact the environment and have health consequences for miners.

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