Science & Environment

Uranium from seawater idea boosted with shrimp shells

Shrimp farming, Madagascar

A happy coincidence in the seafood industry has raised the prospects of harvesting uranium - the fuel source for nuclear power - from seawater.

Oceans hold billions of tonnes of uranium at tiny concentrations, but extracting it remains uneconomical.

A report at the 244th meeting of the American Chemical Society described a new technique using uranium-absorbing mats made from discarded shrimp shells.

A range of improved approaches were outlined at a symposium at the meeting.

The developments are key to a future nuclear power industry. Uranium is currently mined from ore deposits around the world, but there are fears that demand may outstrip the supply of ore as nuclear power becomes more widespread.

At issue is the tremendously low concentration of uranium in seawater: about three parts per billion, so that just 3.3mg exist in a full tonne of water. As a result, extracting it is an inherently costly process.

Much work carried out in Japan in recent decades has sought to address that.

Researchers there came up with 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 They are withdrawn and rinsed with an acid solution that frees the uranium, and the cycle is repeated.

Image caption Billions of tonnes of the uranium required for fuel pellets is found in the world's oceans

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.

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