Organism's ability to distinguish strontium from calcium could help in dealing with nuclear waste.
Common freshwater algae might hold a key to cleaning up after disasters such as Japan's Fukushima nuclear accident, scientists said yesterday at a meeting of the American Chemical Society in Anaheim, California.
The algae, called Closterium moniliferum, are members of the desmid order, known to microbiologists for their distinctive shapes, said Minna Krejci, a materials scientist at Northwestern University in Evanston, Illinois. But the crescent-shaped C. moniliferum caught Krejci's eye because of its unusual ability to remove strontium from water, depositing it in crystals that form in subcellular structures known as vacuoles — an knack that could include the radioactive isotope strontium-90.
Strontium is very similar in properties and atomic size to calcium, so biological processes can't easily separate the two elements. That makes strontium-90 a particularly dangerous isotope: it can infiltrate milk, bones, bone marrow, blood and other tissues, where the radiation that it emits can eventually cause cancer.
"That's what makes strontium-90 one of the dominant health risks of spent fuel for the first 100 years or so after it leaves the reactor," says Krejci. The radioisotope has a half-life of about 30 years.
Unfortunately, reactor waste and accidental spills can contain up to ten billion times more calcium than strontium, making it very difficult to clean up the strontium without also having to dispose of a mountain of harmless calcium. "We need a highly efficient and selective method of separating it," says Krejci.
Enter C. moniliferum. The organism has no particular interest in strontium: it mostly collects barium. But strontium is midway between calcium and barium in size and properties, so any of it that happens to be around gets crystallized as well. Meanwhile, even though calcium is far more abundant than either of the other two elements, it is different enough to barium that it gets left behind.
The result is a crystal that is mainly composed of barium, but is massively enriched in strontium.
How do they do that?
Much of Krejci's research so far1 has focused on trying to work out how the algae generate the crystals, with an eye to making the process even more strontium-selective. For the moment, she knows that the organism isn't purposefully bringing excess barium and strontium through its cell walls. Rather, she says, the crystals appear to form because the vacuoles in which they collect are rich in sulphate. Barium and strontium have relatively low solubility in sulphate solutions, so any barium and strontium that make their way into these vacuoles easily precipitate out to form crystals.
Microbiologists don't know whether the crystals have any function for the organism. Perhaps they are simply waste, forming by accident in vacuoles that serve as storage depots for sulphate, said Krejci.
Whatever purpose the crystals serve, Krejci's research has found that it is possible to enhance the uptake of strontium by tailoring the amount of barium in the algae's environment. This, she says, means that it might prove possible to seed nuclear waste, or a spill of radioactive material, with barium to encourage the algae to grab the strontium — easy to do, she says, because "it would only be a small amount" of barium.
It might also be possible to improve the process by tinkering with sulphate levels in the environment, thereby changing the amount of sulphate in the vacuoles. "Once we learn about how the cells respond to conditions, we can think of more elegant ways to manipulate them," says Krejci.
Once isolated by the algae, the strontium could be sequestered in high-level nuclear waste repositories, while the rest of the waste could go to a less expensive lower-level repository, saving space and money. Currently, Krejci says, there are hundreds of millions of litres of stored nuclear waste in the United States alone, much of which contains strontium. "So we know it's a big problem," she says.
Krejci and her colleagues have not yet tested how well the algae survive in the presence of radioactivity. But even if the organisms respond poorly, she says, they would probably live long enough to start removing strontium, because the process begins quickly. "The cells precipitate crystals within 30 minutes to an hour," she says. And if more are needed, "they are easy to culture".
Gija Geme, a chemist at the University of Central Missouri in Warrensburg, organized the symposium at which Krejci presented her work. Geme, who grew up in an area of Russia not far from Chernobyl and so has a personal interest in nuclear clean-up, was one of the few people at the meeting who knew the significance of Krejci's presentation in advance: the talk's title, focusing on biomineralization, did not mention Japan, radioactivity or nuclear accidents.
"It's a hot topic right now," says Geme. "But when I put this symposium together, there was no tragedy [in Japan]. I was looking for any studies about sequestration of metals that would be of significance to society."
Geme urges Krejci's team not to spend too much time trying to discover precisely why the algae does what it does before they start testing the process with nuclear wastes.
"Sometimes, just getting it out is very, very important," she says. "I would like to see field studies using actual waste as soon as possible."
Krejci, M. R., Finney, L., Vogt, S . & Joester, D . ChemSusChem doi:10.1002/cssc.201000448 (2011).
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Lovett, R. Algae holds promise for nuclear clean-up. Nature (2011). https://doi.org/10.1038/news.2011.195