Alga sponge cleans up nuclear waste

NORTHWESTERN (US) — A common freshwater alga acts like a sponge, soaking up one of the more dangerous radioactive materials created within a nuclear reactor.

Strontium 90 is present in about 80 million gallons of nuclear waste in the U.S.

Closterium moniliferum, one of the bright green algae often seen in ponds, is able to sequester the material and researchers believe it may be possible to use the algae for direct bioremediation of waste or accidental spills in the environment or designing a new process for waste treatment inspired by how the algae work.

Details of the research are published in the journal ChemSusChem.

“Nuclear waste cleanup is a problem we have to solve,” says Derk Joester, assistant professor of materials and manufacturing at Northwestern University, who experienced Chernobyl’s radioactive fallout when he was a teenager living in southern Germany.

“Even if all the nuclear reactors were to shut down tomorrow, the existing volume of waste is great, and it is costly to store. We need to isolate highly radioactive high-level waste from low-level waste. The algae offer a mechanism for doing this, which we would like to understand and optimize.”

Even though strontium 90 doesn’t appear to be a significant environmental threat following the nuclear accident in Japan, the radioactive isotope will need to be dealt with during the power plant and nuclear waste cleanup, Joester says.

Strontium 90 has a half-life of about 30 years and, because it is chemically similar to calcium, is drawn to bone. The cumulative cancer risk from strontium 90 exposure when strontium is bound in bones for many years is extremely high.

The crescent-shaped, single-celled organism naturally makes biominerals that include non-radioactive strontium, and can differentiate strontium from calcium—a rare feat.

Researchers want to learn more about this selectivity because calcium is present in far greater abundance than strontium in nuclear waste, but is harmless. By concentrating the radioactive strontium (Sr-90) in the form of solid crystals with very low solubility, the dangerous high-level waste could be isolated from the rest and dealt with separately.

“Using the algae for direct bioremediation of waste is one approach,” says Joester, “but we also are looking at the basic mechanisms of how the algae sequester strontium so we can engineer a more selective process for waste treatment. We want to isolate and concentrate in the crystals the most strontium possible.”

The algae’s ability to separate strontium from calcium occurs when the crystals are formed inside the cells. The algae first soak up barium, strontium, and calcium from their watery environment.

Strontium then is sequestered along with barium in the crystals, which remain in the cells, while the calcium is excreted from the cells. (Barium must be present for the organisms to take up strontium.)

Joester and graduate student Minna Krejci teamed up with Lydia Finney and Stefan Vogt at the Advanced Photon Source at Argonne National Laboratory to produce maps showing the distribution of barium, strontium, calcium, and several other elements in the cells. At the same time, the composition of the crystals made by the cells was determined. (The crystals are located in the organism’s vacuoles, at the tips of the cells.)

The researchers varied the amount of barium and strontium in the algae’s environment, measured the amount of strontium taken up into the cell and then found the ratio of barium to strontium in the water affected the amount of strontium incorporated into each crystal.

Depending on the medium’s composition, the strontium measured in a crystal ranged from less than 1 percent up to 45 percent, giving the researchers an avenue for making the process more strontium-selective.

“The synchrotron X-ray microscopy available at the Advanced Photon Source was absolutely critical to this study,” Joester says. “It allowed us to visualize where calcium, strontium and barium go inside the cells.”

Nonradioactive strontium, which is chemically identical to the radioactive version, was used in the experiments. The researchers do not know how well the algae would survive in a radioactive environment, although the organisms have proven resistant in other harsh environments.

The U.S. Department of Energy and the Initiative for Sustainability and Energy at Northwestern supported the research.

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