Synthetic clay removes radium from water and soil

April 10, 2001

University Park, Pa. - An inexpensive, synthetic clay may one day help provide radium free drinking water and clean up radium-contaminated mine and mill tailings according to a Penn State researcher.

Radium, a natural decay product of uranium, is often found in the southwestern United States where large deposits of uranium are mined, but is also present in many other areas in rocks and soils. Coal and phosphate processing also produce tailings that contain radium.

"Areas like Pennsylvania, which have a known radon problem, will also have radium in their soils and perhaps in their water supplies," says Dr. Sridhar Komarneni, professor of clay mineralogy both with Penn State's College of Agricultural Sciences and Materials Research Institute. "At least 25 water systems in Wisconsin have had problems with radium in their drinking water." Federal regulations limit the amount of radium in drinking water to 5 pico Curies per liter of water. A pico Curie is a trillionth of a Curie and is a million times less than the radiation produced by the radium on a wristwatch face. Current methods to remove radium are complicated and expensive.

Komarneni, working with Naofumi Kozai, a visiting scientist from the Japan Atomic Energy Research Institute and William J. Paulus, master's degree recipient, now at General Motors Corporation, tested a variety of synthetic micas for radium removal, but found that sodium-4 mica was the best synthetic clay for this purpose. The researchers reported on this work in today's (April 12) issue of the journal Nature.

Natural mica is a mineral containing a combination of aluminum, silica, magnesium and potassium. The mineral is found in sheets and has a structure like the pages of a book. The sheets are bonded to each other to form a solid, layered mass.

Natural mica has a closed structure with all the spaces between layers filled and is not a good ion exchange media, says Komarneni. Sodium-4 mica, like natural mica, contains aluminum, silica and magnesium, but each potassium atom is replaced by two sodium ions, and fluorine is also added. The two sodium ions take up more space than the potassium ion and the layers of mica become offset, creating a space to capture water and radium.

"Sodium-4 mica has an interlayer spacing of 2.6 angstroms, too small to capture ions of hydrated sodium, calcium, magnesium or potassium," says Komarneni. "Radium, however, is less hydrated and therefore small enough to fit between the layers as are barium, copper, nickel and zinc."

These other transition metals are not usually found in great abundance in radium- contaminated water or in tailings containing radium, so they would not compete for space between the layers. When the mica is filled with radium, a shift in the layers occurs and the atoms of radium are trapped between the layered structure.

"Once the radium is trapped, it will not leave the mica," says Komarneni. "Disposal and storage requirements would then depend only on the radioactivity of the material and not whether radium could leach out of the clay. Very low level radioactive clay could simply be buried."

If the mica is only partially filled with radium at the time of disposal, then heating to above 212 degrees Fahrenheit will lock the radium in place.

Sodium-4 mica is easily synthesized by heating kaolinite -- a naturally occurring clay with an equal ratio of silicon and aluminum -- with magnesium oxide and sodium fluoride to about 1500 degrees Fahrenheit.

"Clays are already being synthesized for cosmetics, pigments and catalyst substrates," says Komarneni. "The cost of manufacture is probably around $2 per pound."

Sodium-4 mica could be used in conventional ion exchange columns to remove radium from water, but would first need to be pelletized. To immobilize radium from mine or mill tailings, simply mixing the clay with the tailings is sufficient.

The clay could also line ponds that receive radium containing tailing water to prevent migration from the pond, or clay curtains around tailings could keep the radium inside.
EDITORS: Dr. Komarneni may be reached at (814) 865-1542 or at by e-mail.

Penn State

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