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Of humus, glass beakers and radwastes.

In tanks at several federal facilities, the Department of Energy (DOE) maintains huge stores of highly radioactive defense wastes. The 177 tanks at its Hanford Site in Richland, Wash., for example, hold between 55,000 and 1 million gallons each. Concern over leaks of radioactive wastes at Hanford--totaling some 750,000 gallons--has added urgency to DOE's waste-solidification program. Later this year, the agency plans to begin demonstrating vitrification (incorporation into glass) of sludges distilled from liquid radwastes, according to DOE spokesman Fred Lash. Full-scale waste vitrification may begin next year, he says, initially at DOE's facility in Savannah River, S.C.

DOE chose to stabilize its wastes in borosilicates, Lash explains, because of all solids, these glasses are "probably the most stable and insoluble." Borosilicates will not break apart, even if subjected to large amounts of water and heat, he says. But DOE aims to prevent such aqueous encounters by sealing vitrified wastes in stainless steel drums at very dry hideaways.

Experiments now under way at another DOE facility underscore the wisdom of taking those additional precautions: The tests indicate that natural organic hydrocarbons present in even pristine groundwater can leach radioactive materials from borosilicate glass, dissolving them in the water.

As leaves and other plant materials decay into humus, they create a mix of complex hydrocarbons known as humic acids. At Argonne (Ill.) National Laboratory, Jeffrey S. Gaffney has been studying humic acids' ability to leach radium, thorium and uranium from rocks and sand--and to transport the radionuclides long distances in water. Though ubiquitous at widely varying concentrations in soil, lakes, springs and even groundwater, humic acids "have been ignored in the models to determine whether [borosilicate] glasses will be stable" over the centuries they must bind radioactive wastes, points out Argonne chemist Nancy A. Marley. That oversight could prove important, Gaffney believes, since their work indicates that humic acids "are [chemically] active, even at fairly low concentrations." Indeed, he notes, "they literally etched the [borosilicate] glass of a new beaker in just a few hours."

Marley made that discovery a few years back when she inspected the apparent residue left by bog water boiled in the beaker. Close examination revealed that "natural humics in the water had actually removed silica form the beaker," clouding its surface, she recalls. The finding, which Marley is preparing for publication, initially surprised the two researchers: Beakers are supposed to be nonreactive, and the water in this one had a pH of only about 6, typical of normal rain.

Marley and Gaffney now suspect that carboxylate (COO-) sidegroups are a predominant constituent of the still largely uncharacterized humic acis. Carboxylates would explain the high solubility--and therefore the long lifetime in water-of the organic hydrocarbons, Gaffney asserts. That carboxylates "are fairly active binding agents for free metals" might also help explain their affinity for the radionuclides, he adds.

But could humics actually leach radionuclides from glass? Preliminary findings of tests begun at Argonne last month by Marley, Gaffney and Ken A. Orlandini indicate that the waterborne acids indeed removed uranium from borosilicate glass and held it in the water. This suggests that even if such a process were allowed to proceed very slowly in nature, "over the years it's going to be important," Marley says.
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Title Annotation:Chemistry
Publication:Science News
Date:Mar 14, 1992
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