TechnologyMSolutions A simple way to remediate strontium?
Benjamin. In contrast, materials such as silicotitanates can scavenge strontium from tank waste, but once the strontium is adsorbed, the material For more than four decades, the can no longer be used and has to be United States produced plutonium disposed of as radioactive waste, says for national defense purposes, leaving Korshin. By removing the strontium behind millions of gallons of nuclear from the IOCS, the high-level waste waste. Approximately 53 million galvolume is dramatically reduced, he lons, or 60%, of that waste is now says. stored in underground tanks at the In laboratory experiments, the reDepartment of Energy’s (DOE’s) searchers were able to run a few hunHanford facility in Washington state. dred bed volumes of simulated Because most of the radioactivity in Hanford waste through IOCS-filled the waste is generated by the decay of columns before having to regenerate cesium-137 and strontium-90 them. Because the IOCS re(90Sr), many remediation efmains essentially unchanged forts have focused on finding even after several regeneranew technologies for retion cycles, treatment of at moving these two raleast 20,000 bed volumes of dionuclides. tank waste may be possible Although a variety of using the approach, they say. more complicated and One bed volume is equivacostly adsorbent materilent to the amount of soluals have been developed, tion that would completely researchers at the fill a treatment column, if it University of Washington, contained no packing mateSeattle, believe that somerial. thing as simple and easy to The simulated waste conobtain as iron oxide-coated tained high levels of hydroxsand (IOCS) may provide an ide (1.2 M), sodium (5.5 M), inexpensive way of removing and nitrate (3.7 M), as well 90Sr from radioactive waste in as trace amounts of competunderground storage tanks. ing ions like Ca2+, Al3+, and Some scientists, however, are Cr3+, and complexing agents skeptical that the material like carbonate, sulfate, and will work for remediating ethylenediaminetetraacetic tank wastes like those at acid (EDTA). Under such Hanford, which contain high conditions, IOCS is very selevels of organic complexing lective for strontium, says agents. Benjamin. The composition of Scientists familiar with Hanford’s radioactive tank Hanford tank wastes, howevwaste varies, but it is “horer, say that the waste simurendously alkaline, salty, and lated by the University of complex material,” says Mark Washington researchers did Benjamin, who along with not contain high enough levGregory Korshin and colels of organic complexing A colum n packed w ith iron oxide-coated sand,such asthe one leagues at the University of agents. “They attempt to pictured here,haspotentialforrem oving strontium -90from Washington, Seattle, recently simulate a single-shell tank nuclearw aste tanks.(Inset)Quartzsand,w hich isshow n (left) reported that IOCS-filled (SST) waste. The concentrabefore and (right)afterbeing coated w ith iron oxide,isattractive tion of 90Sr in the Hanford columns can selectively ref orrem ediation purposesbecause itisinexpensive and abundant. SST liquid waste is not of move strontium from simuThe pennyisshow n forsize com parison. lated Hanford tank waste concern,” says Gregg MARK BENJAMIN, UNIVERSITY OF WASHINGTON
(Environ. Sci. Technol. 2001, 35, 4905–4909). Most other adsorbents do not hold up under such harsh conditions, but IOCS does, he says. Another key advantage of IOCS is that it can be regenerated and used over and over again, says Korshin. Once the IOCS-filled column gets saturated with strontium, it can be rinsed with a dilute acid and the strontium will be stripped off. “You end up with a mildly acidic solution containing strontium and very little else, which makes it pretty easy to treat further or dispose of,” explains
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Lumetta, lead scientist of the Separations and Radiochemistry Team at DOE’s Pacific Northwest National Laboratory. The issue of high 90Sr levels in liquid tank waste lies exclusively with certain double-shell tanks (DSTs), which have high concentrations of organic complexants, he says. Although the simulated waste contained EDTA concentrations of up to 2 × 10−4 M, complexed DST wastes contain EDTA and other complexing agents at levels 2 or 3 orders of magnitude higher, he points out. Even with low EDTA levels the researchers saw adverse effects on strontium removal due to the formation of soluble Sr-EDTA complexes, which did not adsorb to the IOCS. To overcome the interference caused by EDTA, they simply added Ca2+, which
preferentially binds with EDTA, displacing any Sr2+ from the EDTA. Such an approach may not be feasible, however, in a real-world situation. “For actual Hanford wastes, this would require a significant addition of calcium, increasing the low-activity waste volume,” says Lumetta. Some scientists question why something as abundant as iron oxide, which is found in soils, tank sludge, and corrosion products on tank walls, hasn’t already adsorbed all the strontium of concern. “In our soils, which are quite often coated with iron hydroxides, hematite, and other iron oxides, the strontium is relatively mobile. It ends up in our groundwater in measurable quantities,” says Miles Denham, a geochemist at DOE’s Savannah River Site, which
also houses large amounts of nuclear waste in underground tanks. “Studies have shown that strontium adsorbs quite easily on surfaces of naturally occurring iron oxides or other minerals,” says Korshin. But under acidic conditions, strontium tends to be quite mobile. “The site where we have the highest concentrations of 90Sr in the groundwater is an acidic site,” says Denham. One might expect strontium to adsorb to iron-oxide coatings on tank walls or in tank sludge under alkaline conditions, but the presence of complexing agents complicates the situation. Once the strontium is complexed, it remains in solution, says Lumetta. “That is the main technical challenge.” —BRITT E. ERICKSON
Continue a Great Tradition The American Chemical Society is soliciting nominations for the Editorship of Environmental Science & Technology. Candidates need not be U.S. citizens or working within the United States, but they should be recognized leaders in environmental research. Send names of nominees, together with their phone numbers and e-mail addresses, to
[email protected] by January 25, 2002, to the attention of the Editor Search Committee.
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