Synthetic Clay Excels in 90 Sr Removal
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90
Sr removal
Sridhar Komarneni, Tatsuya Kodama, and William J. Paulus Materials Research Laboratory and Department of Agronomy, The Pennsylvania State University, University Park, Pennsylvania 16802
C. Carlson Process Technology and Environmental Management Resources, Environmental Technology Division, Pacific Northwest National Laboratory, Richland, Washington 99352 (Received 20 August 1999; accepted 20 March 2000)
Tests with actual ground water from Hanford site, and fundamental studies of 2Na+ → Sr2+ exchange equilibria revealed that a synthetic clay is extremely selective for 90Sr with a high capacity for uptake. Comparative studies with existing Sr selective ion exchangers clearly revealed that the present synthetic clay exhibited the best performance for 90Sr removal from actual ground water collected from three different locations at Hanford. This novel Sr ion sieve is expected to be useful for the decontamination of the environment after accidental release and contamination with 90Sr.
Naturally occurring cation exchangers such as clays and zeolites have been used to decontaminate and dispose of the radioactive species.1–4 There has been a great deal of effort to develop high performance synthetic cation exchangers for the uptake of Sr and Cs5–10 and their immobilization. A need exists for high performance cation exchangers to separate Sr, Cs, and other species from high-level alkaline tank wastes and remediation of process and ground water at the Hanford nuclear site, the latter to prevent contamination of the Columbia River.11 Here we report the discovery of a new synthetic clay, Na2Si6Al2Mg6O20F4 ⭈ xH2O (nominal composition), which excels in 90Sr removal from contaminated ground waters. This phase will also be useful in 90Sr immobilization in the interlayers by modest heating after the Sr ion uptake. The starting precursor for the synthesis method was derived by a sol-gel process and is briefly described here. A single phase or monophasic gel was prepared by dissolving Mg(NO3)2 ⭈ 6H2O and Al(NO3)3 ⭈ 9H2O separately in absolute ethanol and then combining and mixing the two solutions by stirring for 1 h before adding tetraethoxysilane, Si(OC2H5)4. The stoichiometric composition of the three components on an oxide basis was 6MgO–Al2O3–6SiO2. The above mixed sol of the three components was stirred for three hours to achieve homogeneity. The container was then covered with a plastic film and placed in an oven at 60 °C to form a gel through hydrolysis, condensation, and polymerization reactions for three days. This monophasic gel was then dried in an oven at 100 °C, calcined at 475 °C for 12 h, and ground to a powder to pass through a −325 mesh screen. This gel 1254
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J. Mater. Res., Vol. 15, No. 6, Jun 2000 Downloaded: 18 Mar 2015
powder was then mixed with an equal weight of −325 mesh NaF crystalline powder in a platinum crucible and sintered for 18 h at 890 °C in a programmed furnace. These sintering conditions were found to be ideal for synthesis of this type of fluorina
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