Competitive Incorporation of Perrhenate and Nitrate into Sodalite

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• 作者：Johnbull O. Dickson ; James B. Harsh ; Markus Flury ; Wayne W. Lukens ; Eric M. Pierce
• 刊名：Environmental Science & Technology
• 出版年：2014
• 出版时间：November 4, 2014
• 年：2014
• 卷：48
• 期：21
• 页码：12851-12857
• 全文大小：351K
• ISSN：1520-5851

文摘

Nuclear waste storage tanks at the Hanford site in southeastern Washington have released highly alkaline solutions, containing radioactive and other contaminants, into subsurface sediments. When this waste reacts with subsurface sediments, feldspathoid minerals (sodalite, cancrinite) can form, sequestering pertechnetate (⁹⁹TcOb>4b>^{鈥?/sup>) and other ions. This study investigates the potential for incorporation of perrhenate (ReOb>4b>鈥?/sup>), a chemical surrogate for 99TcOb>4b>鈥?/sup>, into mixed perrhenate/nitrate (ReOb>4b>鈥?/sup>/NOb>3b>鈥?/sup>) sodalite. Mixed-anion sodalites were hydrothermally synthesized in the laboratory from zeolite A in sodium hydroxide, nitrate, and perrhenate solutions at 90 掳C for 24 h. The resulting solids were characterized by bulk chemical analysis, X-ray diffraction, scanning electron microscopy, and X-ray absorption near edge structure spectroscopy (XANES) to determine the products鈥?chemical composition, structure, morphology, and Re oxidation state. The XANES data indicated that nearly all rhenium (Re) was incorporated as Re(VII)Ob>4b>鈥?/sup>. The nonlinear increase of the unit cell parameter with ReOb>4b>鈥?/sup>/NOb>3b>鈥?/sup> ratios suggests formation of two separate sodalite phases in lieu of a mixed-anion sodalite. The results reveal that the sodalite cage is highly selective toward NOb>3b>鈥?/sup> over ReOb>4b>鈥?/sup>. Calculated enthalpy and Gibbs free energy of formation at 298 K for NOb>3b>- and ReOb>4b>-sodalite suggest that NOb>3b>鈥?/sup> incorporation into the cage is favored over the incorporation of the larger ReOb>4b>鈥?/sup>, due to the smaller ionic radius of NOb>3b>鈥?/sup>. Based on these results, it is expected that NOb>3b>鈥?/sup>, which is present at significantly higher concentrations in alkaline waste solutions than 99TcOb>4b>鈥?/sup>, will be strongly preferred for incorporation into the sodalite cage.}