Department of Crop and Soil Sciences, Washington State University , P.O. Box 646420, Pullman, Washington 99164, United States.
Environ Sci Technol. 2014 Nov 4;48(21):12851-7. doi: 10.1021/es503156v. Epub 2014 Oct 21.
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 (99TcO4-) and other ions. This study investigates the potential for incorporation of perrhenate (ReO4-), a chemical surrogate for 99TcO4-, into mixed perrhenate/nitrate (ReO4-/NO3-) sodalite. Mixed-anion sodalites were hydrothermally synthesized in the laboratory from zeolite A in sodium hydroxide, nitrate, and perrhenate solutions at 90 °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' chemical composition, structure, morphology, and Re oxidation state. The XANES data indicated that nearly all rhenium (Re) was incorporated as Re(VII)O4-. The nonlinear increase of the unit cell parameter with ReO4-/NO3- 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 NO3- over ReO4-. Calculated enthalpy and Gibbs free energy of formation at 298 K for NO3- and ReO4-sodalite suggest that NO3- incorporation into the cage is favored over the incorporation of the larger ReO4-, due to the smaller ionic radius of NO3-. Based on these results, it is expected that NO3-, which is present at significantly higher concentrations in alkaline waste solutions than 99TcO4-, will be strongly preferred for incorporation into the sodalite cage.
美国华盛顿州东南部汉福德场址的核废料储存罐已将含放射性和其他污染物的高碱性溶液释放到地下沉积物中。当这些废物与地下沉积物发生反应时，可能会形成类沸石矿物（方钠石、钙霞石），从而固定高锝酸盐（99TcO4-）和其他离子。本研究调查了同多铼酸盐（ReO4-），即 99TcO4-的化学替代物，进入混合铼酸盐/硝酸盐（ReO4-/NO3-）方钠石的可能性。在 90°C 下，将沸石 A 分别溶解于含有氢氧化钠、硝酸盐和高铼酸盐的溶液中，反应 24 小时，在实验室中合成了混合阴离子方钠石。采用全元素分析、X 射线衍射、扫描电子显微镜和 X 射线吸收近边结构光谱（XANES）对方钠石的化学组成、结构、形貌和 Re 氧化态进行了表征。XANES 数据表明，几乎所有的铼（Re）都以 Re(VII)O4-的形式存在。随着 ReO4-/NO3-比例的非线性增加，单元晶胞参数增大，表明形成了两种独立的方钠石相，而不是混合阴离子方钠石。结果表明，方钠石笼对 NO3-的选择性高于 ReO4-。计算得出 298 K 时 NO3-和 ReO4-方钠石的形成焓和吉布斯自由能表明，由于 NO3-的离子半径较小，NO3-进入笼内的过程比 ReO4-更容易发生，因此 NO3-进入笼内的过程更有利。基于这些结果，可以预期在碱性废水中，NO3-的浓度要比 99TcO4-高得多，因此，与 ReO4-相比，NO3-将优先进入方钠石笼内。
