Uptake of Np(IV) by C鈥揝鈥揌 Phases and Cement Paste: An EXAFS Study

详细信息    查看全文

• 作者：Xavier Gaona ; Rainer D盲hn ; Jan Tits ; Andreas C. Scheinost ; Erich Wieland
• 刊名：Environmental Science & Technology
• 出版年：2011
• 出版时间：October 15, 2011
• 年：2011
• 卷：45
• 期：20
• 页码：8765-8771
• 全文大小：917K
• 年卷期：v.45,no.20(October 15, 2011)
• ISSN：1520-5851

文摘

Nuclear waste disposal concepts developed worldwide foresee the use of cementitious materials for the immobilization of long-lived intermediate level waste (ILW). This waste form may contain significant amounts of neptunium-237, which is expected to be present as Np(IV) under the reducing conditions encountered after the closure of the repository. Predicting the release of Np(IV) from the cementitious near field of an ILW repository requires a sufficiently detailed understanding of its interaction with the main sorbing components of hardened cement paste (HCP). In this study, the uptake of Np(IV) by calcium silicate hydrates (C鈥揝鈥揌) and HCP has been investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS studies on Np(IV)-doped C鈥揝鈥揌 and HCP samples reveal that Np(IV) is predominantly incorporated in the structure of C鈥揝鈥揌 phases having different Ca:Si ratios. The two main species identified correspond to Np(IV) in C鈥揝鈥揌 with a Ca:Si mol ratio of 1.65 as in fresh cement and with a Ca:Si mol ratio of 0.75 as in highly degraded cement. The local structure of Np(IV) changes with the Ca:Si mol ratio and does not depend on pH. Furthermore, Np(IV) shows the same coordination environment in C鈥揝鈥揌 and HCP samples. This study shows that C鈥揝鈥揌 phases are responsible for the Np(IV) uptake by cementitious materials and further that incorporation in the interlayer of the C鈥揝鈥揌 structure is the dominant uptake mechanism.