Mandaliev Peter, Dähn Rainer, Wehrli Bernhard, Wieland Erich
Laboratory for Waste Management, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.
Environ Sci Technol. 2009 Nov 1;43(21):8462-8. doi: 10.1021/es902016q.
Cement is an important component in repositories for low-level and intermediate-level radioactive waste. Nd uptake by hardened cement paste (HCP) has been investigated with the aim of developing a mechanistic understanding of the immobilization processes of trivalent lanthanides and actinides in HCP on the molecular level. Information on the microstructure of HCP, the Nd distribution in the cement matrix, and the coordination environment of Nd in these matrices was gained from the combined use of scanning electron microscopy (SEM), synchrotron-based micro-X-ray fluorescence (micro-XRF), micro-X-ray (micro-XAS), and bulk-X-ray absorption spectroscopy (bulk-XAS) on Nd doped cement samples. The samples were reacted over periods of time between 15 min and 200 days. SEM and micro-XRF investigations suggest preferential Nd accumulation in rims around "inner"-calcium silicate hydrates (C-S-H). The EXAFS data indicate that the coordination environment of Nd taken up by HCP was dependent on reaction time. Changes in the structural parameters derived from EXAFS support the idea of Nd incorporation into the structure of C-S-H phases. The Nd binding mechanisms proposed in this study have implication for an overall assessment of the safe disposal of trivalent actinides in cement-based repositories for radioactive waste.
水泥是低放和中放放射性废物处置库中的重要组成部分。为了从分子水平上对三价镧系元素和锕系元素在硬化水泥浆体（HCP）中的固定过程形成机理认识，对HCP中钕的吸收情况进行了研究。通过对掺钕水泥样品联合使用扫描电子显微镜（SEM）、基于同步加速器的微X射线荧光光谱（微XRF）、微X射线吸收精细结构光谱（微XAS）和体相X射线吸收光谱（体相XAS），获得了HCP的微观结构、水泥基体中钕的分布以及钕在这些基体中的配位环境等信息。样品反应时间为15分钟至200天。SEM和微XRF研究表明，钕优先在“内部”硅酸钙水化物（C-S-H）周围的边缘处积累。扩展X射线吸收精细结构（EXAFS）数据表明，HCP吸收的钕的配位环境取决于反应时间。从EXAFS得出的结构参数变化支持钕掺入C-S-H相结构的观点。本研究中提出的钕结合机制对于全面评估三价锕系元素在放射性废物水泥基处置库中的安全处置具有重要意义。
