Safety assessment studies of future nuclear waste repositories carried out in many countries predict selenium-79 to be a critical radionuclide due to its presence as anion in three relevant oxidation states (VI, IV, -II) resulting in weak retardation by most common rock minerals. This assumption, however, ignores its potential uptake by AFm phases, positively charged anion exchangers, which are present in significant quantities in the cementitious materials used in artificial barriers. Here we report for the first time wet chemistry and spectroscopic data on the interaction of the most relevant selenium anion species under the expected strongly reducing conditions, i.e. HSe⁻, with two AFm phases commonly found in cement, monocarbonate (AFm-MC) and hemicarbonate (AFm-HC). Batch sorption experiments showed that HSe⁻ is retained much more strongly by AFm-HC (solid–liquid distribution ratio, R_d, of 100 ± 50 L kg⁻¹) than by AFm-MC (R_d = 4 ± 2 L kg⁻¹) at the equilibrium pH (∼12). X-ray absorption fine-structure (XAFS) spectroscopy revealed that the larger d-spacing in AFm-HC (d-spacing = 8.2 Å) provides easy access for HSe⁻ to the AFm interlayer space for sorption, whereas the smaller d-spacing of AFm-MC (d-spacing = 7.55 Å) hinders interlayer access and limits HSe⁻ sorption mostly to the outer planar surfaces and edges of the latter AFm phase. XAFS spectra further demonstrated that Se(-II) prevalently sorbed in the interlayers of AFm-HC, is better protected from oxidation than Se(-II) prevalently sorbed onto the outer surfaces of AFm-MC. The quantitative sorption data along with the molecular-scale process understanding obtained from this study provide crucial insight into the Se retention by the cementitious near-field of a radioactive waste repository under reducing conditions.

中文翻译：

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在强烈减少放射性废物处置库条件下，铝酸钙水合物（AFm）相保留硒†

在许多国家进行的未来核废料储存库的安全评估研究中，由于硒以三种相关氧化态（VI，IV，-II）中的阴离子形式存在，导致大多数常见岩石矿物的缓凝性较弱，因此硒79是一种关键的放射性核素。。但是，该假设忽略了AFm相（带正电的阴离子交换剂）潜在吸收的可能性，AFm相带正电荷的阴离子交换剂大量存在于人工屏障中所用的胶结材料中。在这里，我们报告的第一次湿化学和光谱数据最相关的硒阴离子物种的预期强烈还原条件下的相互作用，即HSe组^-，在水泥中通常会发现两个AFm相，即一碳酸盐（AFm-MC）和半碳酸盐（AFm-HC）。批次吸附实验表明，HSe组^-由AFM-HC更牢固地保持（固-液分配比率，- [R _d的100±50升公斤，^-1）比用AFM-MC（[R _d = 4±2升公斤^{- 1}）在平衡pH（〜12）下。X射线吸收微细结构（XAFS）光谱表明，较大的d在AFM-HC（-spacing d -spacing = 8.2埃）可以轻松访问HSE的^-层间为吸附空间到AFM，而较小的d -spacing AFm-MC（d-spacing = 7.55）阻碍层间访问，并限制HSe组^-吸附大多于外平面表面和后者AFM相位的边缘。XAFS光谱进一步表明，与普遍吸附在AFm-MC外表面的Se（-II）相比，普遍吸附在AFm-HC中间层中的Se（-II）具有更好的抗氧化保护作用。从这项研究中获得的定量吸附数据以及对分子规模过程的了解，为还原条件下放射性废物储存库胶结性近场对硒的保留提供了重要的见识。