Laboratory for Waste Management, Paul Scherrer Institute , 5232 Villigen-PSI, Switzerland.
Environ Sci Technol. 2013 Sep 3;47(17):9816-23. doi: 10.1021/es401530n. Epub 2013 Aug 19.
Cs migration in soils at contaminated sites or in clay-rich backfill of waste disposal sites can take place under partially saturated conditions. To understand the molecular mechanism of Cs migration in partially saturated clays, Grand Canonical Monte Carlo simulations were applied to model adsorption of water films onto external surfaces of Cs and Na montmorillonites as function of partial water pressure. The surface complexation and diffusivity of Cs and Na at different partial water pressure was obtained by molecular dynamics simulations. The results suggest that ion mobility in adsorbed water films on external basal surfaces of clay is similar to that in the near-surface water of a saturated pore as far as the thickness of the adsorbed water film is more than two water layers. At lower partial water pressure (i.e., in thinner water films) the ion mobility dramatically decreases. In contrast, the average water mobility in thin water film is higher than in the water-saturated system due to enhanced mobility of water molecules close to vapor-film interface. The results of the simulations were applied to interpret recent laboratory measurements of tritiated water and Cs diffusivity in Callovo-Oxfordian Claystones under partially saturated conditions.
在受污染场地的土壤中或在废物处置场的富粘土回填物中，Cs 可能会在部分饱和条件下迁移。为了了解部分饱和粘土中 Cs 迁移的分子机制，应用巨正则蒙特卡罗模拟来模拟水膜在 Cs 和 Na 蒙脱石外部表面上的吸附作为部分水压力的函数。通过分子动力学模拟获得了不同部分水压力下 Cs 和 Na 的表面络合和扩散系数。结果表明，只要吸附水膜的厚度超过两个水分子层，吸附在粘土外部基面的水膜中的离子迁移率就与饱和孔隙近表面水中的离子迁移率相似。在较低的部分水压力（即较薄的水膜）下，离子迁移率急剧下降。相比之下，由于靠近蒸汽-膜界面的水分子迁移率增强，薄水膜中的平均水迁移率高于水饱和体系中的平均水迁移率。模拟结果被应用于解释最近在部分饱和条件下对 Callovo-Oxfordian Claystones 中氚水和 Cs 扩散率的实验室测量结果。
