Abstract Diffusive transport and sorption processes of uranium in the Swiss Opalinus Clay were investigated as a function of partial pressure of carbon dioxide p C O 2 , varying mineralogy in the facies and associated changes in porewater composition. Simulations were conducted in one-dimensional diffusion models on the 100 m-scale for a time of one million years using a bottom-up approach based on mechanistic surface complexation models as well as cation exchange to quantify sorption. Speciation calculations have shown, uranium is mainly present as U(VI) and must therefore be considered as mobile for in-situ conditions. Uranium migrated up to 26 m in both, the sandy and the carbonate-rich facies, whereas in the shaly facies 16 m was the maximum. The main species was the anionic complex C a U O 2 ( C O 3 ) 3 2 − . Hence, anion exclusion was taken into account and further reduced the migration distances by 30 %. The concentrations of calcium and carbonates reflected by the set p C O 2 determine speciation and activity of uranium and consequently the sorption behaviour. Our simulation results allow for the first time to prioritize on the far-field scale the governing parameters for diffusion and sorption of uranium and hence outline the sensitivity of the system. Sorption processes are controlled in descending priority by the carbonate and calcium concentrations, pH, pe and the clay mineral content. Therefore, the variation in porewater composition resulting from the heterogeneity of the facies in the Opalinus Clay formation needs to be considered in the assessment of uranium migration in the far field of a potential repository.

中文翻译：

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Opalinus 粘土中扩散铀运输和吸附过程的模拟

摘要 研究了瑞士 Opalinus 粘土中铀的扩散传输和吸附过程，作为二氧化碳分压 p CO 2 的函数、相中的矿物学变化以及孔隙水组成的相关变化。使用基于机械表面络合模型和阳离子交换的自下而上方法在 100 米尺度上的一维扩散模型中进行了长达一百万年的模拟，以量化吸附。形态计算表明，铀主要以 U(VI) 形式存在，因此必须将其视为可移动的原位条件。铀在砂质相和富碳酸盐相中均迁移至 26 m，而在页岩相中 16 m 最大。主要种类是阴离子配合物C a UO 2 (CO 3 ) 3 2 - 。因此，阴离子排除被考虑在内，并进一步将迁移距离减少了 30%。由设定的 p CO 2 反映的钙和碳酸盐的浓度决定了铀的形态和活性，从而决定了吸附行为。我们的模拟结果首次允许在远场尺度上优先考虑铀的扩散和吸附的控制参数，从而勾勒出系统的灵敏度。吸附过程由碳酸盐和钙浓度、pH、pe 和粘土矿物含量按降序控制。因此，在评估潜在处置库远场中的铀迁移时，需要考虑由 Opalinus 粘土地层中相的异质性导致的孔隙水组成的变化。