Surface diffusion may enhance the diffusive fluxes of cationic species in argillaceous materials and lead to an underestimation of radioactive doses emanating from deep geological repositories for radioactive waste to the biosphere. The conceptual understanding of surface diffusion implies that competition effects between different cations would also affect diffusion and not only sorption. The present work provides for the first time diffusion data and simulations supporting this view. The effect of Ca²⁺ on the diffusion and sorption of a ⁵⁷Co²⁺ tracer was measured in compacted illite. The results could be quantitatively explained using an electrical double layer or a cation-exchange model for the uptake of cationic species, with both models involving mobile species at the planar surfaces. The observed effects were related to the presence of different sites for Co²⁺ uptake, exhibiting different surface mobilities and different affinities for Ca²⁺. The strong sorption sites, which dominate sorption at the conditions of the experiments, do not exhibit noticeable surface mobility and are not prone to competition with Ca²⁺. The effective diffusion coefficients are rather dominated by the cation species at the planar surfaces, which are affected by competition with Ca²⁺. The relevance of the illite model system for intact clay rock was demonstrated by a successful application of the illite source data to ⁵⁷Co²⁺ diffusion measurements in Opalinus Clay, in which the equilibrium pore water contained substantial amounts of alkaline earth cations. Our present findings enhance the conceptual understanding of sorption and diffusion of trace elements in compacted clay minerals and clay rocks.

中文翻译：

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阳离子对压实伊利石和蛋白石粘土中强吸附痕量阳离子扩散特性的竞争影响

表面扩散可能会提高泥质材料中阳离子物质的扩散通量，并导致低估从放射性废物的深层地质处置库到生物圈的放射性剂量。表面扩散的概念理解意味着不同阳离子之间的竞争效应也会影响扩散，而不仅仅是吸附。目前的工作首次提供了支持这一观点的扩散数据和模拟。Ca ²⁺对⁵⁷ Co ²⁺扩散和吸附的影响示踪剂是在压实的伊利石中测量的。结果可以使用双电层或阳离子交换模型对阳离子物质的吸收进行定量解释，这两种模型都涉及平面表面的移动物质。观察到的效果与 Co ²⁺吸收的不同位点的存在有关，表现出不同的表面迁移率和对 Ca ^{2+ 的}不同亲和力。在实验条件下主导吸附的强吸附位点不表现出明显的表面迁移率并且不易与Ca ²⁺竞争。有效扩散系数主要由平面表面的阳离子物质支配，受与 Ca ²⁺竞争的影响. 伊利石模型系统与完整粘土岩的相关性通过将伊利石源数据成功应用于Opalinus 粘土中的⁵⁷ Co ²⁺扩散测量来证明，其中平衡孔隙水包含大量碱土金属阳离子。我们目前的发现增强了对压实粘土矿物和粘土岩中微量元素吸附和扩散的概念理解。