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首页> 外文期刊>Environmental Science & Technology >Diffusion of ~(22)Na and ~(85)Sr in Montmorillonite: Evidence of Interlayer Diffusion Being the Dominant Pathway at High Compaction
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Diffusion of ~(22)Na and ~(85)Sr in Montmorillonite: Evidence of Interlayer Diffusion Being the Dominant Pathway at High Compaction

机译：〜（22）Na和〜（85）Sr在蒙脱土中的扩散：层间扩散的证据是高压实的主要途径

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摘要

A mechanistic understanding of transport phenomena in compacted clays is essential for the use of such materials as engineered barrier systems for the safe geological disposal of radioactive wastes. The present contribution is a first step in the development of an integrative treatment of the properties of tracer cations in compacted bentonites with respect to diffusion and sorption. The diffusion of ~(22)Na and ~(85)Sr in highly compacted montmorillonite and kaolinite is investigated as a function of the "external salt concentration" (NaCI0_4), i.e., of the solution in equilibrium with the clay. Consistent results were obtained from through-diffusion experiments and tracer profile analysis. Knowledge of genuine diffusion coefficients of the filter plates turned out to be crucial in cases where the diffusive resistance of the filter plates was similar to that of the clay. Diffusion coefficients formally calculated on the basis of the tracer concentration gradient in the external aqueous phase, and the sorption distribution ratios were found to decrease with increasing external salt concentration in the case of montmorillonite. In a logarithmic representation of these data, a slope of -1 was obtained for the monovalent ~(22)Na, whereas the slope was -2 for the divalent ~(85)Sr. In the case of kaolinite, diffusion coefficients were independent of the external salt concentration. It is postulated that the diffusion of the tracer cation through the interlayer water is the dominant pathway in compacted swelling clays under the experimental conditions tested. Effective diffusion coefficients, based on a tracer concentration gradient in the interlayer water of the clay, were found to be independent of the composition of the external aqueous phase. The latter gradient is assumed to be a function of the external salt concentration, according to a calculated distribution of the tracer cation between free pore water and the interlayer water via cation exchange.
机译：机械地理解压实粘土中的传输现象，对于使用这类材料（例如，工程设计的屏障系统）进行放射性废物的安全地质处置至关重要。本研究成果是对膨润土中示踪阳离子的扩散和吸附特性进行综合处理的第一步。研究了〜（22）Na和〜（85）Sr在高度致密的蒙脱石和高岭石中的扩散与“外部盐浓度”（NaCl0_4）（即与粘土保持平衡的溶液）的关系。从穿透实验和示踪剂谱分析获得了一致的结果。在滤板的扩散阻力与黏土的扩散阻力相似的情况下，了解滤板的真实扩散系数至关重要。根据外部水相中的示踪剂浓度梯度正式计算出扩散系数，在蒙脱土的情况下，发现吸附分布比随外部盐浓度的增加而降低。在这些数据的对数表示中，一价〜（22）Na的斜率为-1，而二价〜（85）Sr的斜率为-2。在高岭土的情况下，扩散系数与外部盐浓度无关。假定在所测试的实验条件下，示踪阳离子通过层间水的扩散是压实膨胀粘土中的主要途径。发现基于示踪剂在粘土夹层水中的浓度梯度的有效扩散系数与外部水相的组成无关。根据计算得出的示踪阳离子在自由孔隙水和层间水之间通过阳离子交换的分布，后者的梯度被认为是外部盐浓度的函数。

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