Importance of Interlayer Equivalent Pores for Anion Diffusion in Clay-Rich Sedimentary Rocks

Anion transport in compacted clay-rich rocks remains poorly understood, despite its importance for subsurface applications like nuclear waste repositories. Prior research has shown that anion movement is restricted in such materials due to electrostatic interactions and pore structure. However, the role of interlayer equivalent pores in this exclusion behavior has not been fully resolved. Existing studies have focused on total porosity and general anion exclusion, but few have examined how pore structure influences anion accessibility. This gap motivated further investigation into how pore geometry and mineral composition affect anion diffusion. The need for precise measurements of accessible porosity under varying ionic conditions remains unmet. Limited data exist on how different clay types respond to changes in pore water salinity. Understanding these interactions could improve predictions of contaminant transport in geological formations. This paper addresses these uncertainties by comparing two distinct clay stones under controlled experimental conditions.

This study aimed to evaluate how anion diffusion is affected by pore structure in two different clay-rich rocks. The focus was on determining how accessible porosity changes with ionic strength and how this relates to anion exclusion. The goal was to compare Opalinus Clay and Helvetic Marl to identify structural differences influencing anion transport. The researchers sought to quantify the fraction of pores accessible to anions across a wide range of ionic strengths. They also aimed to correlate these findings with mineralogical and structural characteristics of the rocks. The study's motivation was to better understand how pore geometry affects anion movement in compacted sediments. By using a through-diffusion technique, the team could isolate the effects of pore structure from other variables. The ultimate purpose was to provide insights into anion transport mechanisms relevant to subsurface applications.

The researchers used a through-diffusion setup to measure anion transport in two clay stones. They selected Opalinus Clay and Helvetic Marl as test materials due to their distinct mineralogical properties. The pore water ionic strength was varied from 0.01 to 5 M to observe its effect on anion diffusion. HTO and ³⁶Cl^- were used as tracers to distinguish between total and anion-specific porosity. The experimental setup allowed for controlled diffusion measurements under different salinity conditions. The team calculated accessible porosity by comparing tracer diffusion rates at different ionic strengths. They also analyzed the mineral composition of the samples to identify structural differences. These methods enabled a direct comparison of anion exclusion behavior between the two clay types.

The total porosity measured via HTO diffusion remained constant across all ionic strengths tested. In contrast, anion accessible porosity increased significantly with higher ionic strength in both samples. For Opalinus Clay, anion accessible porosity rose from 3% at 0.01 M to 8.4% at 5 M. Helvetic Marl showed a smaller increase, from 0.6% to 1.1% under the same conditions. These findings indicate that anion exclusion is more pronounced in Helvetic Marl than in Opalinus Clay. The difference in accessible porosity correlated with the presence of interlayer equivalent pores in Helvetic Marl. These pores are small and compressed, with overlapping electric double layers that restrict anion access. The results suggest that pore structure plays a critical role in anion transport behavior. The study provides direct evidence of how interlayer equivalent pores influence anion diffusion in compacted sediments.

The study found that anion accessible porosity increases with ionic strength in both Opalinus Clay and Helvetic Marl. However, Helvetic Marl exhibited stronger anion exclusion effects compared to Opalinus Clay, even at high ionic strengths. This difference was linked to the presence of interlayer equivalent pores in Helvetic Marl. These pores are small and compressed, making them less accessible to anions due to overlapping electric double layers. The findings suggest that pore structure significantly influences anion transport in compacted sediments. The researchers propose that mineralogical differences between the two clays contribute to variations in anion exclusion. The study highlights the importance of considering pore geometry when modeling anion diffusion in geological formations. These results may inform future work on subsurface transport processes in low-permeability rocks.