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

The anion exclusion behavior in two different clay stones, Opalinus Clay (OPA) and Helvetic Marl (HM), was studied using a well-established experimental through diffusion technique. The ionic strength of the pore water was varied between 0.01 and 5 M to evaluate its effect on the diffusion of HTO and Cl-36(-). The total porosity determined by HTO-diffusion was independent of the ionic strength, while the anion accessible porosity varies with the ionic strength of the pore water. In the case of Opalinus Clay, the anion accessible porosity increases from 3% at low ionic strength (0.01 M) up to 8.4% at high ionic strength (5 M), whereas the anion accessible porosity of Helvetic Marl increases from 0.6% up to only 1.1%. The anion exclusion effect in HM is thus more observation can be correlated to differences in mineralogy and pores. Interlayer equivalent pores are small pores in compressed electric double layers, properties similar to those of interlayers pronounced than that in OPA, even at high ionic strength. This to the fact that HM has a larger fraction of interlayer equivalent clay stones that are small enough to have, because of overlapping and are therefore rather inaccessible for anions.