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. 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 pronounced than that in OPA, even at high ionic strength. This observation can be correlated to differences in mineralogy and to the fact that HM has a larger fraction of interlayer equivalent pores. Interlayer equivalent pores are small pores in compressed clay stones that are small enough to have, because of overlapping electric double layers, properties similar to those of interlayers and are therefore rather inaccessible for anions.

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Manuscript Click here to download Manuscript Wigger_Manuscript_CCM.docx This is a 'pre-publication' version of an accepted article for Clays and Clay Minerals. This version may be subject to change during the production process. The DOI given, which may be used for citation purposes, though which will not be active until the version of record is published,