title: Consistent interpretation of the results of through-, out-diffusion and tracer profile analysis for trace anion diffusion in compacted montmorillonite.
abstract: Literature data for anion diffusion in compacted swelling clays contain systematic inconsistencies when the results of through-diffusion tests are compared with those of out-diffusion or tracer profile analysis. In the present work we investigated whether these inconsistencies can be explained by taking into account heterogeneities in the compacted samples; in particular increased porosities at the clay boundaries. Based on the combined results of out-diffusion, tracer profile analysis and the spatial distribution of the electrolyte anion in the clay, we conclude that the inconsistencies can indeed be resolved by taking into account a heterogeneous distribution of the total and the anion-accessible porosity. This, by definition, leads to a position dependence of the effective diffusion coefficient. Neglecting these effects results in a rather subordinate systematic error in the determination of effective diffusion coefficients of anions from through-diffusion tests with clay thicknesses in the centimetre range. However, stronger errors in terms of absolute values and conceptual interpretation may be introduced in out-diffusion tests and profile analyses of the diffused tracer. We recommend that anion diffusion tests should be accompanied by measurements of the total and anion-accessible porosity as a function of position in the direction of diffusion.
authors.affiliation: Laboratory for Waste Management, Paul Scherrer Institut, CH-5232 Villigen, Switzerland. martin.glaus@psi.ch
authors.forename: Martin A
authors.initials: MA
authors.lastname: Glaus
authors.forename: Sabrina
authors.initials: S
authors.lastname: Frick
authors.forename: Roger
authors.initials: R
authors.lastname: Rossé
authors.forename: Luc R
authors.initials: LR
authors.lastname: Van Loon
mesh_terms: D001546:Bentonite / Q000737:chemistry*; D002712:Chlorides / Q000737:chemistry*; D004058:Diffusion; D016062:Porosity; D011850:Radioactive Waste
chemical_list: D002712:Chlorides; D011850:Radioactive Waste; D001546:Bentonite