“…In Opalinus Clay, diffusion of cations is enhanced, and that of anions diminished, relative to a neutral tracer such as tritiated water (HTO). This is in line with results from many other diffusion experiments with clayrock, bentonite or pure montmorillonite (Shackelford and Daniel, 1991;Sato et al, 1992;Sato, 2005;Kozaki et al, 1998Kozaki et al, , 2005Kozaki et al, , 2008; Molera and Eriksen, 2002;Van Loon et al, 2003a,b;2004a,b;Melkior et al, 2007;Wersin et al, 2007;Descostes et al, 2008). The increased diffusion of cations is explained either by interlayer or surface diffusion of sorbed cations (Ohlsson and Neretnieks, 1998;Eriksen et al, 1999;Bourg, 2004;Bourg et al, 2007;Glaus et al, 2007), or by diffusion in the diffuse double layer (DDL) that surrounds the negatively charged clay surface and contains an excess of cations (Van Schaik et al, 1966;Kemper and Quirk, 1972;Ochs et al, 1998Ochs et al, , 2001Lehikoinen et al, 1999;Leroy and Revil, 2004;Leroy et al, 2006;Appelo and Wersin, 2007;Jougnot et al, 2009).…”