Acquisitions of effective diffusion coefficients (De) for Ni (II), Am (III), Sm (III) and Se (IV) in bentonite by through-diffusion method

Effective diffusion coefficients (De) for Ni2+, Sm3+, Am3+ and SeO32- were measured as a function of the ionic charge of diffusion species to quantitatively evaluate the effect of ionic charge in compacted bentonite. The De measurements for Ni2+ and Sm3+ were carried out for a bentonite dry density of 1.8 Mg·m-3 with a simulated porewater condition of pH5-6 by through-diffusion method. The De values for SeO32- were measured for a bentonite dry density of 1.8 Mg·m-3 with a simulated porewater condition of pH11. The De measurements for Am3+ were carried out for the dry densities of 0.8, 1.4 and 1.8 Mg·m-3 with a porewater condition of pH2 in order to check cation exclusion. Sodium bentonite, Kunigel-V1 was used for those measurements. For the measurements of Am, H-typed Kunigel-V1 which interlayer ion (Na+) was exchanged with H+, was used, because the experiments are carried out for a low pH range. The order of obtained De values was Sm3+ > Ni2+ > Am3+ > SeO32-. These De values were compared to those reported to date. Consequently, the order of De values was Cs+ > Sm3+ > HTO > Ni2+ > anions (I-, Cl-, CO32-, TcO4-, NpO2CO3-, UO2(CO3)34-), showing a tendency of cations > HTO > anions. Only the De values of Am3+ were approximately the same degree as those of anions. The reason that the De of Ni2+ was lower than that of HTO may be because the free water diffusion coefficient (Do) of Ni2+ is about 1/3 of that of HTO. The cause that the De of Am3+ was approximately the same degree as those of anions may be because the Do of Am3+ is about 1/3 of that of HTO and that Am3+ was electrostatically repulsed from the surface of bentonite by cation exclusion. The formation factors (FF), calculated normalizing Do, were in the order, Sm3+ > Cs+ > Ni2+ > HTO > Am3+ > anions, indicating a possibility of surface diffusion for Cs+, Ni2+ and Sm3+, cation exclusion for Am3+ and anion exclusion for anion exclusion including SeO32-. From the calculations of FF values, the degree of surface diffusive effect for a dry density of 1.8 Mg·m-3 was approximately estimated to be 5 times for Sm3+, 3 times for Cs+ and 1.3 times for Ni2+ as much assuming that FF of HTO is the standard. The degree of anion exclusive effect for the same conditions was approximately estimated to be 1/7 for TcO4-, 1/16 for NpO2CO3- and 1/5 for SeO32- as much. The degree of cation exclusive effect for Am3+ for the same density was approximately calculated to be 0.85 times as much as that of HTO and not so much effect on De was recognized. However since the Do of Sm3+ is also about 1/3 of that of HTO, the De values of Sm3+ may have been only a little higher than those of HTO. Based on this, it is presumed that surface diffusive effect on De in bentonite is insignificant. (author)