Measurements of Apparent Diffusion Coefficients (Da)for Ca(I), Ni(II)and Se(IV) in Bentonite with Silica Sand

Sato, Haruo

Apparent diffusion coefficients(Da) of Cs(Cs), Ni(Ni) and Se(SeO) in a Na-bentonite (Kunigel-V1) were measured for a dry density of 1.8 Mgm with silica sand of 30 wt% at room temperature by in-diffusion method to evaluate the effect of the mixture of silica sand on Da in bentonite. The experiments for Cs and Ni were carried out under aerobic condition, but those for Se which is redox sensitive were carried out in an Ar glove-box (O concentration 0.1 ppm). Consequently, no significant effect of silica sand mixture to the bentonite on Da values of Cs and Se was found, and the obtained Da values were approximately the same as those in the system without silica sand reported so far. On the other hand, Da values of Ni in bentonite with silica sand were 2 orders of magnitude lower than those in bentonite without silica sand obtained to date. The Da values of Ni reported so far were obtained using stable isotopic tracer and a tracer solution with fairly high Ni concentration compared with concentration used in this study was introduced. Additionally, it is known that distribution coefficient (Kd) of Ni on Na-montmorillonite which is the major constituent clay mineral of Kunigel-V1 decreases with increasing Ni concentration. Based on this, the abrupt decrease in Da values of Ni for bentonite with silica sand is considered to be due to the difference of sorption caused by the difference of Ni concentration in the porewater of bentonite.