pH effect on Re(VII) and Se(IV) diffusion in compacted GMZ bentonite

In a high-level radioactive waste (HLW) repository, pH has an impact on the solubility, migration, and adsorption of radionuclides. Thus, understanding the effects of pH on the diffusion of radionuclides is essential for long-term disposal of HLW. In this work, the diffusion behaviors of Re(VII) and Se(IV) in compacted Gaomiaozi (GMZ) bentonite at different pH have been investigated by a through-diffusion method. The effective diffusion coefficient, i. e., D-e values of Re(VII) and Se(IV) were in the range of (1.0-2.4) x 10-(11) m(2)/s at pH 3.0-10.0 and (0.38-2.3) x 10(-11) m(2)/s at pH 3.0-9.0. In the case of Re(VII), the D-e values remained almost unchanged probably because ReO4- was the dominant species in the pH range of 3.0-10.0. In the case of Se(IV), whose predominant species were HSeO3- at pH < 9.0 and SeO32- at pH >= 9.0, the D-e values decreased by a factor of 3-6 at pH 9.0, i. e., D-e (pH < 9.0)/D-e (pH 9.0) approximate to 3-6, implying that the species with a higher valence state had a stronger anion exclusion effect. The decrease in D-e values can be explained by the diffusion species of Se(IV). Additionally, the rock capacity factor a decreased with the increase of pH. HSeO3 was absorbed on GMZ bentonite with distribution coefficient K-d values in the range of (1.0-2.5) x 10(-4) m(3)/kg at pH <= 8.0, whereas SeO32- was negligibly sorbed at pH > 8.0. (C) 2016 Elsevier Ltd. All rights reserved.