Experimental and modeling study on diffusion of selenium under variable bentonite content and porewater salinity

Iida, Yoshihisa ; Yamaguchi, Tetsuji ; Tanaka, Tadao 

Effective diffusion coefficients of selenium through bentonite sand mixtures under reducing and anaerobic conditions were obtained by the through-diffusion method. Experiments were carried out under variable bentonite content and porewater salinity. The employed bentonite material was Kunigel V1. The dominant diffusion species were determined to be HSe and SeO under reducing and anaerobic conditions, respectively. The effective diffusion coefficients of selenium species decreased with increasing bentonite content and with decreasing salinity. This tendency is primarily due to anion exclusion which caused by negative surface charge of montmorillonite. A diffusion model for anionic species was developed based on the electric double layer theory and the pore diffusion model, to evaluate diffusion behaviors of HSe and SeO in bentonite porewater. The effective diffusion coefficients of selenium species predicted by this model showed good agreement with experimentally measured ones.