Experimental and Modeling Study on Diffusion of Selenium under Variable Bentonite Content and Porewater Salinity

Effective diffusion coefficients of selenium through bentonite/sand mixtures were obtained by the through-diffusion method under reducing conditions where selenium was dissolved as HSe^? and Se₄ ^2–. Experiments were carried out under variable bentonite content and porewater salinity. Bentonite contents in bentonite/sand mixtures varied from 20 to 70 wt%. The salinities of experimental solutions varied from 0.05 to 0.5 mol dm^?3, simulating brackish groundwater and seawater. Effective diffusion coefficients were also obtained under anaerobic conditions where the dominant selenium species was determined to be SeO₃ ^2–. The effective diffusion coefficients of selenium species decreased with increasing bentonite content and with decreasing salinity. This tendency is primarily due to anion exclusion that was caused by the 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 the diffusion behaviors of HSe^?, Se₄ ^2–, and SeO₃ ^2– in bentonite porewater. In this model, pores in bentonite/sand mixtures are assumed to consist of two types of space; one is the space between mineral particles and the other is the space between layers of montmorillonite. The scope of application of this model is limited to less than or equal to 1,600 kg m^?3 in dry density. The effective diffusion coefficients of selenium species predicted using this model showed good agreement with experimentally measured ones.