Diffusion behaviour of selenite and hydroselenide in compacted bentonite

The apparent diffusivities (D_a) for selenium (Se) in compacted bentonite were obtained at a dry density of 1.6 mg/m³ as a function of silica sand content and temperature under reducing conditions, in which the dominant species of Se is predicted to be HSe^- (hydroselenide) and were compared with D_a values obtained under anaerobic conditions, in which the dominant species of Se is predicted to be SeO₃^2- (selenite). The experiments were carried out by an in-diffusion method and a Na bentonite, Kunigel-V1® with Na smectite content of 46-49 wt.%, was used in a series of diffusion experiments. All diffusion experiments were carried out in a controlled N₂ atmosphere glove box. The effects of silica sand content and temperature on D_a are altogether summarized as follows: (1) The D_a values show a tendency to increase with increasing silica sand content. (2) The D_a values show a tendency to increase with increasing temperature. (3) The increasing rate of D_a with temperature is approximately constant, independent of silica sand content. The D_a values for HSe^- are about one order of magnitude smaller than those for SeO₃^2-. This trend is consistent with that of distribution coefficients obtained by batch method. The D_a values for HSe^- were well correlative with smectite partial density for both temperatures. This indicates that Se diffusion, including retardation processes, is predominantly controlled by the properties in part of smectite. The calculated activation energies for diffusion in the porewater [Δ E _a(porewater)] were in the range of 17.6-31.6 kJ/mol and were slightly higher than that of ionic diffusivity in free water [Δ E _a=15.3 kJ/mol] for HS^-, which takes the same species as HSe^- in solution. Furthermore, the relative partial molar activation energies [?ΔE_aā(relative)] calculated from Δ E_{a(free water)} and Δ E_a(porewater) values were in the range of -16.2 to -2.2 kJ/mol. A reason might be that the properties of the porewater of compacted bentonite are different from that of free water.