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/m 3 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 SeO 2- 3 (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 N 2 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 SeO 2- 3. 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 [ΔE 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 [ΔE a(free water) = 15.3 kJ/mol] for HS - , which takes the same species as HSe- in solution. Furthermore, the relative partial molar activation energies [ΔE a (relative)] calculated from ΔE a(free water) and ΔE 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.