Sorption and diffusion behavior of selenium in tuff

Sorption and diffusion behavior of Se in tuff was studied by batch sorption and through-diffusion experiments, Tuff samples used in the experiments we re obtained from the Toki Lignite-Bearing Formation, Tone area, Gifu, centr al Japan. The experiments were carried out in a nitrogen glove-box with an oxygen level less than 1 ppm at room temperature. Batch sorption experiment s of Se on crushed tuff samples and its constituent minerals were carried o ut in the pH range between 2 and 13. The fraction of Se sorbed on tuff was above 90% in the pH range below 8 and decreased to below 30% with increasin g pH. Comparison of pH dependencies of sorption between tuff and its consti tuent minerals suggested that Fe minerals such as Fe-oxyhydroxide and pyrit e contributed to Se sorption on tuff. Through-diffusion experiments of Se t hrough intact tuff samples were carried out at the pH values of 8 and 11. E ffective diffusion coefficients obtained at both pH values were almost the same, on the other hand, the distribution coefficient obtained at pH 8 was higher than that at pH 11 by one order of magnitude. This result was consis tent qualitatively with the pH dependency of Se sorption by the batch exper iments. However, the distribution coefficients obtained by the diffusion ex periments were lower than those by the batch experiments and the difference s were one order of magnitude, Correcting the difference of specific surfac e areas for crushed and intact tuff samples could not explain the differenc e between distribution coefficients obtained by batch sorption and diffusio n experiments. Consideration of the pore size distribution by the mercury p orosimetry suggested that the microscopic pores with size below several ten s of nanometers might contribute slightly to sorption. The assumption that the pores with size below 20 nm are not concerned with sorption leads to th e consistency of distribution coefficients between batch sorption and diffu sion experiments. (C) 1998 Published by Elsevier Science B.V. All rights re served.