Sorption Behavior of Selenium(—II) on Rocks under Reducing Conditions

Abstract

The sorption of radionuclides on rocks is an important factor for performance assessment of geologic disposal of radioactive wastes. Batch sorption experiments under reducing conditions were carried out to investigate the sorption behavior of selenium onto granodiorite, sandy mudstone, tuffaceous sandstone, and their major constituent minerals and accessory minerals. Rock cores and groundwater employed in the experiments were sampled with special care to avoid exposing the cores and groundwater to air to minimize their oxidation. Selenium was spiked as HSe À and Se 4 2À in the experimental solutions, and reducing conditions were maintained throughout the sorption periods. Distribution coefficients, K d (m 3 kg À1 ), were obtained in the ranges of 2:2 Â 10 À4 to 4:0 Â 10 À3 m 3 kg À1 for granodiorite, 3:3 Â 10 À2 to 5:6 Â 10 À2 m 3 kg À1 for sandy mudstone, and 2:9 Â 10 À2 to 8:2 Â 10 À2 m 3 kg À1 for tuffaceous sandstone at pHs 8.5 to 11.5. The dominant sorbent minerals for selenium over a neutral to alkaline pH range were determined to be biotite for the granodiorite samples and pyrite for the sandy mudstone and tuffaceous sandstone samples.

Se(IV) uptake by Äspö diorite: Micro-scale distribution

“…Sorption values reported for Se(ÀII), under reducing conditions, on granodiorites ranged from 2.2 Â 10 À4 to 4 Â 10 À3 m 3 /kg (Iida et al, 2011). The main advantage of the present study is that the surface distribution coefficients (K a ) are directly measured on the intact rocks, allowing correlating the rock heterogeneity and selenium sorption, thus restricting the range of sorption values and weighing the contribution of high sorbing minerals or other local in-homogeneities.…”

“…Radionuclides predominantly present as anions or oxyanions are assumed to be non-sorbing or to exhibit weak sorption with low K d values (Carbol and Engkvist, 1997). Selenium available sorption data indicate weak but non-negligible sorption on crystalline rocks (Elrashidi et al, 1989;Garcia-Gutierrez et al, 1999;Iida et al, 2011;McKinley and Scholtis, 1993;Papelis, 2001), and it is believed that selenite sorption is generally higher than selenate one (Elrashidi et al, 1989). Sorption of anionic species should only occur on positively charged minerals.…”