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Workreport 2008-62

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Name:

Sorption of Cesium on Olkiluoto Mica Gneiss and Granodiorite in Saline Groundwater; Retardation of Cesium Transport in Rock Fracture Columns

Writer:

Kyllönen, J., Hakanen, M. & Lindberg, A.

Language:

English

Page count:

47

Summary:

Sorption of cesium on Olkiluoto mica gneiss and granodiorite rock types has been studied earlier by batch experiments using crushed rocks and rock slices (Huitti et al., 2000, Huitti et al., 1998). The sorption distribution ratios (Rd) of cesium on the rocks are strongly reduced with increases in salinity and increases in cesium concentration in the solution. In addition, highly nonlinear sorption at elevated cesium concentrations in saline solutions has been found and also very slow desorption of cesium at trace concentrations.

In this work the sorption of cesium on Olkiluoto rocks is studied and a mechanistic cation exchange model is applied for sorption of cesium on Olkiluoto mica gneiss and granodiorite. The sorption of cesium on rock was in accordance with charge balances of cations in a low-salinity groundwater stimulant. Earlier studies have indicated that biotite is the dominating cesium-sorption mineral in fresh mica gneiss, granodiorite and granite. This suggested that a composite additive sorption model for cesium on rocks could be constructed based on the sorption on biotite.

Biotites were separated from the Olkiluoto mica gneiss and granodiorite for biotite specific sorption studies of cesium. The cation exchange capacities of the separated Olkiluoto biotites were almost the same for all the exchangeable cations. The selectivity coefficients of Cs-Na, Cs-K and Cs-Ca cation exchange reactions were determined for the Na-, K- and Ca-conditioned biotites by modelling of the cesium sorption isotherms in the electrolyte solutions. A three-site model is needed to model the sorption over cesium concentration range 1*10-8 M to 1*10-3 M isotherms.

Selectivity coefficients determined for the exchange reactions were used in a reactive transport model for migration of cesium in rock fractures at the laboratory scale. The PhreeqC programme was applied. The model was calibrated for migration of cesium in a mica gneiss fracture under constant chemical conditions. The model was applied to modelling of migration of cesium in granodiorite fractures under constant chemical conditions. In this case the sorption site concentrations were reduced owing to the lower biotite content of granodiorite. Modelling of cesium breakthrough during constant concentration inflow in saline groundwater-conditioned mica gneiss and granodiorite was made using the sorption site concentrations for the rock types.

Cesium breakthrough in mica gneiss for 1*10-6 M Cs in OL-SO was calculated by PhreeqC omitting the sorption on fracture surfaces. The results indicate that in the long run matrix diffusion dominates the breakthrough, but the time of the start of breakthrough is determined by surface sorption. An analytical approach for the same situation with a Kd for sorption and De for matrix diffusion was used for comparison. High sorption, a De value in accordance with 2% porosity, and omitting the formation factors for the rock porosity were needed to reproduce the long term breakthrough of cesium in an OL-SO-conditioned fracture

Keywords:

Sorption of Cesium, Mica Gneiss, Granodiorite, Olkiluoto

File(s):

Sorption of Cesium on Olkiluoto Mica Gneiss and Granodiorite in Saline Groundwater; Retardation of Cesium Transport in Rock Fracture Columns (pdf) (602.2 KB)


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