ABSTRACT

The sorption of alkaline earth cations Ca²⁺, Sr²⁺, Ba²⁺, and Ra²⁺ was investigated on Na-converted biotite, the main sorbing mineral in granitic bedrock at the nuclear waste disposal site in Olkiluoto, Finland. Batch sorption experiments were conducted under CO₂-free conditions by varying the concentration of the alkaline earth cation (Ca²⁺, Sr²⁺ and Ba²⁺ 10⁻⁸ M–10⁻² M; Ra²⁺ 10⁻¹⁰ M–10⁻⁷ M) in 0.01 M, 0.1 M and 1.0 M NaClO₄ solutions buffered to pH 8. Additional experiments were conducted on four Olkiluoto bedrock reference groundwater solutions. The retention of alkaline earth cations on biotite decreased upon their increasing concentration in the liquid phase and on increasing NaClO₄ solution concentration. The K_d values in NaClO₄ solutions and in glacial melt water (ionic strength 0.12 mmol/L) and fresh groundwater (4.3 mmol/L) systematically decreased in the order Ra²⁺ ≥ Ba²⁺ > Sr²⁺ ≥ Ca²⁺. No apparent sorption occurred except a slight sorption activity for Ba²⁺ in brackish (91.4 mmol/L) and saline (515 mmol/L) reference waters. The sorption of Ca²⁺, Sr²⁺, Ba²⁺, and Ra²⁺ was modelled in 0.01 M, 0.1 M, and 1.0 M NaClO₄ solutions and in Olkiluoto reference groundwaters using a mechanistic three-site cation exchange model previously developed for Cs⁺ sorption on biotite. The model gave a generally good fit with experimental results in NaClO₄ solutions and in glacial melt water and fresh groundwater reference solutions. The model, however, gave overestimated values because sorption was not detected in the experimental sorption tests for Ca²⁺ in 1 M NaClO₄ solutions and for the other alkaline earth metals in brackish and saline reference waters.