Cesium retention in powdered granite under saline and alkaline conditions: Role of biotite in deep geological repositories

Natural granite barriers in deep geological repositories can serve as a final barrier against the migration of hazardous radionuclides such as ¹³⁷Cs into the ecosystem. In this study, the Cs retention characteristics of natural granite barrier under evolved repository conditions (i.e., alkaline and saline conditions) were investigated with a particular focus on the role of biotite. The partial dissolution of aluminosilicate minerals and some exfoliation at the edges of biotite were observed in alkaline solutions, whereas no changes were observed in saline solutions. These alterations slightly affected the Cs retention capabilities of granite samples under alkaline conditions during the specified test period. Cs retention in granite mainly occurred via cation exchange with K ions within the interlayers of biotite. During the ion exchange process, Cs ions competed with major cations in both alkaline and saline solutions, such as Mg, K, and Na, varying Cs retention capacity. Moreover, this variation was particularly pronounced in saline solutions. These findings indicate a possible mechanism for the retention of radioactive cations in natural granite barriers under long-term evolved repository conditions. Accordingly, natural granite barriers enriched with biotite can effectively retain radioactive cations and inhibit their migration into the ecosystem.