Selective Cation Exchange in Substituted Tobermorites

Selective cation exchange in tobermorites with three levels of Al³⁺ and Na⁺ substitution for Si⁴⁺ has been investigated. The cation exchange selectivity for a tobermorite with 2 mol% Al³⁺ and 0 mol% Na⁺ substitution increased as follows: Mg²⁺ > Ba²⁺ > Sr²⁺. Cation exchange in the above tobermorite is postulated to take place mainly from edge and planar surface sites and apparently from interlayer Ca²⁺ sites. Tobermorites with 10 and 15 mol% Al³⁺ (and Na⁺) substitution have additional Na⁺ exchange sites in the interlayers and show the following selectivity: Ba²⁺ > Sr²⁺ > Mg²⁺. The cation exchange selectivity in the substituted tobermorites can be explained by the hydrated radii of cations and the steric limitations of the ion exchange cavity in tobermorite; the latter was determined by ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy. These basic selective cation exchange studies of substituted tobermorites are of relevance in nuclear waste treatment and disposal.