Selective Cation Exchange in Substituted Tobermorites

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Abstract
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.