Cation-exchange properties of (Al + Na)-substituted synthetic tobermorites.

In its natural form, tobermorite exhibits little or no exchange for alkali cations; synthetic tobermorites, however, exhibit reversible alkali cation exchange and selective caesium uptake upon a coupled substitution of (Al³⁺ + Na⁺) for Si⁴⁺. Substituted tobermorites were synthesized using aluminosilicate gels, NaOH and CaO, in Parr bombs at 175oC for 4 days. Unsubstituted tobermorite was synthesized using quartz and CaO in a Parr bomb at 175oC for 20 hr. Two (Al + Na)-substituted tobermorites showed c.e.c. of 77 and 71 me./100 g, whereas an unsubstituted tobermorite showed a c.e.c. of 12 me./100 g. The substituted tobermorites exhibited selective Cs exchange from either NaCl or CaCl₂ solutions. For example, one substituted tobermorite showed a Cs-exchange coefficient (K_d) of 15 100, whereas unsubstituted tobermorite showed a K_d of only 90 from a 0.02 N CaCl₂ solution containing 0.0002 moles/litre CsCl. Exchange isotherms for Na⁺ reverse= Cs⁺ showed that Cs⁺ is preferred over Na⁺ throughout the exchange in the (Al + Na)- substituted tobermorites. This group of cation exchangers is expected to find applications in radioactive waste disposal.-R.A.H.