Cation-Exchange Properties of (Al + Na)-Substituted Synthetic Tobermorites

Tobermorite, Ca 5 Si 6 O 16 (OH) 2 ·4H 2 O, is a hydrous calcium silicate that has a layer-type of structure similar to that of the 2:1 clay minerals. In its natural form, tobermorite exhibits little or no exchange for alkali cations; synthetic tobermorites, however, exhibit reversible alkali cation exchange and selective cesium uptake upon a coupled substitution of (Al 3+ + Na + ) for Si 4+ . Substituted tobermorites were synthesized using aluminosilicate gels, NaOH, and CaO, in Parr bombs at 175°C for 4 days. Unsubstituted tobermorite was synthesized using quartz and CaO in a Parr bomb at 175°C for 20 hr. Two (Al + Na)-substituted tobermorites showed cation-exchange capacities (CEC) of 77 and 71 meq/100 g, whereas an unsubstituted tobermorite showed a CEC of 12 meq/100 g. The substituted tobermorites exhibited selective Cs exchange from either NaCl or CaCl 2 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 CaCl 2 solution containing 0.0002 moles/liter CsCl. Exchange isotherms for Na + ⇋ 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.