CATION-EXCHANGE PROPERTIES OF (A1 + Na)-SUBSTITUTED SYNTHETIC TOBERMORITES

Abstraet--Tobermorite, CasSi60~6(OH)2"4H20, is a hydrous calcium silicate that has a layer-type of structure similar to that of the 2:l 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 f (Al 3 § + Na § for Si 4+. Substituted tobermorites were synthesized using aluminosilicate g ls, NaOH, and CaO, in Parr bombs at 175 ~ for 4 days. Un-substituted tobermorite was synthesized using quartz and CaO in a Parr bomb at 175 ~ for 20 hr. Two (A1 + 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 NaC1 or CaClz solutions. For example, one substituted tobermorite showed a Cs-exchange coefficient (Ka) of 15,100, whereas unsubstituted tobermorite showed a K ~ of only 90 from a 0.02 N CaC12 solution containing 0.0002 moles/liter CsC1. Exchange isotherms for Na § = Cs § showed that Cs § is preferred over Na + throughout the exchange inthe (A1 + Na)-substituted tobermorites. This group of cation exchangers is expected to find applications in radioactive waste disposal