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

Sridhar Komarneni1, Else Breval, Michihiro Miyake and Rustum Roy
Materials Research Laboratory, The Pennsylvania State University University Park, Pennsylvania 16802
1 Also associated with the Department of Agronomy.

Abstract: Tobermorite, Ca5Si6O16(OH)2·4H2O, 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 (Al3+ + Na+) for Si4+. 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 CaCl2 solutions. For example, one substituted tobermorite showed a Cs-exchange coefficient (Kd) of 15,100, whereas unsubstituted tobermorite showed a Kd of only 90 from a 0.02 N CaCl2 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.

Key Words: Cation exchange • Cation substitution • Cs-exchange • Synthesis • Tobermorite

Clays and Clay Minerals; October 1987 v. 35; no. 5; p. 385-390; DOI: 10.1346/CCMN.1987.0350509
© 1987, The Clay Minerals Society
Clay Minerals Society (www.clays.org)