Pergamon Cement and Concrete Research. Vol. 24, No, 3, pp. 573-579. 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All fights re, served 0008-8846/94 $6.00 + .00 CESIUM SELECTIVITY OF (AI+Na)-SUBSTITUTED TOBERMORITE ABSTRACT O.P. Shrivastava and Sridhar Komarneni* Materials Research Laboratory The Pennsylvania State University University Park, PA 16802 (Refereed) (ReceivedJune 17; in final form August 8, 1993) Several synthetic tobermorites with different levels of [Al+Na] substitution were prepared from two different types of starting materials and their cation exchange and cesium selective properties were investigated. The substituted tobermorites were found to have high cation exchange capacities and very high selectivities for Cs + ion. Cesium selectivity of the substituted tobermorites was demonstrated in the presence of divalent cations such as Ca 2+, Mg2+, Ba 2+ and univalent cations such as Na +, K+ and Li+ which are one hundred times more concentrated than the cesium ion. The uptake of Cs is maximum in the presence of highly hydrated Mg 2+ and Li+ ions whereas it is minimum in the presence of less hydrated K+ and Ba 2+ ions due to steric limitations of the tobermorite structure. INTRODUCTION Synthetic tobermorites have now been recognized as a new family of cation exchangers mainly in alkaline or nearly neutral medium [1-6]. Hydrated calcium silicates resemble 2:1 clay minerals in some respects [7]. Megaw and Kelsey [7] ascribed to tobermorite a pseudo- orthorhombic unit cell containing identical layers parallel to (001). The [Si04] tetrahedra are in 'puckered' chains with H atoms attached to 02- anions of the tetrahedra parallel to "o.' The Ca-O octahedra show four 02- neighbors in complex sheets (4Ca2Si3 09) and two 02- in the sheet of the next level. The crystal structure of tobermorite was further refined by Hamid [8] who postulated infinite Si3 (O/OH)9 chains running parallel to 'b' and are linked together by Ca atoms. Ca(l), Ca(2), Ca(3) and Ca(4) are identically coordinated with 7 oxygen atoms forming a tetragonal pyramid. The octahedral coordination of Ca(5) and Ca(6) is distorted due to weak Ca-O interaction. The ion exchange properties of unsubstituted and substituted tobermorites fall into two categories: the reversible exchange as shown by alkali and alkaline earth metal cations like Li+ , Na +, K+, Cs +, Sr 2+, Ba 2+, etc., in [Al+Na]-substituted tobermorites [9-11] and the irreversible type reactions shown by divalent cations like Ni, Co, etc., in unsubstituted tobermorite and other calcium silicates [4-6]. The behavior of the unsubstituted tobermorite and xonoflite towards Mg 2+ is also close to the latter type. The reversible exchange is very similar to the exchange exhibited by zeolites where the kinetics are comparatively fast and full exchange results in only a few minutes to hours [9,10]. Although zeolites and clay minerals can be used for Cs separation from radioactive waste solutions, [Al+Na]-substituted tobermorites * Also with the Department of Agronomy. 573