Abstract
Tobermorite minerals, calcium silicate hydrates substituted with Al3+ and alkali, exhibit cation exchange and selectivity properties. The total cation exchange capacities of the Al3+ and alkali substituted tobermorites synthesized here range from 128 to 197 meq per 100g. These substituted tobermorites also have high selectivity for caesium and rubidium. For examples, a tobermorite synthesized from Na2SiO3, AlCl3 and CaO has a caesium adsorption coefficientK d of 7200 while another one synthesized from a zeolite and CaO has a caesium adsorptionK d of 12 850 in 0.02N CaCl2 containing 0.0002N CsCl. Pure tobermorites and tobermorites with only Al3+ substitution do not exhibit much cation exchange capacity or selectivity because of cation hydration effects and steric factors of tobermorites. The cation exchange and selectivity properties of the Al3+ and alkali substituted tobermorites fall between those of clay minerals and zeolites. Unlike clay minerals and zeolites, the new group of cation exchangers is expected to be thermodynamically stable in cement and concrete which have a similar chemical composition. Therefore, the new cation exchangers will be suitable for inexpensive solidification in cement after their use, for example, in decontamination of caesium from low-level nuclear wastes.
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