Recent progress in the field of synthetic inorganic exchangers having a layered or fibrous structure

Abstract

In the first part, recent results on crystalline insoluble acid salts of tetravalent metals having a layered structure are critically examined. The dependence of the interlayer distance of crystalline zirconium phosphate on its water content and counter ions is discussed and an ion-exchange mechanism for monovalent and divalent ions, in both aqueous and molten salt mediam, is proposed. Furthermore, the effect of the degree of crystallinity of zirconium phosphate on its ion-exchange properties and electrical conductivity, the dependence of its selectivity for large cations on steric hindrance and the catalytic effect of small amounts of sodium ions on the exchange of large hydrated cations are also discussed. Other layered exchangers, such as zirconium arsenate and titanium phosphate, are also considered and some general points on their ion-exchange properties as a function of the tetravalent metal ion and polybasic acid groups involved are made. The second part deals with the synthesis and ion exchange properties of fibrous inorganic ion exchangers with special regard to fibrous cerium(IV) phosphate. In the third part, some practical applications of the exchangers examined, in both ion-exchange and electrochemical processes, are reported.