Radiation stability of synthetic organic ion-exchange resins is not sufficient for the processing of very highly radioactive solutions because these solutions cause significant changes in the capacity and selectivity by hydrolysis of functional groups, chain scission and changes in degree of crosslinking1,2. Among the inorganic ion exchangers, zeolites and clay minerals have been used in decontaminating radioactive waste solutions 3-6 because of their high ion-exchange capacity, selectivity and presumably their radiation stability7; however, their use is limited because of their instability in mildly acid solutions1. Another class of materials - insoluble acid salts formed by polybasic acids and certain hydrolysable polyvalent cations - also possess cation exchange properties. These salts include phosphates, arsenates, tungstates and molybdates of zirconium, thorium, titanium and other metals2. Among these insoluble acid salts, zirconium phosphate gels have been investigated8 very extensively. Crystalline phases of zirconium phosphate were first prepared in 1964 and 1968 from gels on refluxing in phosphoric acid by Clearfield and co-workers9,10. We report here the discovery of crystalline γ-zirconium phosphate as a highly selective caesium ion sieve which can be used to separate 137Cs from accident waste water or from circulating water in nuclear reactors.
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