Ion-exchange reaction of Cs+ selective layered γ-titanium and γ-zirconium phosphates

The H+/M-A(+) ion-exchange reaction of layered gamma-titanium and gamma-zirconium phosphates, M-T(HPO4)(2).nH(2)O, was examined, where MA is an alkali metal and MT is Ti or Zr. They showed remarkably high ion-exchange selectivity for Cs+ and Rb+ in acidic media. The selectivity for Na+ was rather low, and Li+ exchange hardly occurred. The H+ ion exchange of both gamma-phosphates with Cs+ and Rb+ apparently occurred in two steps. These ion-exchange reactions occurred first at a pH value of around 2.0 to form a monobasic structure, MTMAH(PO4)(2).nH(2)O, accompanied by dehydration of the water of crystallization. The second step occurred at a pH around 8.0 to form a dibasic structure, M-T(MAPO4)(2).nH(2)O, the interlayer space of both gamma-phosphates swelled again. The lattice of crystals gradually collapsed corresponding to further progress of the ion-exchange reaction with Cs+ and Rb+, and the amorphous phase finally appeared. According to Cs-133 NMR study, Cs+ was found to be rather restricted. The results were compared with those of layered synthetic mica. The high Cs+ and Rb+ selectivity was ascribed to the dehydration property and strong interactions of these ions with the host lamella layers. The high selectivity of two gamma-phosphates for Cs+ in acidic media suggested a potentiality as promising materials for the recovery from radioactive waste. The interfering effect of coexisting Fe2+ was scarcely observed upon Cs+ exchange, and was even positive due to the prevention of lattice coagulation.