New mechanism in deactivation of excited uranyl(VI) ion by lanthanide(III) ions

A new type of quenching effect of excited uranyl(VI) ion by lanthanide(III) ions is reported. With the exception of Eu(III) and Tb(III) ions, the lanthanide(III) ions are able to quench the excited uranyl(VI) ion. In acidic solutions, where inner sphere coordination is not involved, the quenching efficiency indicated a dependence on the number of unpaired 4f electrons in the tervalent lanthanide ions. The maximum value of k(q) = 6.4 x 10⁴ mol dm^-3 s^-1 was determined for Gd(III) in which the number of unpaired 4f electrons is 7, while a small value of k(q) = 0 was found for Yb(III) which has only one 4f electron. The notable feature is that Eu(III) showed absolutely no quenching effect on the excited uranyl(VI) ion up to [Eu³⁺] = 1.25 x 10^-1 mol dm^-3 and, most strikingly, a negative quenching effect was found for Tb(III) ion. The proposed quenching mechanism is based on the interaction involving 4f electrons of lanthanide(III) ions, which has not been reported so far. The mechanism of the energy transfer from the excited uranyl(VI) ion to lanthanide(III) ions is discussed, which is explained based on the electron exchange interaction.