Complexation of U(VI) with iodate ions: determination of stability constants by using spectroscopic methods and capillary zone electrophoresis

The uranyl iodate system has been studied by using three speciation methods: laser-induced time-resolved fluorescence spectroscopy (TRLIFS), UV-visible spectrophotometry and capillary zone electrophoresis with direct UV detection (CZE). Two uranyl iodate complexes were found: UO2IO3- and UO2(IO3)(2) in 0.05 M NaClO4 solutions of pH 2.6, depending on the iodate concentration. The three methods are convenient and efficient for determining the complex formation constants of the two complexes. The variation of the uranyl mobility as a function of the ligand concentration is very simple to analyse and its fitting leads to accurate thermodynamic data. The values obtained for log beta(11)(1.6 +/- 0.1) and log beta(12)(3.05 +/- 0.1) are in good agreement with the previously published values. Both complexes were also characterized by their fluorescence spectra and fluorescence lifetime. From the analysis of the fluorescence spectra, taking into account the dynamic quenching, the complex formation constants were calculated, in reasonable agreement with the ones obtained by capillary electrophoresis: log beta(11)(1.5 +/- 0.2) and log beta(12)(3.6 +/- 0.2).