Temperature-dependent luminescence spectroscopic investigations of U(VI) complexation with the halides F- and Cl-

In the present study we have investigated the complexation of uranyl(VI) with chloride and fluoride using luminescence spectroscopy (TRLFS, time-resolved laser-induced fluorescence spectroscopy). At 25 °C, in the presence of 0 − 0.175 M fluoride, the first single-component emission spectra for all four U(VI)-fluoride complexes, i.e. UO2F+, UO2F2, UO2F3−, and UO2F42− could be extracted. Based on the aqueous speciation derived from the TRLFS data, logK* values at I = 1 M were calculated for all these complexes and extrapolated to infinite dilution using the SIT approach. In the case of chloride, however, quenching of the U(VI)-luminescence hampered the experiments. Thus, U(VI)-complexation was studied with TRLFS at liquid nitrogen temperatures. Samples were prepared at 25 °C with chloride concentrations ranging from 0 to 1.0 M followed by instantaneous freezing and subsequent luminescence spectroscopic measurements at −120 °C. This allowed for the determination of the first luminescence spectra for the UO2Cl+ complex with the TRLFS method. The chloride quench reaction was further studied in the temperature range 1 – 45 °C using Stern-Volmer analysis. By applying the Arrhenius and the Eyring equations we obtained the first thermodynamic parameters for the dynamic quench process, i.e. the activation energy (Ea = 55.0 ± 12.9 kJ/mol), enthalpy (ΔHǂ = 52.5 ± 13.0 kJ/mol), and entropy (ΔSǂ = 103.9 ± 42.8 J/mol∙K).