Studies of aqueous U(VI)-thiosalicylate complex formation via UV-Vis absorption spectrophotometry, TRLFS and potentiometry

Abstract The formation of U(VI)–thiosalicylate (TSal) complexes in aqueous solutions ( I  = 0.1 M) was investigated over a wide range of pH values (from 3 to 9) using UV–Vis absorption and time-resolved luminescence spectroscopy, as well as by a potentiometric technique. It is demonstrated that the formation of U(VI)–TSal complexes, including monodentate, bidentate and ternary complexes, is responsible for the enhanced solubility of U(VI) and the strong U(VI)-luminescence quenching effects by TSal. In this study the stability constants of the complexes as well as two p K a values of TSal (p K a1  = 3.76 ± 0.05; p K a2  = 8.33 ± 0.07) are provided. The spectrophotometric analysis confirms the presence of both 1:1 and 1:2 (U(VI):TSal) charge-transfer complexes (log  β 101  = 7.59 ± 0.04 and log  β 102  = 13.67 ± 0.09). In luminescence quenching studies, mixed (dynamic and static) quenching processes are observed. The dynamic quenching for UO 2 2+ is attributed to bimolecular excited-state processes by both the acidic and monobasic forms of TSal, i.e., H 2 TSal and HTSal − . The static quenching processes are explained by the formation of non-radiative ground-state complexes: UO 2 (HTSal) + (log  β 111  = 12.11 ± 0.09) at pH 3.0; UO 2 (OH)(TSal) − and UO 2 (OH) 2 (TSal) 2− (log  β 1–11  = 3.9 ± 0.1 and log β 1–21  = −3.8 ± 0.1, respectively) at a higher pH. In particular, the formation of the latter ternary species is found to be responsible for the strong static quenching of hydroxo-uranyl species, i.e. (UO 2 ) 2 (OH) 2 2+ and (UO 2 ) 3 (OH) 5 + .