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=0.1M) 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 pKa values of TSal (pKa1=3.76±0.05; pKa2=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 (logβ101=7.59±0.04 and logβ102=13.67±0.09). In luminescence quenching studies, mixed (dynamic and static) quenching processes are observed. The dynamic quenching for UO22+ is attributed to bimolecular excited-state processes by both the acidic and monobasic forms of TSal, i.e., H2TSal and HTSal−. The static quenching processes are explained by the formation of non-radiative ground-state complexes: UO2(HTSal)+ (logβ111=12.11±0.09) at pH 3.0; UO2(OH)(TSal)− and UO2(OH)2(TSal)2− (logβ1–11=3.9±0.1 and logβ1–21=−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. (UO2)2(OH)22+ and (UO2)3(OH)5+.