The effect of siderophores on the aqueous chemistry of uranium VI: a combined experimental and computational approach

Abstract

Understanding aqueous uranium VI (UVI) chemistry in alkaline environments (pH >10) is crucial as radioactive waste can be stored and disposed in these conditions. Naturally occurring organic molecules can interact with UVI, modifying its aqueous chemistry and subsequent groundwater facilitated transport. The aim of this research project is to characterise the effect of the (tris)hydroxamate siderophore desferrioxamine B (DFOB) on UVI aqueous chemistry in alkaline solutions. Initially, the physico-chemical properties of UVI precipitates were characterised in solutions containing 42 µM UVI and 0.1 M NaCl. UVI formed 640 ± 111 and 837 ± 142 nm diameter Na6U7O24 precipitates at pH 10.5 and 11.5 respectively. These were usually physically immobilised in quartz sand columns. When ≥130 µM DFOB was simultaneously added with UVI to pH 11.5, 0.1 M NaCl solutions, UVI quantitatively passes through 0.2 µm filter membranes. This could be due to the formation of an aqueous UVI-DFOB complex as observed below pH 10. To further explore complex formation, a density functional theory protocol was established. The protocol predicts the stability constants (log β) of UVI-organic ligand complexes with root mean square deviation of 1.19 log β units after calibration against experimental data collected in acidic solutions. The relative stability series for UVI complexes with key siderophore functional groups calculated using the fitting equation is: α-hydroxycarboxylate bound via the α-hydroxy and carboxylate groups (log β110 = 17.08), α-hydroxyimidazolate (log β110 = 16.55), catecholate (log β110 = 16.43), hydroxamate (log β110 = 9.00), hydroxy-phenyloxazolonate (log β110 = 8.43) and α-aminocarboxylate (log β110 = 4.73). Finally the DFT protocol was adapted so that the stability of UVI-hydroxamate complexes can be approximated at pH 11.5. This suggests DFOB complexes in a monodentate fashion via one hydroxamate group. These results highlight the significant effect siderophores can play on aqueous UVI chemistry.