Prevention of UVI Precipitation in Alkaline Aqueous Solutions by the Siderophore Desferrioxamine B

In alkaline and saline solutions, uranium VI (U^VI) forms uranyl salts, limiting its mobility in leachates released from nuclear waste repositories into groundwater. However, recent experimental and computational work suggested that natural organic molecules widely present in groundwater such as siderophores could potentially prevent solid precipitation because of the formation of stable UVI-siderophore complexes. It is important we assess the impact of siderophores on aqueous U^VI chemistry as they could mobilise U^VI from contaminated land and radioactive waste storage and disposal sites. Here we test this hypothesis by combining for the first time experimental studies on uranium precipitation in alkaline pH in the presence of desferrioxamine B (DFOB) and electron structure method calculation of uranyl – hydroxamate complexes to assess their stability. Stirred batch experiments containing 0 to 420 µM DFOB, 42 µM U^VI and 0.1 M NaCl were conducted at pH 11.5. DFT was employed to explore the relative stability of different U^VI-hydroxamate complexes, representative of the local binding mode of DFOB. During the stirred batch experiments, 5%, 11-12%, 41-53%, 95-96% and 100% of U^VI passes through the filter membranes (0.2-1 µm pore diameter) after 24 hours when 0, 4.2, 42, 130 and 420 µM DFOB was added to solution. The DFT results suggest one hydroxamate functional group is most likely to complex with U^VI with ∆_rG calculated as +3 kJ/mol and -9 kJ/mol for [UO₂(OH)₃(L_mono)]^2- and [UO₂(OH)₂(L)]^- respectively. Conversion of the experimentally derived log β (-1.2 ± 0.3) through the equation ∆_rG = -2.303RTlogβ provides ∆_rG of +7 kJ/mol, similar to the ∆_rG of these two complexes. The results of our study confirm that U^VI precipitation could be hindered by the formation of a DFOB complex with U^VI complexation through a single hydroxamate functional group as a likely mechanism. These results highlight the mobilising effect siderophores have on U^VI from contaminant sources and need to be incorporated in environmental risk assessment studies.