Amidoximes as ligand functionalities for braided polymeric materials for the recovery of uranium from seawater

The formation constants of the UO22+ cation with the amidoximate ligands bzam (benzamidoxime) and acetam (acetamidoxime) are reported. These are of interest in light of their proposed use as the functional groups of extractants for uranium in seawater. The formation constants of bzam with UO22+ were measured by monitoring the absorbance of the π → π∗ transitions in the UV spectrum of the bzam ligand in the presence of 1:1 UO22+ as a function of pH. This yielded log K1 = 12.4 for UO22+ with bzam, and log K = 6.9 for the equilibrium UO2(bzam)+ + OH− = UO2(bzam)OH at 25 °C and ionic strength zero. The bzam complexes were also studied monitoring the fluorescence of the UO22+ system. Analysis of the intense fluorescence that occurs in 5 × 10−6 M UO22+ solutions between pH 5 and 9 suggested that this was due to the [(UO2)3O(OH)3]+ trimer. Monomeric species such as UO22+ and [UO2(OH)4]2−, and dimers such as [(UO2)(OH)2]2+, fluoresce only weakly. Titration of such solutions with bzam supported the above log K values measured by absorbance, and with higher bzam concentrations yielded log β2 = 22.3. The acetam ligand does not have any absorbance, so that complex-formation was monitored by fluorescence only. Formation constants measured by fluorescence may differ from those measured by other techniques such as absorbance. The agreement obtained between log K values measured by absorbance and fluorescence for the bzam complex of UO22+ supported the log K values measured for the acetam complexes by florescence alone were reliable: log K1 = 13.6, log β2 = 23.7, and log K UO2(acetam)+ + OH− = UO2(acetam)OH = 6.8. The high log K values found for the bzam and acetam complexes of UO22+ were analyzed using DFT calculations. These log K values are related to the ability of polymer-based extractants bearing bzam or acetam type functional groups to extract UO22+ at a concentration of 1.3 × 10−8 M and in the competing 0.0025 M CO32− present in the oceans.

Graphical abstractThe formation constants of the amidoximate ligands bzam (benzamidoxime) and acetam (acetamidoxime) with the uranyl ion were measured by absorbance and fluorescence spectroscopy, which yield at 25 °C and ionic strength = 0, for bzam: log K1 = 12.4, log β2 = 22.3, for acetam: log K1 = 13.6, log β2 = 23.7. These log K values are related to the possible use of amidoximate groups as the functional groups attached to polymeric materials as the basis for recovering uranium from the oceans.Figure optionsDownload full-size imageDownload as PowerPoint slide