Polymer-Supported Bifunctional Amidoximes for the Sorption of Uranium from Seawater

Bifunctional amidoxime fibers are synthesized from commercially available polyacrylonitrile as sorbents for the recovery of U(VI) from seawater. The purpose of introducing a second ligand is to enhance the affinity of the amidoxime (AO) ligand via two possible mechanisms: by acting directly upon the AO to increase its ability to ion exchange or providing additional coordination sites to the uranyl ion. Amines are chosen as the second ligand since they have a high affinity for U(VI) at ppm levels from synthetic seawater. Diethylenetriamine (DETA) is utilized as the amine because it is sufficiently flexible to interact with AO and able to bind metal ions. Along with AO and AO-DETA fibers, the primary NH 2 moieties were modified with phosphonic acid ligands (AO-phon-DETA). All fibers have high affinities (>99% sorption) for the uranyl ion when initially present at 0.90 ppm in synthetic seawater. When contacted with actual seawater at the Pacific Northwest National Laboratory for 20.8 days, the loadings for the fibers are 124 μg/g (AO), 535 μg/g (AO-DETA), and 789 μg/g (AO-phon-DETA). For comparison, a polyethylene fiber on which was grafted polyacrylonitrile and then converted to AO at the Oak Ridge National Laboratory gave a uranyl loading of 2610 μg/g at the same contact time, perhaps due to its optimized support structure and/or the presence of diamidoxime groups. FTIR spectra indicate that the effect of DETA is through a hydrogen bonding interaction with the AO to facilitate dissociation of the acidic N–O–H. Continuing research on optimizing the loading onto the bifunctional fibers emphasizes the effect of phosphorus capacity, amine ligand, and synthesis conditions.