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 NH2 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.
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