3 research outputs found
Synthesis, Development, and Testing of High-Surface-Area Polymer-Based Adsorbents for the Selective Recovery of Uranium from Seawater
The ocean contains uranium with an
approximate concentration of
3.34 ppb, which can serve as an incredible supply source to sustain
nuclear energy in the United States. Unfortunately, technology currently
available to recover uranium from seawater is not efficient enough
and mining uranium on land is still more economical. We have developed
polymer-based adsorbents with high uranium adsorption capacities by
grafting amidoxime onto high-surface-area polyethylene (PE) fibers.
Various process conditions have been screened, in combination with
developing a rapid testing protocol (<24 h), to optimize the process.
These adsorbents are synthesized through radiation-induced grafting
of acrylonitrile (AN) and methacrylic acid (MAA) onto PE fibers, followed
by the conversion of nitriles to amidoximes and basic conditioning.
In addition, the uranium adsorption capacity, measured in units of
g<sub>U</sub>/kg<sub>ads</sub>, is greatly increased by reducing the
diameter of the PE fiber or changing its morphology. An increase in
the surface area of the PE polymer fiber allows for more grafting
sites that are positioned in more-accessible locations, thereby increasing
access to grafted molecules that would normally be located in the
interior of a fiber with a larger diameter. Polymer fibers with hollow
morphologies are able to adsorb beyond 1 order of magnitude more uranium
from simulated seawater than current commercially available adsorbents.
Several high-surface-area fibers were tested in natural seawater and
were able to extract 5–7 times more uranium than any adsorbent
reported to date
Elution of Uranium and Transition Metals from Amidoxime-Based Polymer Adsorbents for Sequestering Uranium from Seawater
High-surface-area amidoxime and carboxylic
acid grafted polymer
adsorbents developed at Oak Ridge National Laboratory were tested
for sequestering uranium in a flowing seawater flume system at the
PNNL-Marine Sciences Laboratory. FTIR spectra indicate that a KOH
conditioning process is necessary to remove the proton from the carboxylic
acid and make the sorbent effective for sequestering uranium from
seawater. The alkaline conditioning process also converts the amidoxime
groups to carboxylate groups in the adsorbent. Both Na<sub>2</sub>CO<sub>3</sub>–H<sub>2</sub>O<sub>2</sub> and hydrochloric
acid elution methods can remove ∼95% of the uranium sequestered
by the adsorbent after 42 days of exposure in real seawater. The Na<sub>2</sub>CO<sub>3</sub>–H<sub>2</sub>O<sub>2</sub> elution method
is more selective for uranium than conventional acid elution. Iron
and vanadium are the two major transition metals competing with uranium
for adsorption to the amidoxime-based adsorbents in real seawater.
Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt, 1 M)
can remove iron from the adsorbent very effectively at pH around 7.
The coordination between vanadiumÂ(V) and amidoxime is also discussed
based on our <sup>51</sup>V NMR data
Investigations into the Reusability of Amidoxime-Based Polymeric Adsorbents for Seawater Uranium Extraction
The ability to reuse amidoxime-based
polymeric adsorbents is a
critical component in reducing the overall cost of the technology
to extract uranium from seawater. This report describes an evaluation
of adsorbent reusability in multiple reuse (adsorption/stripping)
cycles in real seawater exposures with potassium bicarbonate (KHCO<sub>3</sub>) elution using several amidoxime-based polymeric adsorbents.
The KHCO<sub>3</sub> elution technique achieved ∼100% recovery
of uranium adsorption capacity in the first reuse. Subsequent reuses
showed significant drops in adsorption capacity. After the fourth
reuse with the ORNL AI8 adsorbent, the 56-day adsorption capacity
dropped to 28% of its original capacity. FTIR spectra revealed that
there was a conversion of the amidoxime ligands to carboxylate groups
during extended seawater exposure, becoming more significant with
longer exposure times. Ca and Mg adsorption capacities also increased
with each reuse cycle supporting the hypothesis that long-term exposure
resulted in converting amidoxime to carboxylate, enhancing the adsorption
of Ca and Mg. Shorter seawater exposure (adsorption/stripping) cycles
(28 vs 42 days) had higher adsorption capacities after reuse, but
the shorter exposure cycle time did not produce an overall better
performance in terms of cumulative exposure time. Recovery of uranium
capacity in reuses may also vary across different adsorbent formulations.
Through multiple reuses, the AI8 adsorbent can harvest 10 g uranium/kg
adsorbent in ∼140 days, using a 28-day adsorption/stripping
cycle, a performance much better than would be achieved with a single
use of the adsorbent through a very long-term exposure (saturation
capacity of 7.4 g U/kg adsorbent). A time dependent seawater exposure
model to evaluate the cost associated with reusing amidoxime-based
adsorbents in real seawater exposures was developed. The predicted
cost to extract uranium from seawater ranged from 830/kg
U. Model simulation suggests that a short seawater exposure cycle
(<15 days) is the optimal deployment period for lower uranium production
cost in seawater uranium mining