(567f) Phosphate-Functionalized Membranes for the Selective Sequestration of Uranium from Seawater

Phosphate-functionalized
membranes for the selective sequestration of uranium from seawater

Priyanka
Suresh, Christine E. Duval

Department
of Chemical and Biomolecular Engineering, Case Western Reserve University,
Cleveland OH

Increasing global energy demand
and diminishing fossil-fuel resources have led to the exploration of
alternative energy sources. In spite of research advances in renewable energy
sources such as wind, tidal and solar, nuclear energy has captured extensive
attention due to its economic viability and absence of CO₂
emissions. For a sustainable development of nuclear energy, the availability
and accessibility of uranium, the most prevalent nuclear fuel, is vital. Since
terrestrial ores of uranium are limited, there is an interest to sequester some
of the 4.5 billion tons of uranium that exists in seawater. Use of traditional
adsorbents to recover U(VI) from seawater is a challenge because of uranium’s
low concentration (~3.3ppb) and the presence of numerous competing ions.
Amidoxime-based (PAO) sorbents have been extensively studied for this purpose
due to their fast kinetics, high capacity and facile synthesis; however, they
lack uranium selectivity. In fact, when placed in seawater, PAO sorbents bind
greater quantities of magnesium, calcium and vanadium than uranium. One
strategy to improve sorbent capacity is to develop sorbents with high uranium
selectivity. Among the various functional groups, phosphate groups have
attracted increased attention due to their stability, irradiation resistance,
and high complexation stability.

In this work, we present
phosphate-functionalized ultrafiltration membranes that selectively adsorb
uranium from seawater. Membranes were prepared by grafting ethylene glycol
methacrylate phosphate (EGMP) from the surface of polyethersulfone (PES)
membranes by UV-initiated graft polymerization. The surface morphology of the
membranes was studied by scanning electron microscopy before and after
grafting. The presence of phosphate functional groups on the membrane was
supported by Fourier transform infrared spectroscopy (FTIR). The degree of
grafting (DoG) and the number of acidic sites were quantified by acid-base
titrations. Grafting conditions were optimized by evaluating the effect of
multiple solvents on the DoG. Grafted membranes were characterized in terms of
water uptake as well as uranium capacity and selectivity in seawater
conditions. Membrane stability and reuse is demonstrated through uranium
desorption cycles.  This work demonstrates the importance of employing
selective chemistries for the trace-level concentration of analytes in the
environment.