Radiation-Grafted Fabrics for the Extraction of Uranium from Seawater
Much interest has been generated in extraction uranium from the ocean - the world's largest uranium reserve. This dissertation describes the development and seawater testing of a polymeric adsorbent for uranium based on radiation-induced grafting. Among all monomers and polymeric substrates tested, grafting of the monomer bis(2-methacryloxyethyl) phosphate (B2MP) onto Winged nylon fabric was determined to produce adsorbents of the highest degrees of grafting. Degree of grafting was optimized by irradiating at a range of dose rates and total absorbed doses and by varying monomer concentration, solvent, purging gas and radiation source. Both the University of Maryland's Co-60 gamma irradiator and 1-9 MeV pulsed LINAC were utilized. The grafted adsorbents were tested for uranium extraction capacity using a Uranium-233 radiotracer in synthetic seawater at natural (3.3 ppb) uranium concentrations. It was determined that adsorbents of degrees of grafting between 75 and 100% obtained the highest distribution coefficients for uranium. Kinetic studies revealed an increase in Uranium-233 concentration on the adsorbent over the course of 4 hours after which time a steady-state was reached. Correlation of this data with kinetic models indicated pseudo-second order kinetics, suggesting the rate-limiting adsorption mechanism as chemical complexation between Uranium-233 and the phosphate-containing adsorbent. Overall, the highest performing adsorbents obtained distribution coefficients of 1.2 × 104 mL/g and Uranium-233 loadings of 1.0 × 10-2 mg-U/g-adsorbent. These values were a result of performing the extractions at 3.3 Uranium-233, a concentration several orders of magnitude lower than those reported in similar studies. The chemical changes that occurred upon grafting were investigated with FTIR and Raman analysis of virgin, irradiated and B2MP-grafted Winged nylon. Characterization of the grafted adsorbents with SEM revealed a unique morphology for the grafted fabrics that has been attributed to the precipitation of homopolymer from the solvent during irradiation. SEM/EDS analysis of a grafted adsorbent contacted with Uranium-233-spiked synthetic seawater revealed the presence of several elements abundant in seawater, indicating that competition between uranium and other seawater ions is likely to limit the uranium uptake capacity of the adsorbent.