Recovery of Uranium from Seawater: Preparation and Development of Polymer-Supported Extractants

A new series of polymer-supported extractants is proposed for the removal and recovery of uranium from seawater. The objective is to produce polymers with improved stability, load ing capacity, and sorption kinetics compared to what is found w ith amidoximes. The target ligands are diphosphonates and aminomethyldiphosphonates. Small molecule analogues, especially of aminomethyld iphos-phonates, have exceptionally high stability constants for the uranyl ion. The polymeric diphosphonates will have high affinities due to their ability to form six-membered rings with the uranyl ion while the aminomethyldiphosphonates may have yet higher affin ities due to possible tridentate coordination and their greater acidity. A representative set of the polymers to be prepared are indicated below. The preparations to be developed will be from readily availab le starting materials in a minimum number of steps. The structures will be analyzed through FTIR and XPS spectra, supplementing zero point charge determinations and elemental analyses. The uranyl distribution coefficients will be determined from synthetic seawater containing 5 ppm UO{sub 2}{sup 2+} to allow for accurate analysis by ICP. With solutions having higher concentrations of the uranyl ion, adsorption isotherms and loading capacities will be determined. Polymers w ith high distribution coefficients will be evaluated with authentic seawater samples and uranyl levels of 3 ppb using radiotracer techniques. Rate studies will measure the sorption kinetics. Regeneration of the polymers after loading w ith the uranyl ion will be studied w ith a series of regenerants, including sodium carbonate, 1-hydroxyethyl-1,1-diphosphonic acid, and oxalic acid. The optimum ligand w ill be immobilized onto polypropylene, thus allow ing for its application to uranium recovery from seaw ater. The approach taken w ill be the formation of polypropylene fibers grafted with vinylbenzyl chloride using supercritical CO{sub 2} technology. The scCO{sub 2} allows the monomer and free radical initiator (benzoyl peroxide) to enter deep within the polymer to give high grafting yields. The aromatic groups can then be functionalized in exactly the same w ay that the beads were modified. Uniform functionalization is expected because the porosity of the fiber permits accessibility of the reagents. Removal of the scCO{sub 2} solvent is obviously implemented simply by reducing the pressure. Conditions that minimize homopolymer formation and any degradation of the polypropylene have been defined with maleic anhydride as the monomer and should extend to vinylbenzyl chloride