Search

Amidoxime-based polymer adsorbents have attracted interest within the last decade due to their high adsorption capacities for uranium and other rare earth metals from seawater. The ocean contains an approximated 4-5 billion tons of uranium and even though amidoxime-based adsorbents have demonstrated the highest uranium adsorption capacities to date, they are still economically impractical because of their limited recyclability. Typically, the adsorbed metals are eluted with a dilute acid solution that not only damages the amidoxime groups (metal adsorption sites), but is also not strong enough to remove the strongly bound vanadium, which decreases the adsorption capacity with each cycle. We resolved this challenge by incorporating Le Chatelier's principle to recycle adsorbents indefinitely. We used a solution with a high concentration of amidoxime-like chelating agents, such as hydroxylamine, to desorb nearly a 100% of adsorbed metals, including vanadium, without damaging the metal adsorption sites and preserving the high adsorption capacity. The method takes advantage of knowing the binding mode between the amidoxime ligand and the metal and mimics it with chelating agents that then in a Le Chatelier's manner removes metals by shifting to a new chemical equilibrium. For this reason the method is applicable to any ligand-metal adsorbent and it will make an impact on other extraction technologies.

Studies of the complexation of U(vi) with amidoxime-related ligands help in the development of efficient sorbents for the extraction of uranium from seawater. In the present study, the stability constants of the U(vi) complexes with two phthalimidedioxime ligands were determined by potentiometry and spectrophotometry, and compared with glutarimidedioxime previously studied. Density functional theory calculations were performed to identify the most probable protonation sites of the ligand and to help interpret the trend in the binding strength of the ligands. The phthalimidedioxime complexes were found to be 2-3 orders of magnitude weaker than the corresponding glutarimidedioxime complexes, which was attributed to the difference between the ligands in the electronic and structural properties. The incorporation of the aromatic ring into phthalimidedioxime reduces the electron density on the donor atoms of the ligand and makes the imidedioxime moiety less complementary for binding UO2(2+)via its equatorial plane. Though weaker than glutarimidedioxime, phthalimidedioxime still forms fairly strong U(vi) complexes and can still effectively compete with carbonate for the complexation of U(vi) at seawater pH and carbonate concentration. Due to its higher chemical stability in acidic solutions than that of glutarimidedioxime, phthalimidedioxime is a valuable ligand that could have potential use in the extraction of U(vi) from seawater.