Review and Perspective on Technology for Uranium Extraction from Seawater Using Amidoxime-based Materials

The development of technology for uranium extraction from seawater is briefly described with regard to the aspects of material screening and evolution of deployment methods. By evaluating the developed materials of possibly practicable in last seventy years, grafted polymers of amidoxime functionality arc uniquely addressed in a broader perspective of economic interests. Two representative grafting methods for preparing absorbents bearing amidoxime groups with good practical prospects are summarized. The philosophy of approaching more grafting degree and more functional group intensity is scrutinized, finding that the amount and intensity of amidoxime group on the adsorbents are not proportionally related to the extraction capability of uranium from seawater. The contribution made in recent years to the coordination chemistry of some related small molecule ligands with uranyl ion and other coexisting metal ions is reviewed in detail. Especially the complexation of uranyl ion with two small molecules, succinimidc-dioxime and succindiamodoximc which represent the minor structures from less favorablc 'head-to-head' connection of polymerization of acrylonitrile, is comparatively compiled with that of glutarimidedioxime and glutardiamidoxime which represent the major structures from the favorable 'heat-to-end' connection. With similar structure and same functional group, the two cyclic compounds, succinimidcdioxime and glutarimidedioxime present clear difference in complexation with uranyl ion under seawater conditions. The six-member ring glutarimidedioxime favorably forms mononuclear 1 : 2 uranyl complex, while the five-member ring succinimidcdioxime tends to give di-nuclear 2 : 2 uranyl complex. The conversion reactions of the amidoxime functional groups induced by metal ions under different conditions are tapped to emphasize the variety of functional groups of the absorbents, which might provide additional approaches to optimize the adsorbents with more favorable functional groups for economically catching uranium from seawater. The cyclic glutarimidedioxime functional group can be easily prepared from the open chain glutardiamidoxime through the reaction induced with proton and nickel ions together at room temperature instead of the reaction of heating to over 120°C in traditionally used methods. The unreached functional groups of hydroxamic acids derived from the hydrolysis of the major functional groups in previous studies are proposed for examining. The restrictive factors for the uranium extraction from seawater are analyzed in regard with the open environment of the ocean and the very low concentration of uranium. The possible focusing points of the research and development on the technology for uranium extraction is tentatively predicted in respect to the practical employment method and the exact extraction mechanisms. In contract with the philosophy of emphasizing the selectivity of the adsorbents for uranium in previous research, it is suggested that a comprehensive approach of economically extracting uranium might be to take the co-extraction of other valuable metal ions, such as lithium and gold, from seawater into account. © 2023 Atomic Energy Press. All rights reserved.