XAFS Investigation of Uranium Binding by Seawater-Contacted Amidoxime Adsorbents
Extraction of uranium from seawater constitutes an important avenue for ensuring nuclear fuel availability beyond the coming century. With a predicted 100 years of uranium remaining in terrestrial ores, efficient extraction of the 4.5 billion tons of uranium dissolved in ocean water would enable access to an effectively limitless supply. More near-term benefits include establishment of a backstop technology, preventing runaway price inflation for this critical resource. The current state-of-the-art material for seawater extraction of uranium is a polymeric adsorbent composed of a robust trunk material subsequently surface functionalized by copolymerization of polyacrylonitrile and a hydrophilic comonomer. Treatment of this material with hydroxylamine results in the formation of poly-amidoxime functionalities which have been known to bind strongly with uranium since the early 1980's. Despite more than three decades of investigation, many questions remain unanswered regarding how these adsorbents bind uranium in seawater. The physical characteristics of the polymer prohibit many classical characterization techniques, while most spectroscopic approaches are impeded by the relatively low concentration of uranium compared to the polymer itself and other metals extracted by the adsorbent. The traditional approach has been to investigate small molecule analogues of uranyl - dioxouranium (VI), the form of uranium present in seawater - as bound by amidoxime or amidoxime derivatives, and assert the findings to be representative of the larger polymer system. Computational efforts have also been recently leveraged to a similar end. This approach is not without difficulty, as it has been routinely proposed that the treatment with hydroxylamine and subsequent alkaline conditioning results in formation of a cyclic imide dioxime site, which is truly responsible for uranium binding, as opposed to the 'open chain' amidoxime. Further complications ensue from considering the possibility of cooperative binding interactions from the different hydrophilic comonomers which are known to be necessary for optimizing uranium capacity. Finally, application of different surface polymerization techniques has the potential to directly affect the morphology of the surface polymer by influencing the degree of grafting, extent of polymer crosslinking, and polymer density. The impact of these morphological considerations on the uranyl binding environment cannot be easily replicated with small molecule studies or by computation. Ultimately, the precise binding environment of uranium is unknown, but constitutes critical knowledge for the design and engineering of advanced adsorbent materials capable of selective extraction of uranium from seawater. Analysis of the EXAFS fits reveals three of the four polymer morphologies appear to bind uranium in different chemical environments, none of which were predicted by crystallographic study or computational investigation, challenging numerous long-held assumptions. Both AI-8 and AN/HEA spectra possess features which could be attributable to cooperative binding of uranium by comonomers, but this cannot be definitively determined by EXAFS without further investigation. Unlike AF-1, neither AI-8 or AN/HEA require an adjacent μ{sup 2}-oxo-bridged transition metal to obtain a reasonable fit. The fit for Cyclic AF-1 is refined to almost identical parameters to the fit of the original AF-1 adsorbent, with the only difference is the contracting of the light elements in the second coordination sphere. It is thus proposed that the DMSO and heat treatment of the AF-1 fiber does very little to change the uranium binding site, and any differences in uptake are more readily explained by changes in the morphology of the graft polymer. There is no evidence for cyclic amidoxime functionality contributing significantly to the extraction of uranium from seawater, and efforts to maximize the number of these binding sites should be discontinued. While molecular interactions necessarily occur in adsorption processes, these results clearly reveal physical phenomena at the mesoscale play a critical role in adsorbent behavior, even to the extent that the binding environment differs from those predicted by small molecule studies. In contrast to the current paradigm where chemical functionality is varied, improvements in the performance of polymer-based amidoxime-functionalized adsorbents for extraction of uranium from seawater need also be pursued through interrogation and modulation of bulk morphology and improvements in polymer hydrophilicity. (authors)