Abstract
Mining uranium from seawater is highly desirable for sustaining the increasing demand for nuclear fuel; however, access to this unparalleled reserve has been limited by competitive adsorption of a wide variety of concentrated competitors, especially vanadium. Herein, we report the creation of a series of uranyl-specific “hooks” and the decoration of them into the nanospace of porous organic polymers to afford uranium nanotraps for seawater uranium extraction. Manipulating the relative distances and angles of amidoxime moieties in the ligands enabled the creation of uranyl-specific “hooks” that feature ultrahigh affinity and selective sequestration of uranium with a distribution coefficient threefold higher compared to that of vanadium, overcoming the long-term challenge of the competing adsorption of vanadium for uranium extraction from seawater. The optimized uranium nanotrap (2.5 mg) can extract more than one-third of the uranium in seawater (5 gallons), affording an enrichment index of 3836 and thus presenting a new benchmark for uranium adsorbent. Moreover, with improved selectivity, the uranium nanotraps could be regenerated using a mild base treatment. The synergistic combination of experimental and theoretical analyses in this study provides a mechanistic approach for optimizing the selectivity of chelators toward analytes of interest.
Highlights
The realization of net-zero CO2 emissions by 2050, as advocated by the Paris Agreement, can help in stabilizing global warming below 2 °C to avoid pervasive climate damage
The optimization of the selectivity of adsorbents has been mostly accomplished by designing new chelating sites, which are often accompanied by cumbersome synthetic procedures that hinder their practical application.[27−29] The selective recognition and sequestration of specific ions occur efficiently in nature; this regulates the extreme selectivity for specific ions by manipulating the cooperation of binding sites
Approaches involving biomimetic designs have offered inspiration for designing sophisticated artificial materials.[30−36] Inspired by the preorganization of binding sites employed by nature, we previously demonstrated that the affinity of the chelating group in adsorbents toward uranium could be significantly improved by engineering their spatial distribution to facilitate cooperative binding.[37]
Summary
The realization of net-zero CO2 emissions by 2050, as advocated by the Paris Agreement, can help in stabilizing global warming below 2 °C to avoid pervasive climate damage. A zwitterionic tautomer containing four oxygen atoms from four oxime groups was found to bind to each uranyl in 3(UO2) to form an infinite network, confirming the divergent binding modes in these adsorbents (Table S1) This difference was unanticipated; this emergent behavior that occurred due to the influence of the uranyl complexing mode could be a key contributor to the observed discrepancy in uranium sorption performance (Figure 3). Consecutive cycles revealed that the regenerated samples showed noticeably decreased performance, and only 87 and 77% of their initial capacities were retained by POP2AO and POP3-AO, respectively These experiments revealed that the modification of the R group enabled the selective discrimination of amidoxime moieties toward uranium compared to other metal species. The enrichment index of the uranium for POP1-AO was calculated to be 3836, which is considerably close to the all-time seawater uranium accumulation record (see the summary of the reported representative adsorbents in uranium uptake capacity, selectivity of uranium and vanadium, seawater uranium uptake capacity, and the enrichment index in Tables S5 and S6)
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