Construction of an Ion Pathway Boosts Uranium Extraction from Seawater

Uranium extraction from seawater (UES) is critical for the sustainable development of nuclear energy but faces tremendous scientific and technical challenges. In this issue of Chem, Zhu and colleagues propose an advanced UES strategy incorporating an extended electric field that can facilitate the migration of uranium species in the channels of sorption materials, leading to record-breaking kinetics, selectivity, and capacity of UES. Uranium extraction from seawater (UES) is critical for the sustainable development of nuclear energy but faces tremendous scientific and technical challenges. In this issue of Chem, Zhu and colleagues propose an advanced UES strategy incorporating an extended electric field that can facilitate the migration of uranium species in the channels of sorption materials, leading to record-breaking kinetics, selectivity, and capacity of UES. Nuclear power remains one of the most desirable and mature choices of energy sources that can reduce carbon emission.1Hoffert M.I. Caldeira K. Benford G. Criswell D.R. Green C. Herzog H. Jain A.K. Kheshgi H.S. Lackner K.S. Lewis J.S. et al.Advanced technology paths to global climate stability: energy for a greenhouse planet.Science. 2002; 298: 981-987Crossref PubMed Scopus (1100) Google Scholar However, at the current rate of uranium consumption, the known geological reserves of uranium will be exhausted in less than a century—perhaps even sooner given the many new nuclear power plants that are either under construction or planned in several countries.2US Energy Information AdministrationAnnual energy outlook 2015 with projections to 2040. DOE/EIA-0383(2015).https://www.eia.gov/outlooks/aeo/pdf/0383(2015).pdfDate: 2015Google Scholar,3Tabushi I. Kobuke Y. Nishiya T. Extraction of uranium from seawater by polymer-bound macrocyclic hexaketone.Nature. 1979; 280: 665-666Crossref Scopus (124) Google Scholar Alternatively, the ocean contains approximately 4.5 billion tons of uranium, accounting for 99.9% of the total uranium inventory on Earth, but the concentration of 3.3 ppb is unfortunately extremely low, and the uranium coexists with many different types of metal ions in huge excess. This makes uranium extraction from seawater (UES) one of the “seven chemical separations to change the world,” but it comes with intrinsic challenges.4Sholl D.S. Lively R.P. Seven chemical separations to change the world.Nature. 2016; 532: 435-437Crossref PubMed Scopus (1916) Google Scholar The current development of UES is dominated by the search for high-performance sorbent materials that can enrich uranium in the solid phase through a chemical sorption reaction. In order to show real applications to potentially replace the uranium mining industry, a target sorbent material should meet the requirements of a one-run uptake capacity of 30 mg g−1 or a service life of ten adsorption-desorption cycles with a 6 mg g−1 uptake capacity and an average 3% capacity loss per cycle.5Abney C.W. Mayes R.T. Saito T. Dai S. Materials for the recovery of uranium from seawater.Chem. Rev. 2017; 117: 13935-14013Crossref PubMed Scopus (463) Google Scholar,6Xu X. Zhang H.J. Ao J.X. Xu L. Liu X. Guo X. Li J. Zhang L. Li Q. Zhao X. et al.3D hierarchical porous amidoxime fibers speed up uranium extraction from seawater.Energy Environ. Sci. 2019; 12: 1979-1988Crossref Google Scholar Although a significant amount of effort has been paid to this field in the past decade, almost no uranium sorbent materials can meet the above requirements because of their limited extraction efficiencies.5Abney C.W. Mayes R.T. Saito T. Dai S. Materials for the recovery of uranium from seawater.Chem. Rev. 2017; 117: 13935-14013Crossref PubMed Scopus (463) Google Scholar Specifically, in these traditional sorbent materials, where the chemical sorption reaction is the sole thermodynamic driving force, the amount of uranium uptake is predominately determined by the chemical equilibrium between the solid and solution states. Therefore, the extremely low concentration of uranium in seawater directly leads to low uptake capacity in solids, and the complete elimination of uranium from seawater is quite challenging without additional driving forces. Moreover, the small uranium concentration gradient at the solid-solution interface and the extensive sorption competition induced by coexisting metal ions (especially vanadium and iron) result in notably slow uranium-uptake kinetics. These are bottleneck issues of traditional sorbent materials, calling for new UES strategies.7Liu C. Hsu P. Xie J. Zhao J. Wu T. Wang H. Liu W. Zhang J. Electrochemical method for uranium extraction from seawater.Nat. Energy. 2017; 2: 17007Crossref Scopus (289) Google Scholar In this issue of Chem, Zhu’s team proposes an interesting idea whereby a new type of driving force is added to the sorbent material, leading to much elevated UES capabilities.8Wang Z. Meng Q.H. Ma R.C. Wang Z.K. Yang Y.J. Sha H.Y. Ma X.J. Ruan X.H. Zou X.Q. Yuan Y. Zhu G.S. Constructing an ion pathway for uranium extraction from seawater.Chem. 2020; 6: 1683-1691Abstract Full Text Full Text PDF Scopus (62) Google Scholar They inserted conductive chains (poly phenylacetylene [PPA]) into the channels of the channel- and specific-site-abundant porous aromatic framework (PAF) MISS-PAF-1 (PAFs bearing bis-salicyladoxime as adsorption sites) and obtained [email protected] (Figure 1) to potentially introduce an extended electric field that could cover the whole PAF particles. This contrasts sharply with the recently developed electrochemical UES strategy using the ordinary electrode mode that works in Deby length (λD < 1 nm) only.7Liu C. Hsu P. Xie J. Zhao J. Wu T. Wang H. Liu W. Zhang J. Electrochemical method for uranium extraction from seawater.Nat. Energy. 2017; 2: 17007Crossref Scopus (289) Google Scholar The asymmetrical alternating current electrochemical (AACE) method significantly facilitates the migration of uranium into the PAF, leading to a dramatic increase in the uranium-uptake kinetics. In addition, amidoxime is the state-of-the-art ligand for UES, but its affinity to uranium is often challenged by vanadyl ions, especially in the form of polyamidoximes.9Ivanov A.S. Parker B.F. Zhang Z. Aguila B. Sun Q. Ma S. Jansone-Popova S. Arnold J. Mayes R.T. Dai S. et al.Siderophore-inspired chelator hijacks uranium from aqueous medium.Nat. Commun. 2019; 10: 819Crossref PubMed Scopus (66) Google Scholar Interestingly, [email protected] showed excellent selectivity via the AACE method in that it achieved a selectivity coefficient (mass ratio) of 43.9 for UO22+/VO3−, 20-fold higher than those of state-of-the-art UES sorbents.5Abney C.W. Mayes R.T. Saito T. Dai S. Materials for the recovery of uranium from seawater.Chem. Rev. 2017; 117: 13935-14013Crossref PubMed Scopus (463) Google Scholar This excellent selectivity most likely originates from a combination of the unique uranium molecularly imprinted spatial structure of MISS-PAF-110Yuan Y. Meng Q. Faheem M. Yang Y. Li Z. Wang Z. Deng D. Sun F. He H. Huang Y. et al.A molecular coordination template strategy for designing selective porous aromatic framework materials for uranyl capture.ACS Cent. Sci. 2019; 5: 1432-1439Crossref PubMed Scopus (51) Google Scholar and the charge-differentiation capability induced by the extended electric field, both of which exclude the accumulation of VO3− in the sorbent. [email protected] was then exposed to an unspiked seawater sample with an initial uranium concentration of 4.4 ppb. In the AACE method, over 99% of uranium was extracted within just 120 min by [email protected], and the final concentration reached 0.04 ppb, revealing an outstanding depth of uranium removal, contrasting sharply with that of MISS-PAF-1 (∼2.15 ppb) under identical extraction conditions. The mass-weighted distribution coefficient value (Kd), quantifying the affinity between the sorbent material and uranium, of [email protected] reached 2.18 × 107 mL g−1, 53-fold higher than that of MISS-PAF-1. Under the AACE mode, the UES rate constants (k2) for [email protected] were also clearly higher than those of MISS-PAF-1 and were 2–3 orders of magnitude higher than those of reported remarkable polymers and porous sorbents under similar conditions. These results clearly highlight the critical role of the conductive chain in building the extended electric field in the material and the subsequent enhancement of UES capabilities. To evaluate the extraction capacity for uranium, the authors investigated the performance of [email protected] in actual seawater. Very significantly, the extraction capacity reached 13.0 ± 0.5 and 16.5 ± 0.5 mg g−1 after lab-level sea trial testing for 56 and 90 days, respectively. [email protected] could be easily regenerated by being soaked in NaHCO3 solution to desorb uranium. Meanwhile, the decrease in extraction capacity was lower than 0.5% in each cycle, and the total loss was lower than 5% after ten cycles, satisfying the above-mentioned criteria for potential applications. This work presents a record-high uptake capacity of UES among all sorbent materials reported to date, bringing the field of UES one solid step closer to commercialization. Constructing an Ion Pathway for Uranium Extraction from SeawaterWang et al.ChemMay 12, 2020In BriefThe ultra-low concentration (∼3.3 ppb) of uranium in seawater leads to the low adsorption efficiency and long working time of conventional adsorbents. Conductive chains are incorporated into the porous channels of MISS-PAF-1 to serve as the pathway for ion transportation, which achieve a fast migration velocity and large capacity for the extraction of uranyl ions from seawater. Full-Text PDF Open Archive