Reaction: Semiconducting MOFs Offer New Strategy for Uranium Extraction from Seawater

Hui Li received his PhD from Xinjiang University, after which he worked as a postdoctoral fellow at South China University of Technology and Soochow University. He is currently a research assistant in Prof. Shuao Wang’s group at Soochow University, where he studies uranium extraction from seawater using MOFs and COFs. Shuao Wang received his PhD from the University of Notre Dame and then worked as a postdoctoral fellow at Lawrence Berkeley National Laboratory and the University of California, Berkeley. He is currently a distinguished professor at Soochow University. His research interests include coordination, environmental, and material chemistry of critical radionuclides in the nuclear fuel cycle. Hui Li received his PhD from Xinjiang University, after which he worked as a postdoctoral fellow at South China University of Technology and Soochow University. He is currently a research assistant in Prof. Shuao Wang’s group at Soochow University, where he studies uranium extraction from seawater using MOFs and COFs. Shuao Wang received his PhD from the University of Notre Dame and then worked as a postdoctoral fellow at Lawrence Berkeley National Laboratory and the University of California, Berkeley. He is currently a distinguished professor at Soochow University. His research interests include coordination, environmental, and material chemistry of critical radionuclides in the nuclear fuel cycle. Uranium extraction from seawater (UES) is considered one of the seven chemical separations that could change the world.1Sholl D.S. Lively R.P. Seven chemical separations to change the world.Nature. 2016; 532: 435-437Crossref PubMed Scopus (1338) Google Scholar One-round extraction capacity of sorbent materials in the real seawater test of higher than 30 mg/g is a critical research target for showing the potential for commercialization.2Xu X. Zhang H. Ao J. 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 A common question is why the extraction capacity of sorbent materials in the real seawater test (overwhelming majority < 5 mg/g) is significantly lower than their theoretical extraction capacity obtained by the sorption isotherm experiment (for some materials, this could reach higher than 1,000 mg/g).3Yan B. Ma C. Gao J. Yuan Y. Wang N. An ion-crosslinked supramolecular hydrogel for ultrahigh and fast uranium recovery from seawater.Adv. Mater. 2020; 32: e1906615Crossref PubMed Scopus (42) Google Scholar,4Abney 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 (303) Google Scholar This is because in the sorption isotherm experiment, the sorption capacity is often extrapolated from the sorption experiment where the uranium concentration is extremely high (>1,000 ppm). In this case, uranium can be located at any possible site in the sorbent materials regardless of the binding strength. In sharp contrast, in real seawater, the uranium concentration is extremely low at 3.3 ppb, and its coordination is saturated with carbonate anions, which are known to be strong binding ligands for uranyl ions. Principally, UES capacity is determined when the thermodynamic equilibrium is established between the sorbed uranyl ion in the solid sorbent material and the uranyl carbonate in seawater. The equilibrium constant is designated by the difference of binding strength between uranyl-sorbent interaction and uranyl-carbonate interaction in addition to the number of such sites with strong binding strength. From here, it is very clear that grand challenges are present in reaching a high UES capacity when coordination is utilized as the sole thermodynamic driving force. Fortunately, uranium is a redox-active metal, where highly soluble uranyl(VI) ion can be reduced to almost insoluble uranium(IV) (U(IV)) in a number of ways. Semiconductor-based photocatalytic reduction of uranyl(VI) offers an alternative way to make a breakthrough in the field of UES. With the introduction of the redox reaction path, the UES capacity can be improved, which is no longer determined by the sorption equilibrium. In addition, the energy input in this method is not an issue because solar light can be utilized.5Li Z.J. Huang Z.W. Guo W.L. Wang L. Zheng L.R. Chai Z.F. Shi W.Q. Enhanced photocatalytic removal of uranium(VI) from aqueous solution by magnetic TiO2/Fe3O4 and its graphene composite.Environ. Sci. Technol. 2017; 51: 5666-5674Crossref PubMed Scopus (199) Google Scholar However, although some photocatalysts have been studied for the extraction of U(VI), none of them show real utility in the real seawater test. It seems that the efficiency of sole photoreduction of U(VI) is also quite limited, which is most likely because none of these photocatalysts can pre-enrich uranyl from seawater, and the local concentration of photoreduced U(IV) species is too low to be separated from aqueous solution. In recent years, semiconducting metal-organic frameworks (MOFs) have been regarded as new types of photocatalysts and, unlike most traditional photocatalysts, possess high structural designability and chemical functionalizability.6Peng Y. Zhao M. Chen B. Zhang Z. Huang Y. Dai F. Lai Z. Cui X. Tan C. Zhang H. Hybridization of MOFs and COFs: A new strategy for construction of [email protected] core–shell hybrid materials.Adv. Mater. 2018; 30: 1705454Crossref Scopus (174) Google Scholar,7Zhang M. Lu M. Lang Z.L. Liu J. Liu M. Chang J.N. Li L.Y. Shang L.J. Wang M. Li S.L. Lan Y.Q. Semiconductor/covalent-organic-framework Z-scheme heterojunctions for artificial photosynthesis.Angew. Chem. Int. Ed. 2020; 59: 6500-6506Crossref PubMed Scopus (108) Google Scholar We have proposed a new uranium-extraction strategy with combined specific coordination and photocatalytic reduction based on subtly designed functionalized MOFs (PN-PCN-222 and SCU-19). Pre-enriched into the strong-coordination sites by the introduction of U(VI)-recognizing ligands, U(VI) was simultaneously reduced efficiently by the photoinduced electrons from the photoactive MOF host under visible-light irradiation, affording neutral U(IV) species that were stored in the open space. This also regenerated the coordination sites readily for the next round of uranium extraction.8Li H. Zhai F.W. Gui D.X. Wang X.X. Wu C.F. Zhang D. et al.Powerful uranium extraction strategy with combined ligand complexation and photocatalytic reduction by postsynthetically modified photoactive metal-organic frameworks.Appl. Catal. B: Environ. 2019; 254: 47-54Crossref Scopus (96) Google Scholar,9Zhang H. Liu W. Li A. Zhang D. Li X. Zhai F. Chen L. Chen L. Wang Y. Wang S. Three mechanisms in one material: uranium capture by a polyoxometalate-organic framework through combined complexation, chemical reduction, and photocatalytic reduction.Angew. Chem. Int. Ed. 2019; 58: 16110-16114Crossref PubMed Scopus (112) Google Scholar This auto-recycled process offers an ultrahigh uranium-extraction capacity that will not be limited by the number of adsorption sites. More importantly, elevated uptake selectivity of uranium can be achieved over competing ions that are not redox active. Especially for PN-PCN-222, U(VI) can almost be completely separated in high or low concentration over an extremely wide pH range, which is impossible to achieve by sorbent materials relying solely on coordination or photocatalytic reduction. Theoretically, catalysis is an endless process. Once the issue of sacrificial agents is conquered, if water could be used to make up the half-reaction of oxidation, greener and more effective UES would then be possible. Reaction: Porous Organic Polymers for Uranium CaptureCafer T. YavuzChemFebruary 11, 2021In BriefUranium recovery from seawater is a promising uranium-extraction method that could provide needed resources for low-carbon energy production. However, for widescale implementation of uranium recovery, issues of cost and scale must be addressed. In this reaction piece, Yavuz explores new materials that could help further the implementation of uranium recovery. Full-Text PDF Catalyst: Uranium Extraction from Seawater, a Paradigm Shift in Resource RecoveryKushwaha et al.ChemFebruary 11, 2021In BriefNuclear energy, a low-carbon route to lowering worldwide greenhouse gas emissions, could play a critical role in the transition to a clean energy future. However, low terrestrial supplies of uranium ore could limit the potential of nuclear power unless alternative extraction methods are employed. This Catalysis article highlights the development of methods for sustainable uranium extraction from seawater by discussing past progress and future goals for the discovery and implementation of seawater extraction methods. Full-Text PDF Reaction: Goal-Oriented PAF Design for Uranium Extraction from SeawaterGuangshan ZhuChemFebruary 11, 2021In BriefAlthough nuclear power is a promising technology for achieving a clean energy economy, the terrestrial availability of uranium limits implementation of nuclear power. In this reaction piece, Zhu discusses how porous aromatic frameworks, which are especially promising for adsorbent methods given their open framework architectures, chemically amenable fragments, and high surface areas, have advanced uranium extraction from seawater. Full-Text PDF Reaction: Engineer Biology for UraniumSun et al.ChemFebruary 11, 2021In BriefCost and scalability remain major hurdles facing existing technologies for the extraction of oceanic uranium, but repurposing existing biological systems could help overcome these obstacles. In this reaction piece, Sun and He discuss the progress in protein engineering of biological systems to sustainably mine uranium from seawater. Full-Text PDF