Reaction: Goal-Oriented PAF Design for Uranium Extraction from Seawater

Abstract

Guangshan Zhu received his BSc (1993) and PhD (1998) degrees in chemistry from Jilin University. He is now a full professor at Northeast Normal University. Driven by the demands in sustainable energy and environments, his current research interest focuses on the design and synthesis of porous aromatic frameworks for gas adsorption, separation, and uranium capture. Guangshan Zhu received his BSc (1993) and PhD (1998) degrees in chemistry from Jilin University. He is now a full professor at Northeast Normal University. Driven by the demands in sustainable energy and environments, his current research interest focuses on the design and synthesis of porous aromatic frameworks for gas adsorption, separation, and uranium capture. The utilization of nuclear power is a mature technology with a high capacity factor and persistent energy output. Therefore, it is considered one of the top contenders for clean energy sources. Power generation by worldwide nuclear plants has kept a steady pace in the past two decades, and robust growth is expected in the future. In China, nuclear energy has made great contributions to domestic electricity production; its current power-generating capacity is 49 GW, and its target is 150 GW by 2035.1China Nuclear Energy Association (2020). The Report on the Development of China’s Nuclear Energy. http://www.china-nea.cn/site/content/38423.html.Google Scholar The main fuel resource for nuclear plants is uranium, and its supply is one of the key dominating factors in energy production. However, terrestrial uranium reserves are far from satisfying the increasing demands of global energy consumption. Under the circumstances, uranium extraction from seawater (UES) is becoming a potential process to supply sufficient nuclear fuels for durative energy production because the total uranium content in seawater is 1,000 times more than in terrestrial uranium reserves. However, uranium exists in seawater at an extremely low concentration of ca. 3.3 parts per billion (ppb), whereas many other metal cations are at much higher concentrations and would severely affect uranium extraction by competitive adsorption. In their Catalysis piece,2Kushwaha S. Patel K. Uranium extraction from seawater: a paradigm shift in resource recovery.Chem. 2021; 7: 271-274Abstract Full Text Full Text PDF Scopus (4) Google Scholar Kushwaha and Patel have given insight into the UES progress and discussed the criteria for adsorbent design. Uranium-extracting performance is closely associated with multiple factors and their synergy, including the property and accessibility of uranium-capturing sites, the porosity of adsorbents, the tolerance of adsorbents to seawater, and the extrinsic driving forces that speed up uranium diffusion. In order to achieve high UES capacity, the rational design of uranium adsorbents should be carried out with three key goals, namely (1) high selectivity to uranium over abundant competing metals, (2) fast mass transport of uranium species within adsorbents, and (3) the availability and reliability of adsorbents. Porous aromatic frameworks (PAFs) are a family of nanoporous solids with extended networks. Typically, they are constructed from carbon-carbon-linked rigid building units.3Tian Y. Zhu G. Porous aromatic frameworks (PAFs).Chem. Rev. 2020; 120: 8934-8986Crossref PubMed Scopus (122) Google Scholar Because of the insufficient stacking of rigid building units, PAFs exhibit surface area comparable to that of other porous material series, such as covalent organic frameworks and metal-organic frameworks. Meanwhile, the robust carbon-carbon linkages between the building units give PAFs exceptional stability toward the severe conditions of chemical treatment or harsh hydrolytic environments. Diverse functionalities of PAFs are readily obtained either by direct synthesis from designed building units or by post-modification to introduce desired active sites. The rational design of PAFs has provided opportunities to achieve the key UES goals. The majority of UES adsorbents interact with uranium through coordinating groups, especially oximes and amidoximes. But these coordination sites also bind with other competing metals, such as vanadium. In order to achieve uranium recognition, a molecular imprinting strategy has been developed with uranium species as the imprinting molecule. The building units with coordination sites were coupled to form PAFs with specific chelating sites to uranyl. The molecularly imprinted PAFs exhibited a high uranyl capacity of 37.28 mg g−1, whereas the capacities of other interfering metals were below 0.05 mg g−1.4Yuan Y. Yang Y. Ma X. Meng Q. Wang L. Zhao S. Zhu G. Molecularly imprinted porous aromatic frameworks and their composite components for selective extraction of uranium ions.Adv. Mater. 2018; 30: e1706507Crossref PubMed Scopus (134) Google Scholar It also turns out that the construction of imprinting sites within PAFs’ void space is available. The uranyl-templated bis-salicylaldoxime site was immobilized in PAF-1.5Yuan 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 (32) Google Scholar The obtained MISS-PAF-1 was able to bind uranyl ions with a capacity of 79.8 mg g−1, which was at least 100 times more than that of vanadium. Moreover, because of its high porosity, most chelating sites were accessible, and the uranium capacity of MISS-PAF-1 in real seawater was 5.79 mg g−1, which is close to the 6 mg g−1 target for multiple-cycle used UES adsorbents. The intersecting channels in PAFs boost the number of accessible capture sites and promote uranium diffusion. But because of the extremely low concentration of uranium in seawater, there is a lack of concentration gradient that propels uranium movement from the PAF-seawater interface to the PAF inner surface. To speed up uranium diffusion, an electric field was applied to the PAF channels as an external driving force. A conductive polymer was incorporated with MISS-PAF-1 through in situ polymerization.6Wang Z. Meng Q. Ma R. Wang Z. Yang Y. Sha H. Ma X. Ruan X. Zou X. Yuan Y. Zhu G. Constructing an ion pathway for uranium extraction from seawater.Chem. 2020; 6: 1683-1691Abstract Full Text Full Text PDF Scopus (29) Google Scholar The electric gradient on the PAF surface not only attracted uranyl ions but also sped up their migration inside PAFs. As a result, the conductive-media-modified PAF exhibited 2–3 orders of magnitude higher rate constants than other reported UES adsorbents. The synergy of high porosity, uranium-selective sites, and electric gradient leads to record-high uranium capacities in real seawater up to 13.0 ± 0.5 mg g−1 in 56 days and 16.5 ± 0.4 mg g−1 in 90 days. The uranium binding sites of polymer-based UES adsorbents are usually embedded by the polymeric chains, leading to decreased utilization. PAFs are open frameworks with exposed uranium binding sites. Meanwhile, tough carbon-carbon bonds in PAFs guarantee their recyclability and reusability in real seawater. Therefore, PAFs have exhibited significant advantages in UES capacity and kinetics. However, the organometallic catalysts and tailored building units for PAF syntheses increase the cost, thus hindering their practical applications. In view of the commercially affordable UES process, inexpensive building units from benzene derivatives and Lewis-acid-catalyzed Scholl reactions have been employed.7Li Z. Meng Q. Yang Y. Zou X. Yuan Y. Zhu G. Constructing amidoxime-modified porous adsorbents with open architecture for cost-effective and efficient uranium extraction.Chem. Sci. 2020; 11: 4747-4752Crossref Google Scholar The post-modification has introduced numerous accessible amidoxime units in the frameworks, leading to PAF-170-AO with over 60% available binding sites. Furthermore, PAF-170-AO exhibited good compatibility with other matrixes to fabricate ceramic sheets for practical uses. The cost of extracting 1 kg of uranium can be reduced to ca. US$189.77, which is comparable to the cost of terrestrial uranium. To conclude, PAFs feature open framework architectures, chemically amenable fragments, and high surface areas. These advantages ensure the goal-oriented design of promising viable PAF-based UES adsorbents. PAFs with improved adsorptive UES capacity and high kinetic and cost efficiency are anticipated. Some basic principles for the rational design of promising PAFs for effective UES have been established, and new synthetic strategies and mechanisms are awaiting further exploration. The author is grateful for the financial support provided by the National Natural Science Foundation of China (grants 21531003 , 21601031 , and 91622106 ) and the “111” project ( B18012 ). 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: Semiconducting MOFs Offer New Strategy for Uranium Extraction from SeawaterLi et al.ChemFebruary 11, 2021In BriefUranium extraction from seawater is one of the most important chemical separations for society given that oceanic uranium reserves could enable the expansion of low-carbon nuclear energy production. In this reaction piece, Li and Wang discuss the potential of metal-organic frameworks (MOFs) for efficient and sustainable 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