Application And Mechanism Study On Functionalized Covalent Organic Frameworks For Uranium Extraction From Seawater

With the rapid development of society,energy and environment problems become more and more prominent.Nuclear power is expected as an effective way to solve the problem.While all countries in the world have put the development of nuclear power industry on the agenda,the uranium reserves on the earth can't meet the long-term development of human society.As a country with serious uranium deficiency,it is significant for China to find new channels to acquire uranium resources.Although the concentration of uranium dissolved in seawater is low,the total amount of uranium is about 1,000 times more than terrestrial reserves.If the uranyl in seawater can be fully utilized,the current energy crisis will be greatly alleviated.Uranium extraction from seawater has not only brought new hope to human beings,but also brought new challenges.Complex ion environment and biological pollution in seawater have caused great troubles to the existing seawater uranium extraction process,and overcoming these difficulties has become the hotpot of research.Uranium extraction from seawater by adsorption is the most mature method for uranium extraction at present.but the commercial adsorbent materials have shown limitations in the long-term application.It is urgent to develop new adsorbent materials to overcome the challenge of seawater uranium extraction..Herein,a polyarylether-based covalent organic framework functionalized with open-chain amidoxime(COF-HHTF-AO)was synthesized,showing remarkable chemical stability and excellent crystallinity.Impressively,the adsorption capacity of COF-HHTF-AO towards uranium reached up to 5.12 mg g-1 in real seawater,which is 1.61 times higher than that for vanadium.Theoretical calculation revealed that the higher selectivity for uranium over vanadium originated from their different binding mode with the amidoxime sites.COF-HHTF-AO combining enhanced adsorption capacity,excellent selectivity and ultrahigh stability serves as a promising adsorbent for uranium extraction from the natural seawater.Herein,a polyarylether-based covalent organic framework functionalized with open-chain amidoxime(COF-HHTF-AO)was synthesized,showing remarkable chemical stability and excellent crystallinity.Theoretical calculation revealed that the higher selectivity for uranium over vanadium originated from their different binding mode with the amidoxime sites.Study details are listed as follows:(1)This work uses a two-step method.First,The precursor COF-HHTF was synthesized by the ligand 2,3,6,7,10,11-hexahydroxytriphenylene(HHTP)and tetrafluoroterephthalonitrile(TFTPN)through the nucleophilic substitution reaction.Then,the COF-HHTF-AO was successfully modified with hydroxylamine hydroxylamine hydrochloride.The chemical composition and structure were verified by using FTIR spectroscopy and solid-state NMR.The powder XRD pattern was compared with theoretical calculations to prove its crystal structure and stacking method of COF.Nitrogen adsorption and pore size distribution test,high-resolution scanning electron microscope and contact angle meter were used to test the properties of the COF furtherly.The stability of the material under extreme conditions was studied,it was found that COF-HHTF-AO can maintain its structural stability after being immersed in strong acid(12M HC1),strong alkali(14 NaOH)and real seawater conditions.In summary,COF-HHTF-AO has the potential to extract uranium in seawater conditions.(2)The adsorption performance of the material under different conditions was studied,and it was found that COF-HHTF-AO has extremely high adsorption capacity(550.1 mg/g)for uranyl,extremely fast adsorption kinetics,good recycling performance,and extremely high affinity and ion selectivity?Furthermore,real seawater is used for performance testing.COF-HTTF-AO can still maintain fast adsorption kinetics in different concentrations of uranium-added seawater.The adsorption capacity of uranium in real seawater is as high as 5.12 mg/g-1,which is 1.61 times higher than that for vanadium.It is expected to become the next-generation adsorbent material for uranium extraction from seawater.(3)We study the mechanism of interfacial adsorption Firstly,it is explored through experimental methods by using FTIR,EDS spectroscopy and XPS spectroscopy.It's successful to prove the binding of uranyl on the surface of COF.Then we explore the molecular mechanism of COF-HHTF-AO with uranyl ions and vanadium ions through DFT calculations.Result prove that the high affinity of COF-HHTF-AO to uranium is due to their coordination with higher binding energy.And differential charge density analysis and molecular dynamics simulations futher support this view.This work provides a new idea for covalent organic framework material applied in uranium extraction from seawater.Meanwhile,research uses amidoximed COF material as the backing to do theoretical calculation for uranium extraction from seawater firstly.It proves the feasibility of using COF material as the calculation platform to explore the behavior of the adsorption process in seawater,which has certain guiding significance for the subsequent research.