Synthesis Of Sulfonic Acid-based Covalent Organic Framework And Selective Adsorption Of Uranium(Ⅵ),thorium(Ⅳ)and Mercury(Ⅱ)ions

Covalent organic frameworks（COFs）is a class of porous crystalline framework materials connected by covalent bonds.For COFs,these geometric structures have large BET,flexible and well-definite coordinate methods,high designability,abundant active sites and stable physical and chemical properties,which provide widely used in most field,such as gas adsorption/selectivity,fluorescence,drug delivery and catalysis.In this study,the covalent organic framework named COF-SO₃H with sulfonic acid groups in the pores was synthesized by solvothermal method,and post-modification method was performed to introduce ammonium ions into the channel.The COF after modified was named[NH₄]⁺[COF-SO₃-].We used COF-SO₃H and[NH₄]⁺[COF-SO₃-]for the selective adsorption of U（Ⅵ）,Th（Ⅳ）and Hg（Ⅱ）ions in aqueous solution.In this works,the COF-SO₃H with β-ketoenamine structure were synthesized by solvothermal method following the topology principle,and[NH₄]⁺[COF-SO₃-]were prepared using the functionalization strategy of interlayer modification.The structure,phase,morphology and composition of the COF-SO₃H were characterized by PXRD,FT-IR,XPS,SEM,TGA and other techniques.It was proved that the ammonium ion was successfully grafted into the COF-SO₃H,and the introduction of functional groups did not destroy the structure and characteristic functional groups of the COF-SO₃H.A series of static batch methods were used to study the adsorption performance of COF-SO₃H and[NH₄]⁺[COFSO₃-]for capturing U（Ⅵ）,Th（Ⅳ）and Hg（Ⅱ）ions from their aqueous solution.The three primary factors of the pH,reaction contact time and initial concentration were mainly explored on experimental to determine the optimal condition for the adsorbents of COFSO₃H and[NH₄]⁺[COF-SO₃-],and ultimately to discover their maximum adsorbed amount.The geometric structure,phase,morphology,and composition of the COFs before and after loading were confirmed by using PXRD,FT-IR,XPS,SEM and TGA.By fitting the curve of the adsorption isotherms,the adsorption modes and adsorption kinetics process were discussed to reveal the adsorption mechanism.Moreover,the possibility of industrialization of[NH₄]⁺[COF-SO₃-]materials din an industrial environment was also explored through using the dynamic penetration method.The results show that the COF-SO₃H and[NH₄]⁺[COF-SO₃-]exhibit high performance for U（Ⅵ）adsorption at room temperature,and the excellent chemical stability keeps the crystalline state,structure and morphology of the adsorbed material intact.Under the optimal reaction conditions,the maximum adsorption capacity can reach 360 mg·g-1 in COF-SO₃H and 851 mg·g-1 in[NH₄]⁺[COF-SO₃-],respectively.Moreover,the[NH₄]⁺[COF-SO₃-]shows highly chemical stability for applicability in various of solution environment ranging from acidic medium（pH=1）to alkaline medium（pH=8）,which helps to obtain great adsorption performance after 6 cycles of experiments.[NH₄]⁺[COF-SO₃-]also show relative-high adsorption capacity of up to 17.8 mg·g-1 on uranium extraction from seawater,and excellent selectivity in penetration experiments.By fitting the adsorption isotherms of reaction time and initial concentration,the pseudo-second-order kinetic model and Langmuir adsorption isotherm model were found,which indicated that a monolayer adsorption dominated of chemical adsorption process on their reactions.Moreover,the DFT calculations shown ammonia modification can significantly improve the adsorption performance of[NH₄]⁺[COF-SO₃-].And combined with PXRD and FT-IR further demonstrate the adsorption mechanism that is cation exchange,where the uranyl ions replaced ammonia ions and then chelation coordination with the sulfonate.In the experiment of selective extraction of Th（Ⅵ）by the mixed solution of Th（Ⅳ）,U（Ⅵ）,Eu（Ⅲ）and Ce（Ⅲ）ions,the maximum adsorption amount of COF-SO₃H was 192.6 mg.g-1.The[NH₄]⁺[COF-SO₃-]exhibited an ultra-high adsorption capacity of 395 mg·g-1 and a selective adsorption capacity exceeding 9.4.The adsorption behavior of another radioactive metal ion of Th（Ⅵ）for COF-SO₃H and[NH₄]⁺[COF-SO₃-]were also explored in this work.According to our experimental results,the adsorption kinetics and isotherms of COF-SO₃H and[NH₄]⁺[COF-SO₃-]adsorbing Th（Ⅵ）are more in line with the pseudosecond-order kinetic model and Langmuir adsorption isotherm model.Explaining that the adsorption process of loading Th（Ⅵ）ions is monolayer adsorption dominated of chemical adsorption.The[NH₄]⁺[COF-SO₃-]showed good adsorption performance after 4 cycles of experiments,and in the penetration experiments and membrane separation experiments,the[NH₄]⁺[COF-SO₃-]was effectively extracted from the mixed rare earth solution including Th（Ⅳ）,U（Ⅵ）,Eu（Ⅲ）and Ce（Ⅲ）ions.The Th（Ⅳ）solution was isolated with a purity of 93.5%.The penetration and membranes experiments of Th（Ⅳ）from rare earth elements by[NH₄]⁺[COF-SO₃-]show that the[NH₄]⁺[COF-SO₃-]has ultra-high adsorption capacity,excellent selectivity and excellent cycle stability.The separation experiment also proved the great potential of[NH₄]⁺[COF-SO₃-]materials in industrial applications.Meanwhile,we also explored the adsorption behavior of heavy metal ion of mercury for[NH₄]⁺[COF-SO₃-].In our works,the maximum Hg（Ⅱ）adsorption capacity of COFSO₃H material was 1033 mg·g-1,and the maximum adsorption capacity of[NH₄]⁺[COFSO₃-]was as high as 1299 mg·g-1.The monolayer adsorption dominated by chemical adsorption for[NH₄]⁺[COF-SO₃-]capturing Hg（Ⅱ）ions because the fitting-well to pseudosecond-order kinetic model and Langmuir adsorption isotherm model based on the experimental results of contact reaction time and initial concentration.The[NH₄]⁺[COFSO₃-]has fast adsorption speed and strong mercury affinity,resulting in super high selectivity and great cycle stability in Hg2⁺、K⁺、Mg2⁺、Zn2⁺、Cd2⁺and Pb2⁺ion penetration experiments.And[NH₄]⁺[COF-SO₃-]adsorbed Hg0 up to 932.6 mg·g-1 in the mercury vapor adsorption experiment.Characterization analysis of the adsorbed materials by PXRD,IR,and XPS shows that ammonia modification can significantly improve the adsorption performance of COF-SO₃H.The oxygen and nitrogen atoms interact with Hg（Ⅱ）with Hg-O and Hg-N coordination in the framework.At the same time,the ammoniated NH₄⁺replaced the H⁺ion on-OH,so that the-SO₃-group and Hg（Ⅱ）coordinated with each other.Under the synergistic interaction,[NH₄]⁺[COF-SO₃-]showed an ultra-high adsorption capacity for Hg（Ⅱ）.The above research results show that the[NH₄]⁺[COF-SO₃-]has the advantages of fast adsorption rate,large adsorption capacity and excellent recyclability for the adsorption of radionuclides and heavy metal ions,and also shows excellent performance in the adsorption and separation of complex components.[NH₄]⁺[COF-SO₃-]has good application prospects in the recovery and disposal of radionuclides and heavy metal ions.