Fabrication Of Functionalized Polyarylether-based Covalent Organic Frameworks And Removal Of Heavy Metals From Water

Covalent organic framework（COF）material is a new type of crystalline organic porous material.Because of its high porosity,highly ordered and designable structure,and easy functionalization,it is widely used in heavy metal adsorption.The field has good application prospects.In this paper,for different heavy metal ions,a polyarylene ether type COF material（JUC-505）was used as the matrix material,and different heavy metal adsorbents were obtained by grafting and modifying specific functional groups.This is based on the following features of JUC-505:First,the AA-stacked two-dimensional layered structure makes it have open one-dimensional macropores,providing sufficient space for the transport and exchange of heavy metal ions;second,the polyarylene ether type The high stability of COFs enables them to be functionalized in harsh environments and to maintain stable structure and performance during the adsorption process;thirdly,the cyano group（-CN）on the backbone is an active site,which is easy to achieve functionalization;finally,which is also the most important feature that distinguishes it from other COFs.The abundant in-situ oxygen atoms on the JUC-505 skeleton can form a synergistic effect with adjacent functional groups through hydrogen bonds and enhance the interaction between the adsorption site and heavy metal ions.Based on the above considerations,JUC-505-COOH functionalized with carboxyl group（-COOH）was used for the removal of cadmium ion Cd²⁺and lead ion Pb²⁺in water;JUC-505-COOH and amidoxime group（-AO）modified JUC-505-AO was used for the adsorption and enrichment of uranyl ion UO₂²⁺.The specific work is as follows:（1）The synthesized JUC-505 was used as the precursor material to modify-COOH and-AO,respectively,to obtain JUC-505-COOH and JUC-505-AO.The functionalized material maintains high crystallinity and framework structure similar to JUC-505;however,the interlayer spacing of JUC-505-COOH is increased from 3.44（?）to 3.48（?）;the planar structure of JUC-505-AO is slightly distorted,and the layers The spacing increases from3.44（?）to 3.56（?）.Due to the introduction of-COOH and-AO groups,the BET specific surface areas of JUC-505-COOH and JUC-505-AO are lower than that of JUC-505,which are 380 m~2·g^-1 and 330 m~2·g^-1,respectively.and 705 m~2·g^-1.The stability of the three materials in water,acid（HCl,5M）,alkali（Na OH,5M）and seawater was systematically studied.After immersion in water and HCl solution for 30 days,and after immersion in seawater for 14 days,the three materials remained stable.maintain good crystallinity.In contrast,all three materials are slightly less stable under alkaline conditions,but nonetheless,they can still retain good crystallinity after 7 days of Na OH immersion.（2）JUC-505-COOH was used for the removal of Cd²⁺and Pb²⁺in aqueous solution.The adsorption capacities of JUC-505-COOH for Cd²⁺and Pb²⁺were as high as 504 mg·g^-1 and 559 mg·g^-1,respectively;when the initial concentration was 100 mg·L^-1,it showed fast adsorption kinetics,both Adsorption equilibrium was reached within 5 minutes.The common interfering ions(NO^3-,SO₄^2-,Na⁺,Ca²⁺,Mg²⁺)in the water environment hardly affect the adsorption of Pb²⁺by JUC-505-COOH.When the concentration is as high as 50times,the removal efficiency of Pb²⁺can still reach more than 99%;while Divalent cations(Ca²⁺,Mg²⁺)have a relatively large effect on the removal of Cd²⁺.When the concentration of divalent cations is 10 times,the removal rate of Cd²⁺is 83.72%-84.83%.When the concentration is 50 times,the removal efficiency is reduced to 68.13%-73.58%.JUC-505-COOH has good regeneration ability,and the removal rate of Cd²⁺and Pb²⁺can still be maintained at 80%-90%after six cycles of adsorption and desorption.In addition,in order to reveal the adsorption mechanism of heavy metal ions in JUC-505-COOH,taking Cd²⁺as an example,the most stable theoretical adsorption model obtained by DFT calculation is the monodentate coordination mode of cadmium hexahydrate(Cd²⁺·6H₂O)with low binding energy At the same time,there is a hydrogen bond between Cd²⁺·6H₂O and the O atom on the JUC-505 skeleton,and this weak interaction forms a synergistic effect with the coordination of-COOH on Cd²⁺,as the effect of JUC-505-COOH on heavy metal ions adsorption driving force.（3）The study of JUC-505-AO and JUC-505-COOH on the removal of UO₂²⁺in water environment and the enrichment of UO₂²⁺in seawater.The UO₂²⁺adsorption capacities of JUC-505-COOH and JUC-505-AO were 464 mg·g^-1 and 395 mg·g^-1,respectively.In addition,the two adsorbents exhibited different adsorption characteristics due to different functional groups.Under acidic conditions,the UO₂²⁺adsorption capacity of JUC-505-COOH was stronger,and its removal rate was as high as 77.4%at p H=3.Therefore,JUC-505-COOH is suitable as a remediation material for uranium-contaminated water.JUC-505-AO exhibited faster adsorption kinetics,which could remove 93%of UO₂²⁺within 10minutes and reach adsorption equilibrium within 20 minutes.In addition,JUC-505-AO showed a clear advantage in the selective adsorption of UO₂²⁺in the presence of competing cations.It is worth noting that JUC-505-AO has the ability to extract UO₂²⁺from seawater with an adsorption capacity of 4604μg·L^-1.The adsorption mechanism of JUC-505-AO for UO₂²⁺was explored by synchrotron radiation,XPS spectroscopy and DFT theoretical calculations,and 13 possible adsorption models were compared.Among them,the adsorption energies(-8.73 and-7.05 kcal·mol^-1)of theη~2coordination form are the lowest,which is the most probable adsorption configuration;at the same time,in the theoretical configuration,it is found that there is a hydrogen bond between the hydrogen on the hydrated uranyl ion and the oxygen on the framework,which can enhance the chelation site of the adjacent-AO group to the hydrated uranium.The affinity of acyl ions reduces the adsorption energy and improves the adsorption performance.This work shows that the construction of the synergistic effect of COFs framework and functional groups is an effective strategy to design efficient heavy metal adsorbents,and the rational selection of specific functional groups can meet the different adsorption requirements of heavy metals in various aqueous environments.