Preparation Of Ionic Covalent Organic Frameworks And Their Adsorption And Mechanism Study On Hexavalent Chromium

The discharge of heavy metals has caused many environmental problems and has become a potential threat to the safety of the surrounding water environment and human health.The adsorption method is widely used for the removal of heavy metal in water because of its simplicity and recyclability of the adsorbent.At present,many new nanomaterials have been used as highly selective adsorbents,which have the ability to eliminate heavy metal ions in the environmental ecosystem.Ionic covalent organic frameworks（i COFs）are a new type of functional nanomaterials,which have ionic interfaces that can strongly interact with oppositely charged ionic species,and have the advantages of stable framework structure and easy regulation of structure and function.In this thesis,two kinds of i COFs were synthesized by a simple solvothermal method using different precursors and guanidine salts.The former increases the density of functional groups by increasing the specific surface area through keto-enol tautomerism between monomers,and the latter increases the density of functional groups.The hydroxyl group of the precursor is used to provide more active sites,thereby enhancing the adsorption effect of hexavalent chromium.The specific contents are as follows:（1）Using 1,3,5-trialdehyde-based phloroglucinol and 1,3-diaminoguanidine hydrochloride as precursors,guanidine-based i COFs withβ-ketoenamine bonds were synthesized by solvothermal method(Tp-DG_Cl).The prepared Tp-DG_Cl has four advantages,namely easy synthesis,high selectivity（selective adsorption to hexavalent chromium）,excellent adsorption（the theoretical maximum adsorption capacity calculated by the Langmuir model is 360.02 mg/g and good adsorption capacity）.Recyclability（over 85%adsorption capacity maintained after 5 cycles）makes it a reliable adsorbent for wastewater treatment.Due to the stability between the tetrahedral oxyanion and the diiminoguanidine group in the guanidine salt With good and selective binding affinity,Tp-DG_Cl showed excellent adsorption capacity for hexavalent chromium.After five cycles,the adsorption performance of Tp-DG_Cl remained at a high level,and after adsorption Supplementing Cl^-to Tp-DG_Cl can restore the adsorption performance of the material,which shows excellent regeneration ability.Therefore,Tp-DG_Cl has a strong potential for wide application in wastewater remediation.（2）An i COF(BD-DG_Cl)was successfully prepared using 2,4-dihydroxy trimesicaldehyde and 1,3-diaminoguanidine hydrochloride as precursors and applied to the adsorption of hexavalent chromium in water.In this thesis,the structure of the adsorbent was characterized by XRD,BET and XPS,and the performance of the adsorbent was explored through a series of adsorption experiments.The research shows that the adsorption effect of BD-DG_Cl with hydroxyl groups on hexavalent chromium is improved,and the theoretical maximum adsorption capacity reaches 233.64 mg/g;after fitting the adsorption isotherm,it is found that it is in good agreement with the Langmuir isotherm adsorption model,the adsorption kinetics conformed to the pseudo-second-order kinetic model,indicating that the adsorption type was chemical reaction;from FTIR and XPS,it could be speculated that the adsorption mechanism was the synergistic effect of hydroxyl and guanidine groups.