Performance And Mechanism Of PDA@COFs Composite And Its Application In CDI System For Perfluorooctanoic Acid Removal From Water

Perfluorooctanoic acid（PFOA）is a novel persistent organic pollutant which is widely distributed in the aquatic environment.And PFOA poses a huge threat to environment and human health due to its bioaccumulation,toxicological properties,and environmental persistence.The most widely used PFOA removal technology in water is mainly physical adsorption.However,the problems of uncontrollable structure,slow adsorption speed,and low adsorption capacity of traditional porous materials such as activated carbon and resin greatly limit their application in the wastewater.It is urgent to develop new adsorbents that can quickly,efficiently,and selective remove PFOA.Therefore,by compounding polydopamine（PDA）with covalent organic frameworks（COFs）materials,this paper intends to prepared a PDA@COFs composites which possess efficiency and selective adsorption ability for PFOA.The removal performance of PDA@COFS on PFOA in water was investigated,and the structure-activity relationship of PDA@COFs was clarified.Furthermore,the adsorption mechanism of PFOA by PDA@COFs was revealed and its recyclability was evaluated.The PDA@COFs composites were prepared as electrodes and further applied to capacitive deionization（CDI）systems.And a PFOA removal system based on PDA@COFs was constructed.The performance and mechanism of electroadsorption removal of PFOA in CDI system based on PDA@COFs electrode was revealed.The main findings of this paper are as follows:（1）In this paper,PDA was loaded on the COFs by solvothermal method to prepare PDA@COFs composites.The ordered porous structure of COFs provides more sites for PDA loading while facilitating adsorption and mass transfer.On the other hand,the loading of PDA introduces abundant functional groups,which provide excellent adhesion and abundant adsorption sites for the selective adsorption of PFOA.Moreover,the hydrophilicity of PDA@COFs is improved,which is beneficial to the rapid diffusion of water and PFOA.（2）The PDA@COFs composite exhibited fast adsorption kinetics for the removal of PFOA,which could reach the adsorption equilibrium state within 5 minutes and effectively remove 85%of PFOA（5 mg/L）within 20 minutes.The adsorption rate can reach 0.133min^-1,which is 13.4 times and 11.8 times than that of COFs and PDA,respectively.The adsorption capacity within 20 minutes is 6.91 and 6.05 times than that of COFs and PDA,respectively.In addition,the maximum saturated adsorption capacity of PDA@COFs for PFOA is 819.26 mg/g（1.978 mmol/g）.The fast adsorption kinetics and high saturated adsorption capacity of PDA@COFs for PFOA are more excellent to most of the reported PFOA adsorbents.（3）The PDA@COFs composite exhibited good selective adsorption of PFOA.And PFOA was rapidly and efficiently removed under the electrostatic interaction,and the amino groups in the PDA@COFs composites is the main adsorption site of PFOA.The PDA@COFs composites exhibited good regeneration and recycling performance,and have high potential value in practical applications.（4）The CDI system based on PDA@COFs electrode could effectively remove79.5%of PFOA（1 mg/L）in 90 minutes.The adsorption capacity of PDA@COFs electrode is 1.88 times than that of the activated carbon electrode,and the acidic condition is more favorable for the removal of PFOA.The adsorption process of PFOA in the CDI system mainly includes physical and chemical interaction.The transition of the adsorption process indicates the effect of voltage on the adsorption,which could enhance the adsorption capacity of PDA@COFs for PFOA.In addition,electrostatic interactions dominated the adsorption mechanism.（5）The prepared PDA@COFs electrode is easy to regenerate.And the CDI system possesses good regeneration performance and reusability,which further improved the utilization of PDA@COFs composite.And the regeneration of the electrode in the CDI system does not need additional operations or addition of any reagents,which saved operating costs.Furthermore,it is easy to realize the enrichment and recovery of PFOA in wastewater by CDI system.The resource utilization of PFOA could be realized while economically and efficiently removing PFOA in water,which is coincident with the development of green economy.Based on the above research results,in this paper,a new type of PDA@COFs composite which possessed efficient and selective adsorption were prepared and applied to the removal of PFOA in water.And a PFOA removal system based on PDA@COFs was constructed.It provides new ideas for the development of new and efficient PFOA adsorbents.Furthermore,it also provides theoretical support for the construction of CDI system for PFOA removing,which has important theoretical and practical significance for the low-consumption and efficiency treatment of perfluorinated/polyfluorinated compounds in water.