Mechanism And Efficiency Of Nanofiltration Membrane For Removal Of Perfluorinated Compounds In Water

In recent years,Perfluorochemicals?PFCs?are found to be widely distributed in natural acqutic system,which has attracted more attention due to their potential threats to animal and human health.However,due to its chemical stability,it is rather difficult to remove perfluorinated compounds effectively by traditional biochemical treatment methods in wastewater treatment plants.Nanofiltration membrane technology can be applied to remove perfluorinated compounds in water due to its outstanding retention efficiency.However,as the lack of the interaction between PFCs and membranes during nanofiltration,there are some problems such as unstable removal efficiency and membrane fouling in the process of PFCs filtration.In this study,the mechanism of PFCs removal was studied in a cross-flow nanofiltration system,in which the adsorption and retention of PFCs by nanofiltration membranes were quantitatively analyzed.The process of nanofiltration interception was then simulated based on quantum computation theory to reveal the interaction between PFCs and nanofiltration membranes.The retention efficiency of dense nanofiltration membrane?NF90?and loose nanofiltration membrane?NFG?on six typical PFCs were systematically studied.At the same flux,the theoretical removal rates of particle size exclusion of PFCs were compared and analyzed to determine the interaction of membrane-PFCs such as particle size exclusion and adsorption.The mechanism of PFCs nanofiltration was studied by means of analysis,quantification of filtration mechanism and bonding analysis.The results showed that there were significant differences in the rejection of six PFCs by two kinds of nanofiltration membranes,such as particle size exclusion,electrostatic repulsion,hydrophobic interaction and hydrogen bonding.Quantitatively,it was found that the electrostatic and hydrophobic effects contributed less to the removal of PFCs than 10%,while both contributed more to the removal of PFCs by NFG membranes.The effect and mechanism of static adsorption of PFCs without applying pressure perpendicular to the membrane surface were studied.The effects of pH,calcium ion and humic acid on the removal rate of PFOS were analyzed.The surface morphology of the membrane was observed by means of microscopic morphological characterization,and the fouling behavior of the membrane was analyzed.The results show that the adsorption efficiency of NFG membrane with larger pore size is much higher than that of NF90because of the pore size and the charge distribution on the surface of the membrane.In the process of membrane fouling,when PFCs exist alone,hydrophobic interaction,electrostatic repulsion and chemical bonding act together.When Ca²⁺and humic acid coexist,the surface charge of PFCs will change,which will affect the electrostatic repulsion of PFCs and membrane surface.On the theoretical calculation level,the initial structure of polyamide?PA?layer and six PFCs on the surface of NF90 membrane was constructed and optimized by Density Function Theory?DFT?method.The binding sites,solvent environment and product structure of different membrane surfaces and PFCs were investigated.The adsorption mechanism of PFCs on the surface of membrane was determined.The adsorption behavior mechanism was revealed from the angle of energy and the structure of binding products.The results show that PFCs interact with membrane through mainly hydrophobic interaction,accompanied by hydrogen bond and electrostatic repulsion.