Modification Of Polyamide RO Membrane Using Partially Embedded Electrostatic Self-assembly

The surface of polyamide reverse osmosis(RO)membrane is negatively charged.In the process of separating the solution containing cationic pollutants,it is easy to be polluted,which makes the separation and selectivity performance of the membrane decline and reduces the service life.In order to solve this problem,the ethanol-polyethyleneimine(PEI)solution was used as the electrolyte solution,using partial embedded electrostatic self-assembly technology to modify polyamide RO membranes in this paper.The performance of the RO membranes before and after modification were tested,their structure and properties were characterized.Using partially embedded electrostatic self-assembly method modified polyamide RO membranes,the following four factors were controlled: the molecular weight of polyethylenimine,the concentration of polyethylenimine,self-assembly pressure and self-assembly time.Optimum modification conditions were obtained by testing the flux and rejection of NaCl solution of RO membranes before and after modification under different modification conditions.The results showed that when the molecular weight of PEI is 600 and it's concentration is 800 mg/L,the self-assembly time and pressure are 30 min and 0.2 MPa,respectively,the modified membrane has the best permeation separation performance,the flux of modified membrane increased to 1.86 times and the rejection of modified membrane increased by 1.5%.Several different cationic surfactants were used as simulated pollutants to test the anti-fouling performance of the modified RO membranes.The results showed that the anti-fouling performance of the modified membranes was significantly improved.The modified membranes were treated with alkaline and acidic solutions respectively,and then tested the performance of the treated membranes.The results showed that the modified membranes had good acid and base resistance stability.The results of performance test shows that the modified membrane has good stability.By changing the membrane film to the membrane module,the experiment results showedthat the flux of the modified membrane module obtained by this modification method increases to a certain extent.Field emission scanning electron microscopy(FE-SEM),X-ray photoelectron spectroscopy(XPS),contact angle and Zeta potential on solid surface were used to characterize the compositions and structures of the original and modified RO membranes.FE-SEM results showed that the surface morphology of the modified membranes has no change,it means that this modified method has no influence on the structures of membrane surface;XPS results showed that the content of N of modified membrane increased,while the content of N and O of modified membrane decreased,indicating that PEI was successfully assembled on the RO membrane,the further narrow-spectrum scanning of N and O elements was carried out,the results showed that PEI had electrostatic binding with carboxyl groups on the membrane surface;contact angle results showed that the hydrophilicity of the modified RO membrane increased significantly and Zeta potential results showed that negative charge of modified membranes decreased.Based on the above results,a kind of partially embedded electrostatic self-assembly mechanism is proposed.That is to say,PEI-ethanol solution was used to modify the commercial polyamide RO membranes.Because ethanol swelled the polyamide layer of the membrane,the flux of the membrane increased.At the same time,the partial amino(?NH2)of PEI embedded in membrane pores,and polymer chain deposited on membrane surface and electrostatically adsorbed with carboxyl(?COOH),enhancing the surface positive charge and hydrophilicity of the membrane.The increase of positive charge and hydrophilicity were all beneficial to improve the anti-pollution performance of RO membrane to cationic surfactants.