Architecture Of Mixed Charge Layer Via Michael Addition On The Surface Of Polyamide Reverse Osmosis Membrane

Fouling is one of the important problem that must be faced in reverse osmosis process for water treatment.It is very effective to develop anti-fouling reverse osmosis membrane to improve the stability of reverse osmosis.The zwitterion has been proved to be a potential anti-fouling material for reverse osmosis membrane modification,but there are some disadvantages such as narrow selection of raw materials,strict requirements for functional groups,reaction conditions and equipments,which are not conducive to industrial production.In this paper,in order to improve the feasibility of anti-fouling modification on the surface of reverse osmosis membrane,it was proposed to modify the surface with mixed charge layer,which had the similar charge and hydration effect with zwitterions.With polyethylenimine,2-acrylamido-2-methyl-1-propanesulfonic acid(AMPS)and methacryloyloxyethyl trimethyl ammonium chloride(DMC)as the raw materials,a mild "Amination + Michael addition"two-step modification method was designed to build a charge-balanced mixed charge layer on the surface of the polyamide reverse osmosis membrane.Based on the surface hydrophilicity and charge,the reaction conditions were optimized.Then the influence of the modification process on the membrane structure was studied by surface properties and structural characterization.Finally the separation and anti-fouling performance were evaluated and through the evaluation the anti-fouling mechanism was also analysised.Results showed that the mixed charge layer had excellent resistance to contamination.Specific contents and conclusions were as follows:(1)"Amination + Michael addition" two-step modification method:first polyethylenimine was grafted to the original membrane surface by amidation reaction,which offered abundant amino reaction sites for the subsequent Michael addition.Then AMPS which was an anion and DMC which was a cation,were introduced to construct a mixed charge layer by Michael addition reaction with amino groups.The results showed that the mixed charge layer could be successfully constructed on the surface of the reverse osmosis membrane by the two-step method,and the main structure of the membrane was not damaged.(2)Optimization of reaction conditions of " Amination + Michael addition" two-step modification method:based on the surface hydrophilicity and charge,the optimum reaction conditions were as follows:for the amidation reaction,PEI concentration was 3%,reaction temperature was 39 ℃,reaction time was 1 h.For Michael addition reaction,the one-step grafting method was adopted,AMPS/DMC total concentration was lwt%,anion and cation ratio was 1:1,reaction temperature was 29 ℃,reaction time was 150 min.The surface of the optimized membrane was in a nearly neutral state and the hydrophilicity was greatly improved.(3)Characterization of separation and anti-fouling performance:the modified film was used for the separation of 2000 mg/L NaCl solution at 25 ℃ and 1.6 MPa.The results showed that the water flux decreased from 44.8 L·m-2·h-1 to 39.6 L·m2·h-1 and the rejection to NaCl increased slightly.This meant that the separation performance was not affected by the modification.Chlorine-tolerance also did not change significantly.With N,N,N-trimethyl-l-dodecanaminium bromide,sodium dodecyl sulfate,humic acid sodium,bovine serum albumin and lysozyme as pollutants,the dynamic anti-fouling experiments found that no matter it was single or composite pollutants,the modified membrane had shown excellent anti-fouling performance.The flux recovery experiment showed that the modified membrane also had excellent flux recovery rate.(4)Antifouling mechanism analysis:during membrane fouling process,small organic molecules were more likely to be affected by electrostatic interaction and easier to overcome the hydrophilic barrier.The combination of different charge of proteins had no effect on the degree of membrane fouling,but could accelerate it in the early stage of fouling due to electrostatic attraction.The combination of SDS and protein had a significant effect on membrane fouling.In this paper,simply and mildly,the charge-balanced mixed charge layer was constructed on the surface of polyamide reverse osmosis membrane by " Amination + Michael addition "two-step modification method.The mixed charge layer could not only enhance the anti-fouling performance of reverse osmosis membrane,but also overcome the disadvantages of zwitterions such as narrow selection of raw materials,harsh and complex reaction process,which would be more suitable for industrial applications.