Microstructure Control For The Selective Layer Of Thin Film Composite Forward Osmosis Membrane

Membrane separation technology,such as ultrafiltration,nanofiltration,reverse osmosis,and forward osmosis(FO),has been widely used to address water shortage problems.Among them,FO technology has been widelyd applied in desalination and wastewater treatment by virtue of the low energy consumption.However,the key issue in FO technology to be addressed is the membrane with high water flux and excellent anti-fouling performance.Polyamide(PA)-based thin film composite(TFC)membranes are extensively studied for FO process because of their simple preparation process and excellent separation performance.However,the traditional TFC FO membrane is relatively hydrophobic,thus exhibiting low water flux and poor anti-fouling performance.Herein,crown ether and benzenediamine(BDA)derivatives are employed to modify the PA layer of the TFC membrane to solve the above problems.Firstly,crown ether is used as a co-solvent for bulk modification of the PA layer.The introduction of crown ether can promote the degree of interfacial polymerization,which not only increases the roughness of the PA layer,but also improves the free volume of the PA layer.The inherent hydrophilicity of the crown ether also facilitates the hydrophilicity of the PA layer,thereby improving water flux significantly.Moreover,the increase in the degree of interfacial polymerization can increase the crosslinking degree of the PA layer,making the PA layer denser.And the crown ether in the PA layer can complex alkali metal ions.Therefore,the PA layer modified by crown ether exhibits higher the salt rejection.The anti-fouling performance of the modified membrane is significantly improved with the incorporation of crown ether.In addition,the long-term FO performance and crown ether release are also tested to evaluate the stability of crown ether in the PA layer and performance stability with the bulk modification of crown ether.However,when the crown ether concentration is higher,the PA layer becomes thicker,leading to greater resistance,thereby reducing water flux.In the following work,benzenediamine(BDA)derivatives with carboxylic acid functional groups are also incorporated in the PA layer by being mixed into the aqueous phase of the interfacial polymerization,for the fabrication of novel TFC membranes.The introduction of the BDA derivatives makes the PA layer thinner,denser and more hydrophilic,reducing the mass transfer resistance and enhancing the hydrophilicity of the PA layer,thereby significantly improving the water flux and salt rejection.In order to further improve the anti-fouling performance and the separation performance of the membrane,the surface modification of the PA layer is implemented by grafting silver ions on the surface of the membrane,to metallized and mineralized membranes with enhanced anti-fouling and anti-bacterial properties.Additionally,the silver release test proves the stability of the bound silver nanoparticles on the membrane surface.