Modification Of The Substrate Of TFC Membrane For Forward Osmosis

The forward osmosis（FO）process driven by osmotic pressure difference has attracted the worldwide attention in recent years due to its advantages of low energy consumption,high efficiency and low fouling tendency,and been widely used in wastewater treatment,seawater desalination,food processing and other fields.However,the lack of high-performance FO membrane limits its further development and implementation to a large extent.Thin-film composite membranes（TFC）prepared by interfacial polymerization of m-phenylenediamine（MPD）and trimesoyl chloride（TMC）on polymer substrates are favored by researchers due to their excellent chemical stability and separation performance and become the mainstream type of FO membranes currently.Nevertheless,most TFC membranes suffer from serious internal concentration polarization（ICP）,high reverse salt flux and therefore the unsatisfactory FO performance,as the result of the low porosity,poor hydrophilicity of the substrate and nonuniform polyamide（PA）selective layer.In this work,two different modification strategies of the substrate were employed to address above issues.On one side,the substrate with high porosity and ideal hydrophilicity was prepared by incorporating Cu₂（OH）₂CO₃ nanoparticles and etching the surface with hydrochloric acid,which also results in the high cross-linking degree and smooth surface of the selective layer,therefore the high water flux,low reverse salt flux and low fouling tendency of TFC FO membrane.On the other side,different organic phosphoric acids were grafted onto the substrate to optimize its surface charge（draw solution side）.By choosing different draw solution and the substrate with appropriate surface charge in the practical FO process,a reduced reverse salt flux and improved FO performance can be obtained effectively.Firstly,the bulk modification of the polysulfone（PSf）substrate was investigated.A feasible way was proposed to improve porosity and surface hydrophilicity of substrate,and mitigate the ICP impact in the FO process by incorporating Cu₂（OH）₂CO₃nanoparticles in PSf substrate and subsequent etching.The migration of amine monomers during interfacial polymerization（IP）was confined because of promoted absorption by Cu₂（OH）₂CO₃ nanoparticles,contributing to a higher cross-linking degree and smoother PA selective layer.Correspondingly,the superior FO performance of the resultant TFC membrane were achieved.And,the better antifouling properties was also achieved in the dynamic fouling test for 24 hours,demonstrating the great potential of the modified membrane for practical wastewater treatment.Additionally,the surface charge modification of substrate facing the draw solution side was also studied systematically.By grafting PEI and different organic phosphoric acids on the substrate,the charge properties on the membrane surface were optimized,causing the hindered diffusion of draw solutes via electrostatic interactions,and therefore reducing the reverse salt flux.Furthermore,the performance of FO membrane with optimized surface charge was tested with different draw solutions,and the results revealed that,the FO performance was enhanced if the charge property of the membrane surface was the same as those of the solute ions playing the leading role in the separation process.