Design And Application Of High Performance Nanofiltration Membrane With Ultrathin Charged Separation Layer

Nanofiltration membrane is a kind of pressure-driven membrane and its separation performance is between ultrafiltration and reverse osmosis.It can effectively reject molecules with molecular weight above 200 Da and multivalent ions,which has been widely applied in water treatment,food industry,medicine and health,petroleum industry and other fields.At present,polyamide composite nanofiltration membrane prepared by interfacial polymerization is the mainstream product in the market.For the increasingly complex separation system,the construction and regulation of surface ultrathin separation layer is one of the important strategies to improve the performance of nanofiltration membrane.In this paper,based on the structural design of the separation layer,amine monomers are released by catalysis to control the interfacial polymerization to form polyamide separation layer on substrate membrane surface.Morever,with the aid of dopamine,PDA/PEI/PAA multi-charged layers are constructed on the surface of polyamide membrane to reject dianion,dication and high concentration salts.The detailed results are as follows:?1?P-nitroaniline?PNA?was reduced to p-phenylenediamine?PPD?through Au-catalyzed hydrogenation and then polymerized with trimesoyl chloride?TMC?to prepare polyamide composite membrane.The relationship between the concentration of PPD and catalytic time,and the influence of the catalytic time on morphology,chemical composition and separation properties of polyamide composite membrane were studied.The results show that a complete and dense polyamide membrane can be prepared at a low concentration of 0.02 wt%through the slow release of PPD,whose rejection for Na2SO4 is 91.2%.By controlling the catalytic time,we can effectively control the surface morphology and chemical composition of polyamide composite membrane,and control the separation performance from ultrafiltration to nanofiltration.?2?Dopamine was used to modify the surface of PA membrane,so that it had reaction sites.PEI was grafted on the surface of the membrane via Michael addition and/or Schiff-base formation,subsequently,PAA was immobilized by electrostatic self-assembly.We successfully constructed multi-charged layers on the surface of PA membrane.The results show that when the PDA deposition time is prolonged or the concentration of PEI increases,the thickness of separation layer will increase and pore size decrease,giving rise to an increase of salts rejection.The separation layer thickness of optimum composite membrane is 220±5 nm,and the top PEI/PAA polyelectrolyte nanofilm is only 17±5 nm test by SEM.PEI/PAA composite membrane is super high retention of dianion and dication,especially,achieves a high rejection over 90%for high concentration salts(20g L^-1),overcoming the rejection reduction caused by high osmotic pressure and high concentration polarization.Besides,PEI/PAA composite membrane behaves about 60%rejection of NaCl,indicating that it still has selectivity for monovalent ions.