Development Of A Highly Selective Salt-separating Nanofiltration Membrane Based On Monomer Design

For industrial contaminated water containing high concentration of sodium sulfate(Na₂SO₄)and sodium chloride(NaCl),it is necessary to develop appropriate technology to realize the efficient separation of Na₂SO₄/NaCl.Nanofiltration membranes have charged surface with nano-sized pores,thus theoretically,NF membrane can achieve high-efficiency removal of low-molecular-weight organic matter and divalent or multivalent ions,while allowing monovalent ions to pass through.The structure of the separation layer is the core that affects the separation performance of nanofiltration membranes,which is mainly determined by the structure and the stacking state of the polymer chains forming the separation layer.In view of the high-efficiency separation for Na₂SO₄/NaCl,based on the principle of molecular structure design,adjusting the polyamide network structure and appropriately increasing the free volume size of separation layer can effectively improve the selectivity of nanofiltration membrane for Na₂SO₄/NaCl.In this paper,hollow fiber composite nanofiltration membrane was prepared by interfacial polymerization using homopiperazine(HPIP)as aqueous phase monomer and trimethyl chloride(TMC)as organic phase monomer.The effect of key parameters of interfacial polymerization on the structure and properties of nanofiltration membranes was studied.The results show that the flux of nanofiltration membrane prepared with HPIP reaches 54.0 L·m^-2·h^-1,and the rejection for Na₂SO₄ and NaCl is97.5%and 9.8%,respectively.And the selectivity of Na₂SO₄/NaCl was 10.0,which is twice as high as that prepared with piperazine(4.8).Based on the above study,the principle of the correlation between the structure and performance of the nanofiltration membrane is analyzed..The results show that with the increase of HPIP concentration,the crosslinking degree of the separation layer increases from 30.0%to 67.6%,and the surface becomes relatively positive charged.The synergistic effect of steric hindrance and Donnan effect leads that the rejection for Na₂SO₄ and NaCl increases firstly and then remains stable.The thickness of separation layer increases from 65±5 nm to 82±7 nm with the increase of HPIP concentration,which leads to the decrease of permeation flux of composite nanofiltration membrane.The study on the relationship between reaction time,the structure of separation layer,and separation performance shows that with the increase of reaction time,the degree of crosslinking increases from 42.0%to 48.2%and the density increases,which leads to the increase of the rejection for Na₂SO₄ and NaCl.With the extension of reaction time,the thickness of separation layer increases from 54±4 nm to 78±4 nm.The increased density and thicker separation layer results into the decrease of the flux of composite NF membrane.