Construction And Separation Performance Of Chlorinated Polyvinyl Chloride Loose Nanofiltration Separation Layer

To date,it is very difficult to achieve the effective separation of organic dyes and inorganic salts by the traditional printing and dyeing wastewater treatment technology.For membrane separation technology,nanofiltration membrane can effectively separate organic dyes and inorganic salts based on size screening and charging effects.However,the traditional nanofiltration membrane prepared by an interfacial polymerization has some drawbacks such as high operating pressure,low efficiency and high salt rejection rate.Moreover,the separation layer easily falls off in long-term operation under high pressure due to no chemical bond links with the base membrane.In this study,the nanofiltration separation layers with different porosities and positive charges were constructed on the composite membrane surface by a chemical cross-linking method in aqueous solution to achieve the effective separation of dye/Na salts.The styrene-maleic anhydride copolymer（SMA）/chlorinated polyvinyl chloride（CPVC）composite membranes were prepared by the non-solvent induced phase separation（NIPS）method.Composite membranes with different pore morphologies were constructed by changing the content of SMA,the temperature and composition of the coagulation bath.The results show that the addition of SMA would accelerate the phase separation rate to change the pore structure of the composite membrane surface from double continuous pores to circular pores.When the SMA content was 2 wt.%,the circular pores were evenly distributed on the composite membrane surface and finger-like pores appeared on the cross section,which was attributed to the good compatibility between SMA and CPVC.Consequently,the resultant membrane exhibited the best anti-fouling performance.When the coagulation bath temperature was 30℃,the circular pores of the composite membrane surface were very uniform,and the finger-like pores penetrated through the membrane layer,leading to the good anti-fouling performance.When the coagulation bath contained 3 wt.%DMAC,the double continuous pores and circular pores coexisted on the composite membrane surface,the average pore size became smaller,and more finger-like pores appeared in the sub-layer,resulting in the good anti-fouling performance.A series of branched amine nanofiltration separation layers with different loose structures and positively charges were constructed on the surface of the SMA/CPVC composite membrane by the polyethylene imine（PEI）cross-linking reaction with different molecular weight.The results show that the primary amine group in the PEI chains not only reacted with the anhydride on the composite membrane surface,but also reacted with the CPVC chains by alkylation,forming the branched amine nanofiltration separation layer with cross-linked network structure on the membrane surface.The thickness of the nanofiltration separation layer gradually became thinner and looser due to the changes in the length of the PEI segment.Gaussian software is used to calculate and simulate the geometric size and electrostatic distribution diagram of the dye molecule structure,which proves that the rejection mechanism of the branched amine nanofiltration separation layer for Brilliant Blue R（BBR）,Methyl Blue（MB）,Congo Red（CR）,and Acid Fuchsin（AF）was mainly size sieving effect,and all rejections were more than 95%.The rejections of bromocresol green（BTB）and bromothymol blue（BCG）（less than78%）were significantly lower than the CR,and the retention mechanism was mainly the size-electrostatic synergistic effect.Under 0.2MPa operating pressure,the membranes displayed good separation performance for BBR and Na Cl mixed solution,with higher than 98%rejection to BBR and lower than 10%rejection to Na Cl.The branched amine nanofiltration layer with molecular weight of 22k g/mol PEI had a good separation performance for high Na Cl concentration and dye mixed solution,and its maximum resistance pressure is 0.4MPa.In addition,it also had good long-term operation stability for dye/Na Cl separation at 0.2MPa operating pressure.The branched amine nanofiltration separation layer consisting of 22k g/mol PEI was further tuned by small molecule（BDE,EGDE and PTGE）epoxy with different functional degrees,constructing the epoxy cross-linked nanofiltration separation layer.The O/N ratio of epoxy cross-linked nanofiltration separation layer was higher than that of the branched amine nanofiltration layer,the density was high,and the retention rate of Cr,AF,BCG and methyl violet（CV）was also high.The permeation flux of epoxy crosslinked nanofiltration layers to dye solution was in the order of EGDE>BDE>PTGE,and the rejections of BCG followed the tendency like PTGE>EGDE>BDE,which were more than 95.1%.And the rejections of Na Cl and Na₂SO₄ were less than 20%and 30%,respectively.The epoxy nanofiltration separation layer constructed with the double epoxy functional group EGDE exhibited good separation performances for dye solutions with high salinity,and also showed good long-term operation stability.