Preparation And Characterization Of Carboxymethyl Chitosan And Phosphoesterified Polyvinyl Alcohol Composite Nanofiltration Membrane

The molecular weight cut-off?MWCO?of nanofiltration?NF?membrane is between 200-1000 Da,which can efficiently reject the inorganic ions?small organic molecules?bacteria?germ cells and so on.The current mainstream commercial NF membrane is a charged composite NF membrane,according to the charging property can be divided into positively charged,negatively charged and amphoteric composite NF membrane.Carboxymethyl chitosan?CMCS?is a water-soluble amphoteric derivative of chitosan,and its molecular structure contains a large number of COO-.Phosphoesterified polyvinyl alcohol?PPVA?is a product of the esterification of polyvinyl alcohol,and its molecular structure contains a large number of PO3-4.CMCS and PPVA are selected as the active layer material for composite NF membrane in this study,ethylene glycol diglycidyl ether?EGDE?was used to be the cross-linking agents,30000 Da polysulfone ultrafiltration?PSF UF?membrane as the base membrane.Use coating and cross-linking method,two new series charged NF membranes CMCS/PSF and PPVA/PSF were prepared under a certain preparation process.Attenuated total reflection infrared spectroscopy?ATR-IR?,environment scanning electron microscope?ESEM?and static contact angle measuring instrument were employed to characterize the morphology and surface hydrophilicity of the resulting membrane.At the some time,we also study the optimal membrane preparation conditions and the effects of various external operating factors on the sparation performance of the composite NF membrane.The study found that when using 5 mmol/L Na₂SO₄ as the feed liquid,CMCS/PSF series composite NF membrane preparation conditions were optimal: the concentration of casting solution was 2 wt%,the concentration of cross-linking agent was 5 vol%,reaction temperature was 70 ?,the cross-lingking reaction time was 25 min.Under optimized conditions,with the pressure increaseing,the rejection of the CMCS/PSF composite NF membrane increased,firstly,and then decreased,while the permeate flux of the composite membrane almost linear increased with the increased of pressure.Both the rejection and permeate flux of CMCS/PSF composite NF membrane decreased first and then increased with the increase of feed flux.The rejection of CMCS/PSF composite NF membrane to electrolyte solutions decreased in the order of Na₂SO₄?NaCl?KCl?MgSO4?MgCl2.Both the rejection and the permeateflux of the resulting CMCS/PSF composite NF membrane to Na₂SO₄ and NaCl decrease slowly as the concentration of the electrolyte solutions increased.The rejection of the CMCS/PSF composite NF membrane was decreased first and then increased with the increase of glycerol concentration,while the permeate flux was opposite to the rejection.Through the research,we have found that the optimized synthesize conditions of PPVA/PSF composite NF membrane were 1 wt% of casting solution,20 vol% EGDE,75 ? cross-linking temperature and 30 min thermal cross-linking reaction time.With the pressure increased,the rejection of the PPVA/PSF composite NF membrane increased first and then decreased,while the permeate flux of the composite membrane almost linear increased with the increased of pressure.Both the rejection and permeate flux of PPVA/PSF composite NF membrane decreased first and then increased with the increase of feed flux.The rejection of PPVA/PSF composite NF membrane to electrolyte solutions decreased in the order of Na₂SO₄?NaCl?KCl?MgSO4?MgCl2.Both the rejection and the permeate flux of the resulting PPVA/PSF composite NF membrane to Na₂SO₄ and NaCl decreased slowly as the concentration of the electrolyte solutions increased.The rejection of the PPVA/PSF composite NF membrane almost linear increased with the increase of glycerol concentration,while the permeate flux was opposite to the rejection,almost linear decreased with the increase of glycerol concentration.The rejection of PPVA /PSF composite NF membrane to simulated seawater can reach more than 75%,and the permeate flux can reach more than 8.0 kg·m-2·h-1when the outside operating pressure was 0.4 MPa,liquid flow rate was 0.9 L/min.Indicating that in the process of seawater separation,the seawater can be seen as a“comprehensive inorganic salt”.PPVA/PSF composite NF membrane has good separation performance for three typical organic dyes.The rejection and permeate flux for 3 mg/L alkaline fuchsinwas 93.4% and 13.2 kg·m^-2·h^-1,respectively.