Preparation And Characterization Of PQ-10/PA Positively Charged Composite Nanofiltration Membrane

Nanofiltration（NF）has become a promise separation technology for the treatment of wastewater and the industries pharmaceutical and biotechnology.However,most commericially available nanofiltration membranes are negatively.Therefore,it is of great importance to develop positively charged nanofiltration membrane and thus promote the application of NF technology.In this study,positively charged nanofiltration membrane was prepared through surface coating method with polyamide-based（PA）thin-film composite membrane as virgin membrane,cationic polyelectrolyte polyquaternium-10（PQ-10）as functional material and glutaraldehyde（GA）as crosslinking reagent.The effects of PQ-10concentration,GA concentration,heat treatment time and temperature on the separation performance of PQ-10/PA composite NF membrane were studied.The surface structure,charge and chemical composition of the polyelectrolyte NF membrane were characterized by FE-SEM,Zeta potentiometer and ATR-FTIR,respectively.Membrane separation performance to inorganic salts,dyes and characteristic organic pollutants and anti-fouling property to model foulants of dodecyltrimethylammonium bromide（DTAB）,polyacrylamide（CPAM）and bovine serum albumin（BSA）were investigated through cross-flow permeation tests.The conclusions obtained were as follows:（1）ATR-FTIR analysis demonstrated that the cationic polyelectrolyte PQ-10 has been successfully coated on the PA membrane.After coating the PQ-10 layer,the thickness of the activel layer remained nearly constant and the surface became denser and smoother.Zeta potential measurements revealed that the surface of PQ-10/PA composite NF membrane was positively charged within the pH scope of 3-9.（2）The increase of the concentration of cationic polyelectrolyte PQ-10 could enhance membrane surface charge density.With the increase of PQ-10 and GA concentration,heat treatment time and/or heat treatment temperature,the CaCl₂rejection of the PQ-10/PA composite NF membrane ascended while the water flux decreased.The optimum preparation conditions for the cationic polyelectrolyte composite NF membrane PQ-10/PA were as follows:PQ-10 concentration=0.5w/v%,heat treatment temperature=50℃,heat treatment time=135 s and GA concentration=1.0 w/v%.The PQ-10/PA composite NF membrane prepared exhibited a rejection of 53.8%and a water flux of 28.3 l/m²·h to 500 mg/l CaCl₂ aqueous solution under5.0 bar.（3）The molecular weight cut-off of the PQ-10/PA composite NF membrane was 335 Da,which was much lower than that of the PA membrane of 520 Da.The rejections of the PQ-10/PA composite NF membrane to different inorganic salts followed the order of R_MgSO4（92.4%）>R_Na2SO4（91.2%）>R_NaCl（73.2%）>R_MgCl2（66.8%）>R_CaCl2（54.5%）.The removals of the PQ-10/PA composite NF membrane to different dyes followed the order of Vitoria blue B（99.9%）=methyl blue（99.9%）>crystal violet（99.7%）>sunset yellow（98.4%）>neutral red（92.2%）.Compared with the PA virgin membrane,the removals of PQ-10/PA membrane to crystal violet and neutral red of low molecular weight were significantly enhanced.The cationic polyelectrolyte composite NF membrane exhibited a rejection order of R_LAS（85.8%）>R_NPE（79.8%）>R_DTAB（60.3%）>R_DEP（54.9%）>R_DMP（43.8%）to single organic pollutants and a rejection around 64.3%to mixed organic pollutants of 250 ppm.（4）In the fouling experiments,the flux decline ratios of PQ-10/PA composite NF membrane to foulants of DTAB and CPAM were 13.7%and 14.8%,respectively,which are much lower that those values of 24.5%and 42.5%of the PA virgin membrane.The PQ-10/PA composite NF membrane also showed high flux revocery ratios of 93.1%and 91.4%to aqueous DTAB and CPAM solutions,respectively,which were also much higher than those of the PA virgin membrane.On the other hand,to the negatively charged foulant of BSA,the flux decline ratio and recovery ratio of the PA membrane were 15.6%and 95.1%,respectively,while to the PQ-10/PA composite NF membrane,they were 34.9%and 75.2%,respectively.These results indicated that PQ-10/PA composite NF membrane had better anti-fouling properties to cationic foulants than the PA membrane.