Preparation And Performance Of Self-assembly Nanofiltration Membrane Based On Covalent Interaction

Layer-by-layer (LBL) self-assembly technology is a newly developed method toprepare multilayer membranes in recent years. Commonly, LBL self-assembly NFmembranes are prepared based on electrostatic bonding and the bilayer number ismore than5, but such assembled membrane is not stable. Therefore, it is urgent toinvestigate other interactions to enhance the stability of LBL NF membrane.In this work, a kind of covalent NF membrane was prepared by LBLself-assembly technology based on the combination of the electrostatic bonding andchemical crosslinking. Firstly, poly(ethyleneimine)(PEI) and poly(acrylic acid)(PAA)were used as cationic and anionic polyelectrolytes, respectively, and were alternatingassembled base on electrostatic bonding on porous poly(acrylonitrile)(PAN)ultrafiltration (UF) membrane, then the assembled polymer membrane was immersedin glutaraldehyde (GA), the primary amino group of PEI and the aldehyde group ofGA was crosslinked to form–C=N–bonds (schiff base), while the reaction betweenthe secondary amines and GA will result in-C-O-bonds. A double bilayers covalentcrosslinking membrane was prepared by repeating the above operation. The LBLmultilayer NF membranes were characterized by using atomic force microscope(AFM), scanning electron microscopy (SEM) and attenuated total reflectance-Fouriertransform infrared spectrometry (ATR-FTIR), and the separation performance, stableperformance and antibacterial performance of the prepared LBL NF membranes werealso extensively examined.Firstly, various factors were studied in the membrane assembly process. Resultsshowed that alternatevely electrostatic deposition of0.4%PEI (0.5M NaCl) and0.25%PAA for15min and followed by one hour’s crosslinking in1%GA wasappropriate. At1.0MPa, the rejection for Na2SO4of (PEI/PAA/GA)2could reach98.13%with a permeate flux of12.64L m-2h-1, while the rejection for NaCl couldachieve80.89%with a permeate flux of16.78L m-2h-1. The (PEI/PAA)5NF membrane was assembled by alternately depositing PEI and PAA based onelectrostatic interaction, and the rejection of Na2SO4was91.74%, the flux was18.24L m-2h-1, the rejection of NaCl was29.78%, the flux was26.47L m-2h-1, whichshows that chemical crosslinking greatly shortened the membrane process andreduced the membrane bilayer number and the chemical crosslinked NF membranehas better separation performance. SEM shows that holes on the surface of chemicalcrosslinked NF membrane has been completely covered and membrane surface ismore even. AFM shows that the roughness of membrane increases with increasing ofthe number of bilayer, but after crosslinked by the GA, the roughness of themembrane decreases, which is benefit to anti-fouling.Various performances of the crosslinked LBL (PEI/PAA/GA)2NF membranewere studied and the influence of operating pressure, operating temperature and thesolution pH on the separation performance of the prepared composite NF membranewere investigated, and the organic solvent tolerance degree and the antibacterialperformance of membrane were studied. The stability and the ability of softenningwater were comparatively studied, and the research results show that operatingpressure, temperature and pH have a certain influence on the performance of themembrane, but the effect is not quite obvious, and the membrane has good toleranceto some organic solvents. The prepared LBL (PEI/PAA/GA)2NF membrane has goodantibacterial property, and its bacteriostatic rate reached75.92%. The resistance toswelling of the chemical crosslinked (PEI/PAA/GA)2is better than the electrostatic(PEI/PAA)5NF membrane. Compared with commercial DL membrane, the preparedLBL (PEI/PAA/GA)2NF membrane has higher rejection of monovalent ions inseawater softening process,,however, the flux was much lower.