Preparation And Performance Of Layer-by-layer Self-assembly Nanofiltration Membrane Based On Metal-ligand Coordination Interaction

Nanofiltration (NF) membrane has been widely used in seawater desalinationand water treatment applications because of its unique charge property and separationproperties. There are many ways to prepare NF membrane, such as phase inversion,interfacial polymerization, chemical cross-linking and layer-by-layer (LBL)self-assembly. LBL self-assembly technology is a simple, effective method ofpreparing polyelectrolyte multilayer membrane and has been developed in recentyears. Because it possesses the advantages of simple preparation process, rich filmforming material and controllable film thickness, LBL has become an importantmethod to prepare NF membrane.Till now, most researches in LBL membrane preparation were focused onelectrostatic interaction as driving force..In addition to electrostatic interactions,metal-legend coordination interaction, charge transfer interaction, hydrogen bond,covalent function, or the synergistic of the above effects can also be used as drivingforce. Coordination interaction is a kind of relatively stable reaction, and theintroduction of transition metal ion could endow the membrane with some specialproperties. Therefore, we investigate the preparation of self-assembly multilayer NFmembrane based on coordination interaction between transition metal and ligand.In this work, we adopted two kinds of polyelectrolytes, i.e., polystyrene sulfonate(PSS) and polyethyleneimine (PEI), and prepared the LBL self-assembly NFmembranes with modified polyacrylonitrile (PAN) ultrafiltration (UF) membrane assubstrate. The mechanism of the assembly process and the surface morphology wereanalyzed by X-ray photoelectron spectroscopy (XPS), X-ray spectroscopy (EDX),scanning electron microscopy (SEM), atomic force microscope (AFM), as well ascontact angle (CA), and the performance of the membrane was also extensively investigated.In the process of preparing self-assembly NF membrane (PEI/PSS(Cu)1/2)n, theeffects of the types of substrate membrane and sedimentary sequence wereinvestigated. And then the influence of concentration of transition metal ions, the pHof PEI solution and layer number on the performance of self-assembled membranewas researched. The results showed that modification of PAN ultrafiltrationmembrane with2M NaOH solution in35℃for90min is much appropriate. Suitableconcentration of the transition metal ions is between0.2-0.4M, and the appropriatepH is6. The prepared self-assembly (PEI/PSS(Cu)1/2)5NF membranes showed apermeation flux of65L·m2·h1and a rejection of about84%of SO42for2000mg·L-1Na2SO4aqueous solution, and a permeation flux of80L·m-2·h-1and a rejectionof about25%of Cl-for2000mg·L-1NaCl aqueous solution at1.0MPa. The XPSspectrum analysis proved that driving force of the self-assembly process iscoordination interaction. The alternate variation of contact angle reflects theself-assembly process. The SEM photographs showed that holes on the PAN substratemembrane were completely covered and disappeared after five bilayers deposition,and the prepared membrane has a dense surface; The AFM results showed that themembrane has good surface morphology. The antibacterial experiments showed thatthe self-assembled membrane after four bilayers deposition had good antibacterialproperties. And the prepared self-assembled membrane has good stability and can bepreserved for a long time in water. After cross-linking post-processing withglutaraldehyde, the performance of the membrane has a certain improvement, and themembrane becomes dense, which can be seen from the SEM images.To make NF membrane with even better performance, the self-assemblymembrane (PEI/PSS(Fe)1/2)nwas investigated. Compared with (PEI/PSS(Cu)1/2)n, theperformance of the prepared self-assembly (PEI/PSS(Fe)1/2)nis better, and theprepared self-assembly (PEI/PSS(Fe)1/2)4NF membranes showed a permeation flux of39L·m2·h1and a rejection of about88%of SO42for2000mg·L-1Na2SO4aqueoussolution, and a permeation flux of45L·m2·h1and a rejection of about28%of Cl-for2000mg·L-1NaCl aqueous solution at1.0MPa. The effect of the concentration of transition metal ion Fe and layer number on the performance of self-assembledmembrane was researched, and the results showed that suitable concentration of thetransition metal ion Fe is between0.06M and0.08M. The alternate change of contactangle reflects the self-assembly process. The SEM and AFM photographs showed thatthe prepared membrane has a dense surface, small roughness and good surfacemorphology. The prepared membrane has good stability. in aqueous solution. Aftercross-linking post-processing by using glutaraldehyde, the rejection of membrane hasa certain degree, but the flux declines.