| Different from other membrane processes which apply operating pressure as driving force for mass transportation, forward osmosis (FO) process doesn’t need operating pressure but osmotic pressure as driving force for mass transportion. The main advantages of using FO are low energy consumption, high water recovery and low tendency of membrane fouling. FO has attained much attention in the field of water treatment and others.Layer by layer (LBL) self-assembly technology is a fast developed surface modification method, and is a kind of preparation method for multilayer membrane by alternatively immersing charged substrate into cationic and anionic polyelectrolytes, respectively. Coordination is a relatively stable force. LBL membranes which the driving force of deposition is metal-ligand coordination were common used in NF process. Although the LBL technology was beginning to be used for preparing forward osmosis membrane, but the LBL self-assembly forward osmosis membrane which the driving force of deposition is metal-ligand coordination wasn’t proved with good potential.In our research, a kind of LBL-FO membrane has been prepared based on metallic ion’s coordination interaction, using commercial polyacrylonitrile (PAN) ultrafiltration membrane as substrate membrane, polyethylene-imine (PEI) and sodium polystyrene sulfonate (PSS) as cationic and anionic polyelectrolytes, respectively. The mechanism of the assembly process was analyzed by using scanning electron microscopy (SEM), atomic force microscope (AFM), contact angle meter, X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy(FTIR). The surface morphology and the performance of the prepared membranes were investigated.Optimal preparation conditions were obtained in the experiment, the concentrations of both PEI and PSS polyelectrolyte solutions were 1.00g/L, the NaCl concentration in both deposition solutions was 0.50M, the CuCl2 concentration in PSS solution was 0.20M, the pH of PEI solution was 9.0 and a deposition time of 10 min was adopted. Under the optimal preparation conditions, the pure water flux of the prepared 3 bilayers (PEI/PSS(Cu)1/2)3 FO membrane could reach about 16LMH, and reverse salt flux could decreased to 1.5gMH by using 0.50M MgCl2 as draw solution and DI water as feed solution in FO mode; while the water flux and the reverse salt flux of the prepared 4 bilayers (PEI/PSS(Cu)1/2)4 FO membrane were about 11LMH and 1.3gMH, respectively, under the same test conditions. After chemical crosslinking of each bilayer by GA, the bilayer number could be decreased, while the separation performance of the prepared FO membrane could be maintained. Compared with the 3 bilayer FO membrane without crosslinking, the water flux of the prepared 3 bilayer crosslinking FO membrane, named as (PEI/PSS(Cu)1/2)3x, decreased to 12LMH, and reverse salt flux decreased to 1.25gMH in FO mode. And optimal GA concentration and crosslinking time was 1.0% and 30min, respectively. Both the (PEI/PSS(Cu)1/2)3 FO membrane and the (PEI/PSS(Cu)1/2)3x FO membrane had poor rejection with monovalent ions. The SEM photographs showed that the LBL (PEI/PSS(Cu)1/2)3 FO membrane’s surface morphology was good, had no obvious pores, but had high density. The AFM results showed that the surface roughness was larger than substrate membrane, and the specific surface area is favor to water diffusion. The contact Angle was relatively small, which reflects that the prepared FO membrane has good hydrophily. The result of FTIR and XPS showed that the driving force of deposition is metal-ligand coordination interaction between the PSS(Cu)1/2 and the secondary amine groups. The LBL (PEI/PSS(Cu)1/2)3 FO membrane has applied limitation on seawater desalination, However, the performance of LBL membrane will be improved if we increase the bilayers or crosslink the membrane by GA. By using non-rejection with monovalent ions of this membrane, there is relatively high practical value in the oligosaccharide solution concentration. |
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