Layer-by-layer self-assembly of nanofilatration membrane for water and wastewater treatment
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In this study, polyelectrolyte composite membranes were prepared using layer-by-layer (LbL) self-assembly of chitosan/poly(acrylic acid) (chitosan/PAA) on polyethersulfone (PES) substrates. These thin-film-composite (TFC) membranes were used for salt rejection. The performance of the chitosan/PAA composite membranes showed good separation performance for salt solutions. With an increase in the chitosan/PAA bilayers, the salt rejection of the membrane increased and permeation flux decreased, which indicated the growth of the polyelectrolyte thin film on PES substrates. By varying such preparation conditions as polyelectrolyte concentration, deposition time and the outermost layer in LbL assembly, membranes with different separation performances were obtained. Therefore, LbL assembly of polyelectrolytes can be used to tailor the membrane structure with the desired separation performances. Although the chitosan/PAA composite membranes possessed favorable salt retention, these membranes could not afford a long-term operation in salt solutions. Membrane swelling would take place during a long period of nanofiltration (NF) application. To improve the NF performance and stability of the CS/PAA composite membranes in salt solutions, two post-treatment methods (i.e., heat treatment and crosslinking) were used in membrane preparation. An improvement in the membrane selectivity was accomplished by increasing the heating temperature and duration. When the heating temperature reached 150℃, the salt rejection of membranes had markedly enhanced. A heat treatment time of 60 min seemed to be sufficient to produce membranes with high separation performance. In addition, the stability of membrane was also enhanced by the heat treatment. Chemical crosslinking of chitosan/PAA multilayers was also applied in membrane preparation process. Glutaraldehyde was utilized as a crosslinking agent for membrane modification of chitosan terminated composite membranes. The resulting membranes showed improved stability and salt rejection. A 23 factorial experimental design was used in this study to evaluate the main crosslinking effects (i.e., crosslinking temperature, crosslinking time, and glutaraldehyde concentration) and their interactions on the separation performance of the membrane. The crosslinking temperature, glutaraldehyde concentration, and their interaction showed more significant influence on membrane performance than other effects. Moreover, the stability of the chitosan/PAA composite membrane were enhanced considerably by crosslinking of membrane with glutaraldehyde.