Modification Of Reverse Osmosis Composite Membranes By LbL Selfassembly Method

Reverse osmosis is a kind of effective water purification technology, and plays an important role in developing new water sources. However, the conventional polyamide composite membrane has two main problems. They are usually easy to be chloridized and easy to be fouled. In this study, the base membrane MPD-CFIC was fabricated by interfacial polymerization reaction of m-phenylenediamine(MPD) and 5-choroformyloxyisophaloyl chloride(CFIC).Since there is a lot of linear pendant-COOH groups on the surface of MPD-CFIC membrane, this membrane can be modified by the LbL self-assembling method to improve its antifouling property. First the MPD-CFIC base membrane was ionized by NaOH or NaClO solution to get polyanion, and then was self-assembled with the CS polyelectrolyte to prepare the modified membrane 1 or modified membrane 2. After this, the GA was used to solidify the self-assembling layer and adjust the degree of crosslinking of the skin layer of the resultant membrane. On the other hand, the MPD-CFIC base membrane after being ionized by NaOH solution was first be assembled with polycation PDDA and polyanion PSS successively, and then was assembled with the CS polyelectrolyte, finally the GA was used to solidify the self-assebling layer to obtain the modified membrane 3. The LbL self-assembly technologies including ionization condition, CS concentration, assembling time and GA curing condition were investigated. The separation performance of the selfassembling membranes was measured in different concentration of NaCl solutions. The results showed that both of water flux and salt rejection of the modified membranes via self-assembly method were improved compared to the base membrane.The chemical structure of the active separation layer of the membranes was analyzed via the total reflection Fourier transform infrared spectrometer(ATR-FTIR) and X-ray photoelectron spectroscopy(XPS). The surface morphology of the membranes was observed by scanning electron microscope(SEM) and atomic force microscopy(AFM). The hydrophilicity of the membranes was measured by contact angle meter. The surface of the four membranes exhibits the familiar “hill and valley” sturcture, but the modified membranes by self-assembly has smoother and more hydrophilic surface than the base membrane. Therein, the membrane 3 has the smoothest surface, whereas the membrane 2 has the most hydrophilic surface.The antifouling property of the four membranes was tested by static fouling experiments through immersing membrane in the West Lake water. The separation performance of the four fouled membranes was measured, and SEM was further used to observe the change of the surface morphology of the membranes. The results showed that the modified membranes via LbL self-assemble have better antifouling property than the base membrane.