Research On Layer By Layer Assembled Composite Nanofitration Membrane

Nanofiltration is a membrane separation process lying between ultrafiltration and reverse osmosis, which has attracted a wide spread attention extensively applied in softening process of seawater water. Layer-by-layer(LBL) self-assembly is a new type of membrane preparation method developed in recent years, which has good application prospect in the preparation of charged membrane. At present, most of the preparation methods of LBL self-assembly membrane is using electrostatic adsorption of polyelectrolyte. According to recent study, one of the effective methods to improve the rate of making membrane is rely on external pressure driving. Therefore, this paper mainly studied two self-assembly processes including adsorption deposition and pressure deposition to prepare composite membrane. And the separation effect for salt in seawater of the composite membrane was investigated.In this study, two kinds of polyelectrolyte, PEI and PAA, were selected to prepare the multilayer nanofiltration membranes based on Polyacrylonitrile(PAN)ultrafiltration membrane. The surface morphology and chemical structure of prepared LBL membranes were characterized by SEM, contact angles, UV-vis and FT-IR.In the adsorption deposition process, results showed that the prepared composite membranes had high separation performance for MgCl2. The effects of washing mode,polyelectrolyte concentration ratio, number of layers and other membrane preparation conditions and operating conditions on the performance of composite membrane were investigated. The prepared membrane of 1.5 bilayers has a 84.46% rejection of MgCl2 at the flux of 5.98 L/(m2·h). The SEM images show that the adsorption sedimentary methods can uniform deposit the polyelectrolytes on the supporting membrane surface and entirely cover the membrane pores.According to the results of pressure deposition process, high rejection rate can be obtained with less layers. 1.5 bilayers polyelectrolyte multilayer membrane prepared by pressure deposition method has better rejection than 2.5 bilayers polyelectrolyte multilayer membrane by adsorption deposition method, indicates that pressure deposition method has advantages in LBL self-assembly process. The rejection of self-assembled multilayers membrane increases first and then tends to balance withthe increase of the number of layers. The prepared membrane of 1.5 bilayers has a94.29% rejection of MgCl2 and at the flux of 1.56L/(m2·h), while the rejection ratio of NaCl is only 55.33% under the same condition, demonstrates that the self-assembly nanofiltration membrane can be applied for the separation of monovalent cations and divalent cations.