Pervaporation Desalination Membrane Prepared By Layer-By-Layer

The layer-by-layer assembly is a technique of depositing multilayer on support material which are driven by electrostatic interactions, hydrogen bonds, van der Waals interaction force and so on. The polyelectrolyte with opposite charges layer-by-layer assembly leads to a multi-bipolar architecture. The multi-bipolar architecture of the LbL-deposited polycation and polyanion layers results in a step-by-step rejection of positive and negative ions. The effects of this ion rejection accumulate with each polyelectrolyte layer, resulting in a very high overall rejection rate. In particular, polyelectrolyte multilayer films, formed by changing the condition of LbL, have exhibited adjustable porosity, film thickness, surface wettability, surface charge density, and swelling behaviors. A pervaporation membrane with high water flux and (?) will be prepared through this method. To our knowledge, there are no reports detailing the use of polyelectrolyte multilayer membranes for desalination by pervaporation. This article studies the pervaporation desalination performance of PDDA/PSS multilayer membranes with different support, counter-ions (NaCI, MgCl2, NaBr). The feed concentration and temperature affect on Water flux through a (PDDA/PSS)20.5film at30℃. The optimum condition for salt water desalination through pervaporation were obtained in this article. The main studying contents of this article included these aspects:1. The effect of support materials and treatment methods on composite membrane pervaporation performance. PSF, PES, PAN ultrafiltration membrane were used as the support. The result indicates that PAN ultrafiltration membrane hydrolyzed in1mol/L NaOH solution it60℃for1h creates pores with20-30nm aperture leading to low mass ransfer resistance and has enough charge on surface for LbL.2. The effect of bilayer numbers and the outmost layer on bervaporation performance. After depositing8to10bilayers of ’DDA/PSS, most of the pores on the PAN surface were covered by a thin ilm on top of the PAN membrane. However, some nano-pore defects that uld not be favorable for ion blocking remain. Increasing the number of DDA/PSS bilayers to20resulted in a very homogeneous, compact, and smooth surface almost without defects. The water flux of (PDDA/PSS)2o is7.7L/(m2h) and the salt rejection is99.5%. Because that PDDA is more elastic and more hydrophilic than PSS, allowing for greater swelling and higher flux. The observed surface morphology and desalination performance indicate that (PDDA/PSS)20.5is the optimal multilayer with flux10.9L/(m2h) and rejection99.98%.3. This article studies the pervaporation desalination performance of PDDA/PSS multilayer membranes with different counter-ions (NaCl, MgCl2, NaBr). The composite membrane without counter-ions shows high flux but low salt rejection which can’t satisfy the water desalination even though the membrane coated with (PDDA/PSS)80, the flux is12.75L/(m2h) and rejection is46%. When NaCl, MgCl2or NaBr as the counter-ions, the thickness of multilayer film increases obviously and obtains a rejection of99.98%. For instance, the multilayer thickness increases in the order:NaCl>NaBr>MgCl2.4. The effect of feed solution temperature was evaluated by separating a35000-ppm NaCl solution at temperatures from30to70℃. Water flux increased exponentially to almost46.5L/(m2h) with increasing feed temperature to70℃. The salt rejection rate remained nearly constant at all temperatures.5. The effect of feed solution concentration was evaluated by separating a NaCl solution at30℃temperatures from0to50000ppm. The water flux through (PDDA/PSS)20.5for2000ppm NaCl solution was9.12L/(m2h), for35000ppm was8.86L/(m2h) and for50000ppm was8.15L/(m2h). The water flux decreasing only slightly from10.8L/(m2h) at0ppm NaCl to8.5L/(m2h) at5000ppm NaCl. This shows that the salt concentration of the feed solution has only a slight affect on water flux. The applicability of our (PDDA/PSS)20.5films to the desalination of raw, untreated seawater by pervaporation was assessed with water taken directly from the Bohai Sea. Excellent desalination performance was observed, with a high salt rejection rate of99.9%and a water flux of3.63L/(m2h)at30℃.