Study On Preparation And Performance Of Zeolite Loaded Thin Film Nanocomposite Reverse Osmosis Membrane

Up to now, thin-film nanocomposite (TFN) RO membranes have large potential for improving desalination. In particular, the addition of zeolite nanocrystals (A type) into polyamide has proven to be an effective method to tune the properties of polymer membranes. Therefore, in our study, zeolite-polyamide thin film nanocomposite (TFN) reverse osmosis membranes are fabricated by interfacial polymerization of amine and acid chloride monomers in the presence of zeolite nanocrystals.The NaA and MFI type zeolite nanopaticle were successfully prepared. The membrane formation mechanism was proposed, and effect of zeolite crystal structure of zeolites on membrane structure and transport properties was also investigated. Then, the chemical stability and application of TFN membrane was studied. Also the improved dispersion of inorganic nanoparticles in the polyamide matrix was successfully obtained.Firstly, a simple method for synthesis zeolite nanoparticles was proposed. Just one kind of water soluble polymer PVA was used to blending with the zeolite colloidal nanocrystals, as a temporary barrier to isolate as-synthesized nanocrystals during template removal, without chemical reaction, the dicomposed temperature of PVA was about 520℃, and the proper concentration of PVA solution was 2 wt%. This method would promote the industrially application of zeolite nanocrystals, also there was no report about this in the literature.Then, the effects of introducing the NaA zeolite nanocrystals via the aqueous phase or the organic phase on the membrane morphology, surface properties and desalination performance were investigated. When zeolite was added into the aqueous phase, zeolite was covered by the incipient layer of polyamide membrane at the beginning of polymerization, and then polyamide layer would be extended to the support membrane following slow polymerization. And at the bottom layer, there were macrovoids between the polyamide and zeolite because the zeolite surface adsorbed too much water to occur polymerization reaction. For TFN membrane with zeolite in organic phase, the polyamide layer was first formed on the surface as dense core, and then extended from the zeolite surface to other zone, which is as claimed as literature. In addition, NaA in organic phase will be hydrated and release heat when the organic phase contact with aqueous phase, higher reaction temperature may increase MPD diffusivity through incipient membrane, and the NaA zeolite will be enclosed in polyamide very well and the aggregate pores were eliminated by the cross-linked groups. Both the flux and salt rejection increased clearly with the increase of the zeolite loading in organic phase. Therefore, the zeolite should be added in the organic phase.In order to investigate the effect of zeolite properties such as zeolite particle and zeolite type on membrane structure and transport properties, TFN membranes were synthesized through adding NaA zeolite and MFI type zeolite with different Si/Al ratios. The hydration enthalpy and water diffusion coefficient of MFI zeolite were calculated. The surface structure of TFN membrane was relative to the hydration enthalpy of zeolite, and surface of TFN membrane became relative smoother when Silicalite-1 zeolite was used, which has lower hydration enthalpy. The flux of the TFN membrane was determined by the pore diameter and pore structure. The increased order of flux was:Silicalte-1>ZSM-5>NaA, which is the same as the order of water diffusion coefficient. The cross-linked degree of TFN membrane incresead at the order:NaA>ZSM-5>Silicalite-1, and the salt rejection of the TFN membranes were higher than that of the polyamide membrane. For the Silicalite-1 type zeolte loaded reverse osmosis membrane, the flux increased obove 60 L/m2.h with only a little sacrifice of rejection, which was 3-times of the flux for polyamide membrane.The MFI type zeolite was firstly introduced to the TFN membrane, not only the flux of the membrane increased, but also the other properties was enhanced, such as acid-resistance, ion exchange properties, chlorine resistance and separation performance for the organic matter. In our study, the acetic acid was used to detect the acid-resistance of TFN membrane. After immersed in the acid solution, the structure of NaA zeolite was completely destroyed, and the polyamide/NaA membrane showed that flux increased while salt rejection decreased. However, the separation performance of the polyamide and polyamide/Silicalite-1 membranes was steady. Then, the TFN membranes were used to treat the NaCl and CaCl2 mixed solution. Ion exchange would occurre in NaA zeolite, but not in Silicalite-1 zeolite, thus the TFN membrane loaded with Silicalite-1 zeolite showed steady separation performance. So, it has large potential to enhance the chemical stability by adding Silicalite-1 zeolite, and the membrane could be widely applied.As mentioned above, as the zeolite nanoparticle should be added in the organic phase, producing a better organic-inorganic desalination membrane has become quite a challenge, because inorganic materials were hard to well disperse within thin polymer membrane films. Therefore, surface modification of zeolite with silane coupling agents had been proposed to improve the dispersibility of zeolite and interfacial strength in order to enhance the separation performance. The dispersibility of zeolite in the organic phase was improved obviously and subsequently the separation performance of the TFN also was enhanced.In conclusion, TFN membrane loaded with Silicalite-1 zeolite was found with higher separation performace, the flux of the membrane has a three-times increase, at the same time, the salt rejection also increased. In addition, the chemical stability of the membrane was largely improved. The novel membrane has large potential for industrial application.