Thin film composite membranes via layer-by-layer assembly for pervaporation separation
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The aim of this study was to develop LbL membranes based on polyethyleneimine and graphene oxide (PEI/GO) and to investigate them for three different applications, namely the pervaporative desalination of high-salinity water, dehydration of ethylene glycol (EG) and dehydration of ethanol (EtOH) and isopropanol (IPA). Salts are non-volatile, EG has a high boiling point, and EtOH and IPA can form an azeotrope with water. To prepare LbL membranes in this work, a chlorine-treated thin film composite (TFC) polyamide membrane was used as a substrate, and PEI and GO were used as polycation and polyanion, respectively. To the best of our knowledge, it is for the first time the aforementioned LbL membranes were prepared and investigated in pervaporation applications. Chlorine-treatment of TFC polyamide was initially studied to determine the suitable chlorination conditions. It was found that pure water flux was more than doubled after chlorination with sodium hypochlorite at 6000 ppm for 2h at room temperature. The as-chlorinated membrane showed that the water permeation flux was almost tripled (i.e., 1.3 kg/m2h ) while the salt rejection decreased by 2% (i.e., 95.8%) for pervaporative desalination of 20 wt% feed salt concentration. The chlorine-treated TFC polyamide membranes with improved flux were used as substrates throughout this study. First, attempts were made to improve the pervaporative desalination performance. PEI/GO LbL membrane formed on the surface of chlorine-treated TFC polyamide membrane for pervaporation desalination of high-salinity water was investigated for the first time, and for this reason, concentrations of PEI and GO were 0.02 monomol/L and 100 ppm, respectively. It was shown that the incorporating PEI and GO to the chlorine-treated TFC polyamide membranes improved the salt rejection. The PEI/GO LbL membrane was tested for the desalination of aqueous solutions containing NaCl, Na_2 SO_4, MgSO_4, and MgCl_2 salts, and a water flux as high as 8 kg/m^2h with a high salt rejection (>99.9%) was obtained for all the tested salts at various temperatures and feed concentrations. In order to assess the temperature dependence of the permeation flux through the membrane, the apparent activation energy for permeation of water was determined. The water permeation flux increased with an increase in temperature due to the augmented driving force and diffusivity in the membrane. The properties of the membranes surface were studied using Fourier transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), atomic force microscopy (AFM) and contact angle measurements. Based on the experimental data and stability of the PEI/GO LbL membrane, the formation of the membranes through the LbL self-assembly with PEI and GO showed potential for applications in the treatment of high-salinity water such as industrial wastewater and concentrated reverse osmosis (RO) brine. EG is one of the important substances in gas and chemical industries. Therefore, after the efficiency of PEI/GO LbL membrane with one bilayer was found and analysed for pervaporative desalination of salts, the PEI/GO membrane was further modified by increasing the number of bilayers for uses in the dehydration of ethylene glycol (EG) with and without the presence of salts in the feed. The effects of operating temperature and feed concentration on the membrane performance were studied. The nano self-assembly of GO and PEI with three bilayers showed a satisfactory performance; a permeation flux of 114 g/(m2 h) and a separation factor of 213 were achieved at 35 C for a feed water concentration of 2 wt%. The impact of inorganic salt in the feed on the pervaporation properties were tested by using NaCl as a model salt. The permeation flux decreased with feed salt concentrations while permeate water content increased. The effects of the number of PEI/GO bilayers on membrane performance were also investigated. Increasing number of bilayers from 1 to 15 caused separation factor to increase by 148% while the total permeation flux decreased by 38%. It was for the first time in the literature that the resistance per bilayer and substrate resistance in LbL membranes were evaluated based on the resistance-in-series approach. FTIR and AFM were used to study the chemistry and morphology of the surface of the PEI/GO LbL membranes with different bilayers, respectively. Water contact angle measurements showed that the surface of the PEI/GO LbL membranes was hydrophilic (lower than 54°), which is advantageous for dehydration of EG. Following dehydration of EG, the PEI/GO LbL membranes were crosslinked with glutaraldehyde (GA) to further improve the performance of membranes for pervaporation dehydration of EtOH and IPA. A two-level factorial design was used to determine the effects of three main factors in the membrane preparation (i.e., GA concentration, crosslinking time and temperature) on the permeation flux and separation factor. It was found that the GA concentration and crosslinking time were the most significant factors on the performance of the membranes for alcohol dehydration. The effects of operating temperature and feed concentration on the separation performance of the crosslinked LbL membrane were studied. For the crosslinked LbL membrane, total flux increased sharply with operating temperature, while separation factor showed little dependence on temperature. At 60 ºC, the crosslinked (PEI/GO) LbL membrane with seven bilayers had fluxes of 1.8 kg/m2h and 1.5 kg/m2h at 2 wt% water in feed, and the corresponding separation factors were 77 and 197 (respectively for EtOH/water and IPA/water mixtures). It was also showed that the membrane performance can be efficiently adjusted by altering the number of bilayers. The permeance ratio increased to 250 and 620 for water/EtOH and water/IPA systems, respectively, demonstrating that the membrane became much more permselective after deposition of the bilayers on the substrate. FTIR, AFM and contact angle measurements were used to study the surface chemistry, morphology and hydrophilicity of the (PEI/GO) LbL membranes with different bilayers, respectively. The separation performance of the XL(PEI/GO)7 membrane was monitored over an operation time of 210 h at 50 ºC to verify the membrane stability. The long-term data showed there were no significant variations in pervaporation performance, implying the feasibility of the crosslinked membrane for pervaporation processes. For all target applications, the activation energies for permeation of each penetrant based on permeation flux (E_J) and membrane permeance (E_P) were calculated and discussed in detail. The activation energies of the different penetrants were compared as they were affected by the types of PEI/GO LbL membranes and the composition of the feed solutions to be separated. Finally, suggestions for future work include optimization or modification of the PEI/GO LbL membrane preparation to further improve membrane performances for pervaporation applications. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) can be used to look at the PEI/GO LbL membranes with and without crosslinking in more detail in future studies.
Cite this version of the work
Elnaz Halakoo (2019). Thin film composite membranes via layer-by-layer assembly for pervaporation separation. UWSpace. http://hdl.handle.net/10012/15291