| Forward osmosis(FO)membrane separation technique is a new membrane technique which utilizes the osmotic pressure difference between the draw and feed solution sides as the driving force during the FO process.Compared with the traditional pressure-driven membrane separation processes,the FO membrane separation technique has lower or no hydraulic pressures,higher water recovery,and lower membrane fouling,which is considered as a promising method to simultaneously solve the shortage of water resources and energy crisis.However,the existed thin film composite(TFC)membrane showed more severe internal concentration polarization(ICP)for FO process,which was recognized as the main reason why the actual water flux of TFC membrane was lower than the theoretical flux.In addition,the polyamide(PA)active layer of the TFC membrane was sensitive to active chlorine,leading to more serious membrane fouling and decrease of membrane rejection performance.Therefore,preparation of the higher performance,anti-fouling and chlorine resistance TFC membrane was the current research hotspot.In this paper,we take advantage of the hydrophilic Mg-Al layered double hydroxide nanoparticles(LDH-NPs)as the modification material and successfully prepare of the LDH-NPs modified TFC membranes.The modification of different positions of the TFC membrane and the different modified methods were explored.The effects of the formation conditions of membrane,permeability,anti-fouling,chlorine resistance and mechanism of the chlorine resistance were also investigated systematically.The influence of preparation conditions of the polysulfone(PSf)substrate and the resultant FO performance were systematically investigated,especially the key parameters including the PSf polymer concentration,the solvent ratio and the thickness of the PSf substrate.Moreover,the influence of test conditions on the performance of TFC membranes was also studied,especially the membrane surface velocity,cross-flow direction,feed solution temperature and the draw solution concentration.The optimum preparation conditions and test conditions for the performance of the PA-PSf TFC membrane was obtained.With 1 M NaCl as the draw solution and the DI-water as the feed solution,the water flux of the PA-PSf TFC membrane was 12.69 L m-2 h-1,and the reverse salt flux was 5.55 g m-2 h-1.LDH-NPs,a type of inorganic nanofiller,which was added into the PSf substrates via blending modification.The introduction of LDH-NPs largely improved the porosity,hydrophilicity,surface pore diameter,thermal stability and mechanical strengths of the PSf substrate.Moreover,the penneability was enhanced by the optimization of the support layer,which led to a decline in mass transfer resistance and minimization of the ICP.Consequently,the water flux of TFC membrane with 2 wt%LDH-NPs loading was 18.1 L m-2 h-1,about 1.43 times of that of the unmodified TFC membrane.The structural parameters(S value)of the TFC membrane with 2 wt%LDH-NPs loading was 148 μm.Compared with the unmodified TFC membrane,the structural parameters was significantly reduced.The LDH-NPs modified PA active layer of TFC membrane was obtained via polydopamine immobilization.The performances of LDH-NPs modified PA-TFC membranes by dip-coating and deposition methods were compared.The hydrophilicity,zeta potential,chlorine resistance and anti-fouling were improved by the LDH-NPs modified PA active layer.However,the permeability of LDH-NPs modified membranes was decreased.Compared with the unmodified TFC membrane,the chlorine resistance of the LDHad-TFC-2 membrane was improved,and the chlorine resistance time of the LDHde-TFC-3 membrane was about 96 times.The chlorine resistance mechanism of LDH-NPs was discussed.The LDH deposition layer prevented the PA active layer from the attack of active chlorine as a barrier layer,thus to further improve the chlorine resistance of the PA active layer.In order to improve the permeability,chlorine resistance and anti-fouling of the TFC membrane,the PSf substrate and PA active layer of TFC membrane were simultaneously modified by LDH-NPs.According to the aforementioned research,preparation of the LDH@TFC-LDH membrane was based on the 2 wt%LDH-NPs loading blended PSf substrate and the LDH-NPs deposition modified PA active layer.The prepared LDH@TFC-LDH membrane showed higher permeability,chlorine resistance and anti-fouling.Compared with the unmodified TFC membrane,the LDH@TFC-LDH membrane exhibited chlorine resistance properties(both acidic and alkaline)and better anti-organic fouling,and its permeability did not change.The water flux of the LDH@TFC-LDH membrane was evaluated with 3.5 wt%NaCl as the feed solution and 2 M NaCl as the draw solution,and the results were 7.47 L m-2 h-1 or 8.32 L m-2 h-1 in FO or PRO mode,respectively. |
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