Study On Preparation Of Thin-film Composite Forward Osmosis Membranes For Desalination

Forward osmosis（FO）as an emerging technology,is purely osmotically（chemically）driven membrane process where spontaneous freshwater permeation from the feed to the draw solution.FO has led to a bloom of studies in high-polluting wastewater treatment,seawater/brackish water desalination,and pharmaceuticals industry,which due to its significant advantages:1)no external pressure 2)lower membrane fouling propensity 3)low energy cost.Preparation of FO membrane with high-separation performance is the key core in the whole FO process.Currently,commercial FO membranes mainly contained the following two species:cellulose triacetate（CTA）asymmetric membrane,and polysulfone-polyamine（PSf-PA）thin-film composite（TFC）membrane.However,the water flux of commercial FO membranes was low,which due to the serious internal concentration polarization（ICP）phenomenon.Simultaneously,the ICP phenomenon have been related to the physicochemical properties of the substrate such as porosity,thickness,tortuosity,and hydrophilicity.This paper proposed that a novel strategy to inhibit the ICP phenomenon by modified PSf support layers with layered double hydroxides（LDHs）nanoparticles.Firstly,the PSf/LDHs substrates were prepared by the multiple dimensional LDHs blended PSf polymer,which owning to the hydrophilicity and tunability of LDHs.Such as,the zero-dimensionality（spherical-like）of LDHs;two-dimensionality（plate-like）of LDHs and three-dimensionality（flower-like）of LDHs.Then the polyamide active layer was prepared based on the PSf/LDHs substrates by interfacial polymerization.The water flux of FO membranes with the different LDHs dimensionality were systematically investigated.The results indicated that the improvement of water flux,which the flower-like LDH blended the PSf substrate.Consequently,the water flux of TFN/LDH-F3 membrane was improved 52.7%with its specific salt flux（J_s/J_w）maintaining 0.22 g L^-1 at PRO mode.Based on the above researches,a novel strategy was proposed,which based on the dissolution of 3D LDH by HCl.And the 3D LDH as sacrificial template,that promoting the porosity,macropore structure,and connection of PSf substrate.This will help reduce the ICP phenomenon.Meanwhile,the PA layers become thinner and smoother based on as-prepared substrates by LDH-template.Hence,the water flux of TFN_7.5-HCl membrane reaches up to 20.1 L m^-2 h^-1 at FO mode and 47.2 L m^-2 h^-1 at PRO mode.And the salt rejection of TFN_7.5-HCl membrane remaining88%.Furthermore,the performance of water permeation and water-salt selectivity of desalination membrane was also analyzed based on solution-diffusion model.The TFN_7.5-HCl membrane also achieves permeability–selectivity behavior break to the proposed upper bound of desalination membranes.In conclusion,this paper fully demonstrated that the advantages of LDHs to modified FO membrane,and it also provided a new insight into FO membrane modification by inorganic nanoparticles.