Construction And Performance Control Of Thin Film Nanocomposite Nanofiltration Membrane With Interlayer

The construction and the performance regulating mechanisms of thin film nanocomposite（TFN）nanofiltration membrane are important for preparing the good performance TFN nanofiltration membrane and evaluating the use environment of TFN nanofiltration membrane.In order to understand the mechanism of the construction and performance of TFN nanofiltration membranes when nanoparticles as an interlayer,cerium dioxide（CeO₂）nanoparticles with distinct morphologies（sphere,rod and flake）and loading（9mg/m²,28 mg/m²,47 mg/m²,65 mg/m²and 84 mg/m²）were used as an interlayer to construct TFN nanofiltration membranes by preloading interfacial polymerization and aqueous doping interfacial polymerization.Investigated the effects of different CeO₂nanoparticles as an interlayer on the physicochemical properties and membrane performance of TFN nanofiltration membranes under the two membrane fabrication methods,and to investigate the related influence mechanisms.The main research contents and results are as follows:（1）Three morphological CeO₂ nanoparticles were obtained in different ways,investigated the optimal pretreatment conditions for nanoparticles,and the morphological size,crystalline structure,surface chemical elements,N₂adsorption-desorption curves,and BET specific surface area of CeO₂nanoparticles were characterized to determine the physicochemical properties of CeO₂nanoparticles.The results showed that the three CeO₂nanoparticles were best dispersed when the nanoparticle suspension was ultrasonicated at p H=5under acidic conditions for 30 min,and R-CeO₂being the most dispersed in the solution.The physicochemical characterization of the nanoparticles indicated that the physicochemical properties of the three CeO₂nanoparticles were similar except for the obvious differences in shape and size.（2）Different CeO₂ nanoparticles were introduced into TFN nanofiltration membranes as an interlayer by two membrane fabrication methods,and the surface chemical composition,surface wettability,surface charge,morphological structure and surface roughness of these membranes were characterized to reveal the effects of different CeO₂nanoparticles as an interlayer on the physicochemical properties of TFN nanofiltration membranes and to elucidate the formation mechanism of TFN nanofiltration membranes.It was found that:1)preloading CeO₂nanoparticles as an interlayer increased the crosslinking of TFN nanofiltration membranes,with the best crosslinking of66.7%for TFN/R-CeO₂-65 membrane;aqueous phase doping of CeO₂nanoparticles reduced the crosslinking of TFN nanofiltration membranes,with the worst crosslinking of 7.0%for TFN/F₂-CeO₂-65 membrane.2)Both methods introduced hydrophilic CeO₂nanoparticles as an interlayer enhanced the wettability of the membrane surface;the TFC membrane,TFC 2 membrane and TFN/X-CeO₂-65 membrane were covered by hydrophilic polyamide（PA）layer after interfacial polymerization,and the membrane surface wettability was enhanced;while the TFN/X₂-CeO₂-65 membrane surface is less wettable due to microstructural changes.3)The constructed TFN nanofiltration membranes were all negatively charged,and the morphology of CeO₂nanoparticles had no significant effect on the surface charge of the membranes;the lack of cross-linking led to more negative charge density of TFC 2 and TFN/X₂-CeO₂-65membranes than TFC and TFN/X-CeO₂-65 membranes.4)R-CeO₂formed an orderly network structure on the surface of the substrate membrane,and the continuity and denseness of the interlayer gradually increased with the increase of R-CeO₂loading,and a uniform and continuous interlayer was formed on the surface of the substrate film when the R-CeO₂loading amount was≥47 mg/m².5)The retention and distribution of the PIP aqueous solution on the membrane surface as well as the diffusion of the PIP monomer affect the morphological structure of the TFN nanofiltration membrane;the lower PIP aqueous solution content results in smooth,uniform and small nodules on the surface of TFC membranes,and uneven nodules on the surface of TFC 2,TFN/F-CeO₂-65 and TFN/F₂-CeO₂-65 membranes;further increase in the retention of PIP aqueous solution leads to lender ridge-valley structures on the surface of TFN/S-CeO₂-65and TFN/S₂-CeO₂-65 membranes;with the increase of R-CeO₂loading,the continuous homogeneity of the interlayer increased,the retention of PIP aqueous solution increased and its distribution became more uniform,leading to the transformation of nodular and ridge-valley structures on the surface of TFN/R-CeO₂-m membranes,while the transformation of different ridge-valley structures on the surface of TFN/R₂-CeO₂-m membranes.In addition,the apparent thickness of TFN nanofiltration membranes increased after the introduction of different CeO₂nanoparticles as an interlayer.6)The introduction of different CeO₂nanoparticles as an interlayer enhanced the surface roughness of TFN nanofiltration membranes,and the surface roughness of the membranes increased linearly with the increase of R-CeO₂loading.In addition,the surface roughness of TFN nanofiltration membranes prepared by aqueous phase-doped interfacial polymerization was greater.（3）The effects of different CeO₂ nanoparticles as an interlayer on the water permeability of TFN nanofiltration membranes were investigated using a dead-end filtration system,and the related regulatory mechanisms were elucidated.The water permeability of the TFN/X-CeO₂-m membrane was improved by the increase of effective filtration area and the"gutter effect",in which the pure water permeability of the TFN/R-CeO₂-65 membrane was the largest at 8.75±0.30 L m^-2h^-1bar^-1,which was 124%higher than that of the TFC membrane.The lengthening of the water molecule transport path resulted in poorer water permeability of the TFN/X₂-CeO₂-65 membrane,with the TFN/F₂-CeO₂-65membrane having the worst pure water permeability,which was 53%lower than that of the TFC 2 membrane.（4）A dead-end filtration system was used to investigate the effect of different CeO₂nanoparticles as an interlayer on solute rejection in TFN nanofiltration membranes under two membrane fabrication methods and to elucidate the related regulatory mechanisms.The constructed membranes are all negatively charged membranes.The increased cross-linking degree of the membranes hindered solute permeation and led to increased rejection of Mg SO₄and sucrose in TFN/X-CeO₂-65 membranes;the higher charge on the membrane surface led to the stronger rejection of Mg Cl₂and Na Cl in TFC 2 and TFN/X₂-CeO₂-65 membranes,and the higher rejection of four inorganic salts in TFN/R₂-CeO₂-65 membranes;the extension of the pathway enhanced the ability of TFN/R₂-CeO₂-65 and TFN/S₂-CeO₂-65 membranes to reject Na Cl and the four neutrals;the non-selective defect made TFN/F₂-CeO₂-65 membranes possess the worst rejection capacity of solutes.（5）The separation capacity of TFC membrane,TFC 2 membrane,TFN/R-CeO₂-65 membrane and TFN/R₂-CeO₂-65 membrane in the sugar-salt mixed system was investigated by constant pressure staggered flow filtration.The TFN/R₂-CeO₂-65 membrane had the best separation capacity in the mixed system with a separation factor of 7.9.