Rejection And Antifouling Properties Of Advanced Inorganic-Polymeric Composite Membranes

Nanofiltration?NF?membranes have been validated able to effectively remove most kinds of pollutants in the micro-polluted water sources,and therefore ensured with favorable application prospects in advanced treatment of drinking water.Many widely-used commercial thin-film composite NF membranes,however,are enslaved by their innate defects including the vulnerability to membrane fouling and the trade-off between membrane permeability and selectivity.The doping of inorganic materials was proved to improve the water permeabilily and/or antifouling property of NF membranes in previous works,whereas there is a lack of systematic investigations on mechanisms and properties of inorganic-organic composite membranes.In this study,three representative kinds of inorganic-polymeric composite membranes were chosen/fabricated,and their potential merits and drawbacks were investigated through a series of characterization,solute rejection and membrane fouling experiments.Four kinds of commercial ceramic NF membranes?M-Si O₂,M-Ti O₂,LC1 and LC2?were firstly chosen to be representative inorganic-polymeric composite membranes.Their fundamental properties,the rejection performances for pharmaceuticals,the anti-fouling properties as well as cleaning effects were comprehensively evaluated for the first time.Results showed that although these ceramic membranes had higher molecular weight cut-off,low rejection ratios and low water permeability compared to commercial polyamide NF membranes,their high hydrophilicity,low surface roughness,especially low surface carboxylic group density,endowed them with high anti-fouling and easy-to-clean advantages.Thereafter,the atomic layer deposition?ALD?technology was applied in the surface modification of polyamide membranes?SW30 and NF270?according to the results of ceramic NF membranes,to investigate the influences of inorganic material?TiO₂ and Al₂O₃?doping on membrane structure and performance.Above all,the deposition conditions of ALD were optimized,since all of these conditions,including deposition temperature,operation mode?exposure mode and continuous mode?,deposition time?pulse time and exposure time?,had great effects on the ultimate deposition results.After a series of testing experiments on the deposited membranes,the ALD technology was verified with high controllability that could precisely decrease the effective membrane pore size and therefore increase rejection.Although the modified membranes were still not able to break though the trade-off,the deposition process reduced the negative charge density of membrane surface by consuming the carboxylic groups that could react with the precursors and thus increased the membrane anti-fouling property.Then,the membrane active layer modifications were performed with metal-organic frameworks?MOFs?.Three types of hydro-stable MOFs,namely MIL-53?Al?,NH₂-Ui O-66 and ZIF-8,were used to fabricate thin-film nanocomposite?TFN?membranes via both blending?BL?and preloading?PL?interfacial polymerization?IP?methods.Membrane characterization revealed that MOFs decreased the cross-linking degree while increasing the membrane thickness,surface negative charge density and roughness of polyamide active layer.The potential of doping of some typical kinds of MOFs in overcoming the trade-off is highly related to the window size,hydrophilicity of the MOFs as well as the interaction between the MOF nanoparticles and the organic polymers.The systematic investigations on the rejection and antifouling mechanisms of inorganic-polymeric composite membranes in this work provide foundations and references to the membrane modification targeted to typical pollutants or application occasions in the future.