Preparation And Application Of High-performance Nanomaterial Modified Composite Membranes For Forward Osmosis

Forward osmosis shows a broad prospect for water treatment owing to advantages of low energy consumption,excellent separation effect and simple operation.However,forward osmosis membrane subjects the trade-off between selectivity and permeability due to the challenges of membrane fouling,internal concentration polarization and reverse solute diffusion during the separation process.Besides,forward osmosis process is unable to extract water directly.The dilute draw solution increases the cost and reduces the effective driving force,resulting in a reduction in water flux,which seriously hinders FO for further application.Therefore,design and preparation of high-performance forward osmosis membranes and regeneration of draw solution to produce water are key to the forward osmosis technology.In this thesis,forward osmosis membrane with high permeability and selectivity,was constructed by using nanomaterial modification to regulate the surface properties（wettability,roughness and pore size）of the supporting layer and polyamide active layer in thin film composite forward osmosis membrane.On this basis,an innovative forward osmosis-self-heating membrane distillation system was developed to regenerate and reuse the draw solution as well as obtain water.The main research results are as follows:（1）Based on the asymmetric structure of the composite membrane,polydopamine/polyamide（PDA/PA）forward osmosis membrane was fabricated via autopolymerization and interfacial polymerization.The effects of surface wettability and roughness of the supporting and active layer controlled by polydopamine nanoparticles on the performance and anti-fouling of forward osmosis membrane were revealed.As PDA nanoparticles were presented on the supporting layer surface（TFC-I）and active layer surface（TFC-S）of the composite membranes,the water contact angle decreased from 87.7±0.55°（TFC-C）to 50.24±1.61°and 46.2±0.86°,respectively.Moreover,the surface roughness of TFC-S membrane was rougher than that of TFC-I membrane.The water flux of TFC-I membrane（51.71±1.47 LMH）and TFC-S membrane（60.95±3.16 LMH）were increased by 46.45%and72.61%,respectively,compared to TFC-C membrane with 2.0 M NaCl solution as the draw solution and active layer face the draw solution（AL-DS）mode.The loss of draw solute per unit of water produced（Js/Jw）were 0.27±0.03 g L^-1 and 0.18±0.03 g L^-1 for TFC-I and TFC-S membrane,which were 6.9%and 37.9%lower than those of TFC-C membrane.As the hydrophilic PDA nanoparticles formed a hydration layer on the active layer surface through hydrogen bonding with water molecules,which effectively hindered the rapid adhesion of contaminant,the TFC-S membrane presented an excellent fouling resistance and membrane regeneration during the organic fouling cycle experiments.The results indicate that the modification of the polyamide active layer surface by PDA nanoparticles is more beneficial to improve its separation performance and anti-fouling ability.（2）In order to improve rejection rate of draw solute as nanoparticles modify the surface of the supporting layer,graphitic carbon nitride（g-C₃N₄）with two-dimensional lamellar structure was selected as the modified nanomaterial.g-C₃N₄/PA forward osmosis membrane was prepared via vacuum filtration-interfacial polymerization,and the effects of nanostructure and content on the selectivity and permeability of the membrane were investigated.The results revealed that the g-C₃N₄ increased the surface roughness and hydrophilicity of the membrane.The water flux of the unmodified-clustered g-C₃N₄ and protonated g-C₃N₄ corresponding to the forward membranes in AL-DS mode with a draw solution of 2.0M NaCl were 57.22±6.21 LMH and53.26±1.41 LMH,respectively.Js/Jw were 0.26±0.06 g L^-1 and 0.13±0.01 g L^-1,respectively.These results indicate that the protonated g-C₃N₄ with better dispersion and hydrophilicity as well as smaller molecular size overcame the problem of agglomeration of unmodified g-C₃N₄nanosheets,shorten the water transport channels.Furthermore,the protonated g-C₃N₄ also reduced the risk of defective PA layer formation induced by nanosheet agglomeration while shortening the water transport channel,thus enhancing permeability and rejection of the draw solute.（3）In order to alleviate the internal concentration polarization of micro-filtration membrane as using it as the supporting layer,electrospun nanofibers with low internal concentration polarization were selected as the supporting layer and supplemented with a carbon nanotube interlayer to compensate for the difficulty of forming a complete active layer in the microporous supporting layer.Polyacrylonitrile/carbon nanotube/polyamide（PAN/CNT/PA）membranes were constructed by spray-interfacial polymerization.The morphological structure and performance of the membranes were investigated.Besides,the mechanism of the CNT interlayer to achieve the co-enhancement of membrane selectivity and permeability by regulating the pore size of the fiber membrane surface was elucidated.The results indicate that the three-dimensional mesh-like porous structure of the PAN nanofiber supporting layer and the CNT interlayer is conducive to the free diffusion of the solute in the support layer,which effectively inhibited the formation of internal concentration polarization phenomenon.The average pore size of the PAN nanofiber substrate decreased from 0.60μm to 0.31μm as the content of carbon nanotubes in the interlayer increased,providing a suitable reaction platform for the interfacial polymerization process to promote the formation of defect-free,smooth and thin polyamide layer.Optimally,the PAN/CNT/PA2 membrane showed an 83.55%increase in water flux（84.17±2.28LMH）,a 75.58%reduction in reverse salt flux and a 78.45%reduction in Js/Jw compared to the unmodified membrane,breaking the trade-off between membrane selectivity and permeability.（4）To solve the problem of dilution of the draw solution and inability to obtain water directly during the forward osmosis process,an integrate system of forward osmosis-self-heating membrane distillation was proposed,and carbon nanotube/polydimethylsiloxane（CNT/PDMS）hollow fiber membrane for this system was prepared via wet spinning-calcination-impregnation.The impacts and mechanisms of the operating conditions on the regeneration of the draw solution,the extraction of water and the performance of forward osmosis during the distillation of the self-heating membrane were investigated.The results show that the CNT/PDMS hollow fiber membrane has excellent joule-heating performance and hydrophobicity.During the distillation process,the self-heating membrane effectively overcame the problem of temperature polarization and reduced heat loss by combining the characteristics of homogeneous self-heating with the narrow channel of the hollow fiber structure to confine the micro-liquid to a localized space and surround it with heating.At an input power of 2.5 W,the draw solution was concentrated from 35 g L^-1 to 54.9 g L^-1.The water output was 24.27±0.89 kg m^-2 h^-1 with a salt rejection rate of more than 99.9%and a gain output ratio of 0.86.Meanwhile,the superhydrophobicity endowed the self-heating membrane with good salt resistance and stability.Compared to a stand-alone forward osmosis system where the water flux is reduced by 21%due to dilution of the draw solution,the integrated forward osmosis-self-heating distillation system concentrated and then reused then draw solution timely to mitigate the reduction in concentration of the draw solution,thus maintaining a stable forward osmosis process over time.