Preparation Of Thin Film Nanofibrous Composite Membranes By Vertical Melting Treatment And Their Applications In Organic Wastewater Treatment

Nowadays the water shortage and pollution are becoming severe around the world, and the organic contaminants have drawn much attention for their great and wide damage to the environment and living things. The membrane separation technology has been widely used in wastewater treatment due to its high separation efficiency, low energy consuming, environmental friendliness, easy operation, relatively low investment and maintenance costs. The composite membrane with a hierarchical structure exhibited promising applications in organic separation considering their good permeability, high rejection and pressure resistance.Generally, the composite membrane consists of an ultra-thin top barrier layer offering filtration function and a porous substrate with low resistance. Hence, different membrane materials and membrane preparation techniques can be chosen to fabricate the top barrier layer and the porous substrate separately, thus the whole membrane performance can be optimized. Electrospun nanofibrous membranes are prospective candidates as porous substrate for the composite membranes due to their superior properties of large porosity, interconnected pore structures, light weight and so on, which can enable the resultant composite membrane with low energy cost and high permeability in water filtration applications. Usually, hydrophilic film-forming materials are chosen to prepare top barrier layer of the composite membrane to increase membrane permeability and reduce membrane fouling. However, the high permeability of nanofibrous substrate is often overshadowed by easy penetration of the coating solution into the highly porous nanofibrous substrate in typical fabrication process of surface-coated composite membranes.To solve this problem, novel routes were proposed to fabricate high flux thin film nanofibrous composite(TFNC) filtration membrane by combining electrospraying technique and evolving vertical melting techniques. For preparation of TFNC membranes, the hydrophilic materials with good film-forming ability, such as chitosan(CS), polyvinyl alcohol(PVA), sodium alginate(SA) and so on, were firstly electrosprayed onto electropun polyacrylonitrile(PAN) nanofibrous substrate, and then the hydrophilic top layer was melted or softened and then fused insidiously to form the integrated ultra-thin barrier layer on the supporting layer by various post-treatments. Various parameters of the film-forming and post-treatment conditions of the nanobeads were investigated systematically to optimize the structure, morphology and property of the TFNC membranes during the membrane preparation in order to achieve an integrated and compact film. Series of composite membrane formats based on the nanofibrous substrate have been developed for oil/water emulsion system, anionic dye aqueous solutions or proteins separation with high performance at low pressure. The rejection rates, permeability and anti-fouling property of the resulting TFNC membranes were also analyzed in detail to characterize their filtration performance to organic molecules with different sizes.1. Electrospraying technique combined with solvent vapor exposure treatment was used to fabricate high flux TFNC ultrafiltration membrane containing a hydrophilic poly(ethylene oxide)(PEO, facilitating agent for electrospinning) doped CS(CS-PEO) barrier layer and electrospun PAN nanofibrous substrate. CS-PEO beads layer was firstly electrosprayed onto PAN supporting layer, and then swollen to merge imperceptibly into an integrated barrier film by acetic acid/water vapor treatment and finally chemically crosslinked in glutaraldehyde(GA) water/acetone solution to form an integrated and compact barrier film on the PAN supporting layer. It was found that the cold-pressed PAN nanofibrous membrane was more suitable as a substrate of the TFNC membranes due to its little change in morphology, smoothness and compactness. For PAN nanofibrous substrate, its nanofibrous diameter is around 460 nm and its thickness was about 40 μm with porosity of ~ 63.3% and water flux of ~ 4600 L/m2 h at 0.2 MPa. For the preparation of the CS-PEO/PAN TFNC membrane, depositing time of 50 min was chosen for electrospraying 1.0 wt% CS-PEO(mass ratio of 95:5) on PAN substrate, and acetic acid/water vapor treatment from 1.0 wt% acetic acid of 60 min was utilized to smoothen the CS-PEO layer. The thickness of the resulting CS-PEO/PAN TFNC membrane was ~ 250 nm. It was concluded that the CS-PEO/PAN TFNC membrane could perform well in the ultrafiltration of oil/water emulsions with permeate flux of 117.2 L/m2 h and rejection ratio of 99.0% at relatively low operation pressure(0.2 MPa), and exhibited good pressure resistance, since it showed higher permeate flux of 223.5 L/m2 h when the rejection rate was 99.5% at high operation pressure(0.6 MPa).2. A novel and facile strategy combining electrospray technique with hot press treatment inspired by conventional powder coating technology was proposed to fabricate composite membrane in order to realize the precise control of the film-formation process. PVA nanobeads were electrosprayed onto the electrospun PAN substrate and then followed by hot-pressing treatment assisted by moistening absorption. Water molecules from absorbed moisture acted as ―plasticizer‖ and could facilitate PVA melting. The moistened electrosprayed PVA nanobeads could be easily melted or softened to be pressed imperceptibly into an integrated barrier film on the supporting layer. Here, water molecules act as the plasticizer and can improve the melting processability of PVA. The depositing time of 18 min, moistcuring time of 90 s, heating temperature of 60 ℃ and curing time of 4 min were optimized to achieve an integrated, nonporous PVA top layer with thickness of around 300 nm. Finally, the resulting top layer was chemically crosslinked and different GA concentrations were used to finely control its filtration performance by adjusting the crosslinking degree of PVA, and the resulting PVA/PAN composite membrane was applied for bovine serum albumin(BSA) and vitamin b12 filtration. The PVA/PAN TFNC membrane crosslinked by GA concentration of 0.8 wt‰ was used for BSA filtration with flux of 173.0 L/m2 h and rejection above 98.0% at low feeding pressure 0.3 MPa, while the membrane crosslinked by GA concentration of 5.0 wt‰ was chosen for vitamin b12 separation with rejection rate of 91.1% and permeate flux of 21.3 L/m2 h at feeding pressure of 0.6 MPa. Besides, the resultant membrane possessed good structural stability and good anti-pressurizing property.3. The steric effect and Donnan effect are the main two theories to explain the separation mechanism of composite membrane. According to these theories, SA was doped into PVA to prepare negative-charged PVA-SA/PAN TFNC membrane by the similar hot-pressing treatment as that for the PVA/PAN TFNC membrane, in order to retain smaller size molecules(like anionic dyes) than vitamin b12. SA molecules can bond with PVA through intermolecular hydrogen-bond interaction and has great hydroscopicity, the component of SA would affect the film-formation process of PVA-SA layer compared to PVA/PAN TFNC membrane. Therefore the re-optimized depositing time of PVA electrospraying(18 min), moistcuring time(120 s), heating temperature(60℃) and curing time(6 min) were chosen to achieve an integrated, nonporous PVA-SA barrier layer with thickness of ~ 480 nm. Different GA crosslinking systems(GA concentration and volume ratio of water/acetone) were utilized to adjust the filtration performance for anionic dyes separation. And when GA concentration of 4.0 wt‰ and volume ratio of water/acetone of 70:30 were selected, the optimized PVA-SA/PAN TFNC membrane possessed high nanofiltration performance to fast green solution with permeate flux of 57.1 L/m2 h and rejection above 96.8% at low feeding pressure 0.6 MPa. The PVA-SA/PAN TFNC membrane was negatively charged because of the sulfonic groups from SA, so the good nanofiltration performance resulted from the combined action of steric effect and Donnan effect. The resulting membrane also exhibited excellent pressure resistance and anti-fouling property, and it was found SA played an important role in the membrane filtration.4. To further actualize the precise control of the film-formation process and facilitate its industrialization, electrospraying technique, hot pressing treatment and semi-interpenetrating polymer network were combined to fabricate UV-cured CS-PEO-polytriethylene glycol dimethacrylate(PTEGDMA)/PAN TFNC membrane. First, double-layer mat containing an ultrathin electrosprayed hydrophilic CS-PEO-triethylene glycol dimethacrylate(TEGDMA, monomer) nanobeaded top layer and PAN nanofibrous substrate was fabricated. Then the hydrophilic top layer was acidic moistcured followed by hot pressing treatment to form an integrated barrier film on the supporting layer. Here, acidic moisture was utilized to soften the nanobeads and facilitate the CS melting processing. Finally, the top layer was cured under UV-radiation to form CS-PEO-PTEGDMA semi-interpenetrating polymer networks. UV-radiation was used to physically crosslink CS instead of the chemical crosslink in solutions, which would be easier for scale-up development. The deposition time of 30 min, acidic moistcuring time of 60 s, heating temperature of 50 oC and curing time of 4 min were optimized for the CS-PEO-TEGDMA TFNC membrane preparation to achieve an integrated barrier layer on PAN nanofibrous substrate. The optimized TFNC membrane possessed high nanofiltration performance for anionic dyes separation with superior permeate flux of ~117.5 L/m2 h and high rejection of ~99.9% to Direct Red 80 solutions under low operation pressure of 0.2 MPa. An adsorption-assisted nanofiltration process was proposed for the CS-PEO-PTEGDMA membranes to separate anionic dyes: The as-prepared nanocomposite membranes with positive surface charge(owing to the protonated amine groups in chitosan) could adsorb negative charged anionic dyes through electrostatic attractions on the membrane surface and inside the pores, resulting in the decrease of the membrane pore sizes to achieve a compacter CS barrier layer. The negative charged composite membrane repels the negative charged anionic dyes in the solution and leads to higher dye rejection. Solution p H has a significant effect on the uptake of dyes on the composite membrane surface, thereafter the filtration performance. And the optimum p H values for the anionic dyes direct red 80 separation were found to be in the p H range of 5-10. Moreover, the resultant CS-PEO-PTEGDMA nanofiltration membranes exhibited excellent antifouling properties due to their good hydrophility and surface charge, and possessed good reusability over repeated operations with a simple regeneration process. This work may pave the way for other intriguing polymer materials and provide a practical feasibility for water purification.To sum up, series of TFNC ultrafiltation/nanofiltation membranes for various organic molecules separation have been developed by combining electrospraying technique with evolving post-treatment techniques, which could finely control the micro structure and morphology of the top barrier layer. It was concluded that the shortcoming of easy penetration of the coating solution into the porous substrate in typical fabrication of surface coated composite membranes could be overcome, and the thickness of the top barrier layer can be easily controlled by the deposition time of the top polymer electrospray. The low-pressure high-flux performance of resultant membranes was attributte to the excellent permeability of the electrospun nanofibrous substrate and the ultra-thin barrier layer. The strategy for fabricating TFNC membranes described here can be extended easily to fabricate TFNC membranes from many other polymeric membrane materials simply by choosing the suitable vertical melting treatment. It is very desirable for the scale-up of nanofibrous composite membranes in industrial application by utilizing the method explored here since it is easy to operate and control the film-formation process under mild conditions.