| Thin-film composite nanofiltration membrane plays an important role in the field of nanofiltration membrane technology. Compared with the asymmetric nanofiltration membrane, the thin-film composite membranes exhibit higher permeability and selectivity, and therefore they have become the dominant membranes in the area of nanofiltration membrane. However, with the application of nanofiltration membrane in the treatment of industrial fluids expends, membrane fouling and insufficient selectivity are usually occurred with the commonly used commercial nanofiltration membranes, and requirements such as higher permeability and performance stability are necessary for the nanofiltration membranes in the treatment of industrial fluids, especially the fluids of complicated compositions. Therefore, it is of significant interest to study and develop new thin-film composite membranes through using new membrane material and/or adopting modified manufacture technique. Therefore, one of the key means to solve the above problem is to develop resistance fouling composite nanofiltration membrane through the selection of membrane materials.In this study, thin-film composite nanofiltration membrane with improved permeaselectivity and anti-fouling property was developed through the interfacial polymerization of silk sericin (SS), a natural functional material of good hydrophilicity, and the trimesoyl chloride (TMC) on the surface of porous polysulfone substrate. The membrane properties in terms of surface chemical structure, morphology, charged and hydrophilicity of obtained SS composite membrane was characterized through ATR-FTIR, FE-SEM, AFM, streaming potential measurement and surface contact angle measurement. The preparation parameters such as reactant concentration, reaction time, curing condition temperature, curing time, additive and pH of the aqueous phase were systematically studied and resulted in the optimal membrane preparation conditions. The performance attributions including pure water permeability, molecular weight cut-off (MWCO), rejection rates to different electrolytes and anionic dyes were evaluated through cross-flow permeation tests. Finally, the anti-fouling property of the developed SS nanofiltration membrane was also studied through permeation tests with bovine serum albumin (BSA), sodium alginate (SA), sodium dodecyl sulfate (SDS) and humic acid (HA) as the model foulants, respectively. The conclusions obtained from the experimental results were as follows:(1) The results of ATR-FTIR analysis and SEM showed that a thin skin layer with a thickness about0.2μm had been successfully formed on the surface of the PSf ultrafiltration membrane through the reaction of silk sericin (SS) and TMC. The resulting membrane appeared to comprise a smooth thin film with no block or peak structure and a root mean square roughness (Rms) of about12.8nm. The deposition of the skin layer resulted in a relatively lower surface contact angle of the resulting composite membrane (about64.4°) compared with that of the polysulfone support substrate (around69.0°). The obtained membrane comprises an amphoteric surface with an isoelectric point of pH3.84, and the membrane surface was negatively charged at pH>3.84.(2) Parametric study showed that the separation performance of the resulting SS membrane was largely affected by the reaction concentration, reaction time, curing temperature, curing time as well as the additive and pH of the aqueous phase. With the increase of the concentration of SS in aqueous solution, prolongation of the reaction time, escalation of the curing temperature or prolongation of the curing time, the Na2SO4rejection rate of the resultant membrane was increased, while the permeate flux was decreased due to the formation of denser thin film. However, higher curing temperatures or longer curing time resulted in membranes of poor separation performance due to the shrinkage of the substrate. Additionally, the performance of the membrane could also be modulated by adding certain amount of surfactant sodium dodecyl sulfate (SDS) to the aqueous phase or adjusting the pH of the aqueous phase. The optimum preparation parameters for the composite membranes were as follows:silk sericin=0.5w/v%, sodium dodecyl sulfate=0.001w/v%, pH=10.00, trimesoyl chloride=0.037w/v%, reaction time=50s, curing temperature=60℃, curing time=15min.(3) The desired membrane had a molecular weight cut off of around1500Da and exhibited a salt rejection rate of89.8%and a flux of about69.51/m2h for a feed aqueous solution containing500mg/1Na2SO4at0.5MPa. The rejection rate of the resultant membrane to different inorganic salts followed the order of MgCl2(6.1%) CaCl2(6.4%)<MgSO4(21.9%)<NaCl (24.7%)<Na2SO4(88.2%) at neutral pH. An increase of feed salt concentration resulted in a decrease in both salt rejection and flux. As the operating pressure increased from0.3to0.7MPa, the salt rejection rate decreased slowly-to-rapidly, while the water permeation flux increased proportionally. The desired SS membrane could remove the anionic dyes from the aqueous solution effectively with good anti-fouling property. The dye removal rate was9.7%,96.4%and93.9%, to Congo red, Methyl blue and Sunset yellow, respectively, which was decreased with the increase of feed dye concentration and/or inorganic salt concentration.(4) The obtained SS membrane exhibited different anti-fouling properties to different foulants. After filtration with aqueous solution containing100mg/1model foulant and500mg/1Na2SO4, the permeate flux reduction rate of the SS membrane was about35.4%,15.1%and5.3%to BSA, HA and SA aqueous solution, respectively, while the permeate flux of the SS membrane to SDS aqueous solution was increased by5.9%. The relatively good anti-fouling property of the SS membrane was mainly attributed to its smooth, hydrophilic and negatively charged membrane surface. The anti-fouling property of the SS membrane to BSA was better than that of the commercial nanofiltration membrane NF90, while the anti-fouling property to SA was superior to commercial membrane NF270. |
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