Pp Hollow Fiber Membrane Materials Functionalized Modified Its Separation Performance

Polypropylene (PP) microporous hollow fiber membrane has been widely used in separation processes such as membrane distillation, gas separation and wastewater treatment due to its higher membrane area per unit volume, ease of module fabrication, no requirement of feed and permeate spacers as well as less demand for pretreatment and maintenance. However, membrane fouling resulted from its hydrophobicity limits the potential application of PP membrane in the treatment of aqueous solutions.In this study, thin-film composite hollow fiber membrane with the support substrate of polypropylene (PP) hollow fiber membrane and the selective skin layer of sodium carboxymethyl cellulose (CMCNa) was developed through dip-coating technique followed by cross-linking. Preparation parametric studies were conducted and the obtained membranes were characterized in terms of surface and permeation properties, through which the membrane performance was optimized. Then, the separation characteristics of the desired modified PP hollow fiber membrane under different operation conditions were investigated. Finally, the tailor-fabricated membranes were selected to treat the dye aqueous solution in a submerged filtration mode under different conditions and to explore the prospects of their applications in the treatment of dyeing wastewater.The results of FT-IR and SEM showed that a thin skin layer with a thickness less than0.2μm had been deposited on the surface of the polypropylene (PP) hollow fiber ultrafiltration membrane through the reaction of sodium carboxymethyl cellulose (CMCNa) and FeCl3. After modification, the water contact angle of the membrane surface was reduced significantly from104.7°to81.1°indicating that the hydrophilicity of the polypropylene (PP) microporous hollow fiber membrane had been improved by coating CMCNa layer. The surface of the modified PP hollow fiber composite membrane was negatively charged at pHs higher than3.8.The study of the separation properties of the CMCNa/PP hollow fiber composite membrane revealed that the average pore size (molecular weight cut-off (MWCO)) and the salt rejection rate of the resultant membrane could be adjusted by varying the conditions of coating and cross-linking processes. The performance of the resultant modified membrane was largely affected by the CMCNa content of the coating solution and the repeat times of the dip-coating process. The salt rejection rate of the resultant membrane could be increased by increasing the concentration of CMCNa or FeCl3, prolonging the time of dip-coating or cross-linking, or repeating the dip-coating process with the sacrifice of pure water permeability. Furthermore, the salt rejection rate of the resultant membrane could also be enhanced through the addition of sodium polyacrylate (PAAS) to the coating solution. The optimum preparation conditions for the composite membranes were as follows:concentration of CMCNa=2.0%w/v, the coating time of CMCNa solution=30min, the content of FeCl3=8.0%w/v, and contact time with FeCl3=20min. The membrane prepared under the optimum conditions possessed an average pure water permeability of about9.91/m2h bar and exhibited the rejections of87.7%and29.6%to Na2SO4and NaCl, respectively, when tested with a feed aqueous solution containing500mg/1salt under0.3MPa,25℃and pH6.8.The separation characteristics of modified PP hollow fiber membranes under different operation conditions were also investigated. As the operating pressure increased from0.1to0.5MPa, the salt rejection rate increased rapidly-to-slowly, while the water permeation flux increased proportionally. Within the studied concentration range, the increase of feed concentration resulted in a severe decline in salt rejection rate and a slightly decrease in water permeate flux. The rejection of the composite membrane to different inorganic salts followed the order of CaCl2(0.9%)<MgCl2(1.7%)<MgSO4(17.2%)<NaCl (29.6%)<KCl (40.7%)<Na2SO4(87.7%) at neutral pH, the salts with multivalence anions could be well retained by the membrane while the salts with multivalence cations were poorly rejected. The pH of the salt aqueous solution also affected the separation properties of the obtained membrane due to the protonation and ionization of carboxy in sodium carboxymethyl cellulose (CMCNa). After running with aqueous solution containing500mg/1Na2SO4at0.3MPa,25℃and pH6.8for28hours or exposure to1.0g/1NaClO aqueous solution for10hours, the CMCNa/PP hollow fiber composite membrane showed little change in salt rejection, which meaned that the resulting membrane possessed good performance and chemical stability.Submerged filtration of dye aqueous solutions containing different dyes including Neutral red, Methylene red, Rhodamine B, Alizarine yellow R, Methyl red, Sunset yellow, Methyl blue and Congo red were also conducted by employing the optimized CMCNa/PP hollow fiber composite membranes. It was found that the tailor fabricated CMCNa/PP hollow fiber composite membrane could efficiently remove dye molecules from saline aqueous solution. The rejection rates to Congo red, Methyl blue, Sunset yellow, Methyl red, Alizarine yellow R, Rhodamine B, Methylene red and Neutral red were99.9%,99.7%,79.2%,78.1%,76.4%,60.9%,57.1%and55.5%, respectively, while the retention rate to salt NaCl was lower than10%. The influence of salt concentration, feed dye concentration or concentrating rate on dye retention was negligible, while the increase of salt NaCl concentration in aqueous solution resulted in an increase in water flux, and the increase of feed dye concentration or concentrating rate led to a decline in permeate flux. Additionally, long-term submerged filtration also revealed that the developed composite membrane possessed good performance stability and had good prospects in the application to dyeing industry, especially in the treatment of dyeing wastewater.