PP中空纤维膜材料的功能化改性及其分离性能研究

【作者】 吕振华；

【导师】 俞三传；

【作者基本信息】 浙江理工大学 ， 应用化学， 2012， 硕士

【摘要】 聚丙烯（PP）中空纤维膜具有比表面积大、组件无需支撑体、设备小型化、结构简单等优点，被广泛应用于膜蒸馏、气体分离、废水处理等领域。但由于聚丙烯为疏水性材料，在处理水基流体时能耗高且污染严重；另一方面，PP中空纤维膜的孔径通常较大，其选择分离性能相对较差。因而对PP中空纤维膜进行功能化改性，制备分离性能优越的亲水性PP中空纤维复合膜具有重要意义。本论文以羧甲基纤维素钠（CMCNa）为功能材料，结合静态溶液浸泡法及物理表面交联法，对PP中空纤维超滤膜进行功能化改性，获得具有特定分离性能的CMCNa/PP中空纤维复合膜。论文通过PP中空纤维膜的表面性质表征及分离性能测试，对功能化改性工艺进行优化，并研究运行条件对CMCNa/PP中空纤维复合膜分离性能的影响，最后采用浸没式过滤操作方式将其应用于染料分离试验，研究复合膜对染料的分离效果。FT-IR及SEM表征结果表明通过羧甲基纤维素钠与氯化铁的交联反应，成功在PP中空纤维超滤膜表面形成亲水但非水溶的致密涂层，其厚度约为0.2μm。改性后，PP中空纤维膜表面的亲水性得到改善，其水接触角从104.7°下降到81.1°左右；由于改性后膜表面残留羧基，因而在大部分的pH值范围内（pH>3.8），复合膜表面带有负电荷，为高价阴离子的截留创造条件。实验考察了功能化改性工艺对CMCNa/PP中空纤维复合膜分离性能的影响，发现通过改变涂覆条件及交联条件可以有效控制复合膜表层的平均孔径，从而调节复合膜的无机盐截留率。适当增加CMCNa浓度、交联剂浓度以及涂覆次数或延长涂覆时间、交联时间均可以提高复合膜表层的致密度，从而在牺牲一定渗透通量的前提下，提高复合膜对无机盐的截留率。其中，以涂覆液中CMCNa浓度的影响最大，涂覆次数次之，而交联液中FeCl3浓度、涂覆时间及交联时间的影响最小，基本可以忽略。此外，在涂覆液中添加一定的聚丙烯酸钠（PAAS）可以提高复合膜对无机盐的截留率，但效果相对较小。综合考虑复合膜的无机盐截留率及渗透通量，认为较优的功能化改性条件为：在CMCNa浓度为2.0%w/v的涂覆液中浸泡30min，取出晾干后，再在8.0%w/v的FeCl3溶液中交联反应20min。该条件下获得的CMCNa/PP中空纤维复合膜的截留分子量约为720Da，在0.3MPa、25℃及pH6.8下，对500mg/lNa2SO4溶液的截留率为87.7%，对500mg/lNaCl溶液的截留率为29.6%，纯水渗透通量为9.9l/m2hbar。除功能化改性工艺对CMCNa/PP中空纤维复合膜的分离性能产生影响外，运行条件同样影响复合膜的分离性能。当操作压力在0.1~0.5MPa的范围内变化时，渗透通量随压力的增加呈线性增加，无机盐截留率也随操作压力的增加而升高，但截留率的上升速度相对较慢且逐渐趋于稳定；复合膜对无机盐的截留率和渗透通量则随着盐浓度的增加逐渐下降，截留率的下降幅度相对较大；复合膜对不同无机盐的截留顺序为CaCl2(0.9%)<MgCl2(1.7%)<MgSO4(17.2%)<NaCl(29.6%)<KCl(40.7%)<Na2SO4(87.7%)，说明复合膜能有效截留高价阴离子，但对高价阳离子的截留性能则相对较差；在料液pH高于6.6时，膜表面带有较强的负电性，对无机盐的截留率维持在较高的水平，但当料液pH低于6.0时，膜表面负电荷密度降低、CMCNa与FeCl3的络合作用减弱，致使复合膜涂层孔径变大，因而导致无机盐截留率下降、渗透通量升高；中性条件下，CMCNa/PP中空纤维复合膜具有稳定的分离性能，0.3MPa、25℃及pH6.8下，在500mg/lNa2SO4水溶液中运行28h后，Na2SO4的截留率仅下降2.4%。即使在浓度为1.0g/l、pH为8.0的次氯酸钠溶液中浸泡10h后，CMCNa/PP中空纤维复合膜仍能保持初始的截留率。最后将较优工艺下改性得到的CMCNa/PP中空纤维复合膜以浸没式过滤的操作方式应用于染料分离试验研究，发现采用浸没式过滤能有效实现染料溶液中的染料分子与无机盐的分离。复合膜对染料的截留主要受尺寸效应及道南排斥效应的共同作用；复合膜对甲基蓝、刚果红染料的截留率分别高达99.7%、99.9%，而对染料中的NaCl截留率则低于10%。CMCNa/PP中空纤维复合膜对染料的分离性能受染料浓度、染料浓缩倍数及无机盐浓度的影响。实验范围内，染料浓度及浓缩倍数的增加会小幅度降低复合膜的渗透通量，而染料中无机盐浓度的增加，则会提高复合膜的渗透通量。染料浓度、染料浓缩倍数及无机盐浓度对CMCNa/PP中空纤维复合膜的截留率的影响相对较小。中性条件下，改性获得的CMCNa/PP中空纤维复合膜对阴离子染料具有较好的抗污染性能，在含50mg/l刚果红及500mg/lNaCl的染料溶液中运行10.5h后，复合膜的渗透通量仅下降10%，而复合膜的染料截留率则维持在初始值99.8%以上。染料分离试验结果表明，改性获得的CMCNa/PP中空纤维复合膜在染料工业尤其是在染料废水处理中具有潜在的应用价值。更多还原

【Abstract】 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.更多还原