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纳滤(NF)膜是一种分离尺度介于超滤膜和反渗透膜之间的新型压力驱动分离膜[1-2]。由于其操作压力低、条件温和、分离效率高,已在海水脱盐、药物分离、食品加工以及水处理等领域得到广泛应用[3-4]。然而,随着应用的深入,膜处理的环境越来越复杂(如高温、高压),对膜材料提出了更高的要求[5]。与大多数只能在50 ℃下使用的商业化膜相比,耐高温纳滤膜可用于处理各种热流体,无需严格的温度控制,生产率将大大提高[6]。特别是在高温操作条件下,提高运行温度有助于提高膜通量和减少膜污染,这对能量回收和降低成本具有重要意义[7]。因此,开发具有热稳定性的纳滤膜具有重要意义。
凯夫拉(PPTA)具有优异的机械强度、热稳定性以及化学稳定性,是制备超滤膜的合适材料,可用于某些极端条件,如高温废水处理[8-9]。然而,PPTA具有高结晶度,难溶于大部分常规溶剂,制备条件苛刻,在膜领域的相关研究较少[10-11]。Nakura等[12]与Zschock等[13]以PPTA为成膜聚合物分别制备了PPTA超滤膜,前者研究了PPTA成膜条件,渗透性能;后者研究了膜的耐溶剂性能。然而,所得膜分离精度上仍需要进一步完善。Wang等[10]以PPTA为原料制备了PPTA中空纤维膜,所得膜表现出优异的耐热性和抗污染性能,但力学性能较差。此外,PPTA中空纤维膜仅为超滤膜,对于染料、无机盐等物质的分离性能仍有待进一步提高。因此,为了提高膜的分离精度,有必要对PPTA超滤膜进一步改性。
聚吡咯(PPy)具有良好的亲水性、优异的热稳定性、易于聚合,被广泛应用于改善膜的性能[14-15]。目前,电化学聚合法和化学气相沉积法(CVD)是两种常用的PPy合成方法。相较于电化学聚合法,化学气相沉积法操作简单,控制因素相对较少[16-17]。Shao等[18]以过硫酸铵为氧化剂,在水解聚丙烯腈(PAN-H)超滤膜表面原位聚合PPy分离层以制备复合纳滤膜,所得复合纳滤膜对异丙醇的渗透通量为12.1 L·m−2·h·MPa−1,对孟加拉玫瑰红的截留率为99.0%。Liu等[19]采用化学气相沉积法,制备了PVDF/PPy复合纳滤膜,所得膜抗污染性能明显提高。Ji等[20]采用氧化石墨烯(GO)对聚偏氟乙烯(PVDF)超滤膜进行亲水改性,随后采用化学气相沉积法在其表面原位聚合PPy分离层以制备PVDF/GO/PPy复合纳滤膜。与PVDF超滤膜相比,复合纳滤膜亲水性改善,同时对带负电染料具有较高的截留率(˃98.5%)。虽然复合纳滤膜亲水性好、分离精度高及抗污染性好,然而其分离过程都在常温条件下进行,关于气相沉积PPy制备的复合纳滤膜热稳定性研究,鲜有报道。
本文以PPTA为成膜聚合物,PPTA纤维编织管为增强体,采用干-湿法纺丝技术制备同质增强型PPTA 中空纤维膜。随后,通过化学气相沉积法(CVD)在膜表面原位聚合PPy分离层以进一步修饰膜结构,并对所得复合纳滤膜结构和染料脱盐的分离性能进行了研究,为纳滤膜在更高运行温度下处理染料废水提供一定的理论和实验依据。
聚吡咯/凯夫拉中空纤维复合纳滤膜的制备及其染料脱盐性能
Fabrication of polypyrrole/kevlar hollow fiber composite nanofiltration membrane for dye desalination
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摘要: 以同质增强型凯夫拉(PPTA)中空纤维膜为基膜,吡咯(Py)和三氯化铁(FeCl3)分别为反应单体和活化剂,采用化学气相沉积法制备了结构稳定、可控的聚吡咯(PPy)/PPTA中空纤维复合纳滤膜。采用FTIR、SEM、AFM、接触角测定仪以及固体表面Zeta电位仪对基膜和PPy/PPTA中空纤维复合纳滤膜的微观形貌、化学组成、亲水性、表面荷电性进行了表征。结果表明,经PPy气相沉积后,PPy/PPTA中空纤维复合纳滤膜表面形成具有图灵结构特征的分离层,并均匀覆盖膜表面。在0.6 MPa室温下,PPy/PPTA中空纤维复合纳滤膜具有较高的的脱盐性能,其顺序为
${R_{{\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}}} $ (93.59%) >${R_{{\rm{MgS}}{{\rm{O}}_4}}} $ (91.58%) >${R_{{\rm{CaC}}{{\rm{l}}_2}}} $ (83.45%) > RNaCl (54.04%),同时对带负电染料表现出较高的截留率(˃98.82%)。当运行温度从25 ℃升高到90 ℃时,PPy/PPTA中空纤维复合纳滤膜的水通量较明显增加,而截留率几乎保持稳定,表现出优异的热稳定性,为纳滤膜在更高运行温度下处理染料废水提供指导。Abstract: A stable and controllable polypyrrole (PPy)/kevlar (PPTA) hollow fiber composite nanofiltration (NF) membrane with homogeneous reinforced PPTA hollow fiber membrane as the substrate was successfully prepared by chemical vapor deposition method. Pyrrole (Py) and ferric chloride (FeCl3) were used as reaction monomers and activators, respectively. The micro-morphology, chemical composition, hydrophilicity and surface charge of the pristine PPTA membrane and PPy/PPTA hollow fiber composite NF membrane was characterized by FTIR, SEM, AFM, contact angle analyzer, and solid surface Zeta potentiometer. The results showed that the surface of PPy/PPTA hollow fiber composite NF membrane formed a separation layer with Turing structure characteristics after PPy vapor deposition, and it evenly covered the membrane surface. At 25 ℃, 0.6 MPa, the as-prepared PPy/PPTA hollow fiber composite NF membrane exhibited not only a superior rejection (˃98.82%) for negatively charged dyes, but also an excellent salt rejection in the order as:${R_{{\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}}} $ (93.59%) >${R_{{\rm{MgS}}{{\rm{O}}_4}}} $ (91.58%) >${R_{{\rm{CaC}}{{\rm{l}}_2}}} $ (83.45%) > RNaCl (54.04%). When the operating temperature increased from 25 °C to 90 °C, the water flux of PPy/PPTA hollow fiber composite NF membrane increased significantly, while the rejection remained almost stable, showing excellent thermal stability. It provided guidance for NF membrane to treat dye wastewater at higher temperature. -
图 2 SEM形貌图
Figure 2. SEM images of (a) pristine PPTA membrane,(b) Cross-section of pristine PPTA membrane,(c) PPy/PPTA hollow fiber composite NF membrane,and (d) enlarged outer surface of PPy/PPTA hollow fiber composite NF membrane,and AFM images of the outer surface of (e) pristine PPTA membranes and (f) PPy/PPTA hollow fiber composite NF membrane
图 3 (a)化学气相沉积法形成“类图灵”结构的示意图;(b)PPTA基膜孔结构和化学气相沉积过程形成PPy分离层的概念模型[26]
Figure 3. (a) Schematic diagram of “Turing-like” structure formed by chemical vapor deposition method; (b) Conceptual model illustrating the role of pristine PPTA membrane pore structure and PPy separation layer formed by chemical vapor deposition method[26]
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