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兼具高通量和耐油污染的亲/疏水复合膜及其膜蒸馏性能
张燕
Thesis Advisor沈飞
2020-07-01
Degree Grantor中国科学院大学
Degree Name硕士
Degree Discipline环境工程
Keyword膜蒸馏,亲/疏水janus复合膜,脱盐,中间过渡态水,水蒸发
Abstract

膜蒸馏(MD)是一种以疏水多孔膜为物理屏障、以膜两侧蒸汽压差为传质推动力的热驱动型膜分离技术,在海水淡化和高盐废水处理与回用等脱盐领域具有良好的应用前景。但膜通量较低、膜易污染和润湿等问题始终限制着膜蒸馏技术的发展与应用。近年来,在提高膜蒸馏膜通量和增强膜耐污染/润湿能力这两个方面已经各自有了较大进展,但当前的研究方案还难以将这两者兼顾,即在膜通量提升和耐污染性增强之间存在一种trade-off效应,这使得当前膜蒸馏膜的综合性能及其应用稳定性仍需进一步增强。因此,研制兼具高通量和耐污染/润湿性的膜蒸馏膜具有重要意义和应用价值。本论文主要是针对如何突破上述trade-off效应,从新型亲/疏水Janus复合膜的构建角度开展了相关研究。(1)在商业化聚四氟乙烯(PTFE)疏水微滤膜表面依次通过多巴胺(DA)自聚、银纳米颗粒(AgNPs)原位固定、聚多巴胺(PDA)超薄封装的多级修饰,分别制备了P-PTFE、Ag/P-PTFE和P/Ag/P-PTFE三种亲/疏水Janus复合膜,并对膜的表面化学元素、形貌与粗糙度、表面孔径、Zeta电位以及润湿性等物理化学性质进行了表征。与原始PTFE膜相比,复合膜表面孔径略有下降(降幅27.1-39.0 %),但依然保持表面多孔特性,表面平均孔径为72-86 nm;复合膜表面均呈现空气中亲水、水下疏油特性且带有更强的荷负电性。与P-PTFE膜相比,Ag/P-PTFE膜表面生成了大量均匀分布、粒径为36.8 ± 5.1 nm的AgNPs,形成了多级粗糙表面,亲水性和水下疏油性获得显著提升。经过PDA超薄封装,P/Ag/P-PTFE膜的亲水性和水下疏油性未受影响,水接触角和水下油接触角分别为55.7 ± 5.6°和146.4 ± 3.9°,但PDA封装明显增强了AgNPs的稳定性。(2)采用真空膜蒸馏(VMD)操作模式,对原始PTFE膜和Janus复合膜的脱盐性能进行了测试与比较。70℃下处理3.5wt%的NaCl水溶液时(真空度90kPa),修饰前后膜的脱盐率均保持在99.9%以上;原始PTFE膜的VMD通量为19.8 kg m-2 h-1,P-PTFE和P/Ag/P-PTFE膜的通量则显著增至66.9和84.3 kg m-2 h-1,增幅分别达到237.0和324.7 %。60℃下连续处理含油盐水乳液(500 ppm矿物油,3.5 wt% NaCl)时,原始PTFE膜发生快速且严重的污染现象,50小时运行中膜通量由2.24 kg m-2 h-1下降至0.47 kg m-2 h-1;P-PTFE膜的污染速度有所降低,但50小时内其通量由28.99 kg m-2 h-1逐渐降至16.53 kg m-2 h-1,降幅43.0 %;相反,P/Ag/P-PTFE膜表现出稳定的脱盐性能,其初始通量为39.14 kg m-2 h-1,50小时结束运行时的通量为37.65 kg m-2 h-1,通量下降仅为3.8 %,且全过程中保持超过99.9%的稳定脱盐率。由此可见,PDA层在通量增强方面起主导作用,而AgNPs在减轻膜面温差极化以及提升耐油污染性方面也提供了重要贡献,P/Ag/P-PTFE膜的高综合性能应源自上述两方面的协同强化。(3)对亲/疏水Janus复合膜的通量提升机制进行了进一步探讨。通过拉曼光谱测量了受限于膜表面孔内水的状态,发现亲水层内含有大量中间过渡态水(IW)。与自由态水(FW)相比,IW具有更低的蒸发焓,因此推测Janus复合膜的亲水层具有加速水蒸发的功能。另一方面,亲水层的孔结构对液态水的跨亲水层输送(水补给)有重要影响。将亲水性商业化超滤膜(UF)和纳滤膜(NF)物理叠加到PTFE膜表面形成类似于亲/疏水Janus复合膜的“物理叠加膜”,与UF膜相比,即使NF更加亲水且能够提供更多的IW,NF/PTFE膜的VMD通量显著低于UF/PTFE膜,且比原始PTFE膜通量低了55.5%;相反,UF/PTFE膜的通量较原始PTFE则提高了27.6%。这说明,亲水层致密化会导致水补给受限,从而对复合膜的膜蒸馏通量产生负面影响。理论分析也表明,亲水层中加速水蒸发和跨亲水层水补给之间存在竞争关系。(4)初步研究了碳纳米管(CNTs)的PDA修饰及其在不同溶剂中的分散行为。AgNPs对减轻膜面温差极化以及提供多级粗糙度有正面贡献,但AgNPs本身致密无孔。与AgNPs相比,CNTs不仅同样具有高导热性而且管内具有加速水传输效应,有望进一步提升亲/疏水Janus复合膜的膜蒸馏脱盐性能。针对CNTs团聚严重难以分散因而对构建CNTs基亲水层产生负面影响,本论文初步开展了基于PDA修饰的CNTs改性及其在溶剂中的分散性研究。结果显示,PDA可通过物理屏蔽和静电排斥两种效应协同强化CNTs的分散效果;对比改性前后CNTs在水、二甲基乙酰胺和乙醇中的团聚过程可知,95%以上的原始CNTs在超声后静置170分钟内呈现的是小束和团聚形态,而95%以上的改性CNTs(0.5-PDA@CNTs)则呈现出单分散形态,表明PDA改性的CNTs具有优异的分散稳定性。本论文研究表明,在传统疏水多孔膜表面构建结构良好的亲水涂层,能够突破当前在膜通量提升和耐污染性增强之间存在的trade-off效应,获得兼具高通量和耐污染的膜蒸馏膜。针对如何进一步优化亲水层的物理结构和化学性质,结合理论分析本论文也给出了初步的建议。本论文的研究结果有望为制备新一代高性能膜蒸馏膜提供新的视角和方法。 ;Membrane distillation (MD) is a thermally driven membrane separation technique, in which a hydrophobic porous membrane functions as a barrier and the vapor pressure difference across the membrane provides driving force for mass transfer. However, low flux as well as membrane fouling and wetting are hindering its application and development. Research on the enhancement of MD flux or membrane anti-fouling/wetting capacities has made progress in the last few years, while simultaneous improvement in both features has yet to be achieved, namely, there exists a trade-off between increasing flux and improving anti-fouling capacity. The comprehensive performance and stability of MD membranes still need to be improved. As a result, it is of great significance to fabricate MD membranes with simultaneous high flux and anti-fouling/wetting performance. In this work, we mainly focused on how to break through the aforementioned trade-off effect from the perspective of constructing novel hydrophilic/hydrophobic Janus composite membranes.(1) Three hydrophilic/hydrophobic Janus membranes were prepared in sequence based on a commercial polytetrafluoroethylene (PTFE) membrane. Through a multi-step surface modification strategy, which includes polydopamine (PDA) coating, in-situ immobilization of silver nanoparticles (AgNPs) and PDA ultrathin sealing, we obtained P-PTFE, Ag/P-PTFE and P/Ag/P-PTFE membranes. Membrane surface physicochemical features including elementary composition, morphology and roughness, mean pore size, Zeta-potential, and wettability were characterized. Although the mean pore size was 27.1-39.0 % smaller than the pristine PTFE membrane, Janus membranes still maintained a surface porous feature (72-86 nm). Moreover, the surface of Janus membranes was in-air hydrophilic and underwater oleophobic and exhibited stronger electronegativity compared with the pristine PTFE membrane. After the in-situ metallization, a large number of uniformly distributed AgNPs with a diameter 36.8 ± 5.1 nm appeared, endowing the Ag/P-PTFE membrane with a multilevel rough surface, so its hydrophilicity and underwater oleophobicity were further enhanced. The final PDA ultrathin sealing significantly improved the stability of AgNPs but did not impact the wettability of the membrane surface. The in-air water contact angle and underwater oil contact angle of the P/Ag/P-PTFE membrane were 55.7 ± 5.6° and 146.4 ± 3.9°, respectively.(2) The desalination performance of the pristine PTFE membrane and Janus membranes were investigated through vacuum membrane distillation (VMD). When dealing with a 3.5% NaCl solution at 70℃ (vacuum degree 90 kPa), all membranes had the salt rejection rate above 99.9%. The VMD flux of the pristine PTFE was 19.8 kg m-2 h-1. The VMD flux of P-PTFE and P/Ag/P-PTFE membranes soared to 66.9 kg m-2 h-1 and 84.3 kg m-2 h-1, respectively, which was 237.0% and 324.7 % higher than the pristine PTFE membrane. When treating an oily saline emulsion (500 ppm mineral oil, 3.5% NaCl), acute fouling happened on the surface of the PTFE membrane, and its flux reduced from 2.24 kg m-2 h-1 to 0.47 kg m-2 h-1 in 50 hours. Slower fouling happened to the P-PTFE membrane, but there was still a flux decline from 28.99 kg m-2 h-1 to 16.53 kg m-2 h-1, a 43% of flux decline. On the contrary, the P/Ag/P-PTFE membrane exhibited a quite stable desalination performance. Its initial flux and ultimate flux was 39.14 kg m-2 h-1 and 39.14 kg m-2 h-1, respectively, only a decrease in 3.8 %, and the salt rejection was always higher than 99.9 %. It can be seen that the first PDA coating was the main driver of the flux increase, and AgNPs also contributed to the alleviation of temperature polarization and the improvement of anti-fouling capacity. The outstanding comprehensive performance of the P/Ag/P-PTFE membrane came from their synergetic effect. (3) The mechanism of flux increase from the pristine PTFE membrane to Janus membranes was further discussed via experimental results and theoretical analysis. The results of Raman spectra confirmed that there were masses of intermediate water (IW) confined in the hydrophilic PDA coating. Compared with free water (FW), IW possesses lower evaporation enthalpy, so it was speculated that the hydrophilic layer of Janus membranes might have the function of accelerating water evaporation. On the other hand, the pore structure of the hydrophilic layer also has a significant influence on the water transfer across the hydrophilic layer (water recharge). Through physically overlaying a hydrophilic ultrafiltration (UF) or nanofiltration (NF) membrane on the commercial PTFE membrane, we prepared the UF/PTFE and NF PTFE "laminated composite membrane", respectively. Although the NF membrane was more hydrophilic than the UF membrane thus providing more IW, the VMD flux of the NF/PTFE membrane was 55.5 % lower than the PTFE membrane, while the UF/PTFE membrane was 27.6 % higher than the PTFE membrane. This indicated that the densification of the hydrophilic layer would limit the water recharge, thus negatively impacting the MD flux. Theoretical analysis also showed that there was a competitive relationship between water evaporation acceleration and water supply across the hydrophilic layer. (4) PDA modification of carbon nanotubes (CNTs) and their dispersion behavior in different solvents were preliminarily studied. Although AgNPs contributed to alleviating temperature polarization and providing multilevel roughness, it is dense and non-porous. Comparatively, CNTs not only has high thermal conductivity but also can accelerate the water transfer in the tube. Therefore, it is expected that CNTs will further enhance the MD desalination performance based on the concept of hydrophilic/hydrophobic Janus membrane. Given that raw CNTs will always agglomerate severely and have poor dispersibility, which will negatively influence the subsequent application in constructing a hydrophilic layer, we modified the surface of CNTs by coating PDA. Results indicated that the PDA layer could significantly improve the dispersibility of CNTs through the synergistic effect of physical shielding and electrostatic repulsion. By comparing the agglomeration process of CNTs in deionized water, N, N-dimethylacetamide and ethanol before and after modification, it could be found that within 170 minutes after ultrasonication, more than 95 % of the pristine CNTs presented bundles or aggregates, but over 95 % of modified CNTs (0.5-PDA@CNTs) were monodisperse. The results showed that the PDA modified CNTs exhibited outstanding dispersion stability.In summary, we demonstrated in this study that the trade-off effect between increasing flux and improving anti-fouling capacity, especially in the surface modification process for the MD membrane, could be broken through by constructing a well-structured hydrophilic layer on a hydrophobic porous membrane. As a result, we could obtain a high-performance hydrophilic/hydrophobic Janus membrane with both enhanced MD flux and improved anti-oil-fouling ability. In order to further optimize the physical structure and chemical properties of the hydrophilic layer, we also gave some preliminary suggestions based on the theoretical analysis. We hope the findings in this study can provide a new platform and methodology for designing high-performance membranes for MD desalination.

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/49647
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
张燕. 兼具高通量和耐油污染的亲/疏水复合膜及其膜蒸馏性能[D]. 中国科学院大学,2020.
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