应用光学相干断层成像研究渗透压驱动过程的膜污染现象

STUDY ON MEMBRANE FOULING DURING OSMOTICALLY DRIVEN PROCESSES VIA OPTICAL COHERENCE TOMOGRAPHY

陈干

11849076

硕士

环境工程

李炜怡

2020-05-25

2020-05-25

哈尔滨工业大学

深圳

与压力驱动膜分离过程相比，渗透压驱动膜分离过程由于其独特的优势正获得更多的关注。例如，渗透压驱动膜分离过程在水/废水处理以及海水脱盐等领域中有着降低能耗的潜力，并且由于其相对温和的操作条件在生物制品领域也有着广泛的应用前景。然而，膜污染是限制渗透压驱动膜分离过程发展的一个主要障碍。膜污染会严重降低正渗透过程的处理效率，显著增加运行成本（例如，增加分离膜更换频率）。因此，探索开发新型技术用于复杂膜污染现象的表征具有重要意义，而这些表征技术的开发也将为研制减弱膜污染影响的工具提供基础。本研究利用光学相干层析成像（optical coherence tomography ，OCT）技术分析了渗透压驱动膜过程中膜污染现象。基于OCT的表征技术能够对分离膜表面形成的滤饼层进行原位表征；并且通过OCT数据集生成一系列数字化滤饼层，进一步用于评估滤饼层表面覆盖率和平均厚度的变化。本研究中一系列实验均采用由层层自组装技术制备的纳滤型分离膜，即通过静电结合作用使带有相反电荷的聚电解质在多孔聚丙烯腈基膜上交替沉积形成聚电解质层。同时采用包括硫酸钠（Na2SO4）和氯化镁（MgCl2）在内的二价盐作为汲取液的溶质，从而确保在纳滤型分离膜两侧形成有效的跨膜渗透压差。此外，本研究以膨润土颗粒和乳清蛋白作为模型污染物进行了对比分析。以测定临界通量的方式，本研究把基于OCT的表征技术与恒压模式进行有机融合，致力于揭示在不同操作条件下渗透压驱动膜分离过程的污染物行为。结果表明，当二价阳离子存在于汲取液扩散到料液中时（即渗透压驱动膜分离过程中固有的反扩散现象），渗透压驱动膜分离过程的临界通量会明显降低。从数字化滤饼层的演化中可发现，二价阳离子的架桥作用（即金属-配体络合作用）不仅会增强分离膜表面与污染物颗粒之间的相互作用，也会增强污染物颗粒之间的相互作用。此外，通过对分离膜表面滤饼层局域生长速率的测定，揭示了汲取液溶质对边界层传动与传质的耦合作用产生的影响。本研究证实了基于OCT的表征技术是研究渗透压驱动膜分离中膜污染现象的有效方法。所有的表征结果都为探究分离膜与污染物之间复杂相互作用的机制提供了更深入的证据。另一方面，充分开发渗透压驱动膜分离过程还需要更多的研究，并侧重探索和解决二价盐汲取液的使用对渗透压驱动膜分离过程的负面影响。

Osmotically driven membrane processes (ODMPs) have been gaining more attention owing to various advantages that are not available for pressure-driven membrane processes (PDMPs), e.g., the potential to consume less energy during desalination or water/wastewater treatment and mild operating conditions for the treatment of bio products. However, a major obstacle for the application of ODMPs is membrane fouling that could severely decrease the efficiency of forward osmosis and thereby markedly increase the cost of operation (e.g., more frequent replacement of the membranes). It is therefore of great value to explore novel techniques for characterizing the mechanisms underlying the complex fouling phenomena, which will in turn offer powerful tools to mitigate the membrane fouling during ODMPs.This study employed optical coherence tomography (OCT) to analyze the membrane fouling during ODMPs. The OCT-based approach enabled in-situ characterization of the cake layer formed on the membrane surface; the OCT datasets were exploited to generate a series of digitalized cake layers, which were further used to assess the variations in the surface coverage and the average thickness of the cake layer. This study implemented a series of characterization experiments using nanofiltration (NF)-like membranes that were fabricated via the Layer-by-Layer (LbL) technique, i.e., the alternating deposition of oppositely charged polyelectrolytes onto a porous polyacrylonitrile (PAN) substrate. In order to efficiently create the osmotic pressure difference across the NF-like membrane, divalent salts were employed as the solute in the draw solution, including sodium sulfate (Na2SO4) and magnesium chloride (MgCl2). Moreover, both bentonite particles and whey protein were employed as the model foulants to present a comparative study.In a way of measuring the critical flux, the OCT-based characterization was incorporated with the constant pressure mode to reveal the fouling behavior during the ODMPs with various operating conditions. It was demonstrated that the critical flux could be significantly decreased in the existence of divalent cations, which were transferred from the draw solution to the feed solution, i.e., the intrinsic back diffusion phenomenon during ODMPs. The evolution of the digitalized cake layer indicated that not only the foulant-membrane interaction but also the foulant-foulant interaction was substantially increased owing to the bridging effect of the divalent cations (i.e., the metal-ligand complexation). In addition, the distribution of the local growth rates was determined to unravel the role of the draw solutes in varying the coupling effect between the momentum and mass transfer in the boundary layer. This study confirms that the OCT-based characterization is an effective way to exploring the fouling phenomena during ODMPs. All the characterization results provide deeper insights into the mechanisms accounting for the complex interplay between the membrane and foulants. On the other hand, more research is necessary to explore the full potential of ODMPs primarily by addressing the concerns relevant to the negative effects resulting from the use of divalent salts in the draw solution.

渗透压驱动膜分离过程 膜污染 光学相干断层成像 原位观测 纳滤型分离膜

osmotically driven membrane processes membrane fouling optical coherence tomography in-situ characterization NF-like membranes

中文

联合培养

南方科技大学

陈干. 应用光学相干断层成像研究渗透压驱动过程的膜污染现象[D]. 深圳. 哈尔滨工业大学,2020.