Abstract The commonly used commercial hydrophobic membranes are facing great challenges in wetting and low permeation when used in membrane distillation (MD) because of the limited materials and structures. To address these problems, we herein fabricated a kind of carbon nanotubes (CNTs) enhanced membrane with high flux and stable performance in membrane distillation, where a porous hydrophobic network of multi-walled carbon nanotubes (MWCNTs) was spray-coated onto the commercial flat-sheet hydrophobic membrane surface. Then, we systematically investigated the impact of the CNTs loading on the substrate surface properties, membrane structures and MD performance (flux and rejection), and experimentally illustrated the role of CNTs towards improving MD performance. Results illustrate that there is an optimal match of the CNTs spray amount with the substrate pore size and the substrate materials to achieve the maximum specific surface area of membrane, and thus the maximum flux and salt rejection of the resultant CNTs-enhanced membrane, which is governed by the interfacial energy of the CNTs and the substrate, and thus the CNTs packing density. Results find that the CNTs packing density decreases with the decrease of the substrate free surface energy, and the loosest CNTs packing is formed on the polytetrafluoroethylene (PTFE) membrane with the lowest free surface energy. Under the optimized conditions, the CNTs enhanced PTFE membrane exhibited salt rejection of >99.9% and water flux of 33.2 LMH in MD, which was three times as high as that of the pristine PTFE membrane under the temperature difference of 40 °C between hot and cold streams when operated in direct contact membrane distillation. Our research here provides an efficient method to enhance the permeation and stability of commercial membranes by spray-coating CNTs and also supplies some insights on the design of next-generation MD membranes.

中文翻译：

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工程碳纳米管通过喷涂增强疏水膜在膜蒸馏中具有高性能

摘要 由于材料和结构有限，常用的商用疏水膜在用于膜蒸馏（MD）时面临着润湿和低渗透性的巨大挑战。为了解决这些问题，我们在这里制造了一种具有高通量和稳定的膜蒸馏性能的碳纳米管（CNTs）增强膜，其中多壁碳纳米管（MWCNTs）的多孔疏水网络被喷涂到商业平面上-片疏水膜表面。然后，我们系统地研究了 CNTs 负载对基材表面性质、膜结构和 MD 性能（通量和截留）的影响，并通过实验说明了 CNTs 在改善 MD 性能方面的作用。结果表明，碳纳米管喷洒量与基材孔径和基材材料存在最佳匹配，以实现膜的最大比表面积，从而获得所得碳纳米管增强膜的最大通量和盐截留率，即由碳纳米管和基板的界面能控制，从而控制碳纳米管的堆积密度。结果发现，随着基体自由表面能的降低，CNTs堆积密度降低，并且在自由表面能最低的聚四氟乙烯（PTFE）膜上形成最松散的CNTs堆积。在优化条件下，CNTs增强的PTFE膜在MD中表现出>99.9%的脱盐率和33.2 LMH的水通量，在直接接触膜蒸馏操作时，在热流和冷流之间的温差为 40°C 的情况下，它是原始 PTFE 膜的三倍。我们的研究提供了一种通过喷涂碳纳米管来提高商业膜渗透性和稳定性的有效方法，并为下一代 MD 膜的设计提供了一些见解。