海水淡化用疏水性无机分离膜研究

Membrane distillation (MD) is regarded as a low cost alternative to the existing commercial desalination technologies,, because it can run with low-grade or waste heat. In MD, a temperature difference exists between the two sides of the membrane. The water at the hot side of the membrane evaporates and passes through the pores of the membrane, and condenses into liquid water at the cold side of the membrane. Since the surface of the membrane used for MD is hydrophobic, liquid water cannot permeate through the membrane, thus salt ions contained in the seawater are rejected. Currently, the most commonly studied hydrophobic membranes in MD are made of polymers, such as polytetrafluoroethylene, polyvinylidenedifluoride, polypropylene [2-4]. However, the polymeric membranes suffer from poor thermal, mechanical and chemical stabilities, representing a significant challenge for widespread commercialization of MD desalination. It is clear that there is an urgent need for membranes with high MD performance. It is well known that ceramic membranes exhibit excellent chemical, structural and thermal stabilities, and therefore, attempts have been made to explore ceramic membranes for MD applications. Most ceramic membranes are made from metal oxides, such as alumina, zirconia, silica and so on, which are hydrophilic in nature because of the presence of hydroxyl (OH-) groups on the surface. This surface property prevents them from being directly used in MD process. However, the ceramic membranes with hydrophobic surface could be prepared by surface modification. In the present study, we are interested in using fluoroalkylsilanes (FAS) as a surface modifier to promote the membrane surface hydrophobicity for MD application because FAS are shown to be effective in modifying ceramic surfaces. Specifically, FAS are fluorinated organosilanes which are composed of hydrolysable groups and hydrophobic ends. It has been suggested that FAS is attached to the membrane surface through the reaction of the hydrolysable groups with the surface hydroxyl groups of the metal oxide. In this project, the detailed mechanism of surface grafting will be studied, especially in terms of the nature of force linking the FAS and metal oxide surface (chemical or physical). The method to intensify the surface grafting will be explored as well, for example, using the microwave or ultrasonic assisted surface reaction. And the performance of the as-prepared hydrophobic inorganic membrane will be examined under MD water desalination conditions, in particular its chemical stability and durability.

膜蒸馏将蒸馏和膜分离结合起来，可以利用太阳能、工业余热、地热等低品位热源，是一种有良好应用前景的低能耗高效率海水淡化技术。目前主要采用疏水性有机高分子分离膜。以金属氧化物为代表的无机分离膜具有优于有机高分子分离膜的化学和热稳定性、机械强度。通过在无机分离膜表面嫁接硅烷分子，可以将其转化为适合于海水淡化膜蒸馏的疏水性分离膜。为了实现膜蒸馏海水淡化技术的实用化，需要对其中涉及的基础问题和制备化学进行深入细致的研究。本项目拟围绕无机分离膜的表面疏水改性机理、表面嫁接反应强化和膜蒸馏条件下的性能和衰变规律等３个相互关联的问题开展研究，以期为开发膜蒸馏海水淡化技术提供科学基础和材料支撑。

以疏水性多孔膜为基础的膜蒸馏可以利用太阳能、工业余热、地热等低品位 热源，是一种有良好应用前景的低能耗高效率海水淡化技术。常见的疏水膜是有机高分子材料，其耐久性较差，制约了膜蒸馏技术的实用化。无机分离膜具有优于有机高分子分离膜的化学和热稳定性、机械强度，因此本项目拟通过对无机分离膜进行表面修饰，将其转化为适合于海水淡化膜蒸馏应用的疏水性分离膜。本项目采用氟硅烷表面嫁接修饰方法将多孔氧化铝和氮化硅由亲水转化为疏水。XPS分析揭示氟硅烷分子与基底材料表面形成了化学键，这第一次从实验上证实氟硅烷是以化学吸附的方式结合到基底材料表面上的。在研究有机无机杂化膜（如FAS嫁接氧化铝）的同时，本项目提出并实现了全无机疏水膜的制备新方法。将聚二甲基硅氧烷涂敷在多孔氧化铝陶瓷表面，然后在非氧化性气氛（氮气）中热解，生成纳米颗粒。这些纳米颗粒包含纳米晶和无定形相，化学组成为Si，C，O，其表面存在大量的Si-CH3基团。PDMS热解修饰的氧化铝的水接触角达130°，其疏水性可归结于无机纳米粒子表面存在甲基。采用水热法在氧化铝表面负载一层氧化铝纳米颗粒，再经PDMS热解修饰，其水接触角高达160°，为超疏水表面。采用氯硅烷热解方法在多孔氮化硅表面生长SiNCO纳米颗粒，其接触角达142。无机纳米粒子修饰的无机分离膜在pH值为2-12的溶液、有机溶剂中水接触角保持不变，甚至沸水中浸泡24小时仍能保持疏水性。高稳定性无机疏水膜表现出优异的海水淡化性能，对于NaCl浓度高达12-20wt%、含腐植酸的水溶液，仍具有可观的水透量，盐截留率也大于99%。基于这些高稳定性全无机疏水膜，人们可以设想采用膜蒸馏方法从海水中一方面分离制备纯水，另一方面制备盐化工所需的浓缩盐溶液，实现海水淡化的近零排放。本项目研发的高稳定性疏水膜还可望用于含盐废水、液体食品的浓缩处理。