Membrane separation technology is widely explored for various applications, such as water desalination and wastewater treatment, which can alleviate the global issue of fresh water scarcity. Specifically, carbon nanotubes (CNTs)-based composite membranes are increasingly of interest due to the combined merits of CNTs and membrane separation, offering enhanced membrane properties.
Clean water is an essential resource for human life and our ecosystem. However, due to the increasing growth of population and rapid development of economy and urbanization, the increasing pressure on water scarcity issue highly requires cost-effective water treatment technologies to produce high-quality clean water. On the one hand, seawater and brackish water desalination technologies seem to be the most fundamental approaches that possess great promise to effectively increase clean water supply by producing more freshwater. Since the total sources of seawater and brackish water accounts for almost 98% of all water on the earth, capturing a tiny fraction is expected to impart a huge impact on the issue of fresh water scarcity. On the other hand, recovery and recycling technology of wastewater has become a promising trend in the past decades to achieve water sustainability. Wastewater reuse not only decreases the discharge environmental risk, but also relieves the pressure on fresh water resource shortage.
Among various technologies for water treatment, membrane separation technology is widely accepted as an emerging route not only to desalinate seawater and brackish water, but also to reuse wastewater.Furthermore, due to the advantages offered such as high stability and efficiency, ease operation, low operating cost and capital, low energy consumption and also low pollution, it is known as one of the major technologies for substainable envronmental pollution control engineering.
Recent advances in nanotechnology combined with membrane separation have been recognized as some viable and effective approaches to enhance membrane performance with their synergistic effects for water and wastewater treatment. In particular, carbon nanotubes (CNTs) including single-walled carbon nanotubes (SWCNTs)and multi-walled carbon nanotubes (MWCNTs), owing to their high specific surface area, high mechanical strength, excellent chemical inertness and outstanding water-transport property, have received widespread interests in construction of new composite membranes for water treatment application. In addition, CNTs exhibit encouraging adsorption, catalytic and electrochemical properties, which are beneficial to couple adsorption, catalytic or electrochemical function with membrane separation process, thus improving water treatment performances of CNTs-based composite membranes.
1、Water Desalination
Since the sources of seawater and brackish water account for almost 98% of all water on the earth, capturing a tiny fraction is expected to impart a huge impact on the water scarcity issues. Hence, seawater and brackish water desalination through various desalting technologies seems to be the most straightforward approaches that hold great promise to effectively increase water supply by generating more freshwater from sea and brackish water for the world usage. Membrane separation technology has been widely accepted as a promising route to offer sustainable fresh water in a more economical economical and green process. At present, CNTs are interestingly investigated and have attracted significant growing attentions due to their properties to enhance the efficiency and capability of currently available membrane processes such as reverse osmosis (RO), nanofiltration (NF) and membrane distillation(MD).
Son et al. created a novel architecture to produce a better performance thin-film composite (TFC) membrane for seawater reverse osmosis (SWRO) and brackish water reverse osmosis (BWRO) by immobilizing functionalized CNTs in the porous support layer [88]. Beneficial for application, the new membranes showed enhanced water permeability due to their increased hydrophilicity and enhanced pore properties of the support layer without sacrificing NaCl rejection as compared to that of the bare TFC membranes. In SWRO at pressure over 50 bar, the water flux was enhanced 10%–20%, and a 90% enhanced water flux was achieved in the BWRO operating pressure at 30–40 bar. Recently, CNTs-PES mixed matrix membranes have been prepared for NF application. With incorporation of CNTs, the as-prepared membranes showed higher flux and salt rejection than the PES membranes [89]. This study found that when CNTs concentration reached 0.1 wt %, the CNTs-PES membranes obtained the highest water flux (38.91 Lm