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As water molecules permeate ultrafast through carbon nanotubes (CNTs), many studies have prepared CNTs-based membranes for water purification as well as desalination, particularly focusing on high flux membranes. Among them, vertically aligned CNTs membranes with ultrahigh water flux have been successfully demonstrated for fundamental studies, but they lack scalability for bulk production and sufficiently high salt rejection. CNTs embedded in polymeric desalination membranes, i.e., polyamide thin-film composite (TFC) membranes, can improve water flux without any loss of salt rejection. This improved flux is achieved by enhancing the dispersion properties of CNTs in diamine aqueous solution and also by using cap-opened CNTs. Hydrophilic CNTs were prepared by wrapping CNT walls via bio-inspired surface modification using dopamine solution. Cap-opening of pristine CNTs is performed by using a thermo-oxidative process. As a result, hydrophilic, cap-opened CNTs-embedded polyamide TFC membranes are successfully prepared, which show much higher water flux than pristine polyamide TFC membrane. On the other hand, less-disperse, less cap-opened CNTs-embedded TFC membranes do not show any flux improvement and rather lead to lower salt rejection properties.

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During biomass (e.g., waste wood chips) decomposition under an anaerobic fermentation process, organic acids such as acetic acid and butyric acid are continuously produced from controlled microbial activity. Since the accumulation of organic acids hinders the microbial metabolism in the fermentation broths, the organic acids should be removed by using appropriate separation processes. A few separation processes such as extraction, electrodialysis, and distillation have been reported, but they still have many limitations such as high energy input and environmental problems (e.g., toxic chemical effluents). The integrated membrane processes proposed here, including the three steps of (1) clarification of fermentation broth, (2) organic acid separation, and (3) dewatering, can be applied to achieve energy-efficient and environmentally friendly organic acid removal and recovery. First, microorganisms and large insoluble particles in fermentation feed can be mostly removed by clarification steps using microfiltration or ultrafiltration processes. In this study, we focused only on organic acid separation and dewatering processes using nanofiltration and forward osmosis membrane processes. Using nanofiltration (or high-flux reverse osmosis) membranes, aqueous organic acids can be selectively separated from pretreated fermentation feed solutions while other organics and many salts can be rejected using these processes by varying pH conditions in the feed. Finally, a low-energy-consuming forward osmosis process was applied for dewatering in the aqueous organic acid solutions to concentrate organic acid. The concentrated organic acid was successfully obtained by using conventional desalination and/or commercial forward osmosis membranes.