Improving the performance of polyamide reverse osmosis membrane by incorporation of modified multi-walled carbon nanotubes
Graphical Abstract
Modified multi-walled carbon nanotubes have been incorporated into polyamide reverse osmosis membrane to improve the separation performance.
Introduction
Reverse osmosis (RO) processes are economical for the production of potable water from brackish water or seawater. These processes have several advantages over traditional production methods, such as high permeation flux, ease of operation, minimal chemical addition, and low energy requirement [1], [2]. Currently, over half of the 15,000 desalination plants worldwide are using RO processes, and the growth of RO desalination capacity is expected to continue [3], [4], [5].
Although many RO plants have been established by now, research effort to increase the membrane flux is still needed. Based on this demand, the present major researches focus on developing and fabricating high-flux membranes. Several reviews covering different aspects of thin film composite (TFC) RO membranes have been published previously [6], [7], [8], [9]. Most of these researches concentrated on either the optimization of membrane fabrication processes or the exploitation of new monomers and additives. For example, an aqueous solution composed of poly(vinyl alcohol) and a buffer solution was used in post-treatment during the preparation of TFC membranes to improve their physical properties (e.g., improved resistance to abrasion) and to enhance flux stability [10], [11]. Additionally, Zhou et al. [12] fabricated RO membranes with excellent performance (both flux and rejection) using newly synthesized functional monomers (e.g., m-phenylenediamine-5-sulfonic acid and 5-chloroformloxy-isophthaloyl chloride). In our previous work [13], phase transfer catalysts, alcohols, and phenols were added to metaphenylene diamine (MPD) solution during membrane formation. The presence of these additives changed the micro-structure of the separation layer, and the performance of the resultant composite membrane can be improved to some extent. However, empirical evidence suggested that increasing the water permeability of these RO membranes further without sacrificing selectivity is quite challenging [14].
Even enhanced water flux is preferable in membrane application, however, fouling is a more severe issue needed to be addressed. Lots of strategies have been proposed to develop antifouling RO membranes in recent years, including the selection of new monomers, improvement of fabrication process, surface modification of RO membrane by physical and chemical methods as well as the hybrid organic/inorganic RO membrane [15].
With the advancement of nanotechnology nowadays, porous nano-materials may be produced as a new class of additives, especially zeolite [16], [17], [18], [19], [20], [21] and carbon nanotubes [22], [23], [24], [25], [26], [27], and this is expected to provide a chance to further improve the performance of RO membranes. Mixed matrix membranes were developed to take advantage of the tempting properties of nano-materials [28], [29] and this method was successfully introduced to the area of RO membranes by Jeong et al. [21], who used zeolite nanoparticles in the preparation of mixed matrix membranes and managed to double the water flux of RO composite membranes without affecting the salt rejection. In addition, according to Kalra's simulation [23], due to the super-hydrophobic and tubal structure of carbon nanotubes, the water flux can be enhanced by orders of magnitude if the carbon nanotubes are incorporated into mixed matrix membranes [24]. Before long, these simulation predictions were tested in experiments by Holt et al. [24], who used a silicon nitride matrix, and Hinds et al. [25], who used a polymer matrix. Yu et al. [26] prepared vertically aligned CNT membranes and declared that the prepared membrane showed unparalleled gas permeability and could be used to separate larger molecules from smaller ones. However, these membranes are still not suitable to desalinate sea water because it is quite difficult to produce CNTs with a diameter smaller than that of a water molecule. Even so, aligned CNT membranes have been predicted to be the most promising so far for increasing the water permeability without sacrificing the rejection [1]. This motivated us to incorporate CNTs into the separation layer of polyamide composite membrane prepared by interfacial polymerization to shorten the water transport route. In our previous report [27], the acidifying MWNTs were incorporated in the polyamide layer of RO membranes with a slightly improved dispersion, resulting in a prominent increase in water flux but an accompanying decrease in salt rejection. Amini et al. [30] fabricated novel thin film nanocomposite (TFN) forward osmosis membranes using functionalized MWNTs, which have an obvious impact on the hydrophicity, morphology and forward RO performance of the polyamide/MWNTs membrane. Very recently, Dumée et al. [31] fabricated ultra-thin membranes on top of porous CNTs bucky-papers and showed a new approach for exploiting CNTs in membrane applications.
In this work, multi-walled carbon nanotubes (MWNTs), which were grafted with abundant long chains of aliphatic acid to greatly facilitate the compatibility of MWNTs in the polyamide layer, were used to improve several properties of RO membranes. In view of the chemical inertness of the pristine MWNTs and the difficulty of dispersion, the MWNTs were chemically modified before being embedded into the membranes. The chemically modified MWNTs could be readily dispersed and could serve as a negatively charged one-dimensional molecular tunnel in the membranes. The separation performances of the membranes were investigated, and the improved antifouling and antioxidative properties of the membranes were demonstrated.
Section snippets
Materials
The pristine MWNTs (length 1–5 μm, diameter 20–40 nm and inner diameter 5 nm, purity 95%) were manufactured by Nanotech Port Co. Ltd, Shenzhen, China. Sulfuric acid, nitric acid, and n-hexane were purchased from Fisher Scientific, Canada. 1, 3-Phenylenediamine (MPD) and trimesoyl chloride (TMC) were obtained from Sigma-Aldrich Chemical Company, USA; sodium chloride (NaCl) was purchased from EMD Chemicals Incorporation, Germany. All these chemicals were of analytical grade and were used directly.
Characterization of functionalized MWNTs
The morphologies of pristine and modified MWNTs are shown in Fig. 3. The pristine MWNTs were present as bundles, whereas the carboxylic MWNTs were much shorter than the original ones because of ultrasonic treatment and modification. More importantly, compared to the smooth walls of crude MWNTs, the modified MWNTs had rougher walls and open ends. Raman spectroscopy was employed to identify the formation of defects on the MWNTs.
In Fig. 4a, the FTIR spectrum of pristine MWNTs shows weak sp2 C3H
Conclusions
Composite reverse osmosis membranes incorporating modified multi-walled carbon nanotubes were prepared by interfacial polymerization. Compared to bare polyamide composite membranes, the performance of prepared membranes was enhanced by incorporating MWNTs into the membrane. With an increase in the amount of MWNTs in the membrane, the water flux increased from 14.86 to 28.05 L m−2 h−1 while the salt rejection decreased slightly. The polyamide/MWNTs composite membranes showed better antifouling and
Acknowledgments
This work was supported by the National Science Foundation of China (21076176 and 21076190), the National Basic Research Program of China (2009CB623402), and the Scientific & Technological Innovation Team of Zhejiang Province (2009R50045), and Important National Science & Technology Specific Projects (2009ZX07424-001). The authors thank Xianshe Feng (Department of Chemical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada) for his kind help and also thank Ying Xu and Hua Wang
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