Surface modification of UF membranes with functionalized MWCNTs to control membrane fouling by NOM fractions
Graphical abstract
Introduction
Ultrafiltration (UF) has been increasingly used in water purification because of its broad range of contaminant-removal capability and relatively low cost compared with nanofiltration and reverse osmosis [1]. However, fouling-related problems, including permeate flux reduction, diminished selectivity, diminished lifespan, and frequent chemical cleaning, pose a serious threat to the application and popularization of this membrane separation technology [2], [3]. One common method of controlling membrane fouling is to increase the hydrophilicity of the membrane surface, which has been verified to positively affect the antifouling property and permeability recovery of membranes [4], [5], [6]. The widely adopted methods for modifying membranes include bulk modification, surface modification and blending, and these methods share a common approach, i.e., localizing hydrophilic materials into/on the hydrophobic membranes. The goal of membrane modification is to achieve a suitable compromise between hydrophobicity and hydrophilicity, which are associated with low water solubility and high resistance to fouling, respectively [7].
Among various materials used for modification, MWCNTs have a unique, one-dimensional tubular structure and extremely high surface area-to-volume ratios [8] making them attractive candidates in membrane modification to control fouling and improve contaminant removal. Noticeable changes in the characteristics of the membrane surface, such as its hydrophilicity and roughness, were observed due to the existence of functionalized MWCNTs on the membrane [9], [10], [11]. Yin et al. [12] used oxidized MWCNTs as fillers at concentrations ranging from 0 to 1 wt% to improve polysulfone hollow-fiber membranes. Their results suggested that the pure water flux of membranes initially increased and then gradually decreased with increasing nanotube concentration. Zhang et al. [3] investigated the modification of polyvinylidene composite membranes with graphene oxide and oxidized MWCNTs; they observed that the modified membrane had less fouling potential because of the stronger affinity of oxidized low-dimensional carbon nanomaterials to water. Vatanpour et al. [13] demonstrated that the embedment of acid-oxidized MWCNTs into PES nanofiltration membranes could improve their flux and salt rejection. However, although the membrane surface properties can be improved by modification, two issues should not be neglected. One issue is that the mixed matrix membrane still has direct contact with contaminants during the filtration process, leading to unavoidable fouling. The other issue is related to the probable release of MWCNTs from the composite membrane, which would raise health and safety concerns [14], [15], [16].
To solve the aforementioned issues of membrane fouling and MWCNTs leakage, UF membrane modification by surface coating is an attractive alternative. Membrane fouling can be controlled to a maximum extent because membrane-contaminant contact is prevented by the coating layer. Furthermore, MWCNTs can be completely intercepted by the UF membrane because of the nanoscale size of the membrane pores. Modified membranes can be used without leaking unwanted MWCNTs into the permeate. Ajmani et al. [17] modified low-pressure, flat-sheet membranes by coating them with CNTs with different physiochemical properties; they observed that membranes coated with larger-sized CNTs exhibited a greater macromolecule removal capacity and greater resistance to fouling. In addition, CNT mats were generated on the inner surface of hollow fiber membranes; the modified membranes performed better and exhibited greater sustainability in the filtration process [18].
However, previous studies on membranes coated with MWCNTs have usually been performed from the perspective of materials science, with emphasis placed on the physicochemical and morphological characteristics of MWCNTs, functionalized MWCNTs and modified membranes. With respect to water treatment, only some simple indexes, such as clean water flux and a TMP increase, have been adopted. Insufficient attention has been devoted to the complexity of the organic matrix in natural water and to the complicated relationships among the functionalized MWCNTs’ properties, the characteristics of the modified membranes and the reversibility of membrane fouling. As a result, a gap exists between membrane modification research and the actual application of modified membranes.
This study focused on the following: (1) analyzing the physicochemical properties of raw and functionalized MWCNTs before coating them onto membranes; (2) characterizing the surface morphologies and pure-water permeability of membranes modified with raw MWCNTs, MWCNTs-COOH, and MWCNTs-PEG; (3) evaluating the effects of the modified membranes on fouling control using three NOM foulant models, HA, BSA and SA; and (4) assessing the stability of the modified membranes during pure-water backwashing as well as after NOM filtration.
Section snippets
Materials
Commercial MWCNTs (95% purity) were purchased from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Science (Chengdu, China). According to the manufacturer’s specifications, the MWCNTs had an average outer diameter of 8 nm and lengths ranging from 10 to 30 μm. PES membranes with a nominal molecular weight cutoff (MWCO) of 100 kDa were acquired from Shanghai Mosu Science Equipment Co., Ltd., (Shanghai, China). The diameter of the flat-sheet membranes was 76 mm, corresponding to a filtration
Characterization of the functionalized MWCNTs
TEM images of the raw and functionalized MWCNTs are shown in Fig. 2. As shown in Fig. 2a, the dispersion of raw MWCNTs was not uniform; significant MWCNT agglomerations consisting of an interwoven matrix of long, intact tubes with predominantly closed ends were observed. However, the morphology of the raw MWCNTs changed considerably after the acid treatment (shown in Fig. 2b). The dispersion of MWCNTs-COOH was greatly improved, and the nanotubes were characterized by the presence of short tubes
Conclusions
In conclusion, modified membranes with raw and functionalized MWCNTs coating their surface were prepared and characterized, and the fouling potentials of the three NOM fractions as well as the leakage of the MWCNTs were evaluated and discussed. The following conclusions were drawn:
- (1)
Hydrophilic functional groups were successfully introduced in MWCNTs-COOH and MWCNTs-PEG. Functionalized MWCNTs (MWCNTs-COOH and MWCNTs-PEG) were endowed with better dispersion abilities and stronger negative charges
Acknowledgment
This study was jointly supported by the National Natural Science Foundation of China (51308146), Program for New Century Excellent Talents in University (NCET-13–0169), Open project of State Key Laboratory of Urban Water Resource and Environment (ES201511-02) and Heilongjiang Postdoctoral Fund (Grant LBH-Z13083). Anonymous referees are greatly acknowledged for their constructive and helpful suggestions.
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