Synergetic effects of oxidized carbon nanotubes and graphene oxide on fouling control and anti-fouling mechanism of polyvinylidene fluoride ultrafiltration membranes
Introduction
Despite its high hydrophobicity, poly (vinylidene fluoride) (PVDF) is still a popular membrane material due to its good chemical resistance, thermal stability and mechanical properties [1], [2], [3]. Nevertheless, PVDF membrane fouling results in substantial flux decline that necessitates frequent membrane cleaning and replacement. Consequently, efficient application of membrane technology in wastewater reclamation is significantly hampered by the phenomenon of organic fouling as wastewater effluent contains a considerable amount of organic substances. In recent years, in order to further reduce the susceptibility of PVDF membranes to biofouling or to enhance their anti-fouling performance, various methods to increase their surface hydrophilicity have been described, such as coating [4], adsorption [5], surface graft polymerization [6] and chemical modification of pristine PVDF [7]. Besides, blending modification is another practical method without any pre-treatment or post-treatment procedures [8].
In recent years, several authors have shown the successful preparation of carbon nanotubes blended polymeric composite membranes and they have mainly studied the effects of carbon nanotubes on the performance of membranes [9], [10]. In addition, former reports (including our previous work), have simply investigated the effects of graphene derivatives on the performance of composite membranes [11], [12], [13]. And results indicated that introducing low-dimensional carbon nanomaterials in the membrane matrix can improve the hydrophilicity, water permeability and the anti-fouling performance of polymer based nanocomposite membranes. However, like carbon nanotubes the problem of dispersion is strongly present for graphene derivatives as well due to strong Vander Waals forces and inter-planer stacking [14], [15], [16]. Consequently, the poor dispersion of carbon nanotubes and graphene in polymeric matrices may limit the extent of realizing potential improvements of composite membranes and the performance of low-dimensional nanomaterials-based composite membranes is hampered by the aggregation and stacking of either carbon nanotubes or graphene. It is known that by bringing together two nanofillers like carbon nanotubes and graphene derivatives they form a co-supporting network of both fillers like a hybrid net structure in which the platelet geometry shields the tube fillers from fracture and damage during processing whilst still allowing full dispersion of both during high-power sonication, thus causing improved properties [17]. Hence, we can expect that integrating one-dimensional oxidized carbon nanotubes (OMWCNTs) and two-dimensional graphene oxide (GO) resulted in a strong synergistic effect between the two materials, consequently leading to a superior ultrafiltration membrane with higher anti-fouling performance compared with the membranes modified by either GO or OMWCNTs. Furthermore, as is known in the art, the synergetic effect of OMWCNTs and GO on the membrane−foulant adhesion forces and membrane-fouling behavior as well as on the anti-fouling mechanism of PVDF ultrafiltration membranes has not yet to be systematically studied. And it is an open question to completely understand the synergistic effect brought about by the nanomaterial mixture of different ratios.
Based on these considerations and the body of previous research, the objective of this work is to synthesize composite membranes of different nanofiller ratios using a non-solvent induced phase separation method and determine the synergistic effects on the fouling control and anti-fouling mechanism of two highly-potent nanomaterials in the matrix. These results offer a novel yet simple and effective way of designing composite ultrafiltration membranes with extraordinary performance by incorporating two different low-dimensional carbon nanomaterials neither one of which alone might be essentially perfect for the required applications.
Section snippets
Materials
The PVDF (FR904) was purchased from Shanghai 3F New Materials Co. Ltd. China. N, N-dimethylacetamide (DMAc, >99.5%, reagent) and polyvinyl pyrrolidone (PVP) were purchased from Tianjin Weichen Chemical Reagent Co. Ltd. China. Multi-walled carbon nanotubes (MWCNTs, with diameters of 10–50 nm and length of 1–30 μm) were obtained from Nanjing XF Nanomaterial Science and Technology Co. Ltd. The purity of received MWCNTs is 95%. Oxidized carbon nanotubes (OMWCNTs) were synthesized by the previous
UV–vis spectra and zeta potential of OMWCNTs, GO and OWMCNTs/GO
UV–vis spectra have been used for the characterization of the stabilization and dispersion of carbon nanomaterial suspension [23], [24]. Fig. 1 presents the UV–vis spectra of GO suspensions, OMWCNTs suspensions and the mixture suspensions of GO and OMWCNTs. The peak in the UV–vis spectrum of GO in the region of 227–231 nm determines the degree of remaining conjugation (π–π⁎ transition) [16], [19]. The shoulder around 300 nm can be ascribed to the n–π⁎ transition of carbonyl groups [19]. While
Conclusions
In conclusion, we have demonstrated the feasibility of designing a new generation of composite ultrafiltration membranes for investigating fouling control and anti-fouling mechanism based on the synergistic behavior of GO/OMWCNTs nanofiller. Major findings from this study are as follows:
- (1)
UV–vis spectra and zeta potential validated that the mixture of GO/OMWCNTs showed better dispersion than either of that, which was beneficial to the permeability and anti-fouling performance of membranes.
- (2)
The
Acknowledgments
The work was funded by the National Natural Science Foundation of China (11175130), Natural Science Foundation of Tianjin, China (10JCYBJC02300), China Postdoctoral Science Foundation (2012M520578) and the Jiangsu Planned Projects for Postdoctoral Research Funds (1202067C).
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