Enhancing water permeability and fouling resistance of polyvinylidene fluoride membranes with carboxylated nanodiamonds
Graphical abstract
Introduction
Polyvinylidene fluoride (PVDF) membranes has been extensively used for microfiltration and ultrafiltration due to their excellent thermal stability, mechanical strength and chemical resistance [1], [2]. Such membranes can reject contaminants with large molecule sizes, such as proteins, bacteria and natural organic matters, and thus widely used as the pre-treatment of reverse osmosis [3]. However, PVDF membranes still face some challenges, such as a tendency to severe membrane fouling and relatively low water permeability. The hydrophobic nature of PVDF makes the membranes susceptible to organic pollutants (e.g. proteins and polysaccharide), which causes undesirable permeation flux decline [1].
Considerable efforts, such as surface modification [4], [5], chemical grafting [6], [7], [8], [9], and blending organic [10], [11] or inorganic hydrophilic additives [12], [13], [14], have been made to improve the surface hydrophilicity, maximize the water flux and minimize fouling of PVDF membranes. Among these strategies, blending nanoparticles into the polymeric bulk to form organic/inorganic nanocomposite membranes has shown its effectiveness and popularity in improving membrane separation performance, particularly in enhancing antifouling performance of the membrane [15], [16]. Many nanomaterials, such as graphene oxide (GO) [17], [18], [19], carbon nanotubes (CNTs) [20], powdered activated carbon [21], Ag [22], Cu [23], TiO2 [24], [25], [26], SiO2 [27], Al2O3 [28], and Fe3O4 [29] have been incorporated into various polymeric membranes. However, blending nanoparticles into polymeric membranes face the major problem of aggregation. The inherent interactions among nanoparticles tend to cause non-uniform particle distribution and instability of the casting solution, resulting in imperfect structures and low membrane separation performance. Therefore, nanoparticles with excellent dispersion capacities and high specific surface areas are attractive for PVDF membrane modification.
Nanodiamonds (NDs) have sp3 carbon-carbon bond structures that can be attached by many functional groups, such as hydroxyl, carboxyl and ketones by surface modification. These functional groups are the distinct properties of functionalized NDs compared with other carbon based nanoparticles (e.g. CNTs and GO), offering them great potential to minimize aggregation [30]. Importantly, NDs are not expensive and are biocompatible and non-toxic [30]. Additionally, NDs has excellent mechanical and chemical stabilities, high surface areas and tunable surface structures [30], [31]. Most recently, the mechanical, thermal, antibacterial properties of NDs were demonstrated in cellulose acetate membranes [32], [33]. However, there are still very limited studies on functionalized NDs in membrane development, particularly their effects on the separation performance and antifouling behaviors of hydrophobic membranes. Surface functionalization of NDs can bring massive oxygen-containing functional groups, high specific surface area and excellent dispersity, which may help solve the problems of particle aggregation and membrane fouling when modified NDs are incorporated into membranes.
In this study, we first created hydrophilic functional carboxyl groups on the surfaces of NDs by simple thermal treatment, which significantly increase the dispersity of the NDs. The carboxylated NDs (CNDs) were incorporated into PVDF membranes during phase inversion. The pure PVDF and NDs incorporated membranes were used as controls for comparison. The prepared membranes were characterized by scanning electron microscope (SEM), atomic force microscopy (AFM) and water contact angle analysis. The bulk porosities and pore sizes of the membranes were evaluated by the gravimetric method and the filtration method, respectively. The membrane surface pore sizes were evaluated from SEM images using ImageJ software. The filtration performance and antifouling behaviors of the membranes were also investigated.
Section snippets
Materials
PVDF polymer (FR904) was supplied by Dongguan plastic Co. Ltd. China. The ND nanoparticles (40–50 nm, purity > 99.5%) were produced by the non-detonation (milling) process and purchased from Tong Li Technology Co., Ltd., China. N,N-dimethylacetamide (DMAc) was purchased from Aladdin Co. China. Bovine serum albumin (BSA, Mw = 67,000, purity > 96%) was supplied by Genview Co. USA. All chemicals in the experiments were analytical grade.
Preparation of carboxylated nanodiamonds
Carboxylated nanodiamonds (CNDs) were prepared via thermal
Characterization of nanodiamond nanoparticles
XRD results (Fig. 1) show that both NDs and CNDs have the same diffraction peaks at 43.8° and 75.2°, corresponding to the (111) and (220) planes of the diamonds, respectively [35], confirming the cubic shape of the crystalline phase in the nanodiamond particles. Compared with the NDs, the CNDs had weaker peaks at 43.8° and 75.2°. This phenomenon was mainly caused by the reduced size of the crystallite after thermal oxidation, likely leading to increased surface areas, stabilities and dispersion
Conclusion
Nanodiamonds were carboxylated by thermal oxidation and showed significantly improved dispersion properties (i.e. reduced aggregation), which would help solve the problem of aggregation in developing nanoparticle blended membranes. CNDs were incorporated into PVDF membranes to improve the membrane performance. Compared with the pristine PVDF membrane, CND blended membranes had lower water contact angles larger porosities and surface pore sizes, leading to improved membrane permeabilities.
Acknowledgements
This research is financially supported by the National Natural Science Foundation of China (grant numbers: U1701243 and 51572089), and Research Project of Guangdong Provincial Department of Science and Technology (Grant numbers: 2016B020240002 and 2015B020215004).
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