Fabrication of a novel and green thin-film composite membrane containing nanovoids for water purification
Graphical abstract
Introduction
Thin-film nanocomposite (TFN) membranes, which incorporate nanomaterials in a salt-rejecting polyamide (PA) layer, have attracted tremendous attention in the past decade [1], [2], [3]. Compared to thin-film composite (TFC) membranes, TFN can dramatically improve membrane water permeability (e.g., up to 200% enhancement), while maintaining similar salt rejection [1]. Notably, the embedded nanomaterials with micro or mesopores, such as zeolite [4], [5], mesoporous silica [6], [7], metal-organic-framework (MOF) [8] and carbon nanotubes [9] could further enhance membrane water permeability compared to their solid counterparts. The higher water flux can be ascribed to the preferential water pathway in the nanovoids of these porous nanomaterials [6]. Despite the enhanced membrane separation performance, conventional TFN membranes still have some obstacles, such as potential leaching [10] and nanomaterials toxicity [11].
The improved porosity of polyamide rejection layer can significantly improve membrane separation performance [12], [13]. As an alternative approach to conventional TFN, Livingston and co-workers [14] applied microporous monomers for interfacial polymerization, resulting in the formation of a microporous cross-linked rejection layer that showed excellent permeability and selectivity compared to commercial membranes. In a more recent study [13], we reported series of approaches to generate nanovoids in the polyamide rejection layer by the formation of nanosized gas bubbles (e.g., by the addition of bicarbonate or ultrasound application) during the interfacial polymerization. These nanovoids significantly increased both membrane water permeability and salt rejection. Several studies also highlighted the importance of the microporous structure of membrane rejection layer [15], [16], [17], [18], [19], [20].
In this study, we report a facile method to fabricate novel high performance TFC membranes, where nanovoids were generated through the etching of nanoparticles incorporated in the PA-Polysulfone(PSF) interface. Specifically, we first synthesized copper nanoparticles (CuNPs) loaded TFN membranes, followed by their removal using an HNO3 solution to create an “arch-like” polyamide morphology. We hypothesize that these nanovoids could significantly decrease the membrane hydraulic resistance, thus greatly enhancing water permeability. This work provides a novel insight into the fabrication of a high performance nanovoids-enhanced TFC membrane.
Section snippets
Materials and reagents
Chemicals for the preparation of membrane substrates and interfacial polymerization, including polysulfone (PSF, Mw 35,000), N,N-dimethylformamide (DMF, anhydrous 99.8%), m-Phenylenediamine (MPD, flakes, 99%) and hexane (HPLC grade, 95%) were all obtained from Sigma-Aldrich. Trimesoyl chloride (TMC, 99%) was purchased from J&K Scientific Ltd. Cupric sulfate-anhydrous (CuSO4, AR, Dieckmann, Hong Kong) and sodium borohydride (NaBH4, 98%, Sigma-Aldrich) were used to generate CuNPs in situ on
Membrane characterization
Fig. 2 presents the SEM micrographs (plan view) of the control (PSF) and CuSO4/NaBH4 modified polysulfone (PSF-Cu) substrates. The control polysulfone substrate had a relatively flat surface with a root-mean-square (Rq) roughness of 9.3 ± 1.8 nm. With the increased concentration of the applied CuSO4 and NaBH4 solution, the surface roughness of the modified substrate increased up to 14.9 ± 3.4 nm (Supplementary material, Fig. A1). Elemental analysis of EDS (inserts of Fig. 2) further confirmed
Acknowledgement
This study receives financial support from the Seed Funding for Strategic Interdisciplinary Research Scheme, the University of Hong Kong.
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