Elsevier

Journal of Membrane Science

Volume 551, 1 April 2018, Pages 172-179
Journal of Membrane Science

High-performance, recyclable ultrafiltration membranes from P4VP-assisted dispersion of flame-resistive boron nitride nanotubes

https://doi.org/10.1016/j.memsci.2018.01.030Get rights and content

Highlights

  • First demonstration of regenerable ultrafiltration membrane using flame-resistive nanotubes.

  • Simple fabrication of ultrafiltration membranes having high filtration performance and stability.

  • First demonstration of highly stable boron nitride nanotube (BNNT) dispersion using a cheap polymer.

  • Simulation-supported mechanism study on the BNNT stabilization.

Abstract

Regenerable ultrafiltration membranes were fabricated via filtration of thermally-stable, highly dispersed boron nitride nanotubes (BNNTs). The highly debundled BNNTs were produced by employing judiciously-chosen poly(4-vinylpyridine) (P4VP) as an efficient steric stabilizer. Density functional theory calculations showed strong adsorption energies of P4VP monomers on top of the BNNTs, illustrating the role of P4VP stabilizers. High performance of the BNNT ultrafiltration membranes with large permeation flux was demonstrated by exclusion of polystyrene and gold nanoparticles (~ 25 nm) with higher than 99% removal efficiency. The BNNT membranes were successfully regenerated and recycled continuously at 450 °C due to their excellent mechanical and thermal properties, demonstrating a stark contrast to the membranes from the carbon nanotubes. Our work indicates that unlike the membranes from carbon nanotubes, BNNT membrane can be a promising candidate for the application in cost efficient industrial ultrafiltration.

Graphical abstract

Recyclable ultrafiltration membranes having high water flux and excellent size exclusion performance were fabricated from P4VP-assisted dispersion of boron nitride nanotubes (BNNTs). The concept of thermally durable and reusable BNNT membrane suggests great potential to resolve important problems such as clogging (or fouling) and corresponding increase in energy consumption, which are common in water purification and ultrafiltration applications.

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Introduction

Ultrafiltration using membranes with pore diameters of 10–100 nm has been an emerging technology in desalination of drinking water and food processing, owing to its high selectivity and low operation cost [1], [2], [3], [4], [5]. However, the ultrafiltration membranes are vulnerable to membrane fouling by excluded particulates and micro-organisms, resulting in loss of permeation flux and corresponding increase in energy demand [6], [7], [8]. Therefore, cleaning and recycling these membranes [9], [10], [11], [12], [13], [14] are critical issues that need to be addressed prior to their practical usage including water purification. Often, calcination at high temperature (i.e. 400–500 °C) can be an effective way to regenerate the membranes by removing all of the filtered organic impurities and, as such, the thermal stability of the membranes is a critical requirement for their successful long-term usage [15].

Most ultrafiltration membranes are fabricated with polymers such as polyacrylonitrile, polysulfone, cellulose acetate, and regenerated cellulose due to simple processing and low cost in achieving nanometer-sized pores [16]. However, the polymeric membranes are mechanically, chemically, and thermally unstable during membrane regeneration involving either acid/base cleaning or calcination process to remove any of filtered organic matters. Inorganic membranes mainly composed of metal oxides (including anodic aluminum oxide (AAO)) can be used alternatively [17], [18], [19], but there are other detrimental issues such as high production cost and dissolution/degradation of metal oxide membranes during operation [20]. For all these reasons, the development of a simple protocol to fabricate ultrafiltration membranes with high mechanical strength, chemical stability, and thermal durability against calcination remains a key challenge.

Recently, boron nitride nanotube (BNNT) has attracted a growing amount of attention as an inorganic analogue of carbon nanotube (CNT) [21] with its excellent properties such as high mechanical strength (elastic modulus ~ 1.18 TPa) and chemical inertness against strong acid/base. More importantly, unlike the CNTs, the BNNTs have high temperature tolerance up to 900 °C in air, [22], [23] which makes them perfectly suitable for the fabrication of reusable membranes by calcination. Despite these advantages, the use of BNNTs has been limited because they are typically synthesized into the form of aggregates bundled by van der Waals attractions among the sidewalls, which hinder their solubilization in most solvents [24]. Therefore, for potential applications including membranes, the BNNTs have to be debundled and dispersed individually in solvents [25], [26], [27]. To improve the dispersion stability of BNNTs in solvents, significant efforts have been exerted on the functionalization of BNNTs by covalent attachment using chemical reactions such as harsh acid oxidation, physico-chemical bombardment of ammonia plasma, and hydroxylation with peroxides [28], [29], [30], [31], [32], [33], [34], [35]. However, such processes under harsh conditions often damage the outside wall and degrade the intrinsic properties of BNNTs. By contrast, non-covalent functionalization approaches using physical adsorption or wrapping of stabilizing species on BNNTs provide simple and environmentally friendly protocols for stable dispersion of BNNTs without damaging the BNNTs [27], [36], [37], [38], [39]. Nonetheless, previously reported approaches use expensive conjugated (or bio-) polymers and have limited number of available solvents for dispersing the BNNTs [40], [41], [42], [43], [44].

Here, we demonstrate a powerful one-step method to prepare highly robust and efficient ultrafiltration membranes using a sequential process of filtration and re-assembly of highly debundled BNNTs. The membranes are mechanically strong and highly robust against chemical/thermal oxidation and, thus, readily regenerable and recyclable by calcination. A stable dispersion of BNNTs in an organic solvent is prepared in the presence of polymeric stabilizer poly(4-vinylpyridine) (P4VP) (Scheme 1, figures on the top). The favorable adsorption of P4VP chains on BNNTs was experimentally investigated and also supported by density functional theory (DFT). The excellent long-term stability of P4VP-assisted BNNT dispersion was demonstrated by UV–vis spectroscopy and Turbiscan analysis. The thermally regenerable ultrafiltration membranes were fabricated using simple filtration of BNNT dispersion on glass-fiber prefilter (Scheme 1, figures on the bottom). Interestingly, the thin BNNT membrane with only about 2.6 mg BNNT per centimeter square was enough to exclude over 99% of polymer particles and inorganic Au nanoparticles with the size of about 25 nm in diameter. Most importantly, we demonstrate that the used BNNT membrane could be regenerated and recycled continuously using a simple calcination process, attributed to the excellent thermal inertness and mechanical properties of the BNNTs. In contrast to the BNNT membranes, the control membranes consisting of CNTs could not be recycled due to their thermal instability.

Section snippets

Results and discussion

Steric stabilization by polymer ligands with favorable adsorption on particles provides a simple pathway to disperse the particles without involving any chemical reactions that often damage the particles [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55]. We chose P4VP as dispersing agents for BNNTs in solvents (i.e., alcohols) because the pyridine group in P4VP is a well-known Lewis base and, thus, P4VPs strongly interact with outer surface of BNNTs. Fig. 1 shows the superior

Conclusion

In this work, stable dispersions of BNNTs in organic solvents were prepared via P4VP-assisted steric stabilization of BNNTs. The excellent efficiency of P4VP in dispersing BNNTs with long term stability was clearly shown by both theoretical calculations and systematic experiments. The highly individualized BNNTs by P4VP were employed to fabricate an ultrafiltration membrane having nanopores by simple vacuum filtration of the dispersion. The membrane had high permeation flux with excellent size

Purification of BNNTs and preparation of BNNT dispersions

As-received BNNTs (BNNT LLC.) were calcined at 650 °C for 6 h in air to oxidize amorphous boron impurity (α-boron). The oxidized α-borons were washed out by a three times of bath sonication and filtration cycle with methanol. During the filtration process, additional attention was paid to avoid complete drying of the BNNTs as a form of bucky paper because it was much harder to re-disperse the completely dried BNNT mats compared to as-received BNNT cocoons. Then, the washed BNNTs were freeze dried

Acknowledgements

This work was supported by grants from Korea Institute of Science and Technology open research program (KIST ORP), Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905619), and the Graphene Materials/Components Development Project (10044366) through the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea.

Author contributions

B.J.K., J.K. and S.G.J. planned and designed the experiments and prepared the manuscript. M.J.K. provided purified BNNTs. H.Y. provided scientific discussion on colloidal dispersion and revised the manuscript. H.L. prepared BNNT dispersion and carried out characterization. H.L. and S.G.J. prepared BNNT membranes and characterized the filtration performance. B.L.S. performed theoretical calculation. The corresponding authors J.K., B.J.K., and S.G.J supervised this work. All authors discussed the

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