High-Performance Boron Nitride-Based Membranes for Water Purification

Abstract

In recent years, nanotechnology-based approaches have resulted in the development of new alternative sustainable technologies for water purification. Two-dimensional (2D) nanomaterials are an emerging class of materials for nanofiltration membranes. In this work, we report the production, characterisation and testing of a promising nanofiltration membrane made from water-exfoliated boron nitride (BN) 2D nanosheets. The membranes have been tested for water purification and removal of typical water-soluble dyes such as methyl orange, methylene blue and Evans blue, with the water-exfoliated BN membranes achieving retention values close to 100%. In addition, we compared the performance of membranes made from water-exfoliated BN with those produced from BN using sonication-assisted liquid exfoliation in selected organic solvents such as 2-propanol and N-methyl-2-pyrrolidone. It was found that membranes from the water-exfoliated BN showed superior performance. We believe this research opens up a unique opportunity for the development of new high-performance environmentally friendly membranes for nanofiltration and new sustainable separation technologies.

Keywords: 2D nanomaterials; membranes; nanofiltration; separation technologies; sustainable; water purification.

Figure 1

(A) UV-Vis spectra of exfoliated BN in NMP (red), IPA (blue) and water (green); (B) photograph of original solutions of BN exfoliated in the three solvents (3 mg/mL); (C) Tyndall effect on BN exfoliated in IPA, NMP and water (0.03 mg/mL).

Figure 2

SEM images of exfoliated BN in (A) NMP, (B) IPA and (C) Millipore water where the nanosheets are present.

Figure 3

TEM (A,B,D,E,G,H) and STEM (C,F,I) images of 2D BN nanoflakes produced in NMP (A–C), IPA (D–F) and water (G–I).

Figure 4

(A) Normalised PXRD patterns of bulk BN and exfoliated BN-NMP, BN-IPA and BN-Water. Miller indices are shown for the bulk material. (B) Comparison of (002), (010) and (011). (C) Raman spectra of h-BN in bulk form (black) and exfoliated BN-NMP (red), BN-IPA (blue) and BN-Water (green).

Figure 5

AFM images of (A) single BN flakes obtained from water exfoliation, with corresponding line profile; (B) BN flakes from NMP exfoliation, with corresponding line profile. Three-dimensional AFM images of single BN flakes obtained from (C) water exfoliation and (D) NMP exfoliation (images taken using 3× magnification on z-axis).

Figure 6

Schematic representation of BN membrane formation using vacuum filtration.

Figure 7

SEM of BN membranes with BN-NMP-Mem top-down view (A) and cross-sections (B,C); BN-IPA-Mem top-down view (D) and cross-sections (E,F); BN-Water-Mem top-down view (G) and cross-sections (H,I).

Figure 8

Comparison of mercury intrusion characteristics of the BN membrane samples (A,B) comparison of mercury pore size distributions of the BN membrane samples.

Figure 9

UV-Vis spectra showing the average retention of (A) 20 mL of Evans blue (15 µM) through BN membranes made from exfoliation in NMP, IPA and Millipore water. (B) Methyl orange (50 µm) and methylene blue (27 µm) through BN membranes exfoliated in Millipore water.