Self-Assembly in "Matrix-Free" Functionalized Boron Nitride Sheets as Free-Standing Thin Film Sieves for Stable Forward Osmosis and Robust Dye Removal Applications

Abstract

Herein, a facile "matrix-free" thin film membrane strategy is reported that exploits polymer brushes grafted hydroxyl-functionalized hexagonal boron nitride nanosheets (h-BNOH) toward forward osmosis desalination and dye rejection application. The functionalized nanosheets are characterized using Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Nuclear magnetic resonance (NMR), UV–visible spectrophotometry, and thermogravimetric analysis (TGA). Covalently functionalized nanosheets of poly methyl methacrylate (PMMA) termed as boron nitride grafted PMMA (BN-g-PMMA) obtained via RAFT polymerization are deployed toward "matrix-free" nanocomposite membranes. The membranes exhibit ≈ 5260 L/m⁻² h⁻¹ at a moderate pressure of 0.21 MPa in cross-flow mode and a 70% of flux retention ratio against bovine serum albumin as protein foulant. The matrix-free membranes exhibit ≈ 93% salt rejection for NaCl and > 99% rejection for MgCl₂ and Ca(NO₃)₂·4H₂O for 2000 ppm draw solution (water TDS benchmark for domestic purifiers) while 90% NaCl, and > 96% rejection for MgCl₂ and Ca(NO₃)₂·4H₂O for 5000 ppm draw. In simulated domestic water salinity, 97.8% rejection is observed for 2000 ppm and 94.5% for 5000 ppm draw solution, respectively. The matrix-free membranes also achieve almost 100% dye rejection for eight model dye pollutants at 1 ppm and > 96.6% for 10 ppm concentrations, respectively.

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