Modifying cellulose nanocrystal dispersibility to address the permeability/selectivity trade-off of thin-film nanocomposite reverse osmosis membranes

Abstract

Thin-film nanocomposite reverse osmosis (TFN-RO) membranes with hydrophilic nanoparticles usually face a trade-off between water permeance and salt rejection. In this study, TFN membranes were produced by embedding different loadings of cellulose nanocrystals (CNCs) into the polyamide (PA) active layer by dispersing the CNCs in an organic monomer solution as opposed to the traditional approach where hydrophilic nanoparticles are added to the aqueous monomer solution. To facilitate the addition of the hydrophilic CNCs to the organic monomer solution, the CNCs were acetylated to form ACNCs with different degrees of substitution (DS = 0.7 and 1.7) of the hydroxyl groups. The successful incorporation of the CNCs and ACNCs into TFN-RO membranes was confirmed with ATR-FTIR spectroscopy, surface zeta potential, scanning electron microscopy, atomic force microscopy and water contact angle measurements. The NaCl rejection of ACNC-based TFN membranes remained stable (98–99 %) while water permeance increased by 40 % in comparison to CNC-TFN membranes. Compared to a reference thin-film composite (TFC) membrane, a two-fold water permeance increase was achieved (2.60 vs. 1.27 L/m2.h.bar) with a 0.4 wt% loading of ACNC2 in the TFN membrane. The ACNC-TFN membranes were significantly closer to the so-called upper-bound line than the best CNC-TFN membranes and the reference TFC membrane. This study shows the potential of using hydrophilic green nanoparticles in overcoming the permeance-selectivity trade-off of TFN-RO membranes. Hydrophobic functionalization of hydrophilic nanoparticles was demonstrated to be an effective approach in overcoming the limit of CNC nanoparticle incorporation in CNC-TFN membranes.