Surface modification of thin film nanocomposite membranes using plasma enhanced chemical vapor deposition method for desalination process

Abstract

Thin film composite reverse osmosis (TFC RO) membrane has been commercially used to desalinate salty water since the 1970s to produce clean water to address water scarcity issue in many countries. However, the commercial TFC RO membranes are still associated with several drawbacks including permeability/selectivity trade-off and scaling problem. Thin film nanocomposite (TFN) membrane incorporating titania nanotube (TNT) as previously reported, was found to offer outstanding features as the incorporation of TNT in polyamide (PA) layer could improve membrane pure water flux (PWF) and salt separation performance. However, TNT tends to have high aggregation ability and low dispersion in the organic solvent, which reduces its practicability for TFN membrane development. Therefore, this study aimed to develop a new type of TFN membrane incorporated with TNT functionalized using an environmentally friendly plasma-enhanced chemical vapour deposition (PECVD) method. The surface of TNT was respectively functionalized with 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) at different plasma deposition times (5 and 10 min). Results showed that the incorporation of 0.05 w/v% MMA-modified TNT (5 min) into the membrane outperformed the HEMA-modified TNT (5 min) with respect to PWF and NaCl rejection, achieving 52.5 L/m2.h (measured at 15 bar) and 97.6% (tested at 2000 ppm NaCl feed solution at 15 bar), respectively. This is due to the even distribution of MMA-modified TNT throughout the PA layer which increased the membrane water affinity. The TFN membrane incorporated with MMA-modified TNT was further coated using hydrophilic acrylic acid (AA) via PECVD method as a strategy to heal the surface defects of the selective layer caused by TNT incorporation. The NaCl passage was observed to reduce from 2.43% to 1.50% (tested at 2000 ppm NaCl feed solution at 15 bar) without significantly altering PWF. This membrane also exhibited extraordinary anti-scaling performance with a higher flux recovery rate (FRR) (>85%) compared to the unmodified TFC membrane (74.8%), which is mainly attributed to its enhanced surface negative charge and improved hydrophilicity. Likewise, the developed AA-modified TFN membrane could effectively mitigate the silica scaling by achieving higher FRR (88.1%) than the unmodified membranes. In conclusion, this work demonstrated the potential of using PECVD method to rapidly modify not only the surface properties of nanomaterials but also the top PA selective layer of TFN membranes, overcoming the drawbacks associated with the TFC membrane and improving the TFN membrane for enhanced salt rejection and anti-scaling performance without trading-off its water permeability.