Polyethyleneimine interlayered thin film nanocomposite reverse osmosis membrane for improved water desalination

Abstract

Water desalination is the most effective strategy in dealing with global water crisis. However, the current thin film composite (TFC) reverse osmosis (RO) membranes which dominate the desalination process are still susceptible to permeability and selectivity trade-off, fouling and chlorine attack. These issues were resolved in this work by optimizing the synthesis conditions of interfacial polymerization (IP) technique for TFC membrane fabrication followed by adopting interlayer-assisted IP technique and graphene oxide (GO) incorporation for the fabrication of thin film nanocomposite (TFN) membrane. Polyethyleneimine (PEI) was used as interlayer for TFN membrane fabrication to avoid defects caused by GO incorporation in the polyamide (PA) layer. The effects of post IP rinsing on the TFC membrane were first investigated prior to the TFN membrane fabrication. It was found that the rinsing solution properties such as boiling point, surface tension and miscibility could affect the efficiency of unreacted monomers removal, altering the physicochemical properties of PA layer and yielding reproducible TFC membrane with higher water flux and least deteriorated salt rejection. Aqueous solution rinsing was found to be able to enhance membrane pure water flux (PWF) from 17.53 to 22.56 L/m2·h at 15 bar without significantly trading off its promising sodium chloride (NaCl) rejection (97.70%) when compared to the control membrane and organic solvent-rinsed membrane. For the TFN membrane, the presence of PEI interlayer was found to improve the distribution and orientation of GO in the PA layer which minimized the defects formed. Compared to the typical TFN membrane fabricated using conventional IP, the PEI-interlayered TFN membranes containing the same amount of GO (0.015 wt/v%) were found to exhibit a relatively thinner but rougher PA. As a result, almost all PEI-interlayered TFN membrane exhibited better desalination performances than the typical TFN membrane. It was also discovered that the substrate of membrane coated with a single layer of 0.05 wt/v% PEI followed by 60-min drying produced promising TFN membrane (i.e., iTFN-C0.05-T60-L1), achieving 96.66% NaCl rejection and 2.24 L/m2·h·bar PWF. The experimental results also revealed that the use of optimum GO loading (0.01 wt/v% GO) in the PA layer fabricated via interlayer-assisted IP could further improve TFN membrane performance, leading to the highest PWF (2.66 L/m2·h·bar) achieved without compromising NaCl rejection (~97.5%). This was caused by the improved membrane surface hydrophilicity and roughness paired with the nanochannels created by GO. The optimized TFN membrane also showed improved resistivity against alginate and least deteriorated desalination property after chlorination. Although the antibacterial property of GO was hindered by the PA layer, the membrane still exhibited better antibacterial property than that of commercial RO membrane. The outcomes of this study suggested that properly arranged GO in PA layer is necessary to minimize the formation of defects that could be detrimental for membrane separation. The position of GO in PA layer is particularly important to optimize its functionality. As a conclusion, the PEI-interlayered TFN membrane fabricated in this study portrayed a great potential in addressing the drawbacks of commercial TFC membrane for seawater or brackish water desalination.