Thin film nanocomposite reverse osmosis membrane with layer by layer assembled titania nanosheets for produced water desalination

Abstract

Reverse osmosis (RO) is an emerging desalination technology that holds great potential to provide an effective approach for solving global water scarcity issues. In the blooming oil and gas industries, produced water (PW) desalination is a viable option to resolve the oily water disposal issue. However, one great challenge of PW desalination is the membrane fouling caused by the presence of hydrocarbon contents in the PW. In this study, thin film nanocomposite (TFN) membrane for RO desalination was fabricated by depositing positively charged titania nanosheet (pTNS) and negatively charged titania nanosheet (nTNS) on the surface of polyamide (PA) layer via layer by layer (LbL) assembly. The pTNS was synthesized through solidstate calcination and acid ion-exchange. Through the additional step of exfoliation, single nTNS with improved hydrophilicity property was obtained. The formation of pTNS/nTNS assembly that created as TNS was formed atop PA layer with the number of bilayer ranged from 0 to 4. The hydration layer created on the surface of TNS-PA could hinder direct contact of foulants with the membrane surface, hence significantly enhanced the water permeability and salt rejection as well as foulant resistance. The characterization findings revealed that the membrane surface hydrophilicity was improved while surface roughness was decreased by increasing number of bilayer. However, the excessive TNS bilayer coating has imposed additional hydraulic resistance, resulting in the reduction of water permeability. The highest water permeability of 0.97 L·m-2·h-1·bar-1 (33% improvement) was achieved with the 2- bilayer of TNS-PA TFN membrane compared to the pristine PA membrane. The sodium chloride (NaCl) rejection was >98% which was also higher than pristine thin film composite (TFC) membrane of 96%. Furthermore, the 2-bilayer of TNS-PA TFN membrane achieved >99% for oil rejection. On the other hand, manipulating nanomaterials loading (0-0.10 wt.%) of 2-bilayer could lead to more positive features on the resultant TFN membrane. The effect of 0.05 wt.% TNS loading in tandem with controlled 2 bilayer achieved the highest permeability and solute rejection, recording 0.98 L·m-2·h-1·bar-1 and >98%, respectively. This study also found that the 2TNS-PA TFN membrane outperformed the pristine TFC membrane by exhibiting higher permeability and much lower fouling propensity for low to high concentration of saline oily water (2000 ppm, 5000 ppm and 10,000 ppm) and oily saline water (1000 ppm, 5000 ppm and 10,000 ppm) over a 960 min operation. With saline oily feedwater, the permeability relative rate became low with 84.48%, 78.82% and 78.82%, respectively. However, for oily saline feedwater, the 2TNS-PA TFN membrane achieved almost 100% flux recovery for three cycles by hydraulic washing. While the average permeability of uncoated TFC membrane could only be recovered by 95.7%, 89.1% and 82.9% for 1000 ppm, 5000 ppm and 10,000 ppm of the oily saline feedwater, respectively. The 2TNS-PA TFN membrane suffered a less significant decline and exhibited a relatively higher average permeability recovery rate compared to the TFC membrane due to the better resistance to the oil adsorption on the skin layer. Overall, the surface modification of TNS on the TFC membrane can provide hydrophilic, stable and effective thin water layer to tackle the fouling problem encountered by the TFC membrane.