Roughly two-thirds of the world’s population lives in water-scarce regions, causing access to safe drinking water to continue to be a common issue worldwide. Seawater or brackish water desalination is one of the most effective and increasingly common methods of producing potable water. For reverse osmosis water purification, fully aromatic polyamide thin film composite (TFC) membranes are a common and effective material for desalination. These TFC membranes are often modified to include nanoparticles making thin film nanocomposite (TFN) membranes, which can improve their performance as the need for high flux and high salt rejection is balanced. The effects of the modified pH in the feed water on the performance of TFN membranes remain relatively under-investigated. The typical pH of brackish water is slightly evaluated as compared to fresh water, while wastewater can be as high as 13 in purification applications in the mining and paper industries. We have modified our feed solution with both elevated and reduced pH to understand the effects of these conditions on the performance of our TFC membranes as well as TFNs that contain cellulose nanocrystals (CNCs) as well as functionalized CNCs.
CNCs are desirable nanoparticles for TFN membrane fabrication due to their high aspect ratio, low cost, availability, and sustainability. The CNCs used for this study were modified to produce 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanocrystals (TOCNs). TFNs were fabricated via interfacial the interfacial polymerization of an organic solution of tri-mesoyl chloride with an aqueous solution of m-phenylene diamine containing TOCNs. The flux and salt rejection were investigated for TFN membranes with CNCs and TOCNs at a 0.1% loading level. For feed preparation, pH modification was performed with NaOH or HCl added to the solution to reach the desired pH conditions of 4 and 10 for comparison to the typical neutral conditions of pH 6. The pKa of the fabricated TOCNs was found to be around 2.3 and these values were selected to subject the material to extreme pH conditions within the stability limitations of the polyamide matrix.
