Fabrication of Chlorine-Resistant Forward Osmosis Membrane with High Permeability via Grafting of Novel Sulfonamide Monomers

Abstract

Forward osmosis (FO) is a membrane-based filtration technology that capitalises on the osmotic pressure disparity between the feed side and the draw side of the system. The draw side has a higher osmotic potential, making FO an attractive option for wastewater treatment due to its ability to remove contaminants, concentrate wastewater, and potentially recover nutrients and energy. However, FO membranes face challenges related to low chlorine resistance, low ammonia rejection, and the existence of reverse solute flux (RSF). This PhD thesis aims to comprehensively investigate strategies to overcome the challenges of FO membranes, optimise the FO processes, and promote resource recovery in water and wastewater treatment.

The initial study employed a layer-by-layer interfacial polymerisation approach using sulfonamide monomers, 3,5-dihydroxybenzoic acid (DHBA) and 4-amino-benzene sulfonamide (4-ABSA). The introduction of the stable sulfonamide structure, featuring the N-H group, enhanced chlorine resistance by shielding the active layer from chlorine attack. The hydrophilic groups in ABSA and DHBA facilitated the formation of a hydration layer through hydrogen bonding, improving FO membrane permeability. Subsequent research aimed to further enhance chlorine resistance and permeability by introducing stronger sulfonamide structures. The modified membranes exhibited a significant increase in water flux (148.2%) and a decrease in RSF (35.7%) compared to conventional membranes. They also demonstrated excellent chlorine resistance and stability under various pH conditions.

Subsequent studies focused on addressing challenges related to low ammonia rejection and RSF in FO membrane treatment of real wastewater. The proposed solution involved utilising unrejected ammonium ions in the draw solution and adding sodium hypochlorite to generate chloramines, which diffused back to the feed solution, demonstrating self-cleaning to prevent severe fouling. Chloramine addition resulted in significant improvements in water flux and reduction of RSF. Monochloramines were found to be more effective than dichloramines in reducing fouling, highlighting their potential as cleaning agents. These studies provide insights into the mechanisms and dynamics of chloramine formation and decay, offering guidance for optimising FO processes.

Overall, this PhD thesis contributes to the advancement of FO membrane technology by enhancing chlorine resistance, water permeability, and fouling mitigation. The findings have implications for improving the performance and expanding the application potential of FO membranes in water and wastewater treatment processes.

Date of Award	19 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kyoung Jin Alicia AN (Supervisor)