Evaluation and performance of chitosan and chitosan-graphene oxide coated UF membranes for drinking water treatment
Creators: name: Liu, Teng
Publisher: name: Imperial College London
Year: 2021

description: Ultrafiltration (UF) membrane-based technology has been applied in the water purification industry since the 1980s and employed widely to relieve the crisis of global water scarcity. However, membrane fouling is the major obstacle to the further application of this membrane-based technology. There are three main methods to alleviate membrane fouling, namely the pre-treatment of raw water, membrane modification, and membrane cleaning. As the most cost-effective approach is likely to be the prevention of fouling by modification of the membrane, this was selected for investigation in this study. Consequently, two novel, low cost and widely available hydrophilic materials, namely chitosan (CS) and graphene oxide (GO), were chosen to coat a commercial PVDF UF membrane in order to produce a superior membrane surface with a reduced propensity to fouling. Therefore, this study aimed to evaluate and compare the filtration performance of uncoated PVDF and coated PVDF membranes (PVDF-CS and PVDF CS-GO) in terms of permeability, separation, and anti-fouling performance. In the short-term filtration section of the experimental work in this study, chitosan alone, and chitosan combined with graphene oxide were applied to coat a commercial 100 kDa PVDF membrane. The comparative performance of chitosan with three different molecular weight ranges, specifically 50k-190kDa, 100k-300kDa, and 600k-800kDa, was studied. The concentration of CS solution was 0.01%wt and 10 mg CS was added to fabricate PVDF-CS membranes. The concentration of CS solution was 0.01%wt and GO was mixed in 50 ml DI water at a concentration of 20 mg/L. Filtration tests were conducted using four types of water samples, comprising solutions of three representative natural organic substances (BSA, SA, HA solutions), and a surface water. The Jingmi (JM) River water with a TOC of less than 3 mg/L was used for PVDF-CS membrane tests and the Beijing Olympic Park (OP) Lake with a TOC of approximately 8 mg/L was used for CS-GO membrane tests. The results demonstrated that PVDF-CS membranes exhibited greater anti-fouling and separation performance compared to the uncoated PVDF membrane, and the molecular weight of the chitosan did not appear to affect the properties and performance of the PVDF-CS membranes. However, the coating of chitosan led to a substantial decrease in the water flux of the membrane. This reflects the common trade-off that needs to be made between water permeability and separation performance when fabricating the membranes. It was decided to further improve the separation performance as the primary objective, rather than permeability in the subsequent experiments. The results demonstrated that the employment of GO significantly improved the separation performance of CS-GO membranes compared to the PVDF-CS membranes tested in the first phase of experiments. Specifically, the CS-GO membranes exhibited an almost complete removal of BSA, SA, and HA as model natural organic matter (NOM) components. For the surface water collected from OP Lake, the CS-GO membranes removed up to approximately 50% of NOM. In addition, the interlayer spacing of CS-GO composite membranes was studied by changing the MW and the dosage of CS. Two CS-GO mass ratios (0.2:1 and 0.5:1) were investigated. In the final phase of the experimental work in this study, the uncoated PVDF and coated PVDF-CS membranes were evaluated and compared under long-term continuous-flow conditions, focusing on the feasibility and potential for application in small community systems. Gravity-driven filtration (GDF) tests were conducted using samples of a real surface water. The stabilized flux of the PVDF-CS membrane was almost double that of the PVDF membrane from day 7 till the end of the 30-day operation, indicating that the water output of the PVDF-CS membrane module should be almost double that of the PVDF membrane module. The results demonstrated that the PVDF-CS membrane was a feasible technology with a promising potential for applications in decentralized drinking water treatment systems in rural areas of developing countries. Overall, this study has been the first to evaluate and compare the performance of chitosan-coated membranes from the perspective of water treatment and the first to investigate the benefit of combining chitosan with graphene in a composite membrane. Future research is recommended to explain further the complexity of NOM by the membranes in real waters, the microbial communities governing the phenomenon of flux stabilization under the GDF arrangement, and pilot-scale testing of the PVDF-CS membrane to demonstrate its performance in practice.