Improving the Performance of Cellulose Acetate Reverse Osmosis Membranes

Author: Clare Helen Worthley

Worthley, Clare Helen, 2012 Improving the Performance of Cellulose Acetate Reverse Osmosis Membranes, Flinders University, School of Chemical and Physical Sciences

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Reverse osmosis membrane desalination technology has come a long way since the Loeb and Souririjan type membranes of the 1960s. Advances in production and engineering processes have overcome many inherent system limitations, however some remain. Truly biofouling-resistant membranes are yet to be realised, and membrane compaction remains an issue. In this thesis methods for mitigating these two membrane limiting factors have been investigated; polymeric modification and nanoparticle inclusion. In the first part of the project, a detailed study of the polymerisation of poly(2hydroxyethyl methacrylate) via activators regenerated by electron transfer atom transfer radical polymerisation was undertaken. Conditions were chosen to be commercially attractive. The modified membranes were studied with fourier transform infrared spectroscopy (FTIR), x-ray photospectroscopy, nuclear magnetic resonance, and thermogravimetric analysis. Results showed that by varying the initial monomer volume and/or the polymerisation reaction time it was possible to create a series of modified membranes with a range of polymer graft densities, thus indicating the livingness of the polymerisation reaction. In order to evaluate the ideal graft density, the properties of the modified membranes were further investigated. The polymer coating was clearly visible using scanning electron microscopy and an increase in surface roughness was observed with atomic force microscopy, in both cases confirming the increase in polymer graft density. Water contact angle studies explored the relationship between surface morphology and wettability, indicating conformational changes in the polymer. Hydrolysis had little effect on modified membrane filtration properties when soaked at pHs outside the recommended range for pristine cellulose acetate membrane (CAM). Unique aquarium biofouling tests were performed, and showed a decrease in biofouling for the modified membranes. Stirred-cell experiments were used to evaluate the filtration properties of the modified membranes. From the results it was possible to determine optimum conditions for membrane modification to obtain a polymer graft density with maximum biofouling resistance and minimum loss of filtration properties. In the second part of the project, aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS) was investigated as a nanocomposite additive. Since nanoparticle agglomeration and leaching were identified as issues in nanocomposite materials, an anchored nanoparticle was synthesised using isocyanate chemistry to attach POSS to cellulose acetate (CA). This anchored nanoparticle was compared to un-anchored POSS as an additive in CA membrane casting solutions at loadings of 0.5, 1.0 or 5.0 wt%. FTIR and energy dispersive X-ray microanalysis showed the anchored nanoparticle to have better dispersion in the resulting membranes. The nanocomposite membranes showed greater flux of water and salt than an unmodified CA control. Membrane compaction was mitigated at low nanoparticle loadings. Dynamic Mechanical Analysis (DMA) results suggest POSS has a plasticisation effect on the CA matrix.

Keywords: Desalination,reverse osmosis,membrane,nanocomposite

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2012
School: School of Chemical and Physical Sciences
Supervisor: Dr Stephen Clarke, Dr Milena Ginic-Markovic, Dr Kristina Constantopoulos