Fabrication and positron annihilation spectroscopy (PAS) characterization of cellulose triacetate membranes for forward osmosis

Rui Chin Ong, Tai Shung Chung*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

69 Scopus citations


Cellulose triacetate (CTA) membranes for forward osmosis (FO) were fabricated in this study. The fundamental engineering and science of CTA membrane formation were explored by casting the membranes from different solvent systems and characterizing the membranes' morphology using advanced tools such as positron annihilation spectroscopy (PAS). N-Methyl-2-pyrrolidone (NMP) and dioxane were used as the solvents for CTA in the membrane fabrication process. It was found that the choice of solvents for membrane fabrication significantly affects the morphology of as-cast membranes and their FO performance. The CTA membrane cast using NMP as the main solvent has shown poor NaCl rejection but high water flux, whereas the CTA membrane cast using dioxane as the main solvent has excellent NaCl rejection but low water flux. SEM and PAS data reveal that the sublayer of CTA membranes cast from dioxane has a close-cell and much denser structure compared to that cast from NMP. As a result, the former has a significantly lower water flux than the latter in the FO process (5 vs. 27 LMH) using a 2. M NaCl draw solution. Surprisingly, with the addition of acetic acid into the dioxane/acetone solvent system, the resulting membrane has a significantly more porous and open-cell sublayer structure. In addition to the pore forming ability of acetic acid, FTIR spectra confirm the formation of acetic acid/dioxane complexes in the casting solution. Thus, as validated by PAS spectra, the free volume of the active layer of the resultant CTA membranes increases after the addition of acetic acid into the dioxane/acetone casting solution and the water flux increases to 23 LMH using a 2. M NaCl draw solution. Molecular simulations were also conducted to examine CTA polymeric chains in different solvent systems and to witness different CTA chain behaviors in these solvents that leads to significant differences in the as-cast membranes' morphology.

Original languageEnglish (US)
Pages (from-to)230-240
Number of pages11
JournalJournal of Membrane Science
StatePublished - Mar 15 2012
Externally publishedYes

Bibliographical note

Funding Information:
The authors would like to thank the Singapore National Research Foundation (NRF) for support through the Competitive Research Program for the project entitled, “New Advanced FO membranes and membrane systems for wastewater treatment, water reuse and seawater desalination” (grant number R-279-000-336-281 ) as well as Eastman Chemical Company, USA , for providing these CA materials. Special thanks are due to Ms. Zhang Sui, Dr. Wang Kaiyu, Dr. Su Jincai, Dr. Teoh May May, Dr. Natalia Widjojo and Dr. Xiao Youchang for their valuable advice given. Thanks are also due to Ms. Lin Xiaochen and Ms. Liang Jiayue for their kind assistance in this work.


  • CTA membranes
  • Cellulose triacetate membranes
  • Forward osmosis
  • Forward osmosis membranes
  • Positron annihilation spectroscopy (PAS)

ASJC Scopus subject areas

  • Biochemistry
  • General Materials Science
  • Physical and Theoretical Chemistry
  • Filtration and Separation


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