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Membranes 2013, 3(4), 354-374; doi:10.3390/membranes3040354
Article

Gypsum (CaSO4·2H2O) Scaling on Polybenzimidazole and Cellulose Acetate Hollow Fiber Membranes under Forward Osmosis

1,2
, 2,3
, 2
, 4
 and 1,2,5,*
1 NUS Graduate School for Integrative Sciences & Engineering (NGS), National University of Singapore, 28 Medical Drive, 117456, Singapore 2 Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576, Singapore 3 Mann+Hummel Ultra-Flo Pte Ltd., 18 Tuas Avenue 8, 639233, Singapore 4 Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742, USA 5 Water Desalination & Reuse (WDR) Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
* Author to whom correspondence should be addressed.
Received: 12 October 2013 / Revised: 29 October 2013 / Accepted: 1 November 2013 / Published: 8 November 2013
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Abstract

We have examined the gypsum (CaSO4·2H2O) scaling phenomena on membranes with different physicochemical properties in forward osmosis (FO) processes. Three hollow fiber membranes made of (1) cellulose acetate (CA), (2) polybenzimidazole (PBI)/polyethersulfone (PES) and (3) PBI-polyhedral oligomeric silsesquioxane (POSS)/polyacrylonitrile (PAN) were studied. For the first time in FO processes, we have found that surface ionic interactions dominate gypsum scaling on the membrane surface. A 70% flux reduction was observed on negatively charged CA and PBI membrane surfaces, due to strong attractive forces. The PBI membrane surface also showed a slightly positive charge at a low pH value of 3 and exhibited a 30% flux reduction. The atomic force microscopy (AFM) force measurements confirmed a strong repulsive force between gypsum and PBI at a pH value of 3. The newly developed PBI-POSS/PAN membrane had ridge morphology and a contact angle of 51.42° ± 14.85° after the addition of hydrophilic POSS nanoparticles and 3 min thermal treatment at 95 °C. Minimal scaling and an only 1.3% flux reduction were observed at a pH value of 3. Such a ridge structure may reduce scaling by not providing a locally flat surface to the crystallite at a pH value of 3; thus, gypsum would be easily washed away from the surface.
Keywords: forward osmosis; fouling; gypsum scaling; polybenzimidazole; polyhedral oligomeric silsesquioxane; cellulose acetate forward osmosis; fouling; gypsum scaling; polybenzimidazole; polyhedral oligomeric silsesquioxane; cellulose acetate
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Chen, S.C.; Su, J.; Fu, F.-J.; Mi, B.; Chung, T.-S. Gypsum (CaSO4·2H2O) Scaling on Polybenzimidazole and Cellulose Acetate Hollow Fiber Membranes under Forward Osmosis. Membranes 2013, 3, 354-374.

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