(146e) Calcium Ions in Gypsum (CaSO4·2H2O) Scaling in Forward Osmosis Processes and a Novel Design Strategy to Fabricate Polybenzimidazole Anti-Scaling Hollow Fiber Membranes

Calcium
ions in gypsum (CaSO₄°¤2H₂O) scaling in forward osmosis
processes and a novel design strategy to fabricate polybenzimidazole
anti-scaling hollow fiber membranes

Si Cong Chen^a,b, Jincai
Su^b, Feng-Jiang Fu^b , Baoxia Mi^c, Tai-Shung
Chung^a,b,*

^a NUS Graduate School for
Integrative Sciences & Engineering (NGS), National University of Singapore,
28 Medical Drive, Singapore 117456, Singapore

^b Department of Chemical and
Biomolecular Engineering, National University of Singapore, 4 Engineering Drive
4, Singapore 117576, Singapore

^c Department of Civil and
Environmental Engineering, University of Maryland, College Park, MD 20742, USA

* Corresponding author: chencts@nus.edu.sg

Abstract

Gypsum
(CaSO₄°¤2H₂O) scaling phenomena have been studied on
hollow fiber membranes with different physicochemical properties in forward
osmosis processes. Three types of hollow fiber membranes were demonstrated
i.e., (1) cellulose acetate (CA), (2) polybenzimidazole (PBI)-polyethersulfone
(PES) and (3) PBI-polyacrylonitrile (PAN) -
polyhedral oligomeric silsesquioxane (POSS). For the first time, we have found
that calcium ions not only dominate but also take a leading role on gypsum
(CaSO₄°¤2H₂O) scaling through ionic interactions with the
membrane surface. A high level of calcium ions in scaling solutions can
initiate gypsum crystallization more effectively than a high level of sulfate
ions. Based on this finding, for the first time, a novel strategy to
molecularly engineer PBI hollow fiber membranes with superior anti-scaling
properties has been proposed and demonstrated. As shown in Figure 1c, the newly
designed PBI-PAN (POSS) membranes have a positively charged and smooth membrane
surface at the nanoscale level but possess a ridge and valley structure at the
micrometer level. It shows slightly positive charge at a low pH and exhibits minimal
scaling of only 5% flux reduction in contrast to CA membranes with 63% flux
reduction. The adhesion force measured by atomic force microscopy (AFM)
confirms a strong repulsive force between gypsum particles and PBI (Figure 1b) at
pH values lower than the isoelectric point of 5.7. The repulsion force leads to
the anti-fouling properties, while the novel ridge and valley morphology in the
micrometer scale facilitates surface turbulence flow and effectively remove
foulants.

Figure 1: Fouling mechanisms of gypsum scaling on membranes