Gypsum scaling in forward osmosis: Role of membrane surface chemistry

• Published in: Journal of membrane science 2016-09, Vol.513, p.250-259
• Main Authors: Xie, Ming, Gray, Stephen R.
• Format: Article
• Language: English
• Subjects: Cellulose triacetate; Flux; Forward osmosis; Fourier transform infrared spectroscopy; Functional groups; Gypsum; Gypsum scaling; Membranes; Morphology; Osmosis; Polyamide; Stretching; Wavenumber; X-ray photoelectron spectroscopy
• ISSN: 0376-7388; 1873-3123
• DOI: 10.1016/j.memsci.2016.04.022

Forward osmosis (FO) membranes with varying surface chemical functionalities respond differently to gypsum scaling. Using a real-time monitoring system, gypsum scaling was quantified between an asymmetric cellulose triacetate (CTA) and a thin-film composite (TFC) polyamide membrane in terms of water flux decline, gypsum surface coverage and gypsum crystal morphology. At the same initial water flux, the TFC membrane was subjected to more severe gypsum scaling than the CTA membrane in terms of water flux decline and gypsum crystal surface coverage. The gypsum crystal morphology on the CTA membrane featured with slender platelets; in contrast, that on the TFC membrane demonstrated the formation of rosette arrangements. Fourier transform infrared spectra and X-ray photoelectron spectroscopy proved that the gypsum scaling on CTA membrane was dominated by bulk crystallisation with subsequent deposition; while that on the TFC membrane was driven by surface crystallisation via specific interaction between carboxylic functional groups and calcium ions. No interaction between gypsum and CTA membrane surface was demonstrated by the largely unchanged ratio of wavenumbers 1740cm−1 (C=O stretching) to 1366cm−1 (C–O stretching), as well as binding energy of C1s on the CTA membrane. In contrast, specific interaction between carboxylic functional groups with calcium ions during gypsum scaling was revealed by a gradual increase in the ratio of absorbance wavenumber 3400cm−1 (O–H stretching) to 2970cm−1 (C–H stretching), and the occurrence of the carboxylate functional group at binding energy of 288.1eV on the TFC membrane during the formation of gypsum scaling. [Display omitted] •Gypsum scaling in FO was compared between CTA and TFC membranes.•Gypsum scaling of CTA membrane was dominated by bulk crystallisation.•Gypsum scaling of TFC membrane was driven by surface crystallisation.•No interaction –OH on CTA membrane was demonstrated in gypsum scaling.•FTIR and XPS show interaction between –COOH on TFC membrane with Ca2+.