Gypsum scaling in forward osmosis: Role of membrane surface chemistry

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 1740 cm^-1 (C=O stretching) to 1366 cm^-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 3400 cm^-1 (O-H stretching) to 2970 cm^-1 (C-H stretching), and the occurrence of the carboxylate functional group at binding energy of 288.1 eV on the TFC membrane during the formation of gypsum scaling.