Novel quasi-symmetric thin-film inorganic membrane for elimination of Cd²⁺ in aqueous solution by forward osmosis

Forward osmosis (FO) is an emerging membrane process for desalination driven by the difference of osmotic pressure between feed solution and draw solution (DS). We previously developed a novel quasi-symmetric thin film inorganic (QSTFI) membrane with several advantages compared with conventional polymeric FO membranes. In this paper, elimination of Cd²⁺ in aqueous solution was investigated by using FO process with QSTFI membrane. Scanning electron microscope (SEM) was used to characterize membrane micro-scale morphology, and energy dispersive spectrometer (EDS) and Fourier transform infrared spectroscope (FTIR) were used to characterize chemical properties of the membrane. Besides, atomic force microscopy (AFM) was used to identify membrane surface electrical potential. The effects of Cd²⁺ concentration, DS concentration and membrane surface potential on Cd²⁺ rejection were examined and discussed. The surface of QSTFI membrane was negatively charged, which promoted formation of electric double layer structure through interacting with Cd²⁺ in bulk solution. The Debye length of electric double layer was positively correlated to Cd²⁺ rejection by the membrane. The FO experiments showed that the QSTFI membrane was able to successfully reject Cd²⁺ with overall efficiency up to 99%, at the same time achieving water flux of 69 L·m^-2·h^-1 at initial Cd²⁺ concentration of 10 mg·L^-1 and DS concentration of 2.0 mol·L^-1 NaCl. This study provides a promising approach to using FO process for elimination of heavy metals in waste water in practical applications.