Salt rejection behavior of carbon nanotube-polyamide nanocomposite reverse osmosis membranes in several salt solutions

The present paper describes the desalination performance of reverse osmosis (RO) membranes of multi-walled carbon nanotube-polyamide complex (CNT-PA) and commercial polyamide membranes in NaCl, MgCl2, MgSO4 and Na2SO4 aqueous solutions. The permeate flux, salt rejection, and salt flux were determined in a cross-flow experiment. The CNT-PA and commercial RO (PA) membranes (SWC5, Nitto Denko Co.) showed 96.0% and 99.7% salt rejection, respectively, for 0.2% NaCl aqueous solution at 0.7 MPa. The calculated salt flux was 0.38 g·m−2·h−1 (CNT-PA) and 0.07 g·m−2·h−1 (SWC5). The salt rejection increased with increasing running pressure and decreasing salt concentration. The zeta potential measurement of CNT-PA demonstrated that it is negatively charged due to the presence of CNT. Accordingly, it showed salt rejection performances against the four salt solutions (Na2SO4 > MgSO4 > NaCl > MgCl2) that differed from that of the usual PA membranes (Na2SO4 > MgSO4 > MgCl2 > NaCl). These data are explained based on the Donnan model (CNT-PA) and steric hindrance pore model (SWC5), except for the case of chlorides under low-flux or high ionic strength conditions, where the diffusion and molecular size exclusion of the salts dominate over their mas-transport. Positron annihilation lifetime spectroscopy (PALS) enabled the estimation of the pore diameters of these membranes: 0.55 nm (CNT-PA) and 0.58 nm (SWC5).