Pervaporative seawater desalination using NaA zeolite membrane: Mechanisms of high water flux and high salt rejection

The pervaporative salts rejection and water flux characteristics of the NaA zeolite membrane in seawater were investigated. The upper limit of the non-zeolitic pore diameter was 8 Å. High salts rejection (more than 99.9%) was shown for all of the ions in the seawater, even, and to a considerable extent, for boron (79.2% at 69 °C). The high rejections were attributed to a joint size exclusion/charge exclusion/surface evaporation mechanism. High water flux in the seawater (1.9 kg/m2 h at 69 °C) also was manifested, and was much higher than that in pure water at temperatures lower than 100 °C. Moreover, the apparent activation energy of the water permeation in seawater was always lower than that in pure water (37.39 kJ/mol). The higher water flux and low activation energy were explained by the reduced electrostatic interaction between the positive surface charge and the polar water. The positive surface charge was induced by the charge reversal that was incurred by the adsorption of metal species on the zeolite surface. The water flux in the NaOH, NaCl, NaNO3 and Na2SO4 solutions also was investigated, and could be understood as a function of the surface charge. It was clear that the surface charge was an important distinction governing the water flux. The excellent salts rejection and water flux data indicated that the pervaporation process incorporating the NaA zeolite membrane is a promising candidate for desalination applications.