Cellulose esters for forward osmosis: Characterization of water and salt transport properties and free volume

Abstract

The fundamental salt and water transport properties of cellulose ester polymers have been studied and correlated with their hydrated free volume, as probed by positron annihilation lifetime spectroscopy (PALS), to provide scientific insights for material development in seawater desalination. It has been found that the hydrated free volume is strongly dependent on water uptake. The water sorption in polymeric films reduces the fractional free volume (FFV) at low degrees of hydration due to the overwhelming hole-filling effect, but gradually increases FFV when more water sorption takes place as a result of the swelling effect. The diffusivity and permeability of both water and salt vary with changes in wet-state FFV. Diffusion selectivity of H2O/NaCl increases dramatically as the hydrated FFV decreases, while solubility selectivity of H2O/NaCl is dependent on both hydrated free volume and chemical structure. Moreover, thin freestanding films with no sub-layer support have been prepared by spin casting these polymers on silicon wafers for the purpose of eliminating internal concentration polarization (ICP) in the forward osmosis (FO) process. Based on the theoretical prediction from the solution-diffusion model, the performance ratio (i.e., the ratio of the experimental to theoretical water flux) of the sub-layer free films can be calculated. It can reach as high as around 50%, which is 2–3 times of the values reported elsewhere when testing traditional asymmetric membranes. Using a model 3.5 wt.% NaCl feed solution and a 2M NaCl draw solution, a water flux of 21.8 LMH has been observed, which is the highest value ever reported. The high water flux in the FO desalination process indicates that the concept of sub-layer-free thin films is promising for the next generation FO membranes.