Formation of Hybrid Membranes for Water Desalination by Membrane Distillation

Abstract

A method has been developed for the formation of hybrid membranes consisting of a hydrophilic microporous substrate and a hydrophobic nanofibrous polymer layer deposited by electrospinning. A track-etched poly(ethylene terephthalate) membrane has been used as the hydrophilic microporous substrate, onto the surface of which a thin layer of titanium is deposited by magnetron sputtering to provide the nanofibrous layer with adhesion. Simultaneously, this layer has been used as an electrode of a deposition collector for the electrospinning formation the nanofibrous coating. It has been shown that the application of this method for the preparation of polymer coatings using poly(vinylidene fluoride) as a starting material for the formation of nanofibers makes it possible to obtain a highly hydrophobic layer, the surface of which has an average water contact angle of 143.3 ± 1.3° depending on the deposition density. The morphological study of the nanofibrous coating has shown that its microstructure is typical of nonwoven materials. The nanofibers that form the porous system of this layer have a wide scatter of sizes. FTIR spectroscopic and X-ray diffraction investigations of the molecular structure of the nanofibrous layer have shown that the β-phase prevails in its structure, with this phase being characterized by the maximum dipole moment. It has been shown that the elaborated hybrid membranes ensure high separation selectivity of desalinating an aqueous 26.5 g/L sodium chloride solution by the membrane distillation method. In the studied regime of the membrane distillation, the salt rejection coefficient for membranes with nanofibrous layer densities of 20.7 ± 0.2–27.6 ± 0.2 g/m² is 99.97−99.98%. It has been found that the use of a highly hydrophobic nanofibrous layer with a developed porous structure in combination with a hydrophilic microporous substrate makes it possible to increase the productivity of the membrane distillation process. The value of the maximum condensate flow through the membranes is, on average, 7.0 kg m²/h, and its value depends on the density of the deposited nanofibrous layer.