Surface-modified halloysite nanotube-embedded polyvinyl alcohol/polyvinyl amine blended membranes for pervaporation dehydration of water/isopropanol mixtures

Surface modification of a halloysite nanotube (HNT) was performed by piranha etching of the HNT surface to generate hydrophilic moiety on the surface. The HNT and surface-modified HNT (MHNT) were embedded in polyvinyl alcohol/polyvinyl amine (PVA/PVAm) membranes. Surface modification of the HNT was confirmed using X-ray photoelectron spectroscopy, Fourier transform infrared, and transmission electron microscopy analyses. The 5 wt% MHNT-embedded membrane showed the best pervaporation performance (flux = 0.13-0.031 kg/m(2) h and separation factor = 427-93,313) among all membranes at 40 degrees C with 80-90 wt % of isopropanol (IPA) in the feed solution. Up to 5 wt% loading of HNT and MHNT in the membrane, the flux decreased, while the separation factor increased and subsequently decreased continuously with further HNT and MHNT loading. This was attributed to the hydrophilicity arising from HNT inclusion in the membranes. The membrane performance was evaluated under various pervaporation-operating conditions. Based on the solution diffusion model, with increase in water content, flux increased, while separation factor decreased. The temperature dependence of flux followed the Arrhenius equation. The energy of activation for the permeation of water (25.99 kJ/mol) was less than that of IPA (143.49 kJ/mol for the 5 wt% MHNT-loaded membrane at an 85 wt% of feed IPA composition). Overall, the pervaporation data suggested that the presence of MHNT in the PVA/PVAm membrane benefited pervaporation dehydration.