Polydopamine-modified halloysite nanotube-incorporated polyvinyl alcohol membrane for pervaporation of water-isopropanol mixture

Abstract

In reference, the mussel-encouraged surface adhesion chemistry, dopamine oxidative auto-polymerization on the surface of halloysite nanotubes (HNTs) was performed to confer hydrophilicity to the HNTs. The additional hydrophilic moieties with surface coating by polydopamine (PDA) was assured with transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) techniques. A PDA/HNT-incorporated polyvinyl alcohol/polyvinyl amine (PVA-PVAm) membrane was fabricated for the pervaporation-dehydration of isopropanol/water (IPA/water), and the filler dispersion, crystallinity, and hydrophilicity of the membrane were confirmed by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and contact angle measurements, respectively. The pervaporation test was performed with membranes having different PDA/HNT contents, where the flux increased from 0.19 to 0.39 kg/m2h, and the separation factor decreased from 479 to 63 with the 80/20 (w/w, IPA/water) feed mixture at 40 °C. Compared to the unmodified HNT-incorporated membrane, the membrane with 5 wt.% PDA/HNT exhibited superior performance. Upon increasing the feed (85/15 IPA/water w/w) temperature from 40 to 70 °C, the flux of water and IPA increased from 0.062 to 0.13 kg/m2h and 0.000063 to 0.00144 kg/m2h, respectively. The apparent energy of activation calculated using the Arrhenius equation was positive for both IPA and water. The higher energy required for permeation of IPA (99.65 kJ/mol) compared to water (22.69 kJ/mol) is attributed to facile permeation of water compared to IPA through the hydrophilic channel created by PDA-HNT. In long-term operation (120 h), the IPA and water flux of the 5 wt.% PDA/HNT-incorporated PVA-PVAm membrane was maintained using a feed composition of 80/20 (w/w %) IPA/water at 40 °C, indicating remarkable reusability of the membrane.