Enhanced Removal of Aqueous Phenol with Polyimide Ultrafiltration Membranes Embedded with Deep Eutectic Solvent-Coated Nanosilica

Abstract Novel composite hydrophilic ultrafiltration (UF) membranes, comprising a polyimide (PI) matrix embedded with silica-based nanofillers are established for the enhanced vacuum-filtration removal of small polar organics such as phenol from water. Spherical silica nanoparticles are first prepared by the surfactant-templated sol-gel approach, then coated with choline chloride-ethylene glycol deep eutectic solvent (DES) by post-impregnation. The modified silica nanofillers are introduced into the PI matrix through the casting solution, which is then used to develop the composite UF membranes by phase inversion. The spectroscopy and high-resolution electron microscopy measurements evidence the presence of DES on the surface of the modified silica nanofillers. The topographical atomic force microscopy studies, dynamic water contact angle, and zeta potential measurements conducted on the native polymer membrane and those embedded with bare-silica and DES-coated silica confirm the successful incorporation of nanofillers into the PI matrix. An optimum nanofiller loading of 2 wt% is found to preserve the membrane structure and give the best mechanical properties as well as for the UF performance characteristics in vacuum-filtration studies at 85 kPa. Consistently, the DES-silica modified PI UF membranes (DES@SiO2@PI) exhibit exceptional water permeate flux (300 L m-2 h-1) and removal efficiency (96 %) for 30 mg L-1 aqueous phenol compared with the native polymer and SiO2-modified membranes. The DES-engineered composite membranes can be operated within a wide feed pH window (pH: 2-10) without displaying significant loss in their removal efficiency. The pH and zeta potential results suggest that the interaction of phenol with the optimized membranes is governed by cooperative adsorption/Lewis acid-base mechanisms via hydrogen bonding and assistive ion-exchange interactions with the ethaline coating.