Advanced surface-engineered fluorinated polyimide membranes for sustainable high-salinity water desalination

Abstract

The escalating global demand for freshwater has intensified the development of efficient and sustainable membranes for water purification. Membrane distillation (MD) is a promising technology for the treatment of highly saline seawater and wastewater. However, MD membranes require significant improvements to ensure excellent separation efficiency. This study presents the fabrication of novel recyclable fluorinated polyimide-based membranes (i.e., 6FDA-TMPD), prepared from 4,4′-(hexafluoroisopropylidene) diphtalic anhydride (6FDA) and 2,3,5,6-tetramethyl-p-phenylenediamine (TMPD), with tailored morphologies through vapor-induced phase separation (VIPS) technique. This process involves controlled exposure of polymer solutions to vapor, to precisely tune membrane porous structure and surface characteristics without chemical modifiers or nanoparticle incorporation. Notably, controlling the vapor type (ethanol vs water) and exposure duration transforms the membrane's surface structure from a spherical, nodule-like to a highly porous and open interconnected structure, resulting in a significant increase in hydrophobicity, yielding a water contact angle of 141.6°. Moreover, the flux increased by 43% from 14.59 to 20.85 kg m⁻² h⁻¹, while maintaining excellent salt rejection of ∼99.7% when treating high-salinity solutions (70,000 ppm) in air-gap membrane distillation (AGMD). These findings provide valuable insights into the synthesis and optimization of high-performance 6FDA-TMPD-based membranes and propose a sustainable and facile alternative to conventional fabrication methods, paving the way for practical solutions for desalination and various wastewater treatment applications.