Facile post-treatment of PSf membranes with hexane for fractionation and concentration of alkaline textile wastewater

To treat dyeing and printing industrial wastewater with zero liquid discharge, developing a straightforward technique for fabricating membranes with sufficient alkaline and salt resistance is crucial. This study presents a facile n-hexane post-treatment method to enhance the ability of polysulfone (PSf) membranes to fractionate textile wastewater. The optimal membrane exhibits minimal salt rejections, with only 5.1 % for NaCl and 9.8 % for Na₂SO₄. Additionally, it demonstrates exceptional dye rejection rates, reaching 99.87 % for Direct Red 23 and 99.81 % for Congo Red while maintaining high-water permeance (∼52 L m⁻²h⁻¹ bar⁻¹) at the alkaline environment (pH = 13) and high salt (30 g/L). Furthermore, a combined diafiltration process was devised for the separation of a mixture containing Congo Red and Na₂SO₄. This process achieved an impressive desalination efficiency of 98 % while ensuring a remarkable dye recovery of 99.8 %. The outstanding performance can be attributed to the favorable solvency capability of n-hexane, which was determined based on the analysis of Hansen solubility parameters. Through a two-minute solvent treatment of PSf membranes, the flexibility of the polymer chains within the membranes was enhanced, leading to a denser structural arrangement. This arrangement resulted in the formation of highly selective membranes. The extent of densification is influenced by several factors, such as solvent type, immersion duration, and polymer concentrations. These variables directly affect the level of solvation and chain rearrangement that occurs during the process. This study underscores the potential significance of solvent treatment as a post-treatment step in membrane fabrication, demonstrating its utility as a tool for fine-tuning separation performance in applications that involve dye/salt mixtures, and highlighting its importance in the field of membrane technology.