Performance of Solvent-Pretreated Polyimide Nanofiltration Membranes for Separation of Dissolved Dyes from Toluene

The major problem reported for commercially available membranes is their limited chemical stability in organic solvents. This article reports the influence of organic solvents with different polarity (methanol, acetone, acetic acid, toluene, and n-hexane) on the filtration performance of STARMEM membranes. These membranes are specified to be compatible with the solvents used. The pure toluene flux of these membranes and the rejection of azo dyes (Sudan Black and Rhodanile Blue) were measured before and after a week of exposure to one of the above-mentioned solvents. The results show that the structure of the membrane changed after pretreatment with polar solvents (methanol, acetone, acetic acid) and that the membrane performance shifts toward lower rejections with higher solvent flux. Nonpolar solvents do not change the polyimide membrane performances significantly. Contact angle measurements on new and solvent-treated membranes show that organic solvents change the hydrophobicity of polymeric membrane surfaces, leading to different toluene permeabilities before and after solvent treatment. Swelling measurements confirm the theory that a reorganization of the membrane material takes place, which leads to differences in porosity and changes in rejection. The study also shows that pretreatment of polyimide membranes with polar solvents increases the recovery of the membrane system when the solute size is large enough.