Separation mechanisms of binary dye mixtures using a PVDF ultrafiltration membrane: Donnan effect and intermolecular interaction

Abstract

This study aimed to explore the separation behaviors of various organic dyes across an ultrafiltration membrane. Herein, a poly(vinylidene fluoride) (PVDF) layer was fabricated on a nonwoven support to form microporous membranes for separating dye solutions. Zwitterionic, cationic and anionic dyes in single and binary mixtures were tested for dye rejection, total organic carbon (TOC) rejection and filtrate permeance. The Donnan exclusion mechanism played a major role for dye separation. The PVDF membrane had extremely low rejection for zwitterionic rhodamine B (RhB, 1.1%) and medium rejection for cationic methylene blue (MB, 45.6%), but high rejections for anionic dyes, eriochrome black T (EBT, 83.5%) and naphthol blue black (NBB, 89.1%). The mixed zwitterionic or same-charged dye solutions (RhB-MB, RhB-NBB and EBT-NBB) exhibited dye removal rates similar to their constituents’ single dye solutions. The filtrate permeation rates of these mixtures are in the range of the components’ single dye solutions. Conversely, the mixed solution of opposite-charged dyes (MB-NBB) formed micron-sized aggregates due to intermolecular interaction, leading to almost complete dye removal and enhanced permeance value. The stable separation performance for the dye solutions was demonstrated during multi-cycle filtration tests.