Tailoring nanofiltration membranes for effective removing dye intermediates in complex dye-wastewater

Despite the extensive studies on rejection of large molecular dyes by nanofiltration (NF), their small molecular weight intermediates have got limited concern. Taking reactive black 5 (RB-5) as an example, the separation behaviors of the dye, the intermediate (H-acid) and salts (NaCl) were investigated by preparing two types of NF membranes featuring different charge and pore structural properties. It was found that optimum selectivity could be achieved by the combined effects between aperture sieving and Donnan repulsion. Both membranes possess reasonable long-term stability and more than 95% rejection of large molecular RB-5. But they feature complicated and different rejective behavior for H-acid with small Mw in different aqueous environment, such as different solute concentration and different pH value for feed solution. The results indicate that (1) small pore size of M-PEI may be the dominant factor for high retention of H-acid; (2) highly selective separation of dye intermediate and sodium chloride may be achieved at high pH with positively charged membranes. This study may provide insightful guidelines for developing next-generation NF membranes for treating textile wastewater with complex compositions.