Fractionation of direct dyes and salts in aqueous solution using loose nanofiltration membranes

In the textile industry, high salinity waste streams are a challenge urging for the recovery and purification of dyes and salts (e.g., NaCl), requiring a treatment going beyond the classical filtration by e.g., reverse osmosis to produce pure water. In this work, two commercial loose nanofiltration (NF) membranes (Sepro NF 6 and NF 2A, Ultura) are proposed to fractionate dye/salt aqueous mixtures. It was observed that both NF membranes have a salt rejection <33.3% in solutions with 0.1–40.0 g L−1 of NaCl at 6 bar. Furthermore, both membranes have >99.6% retention of direct dyes (direct red 80, direct red 23, and congo red), even though 40.0 g L−1 NaCl is present, indicating salt addition has no obvious impact on the dye retention. The combination of a low salt rejection and a high dye rejection indicates the feasibility for the reuse of salt from fractionation in forward osmosis and bipolar membrane electrodialysis. Application of diafiltration for an aqueous mixture containing direct red 80 (1000 ppm) and NaCl (~20 g L−1) by both membranes demonstrates that above 95% of NaCl is removed from aqueous mixture, and <0.9 g L−1 NaCl remains after the addition of pure water with a volume factor of 4.0 in the feed solution. At the premise of excellent diafiltration performance, concentration as the post-treatment for dye recovery expectedly indicates direct red 80 is concentrated by a factor of 4.0 for both membranes while keeping the salt concentration with a very slight increase. Over 99.9% dye retention in both diafiltration and concentration procedures yields a very high recovery since <0.045% of dye is permeated. These results indicate that loose nanofiltration membranes have potential for dye/salt fractionation.