Tight ultrafiltration and loose nanofiltration membranes by concentration polarization-driven fast layer-by-layer self-assembly for fractionation of dye/salt

In continuation of previous work on the construction of molecular selective membrane via layer-by-layer (LbL) assembly (J. Membr. Sci. 641, 2022, 119908), herein, we report an extremely fast concentration polarization driven LbL (CP-LbL) self-assembly of polyelectrolytes (PEs) on a new partially cross-linked support membrane to obtain molecular selective tight ultrafiltration (TUF) and loose nanofiltration (L-NF) membranes. These membranes have been prepared using low concentration of PEs (2 mM) and with only total three and five layers of self-assembly. The pure water permeances by the TUF and L-NF membranes are 38 Lm(-2)h(-1)bar(-1) and 25 Lm(-2)h(-1)bar(-1) respectively. The TUF membrane showed 98.9 to >99% rejections of reactive black 5, rose Bengal, Congo red and other dyes (0.2(-1) g L-1). The L-NF membrane exhibits >99.5% rejection of these dyes irrespective of ionic salt concentration. The membranes show very good salt to dye separation efficacy. The L-NF membrane intercepts 96-97% of dyes from real textile wastewater containing mixture of dyes and salt. High dye interception by the membranes is beneficial for the efficient fractionation and resource recovery. This work provides an important strategy for the quick LbL self-assembly using low concentration of PEs to obtain molecular selective membranes for the quick fractionation (similar to 3.5 diavolume) of dye/salt mixture via diafiltration operation and subsequent concentrating the dye solution.