Scaling-up of robust nanofiltration membrane of ultrathin-film-composite structure

Abstract

Ultrathin film composite nanofiltration membrane was prepared in large-scale (1 m × 50 m) using indigenously developed casting and coating units. A spongy, nano-porous, compaction tolerant substrate layer (middle layer) was formed through judicial selection of the concentrations of base polymer (PSF) and its relative proportion with the pore former (PVP). The substrate with symmetric surface pore morphology (uniform distribution of innumerable nano-pores devoid of macrovoids) facilitated the formation of ultrathin polyamide rejection layer much in the form of laminate (of near uniform thickness) with very limited penetration to the substrate pores. The polyamide layer thickness could be down to 40 nm for the NF/PS/8PVP membrane. XPS study revealed a co-existence of polyamide cross-linked network and linear portion comprised of many sub-units with terminal carboxylic acid groups, which impart surface hydrophilicity and potential. The membranes exhibit a trade-off relationship between flux and selectivity with a flux of 158 L M−2H−1 at 50 psi and Na2SO4 rejection of >99% for NF/PS/8PVP membrane.