A comprehensive physico-chemical characterization of superhydrophilic loose nanofiltration membranes

Abstract

Nanofiltration (NF) membranes, especially loose NF membranes, trigger a growing interest for the fractionation of concentrated organic matters/salt mixtures in addition to the production of pure water. This study presents an in-depth characterization of two superhydrophilic loose NF membranes (Sepro NF 6 and 2A, Ultura). The physical characterization included the determination of the molecular weight cut-off (MWCO), pore size distribution, membrane morphology, surface charge, roughness and hydrophilicity. This was combined with a chemical characterization, i.e., by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), to determine the intrinsic membrane properties. The chemical characterization demonstrates that both Sepro NF membranes are poly(piperazineamide) based, showing the modification chemistry for the top layer through XPS measurement. Specifically, Sepro NF 6 and NF 2A membranes were found to have a superhydrophilic surface (contact angle for Sepro NF 6: 14.3±0.9°; that for Sepro NF 2A: 21.7±1.4°) with a low roughness, offering a potential advantage over conventional NF membranes in minimizing membrane fouling. Sepro NF 6 and NF 2A membranes had a mean effective pore size of 0.64±0.03 nm and 0.52±0.01 nm (corresponding to MWCOs of 862±80 Da and 493±53 Da), respectively. In terms of filtration performance, Sepro NF 6 showed a high permeability of 16.7 L m−2 h−1 bar−1 with 88.9% salt transmission for 0.01 mol L−1 NaCl solution, and a slightly lower permeability and salt transmission was obtained for Sepro NF 2A, which is desired for an effective fractionation of target organic matter/salt mixtures.