Coating layer effect on performance of thin film nanofiltration membrane in removal of organic solutes

• Hydrophilic layer on NF270 decreased rejection to hydrophilic solutes.
• Hydrophilic layer on NF270 increased rejection to hydrophobic solutes.
• Hydrophilic layer on NF270 did not affect rejection of salts and non-interacting molecules.
• An explanation for improved/reduced rejection has been proposed.

High affinity of non-ionized hydrophobic organics towards hydrophobic nanofiltration (NF) membranes is often the main cause for their poor removal by NF. Rejection of organics can be increased mostly by making membrane surface hydrophilic and less prone to adsorption. Although numerous studies reported benefits of such surface modifications, very few of them focused on revealing the mechanisms of increased rejection. The present paper aims at comparing measured rejections of various organic solutes of different hydrophilicity/hydrophobicity and sodium chloride on extremely thin NF film membrane, DowFilmTec NF270, before and after coating with poly(vinyl alcohol) (PVA). PVA coating increased the rejection of hydrophobic solutes from 5% to 30%, decreased rejection of hydrophilic solutes from 6% to 50% while the rejections of low interacting solutes, dioxane, and NaCl remained unchanged. Additionally, the rejections of organics of identical molecular mass and similar Stokes radii were compared and found to be governed by the solute/membrane energy of interaction, which correlated with corresponding log Kow values and dipole moments. The removal mechanism of NF270/PVA composite has been proposed based on the difference in measured rejections of solutes and estimated energy of interactions before and after modification.