Temperature effects on sieving characteristics of thin-film composite nanofiltration membranes: pore size distributions and transport parameters

Crossflow filtration experiments were performed to measure transport of water and hydrophilic neutral organic solutes spanning a range of molecular sizes across two commercial thin-film composite nanofiltration (NF) membranes in the temperature range 5-41 degreesC. Non-viscous contributions to activation energies of pure water permeation across these polymeric membranes were calculated to be 3.9 and 6.4 kJ mol(-1). Analysis of solute rejection using a phenomenological model of membrane transport revealed that sizes of pores that contributed to rejection followed a lognormal distribution at any given temperature. Additionally, increasing temperature increased mean pore radii and the molecular weight cutoff suggesting changes in the structure and morphology of the polymer matrix comprising the membrane barrier layer. Consistent with the free volume theory of activated gas transport, activation energies of neutral solute permeability in aqueous systems also increased with Stokes radius and molecular weight indicating their hindered diffusion in membrane pores. All activation energies for pore diffusion calculated in this study were greater than just the viscous contribution to bulk diffusion demonstrating hindered transport across the nanofiltration membranes. Finally, similar to gas transport across zeolites and rubbers, the activation energy and the Arrhenius pre-exponential factor for hindered diffusion coefficients increased with solute size and were highly correlated with each other. (C) 2003 Elsevier B.V. All rights reserved.