(430b) Temperature Effects On Sieving Characteristics Of Thin-Film Composite Nanofiltration Membranes: Pore Size Distributions And Transport Parameters

Even though polymeric nanofiltration (NF) and reverse osmosis (RO) membranes often operate on surface waters and surficial groundwaters whose temperature varies over time and with season, very little detailed mechanistic information on temperature effects on membrane selectivity is available to date. Hence, a study was undertaken to investigate the effects of operating temperature on the morphology and structure of two commercially available thin film composite NF membranes.

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 membranes in the temperature range 5 ? 41 ºC. 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. Analysis of solute rejection using a phenomenological model of membrane transport revealed that sizes of pores in the thin barrier layer of the thin-film composite membranes 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.

Application of hydrodynamic models to experimental rejection of dilute solutions of hydrophilic neutral alcohols, sugars, and polyethylene glycols revealed changes in both the sieving coefficient and permeability of solutes below the membrane glass transition temperature. The vast majority of pores were smaller than 2 nm for both membranes (network pores) even though evidence for a small fraction of larger aggregate pores (~ 30 nm) was also obtained for one membrane. Increasing temperature appears to cause structural changes in network pores by increasing its pore size while simultaneously decreasing pore density. These increases in pore sizes partially explain reported reductions in contaminant (e.g. arsenic, salts, natural organic matter, hardness, etc.) removal by NF and RO membranes with increasing temperature.

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.