engleski

Effective diffusivity of organics through reverse osmosis membrane - test of hindrance factors correlations

A molecule moving through a narrow pore experiences friction with pore walls, which is usually expressed through diffusive and convective hindrance factors depending on the ratio between the pore and solute radii, λ. Parameter λ for a given solute may be calculated from the effective pore radius (e.g., estimated from water flux and Hagen-Poiuseuille equation) and the radius of the molecule that is usually taken to be the Stokes radius. Since the effective diffusivity of organic solutes through the membrane is rarely known, its value is often estimated based using popular correlations, such as ones proposed by Deen, Ferry or Bungay and Brenner. These correlations were originally developed for ideal straight cylindrical pores, yet their validity for complex random media such as polyamide reverse osmosis (RO) membranes has not been directly examined. Effective diffusivity on of n-alcohols within aromatic polyamide layer of commercial RO SWC4+ membrane was measured in this study. The measurements were performed using ATR-FTIR ; the model first developed by Fieldson and Barbari was used to deduce the sorption and diffusion of alcohols using from the intensity of the COH band, located in the 1000-1050 cm-1 region where polyamide and water do not show significant IR absorption. The thickness and swelling of the polyamide films necessary for the calculations were estimated using atomic force microscopy by measuring the thickness of dry and wet films. These results are compared with known correlations of diffusive hindrance factors (calculated effective diffusivities) and their validity is critically discussed.

reverse osmosis; hindrance factor; effective diffusion; polyamide film

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