Transport mechanisms of dissolved organic compounds in aqueous solution during nanofiltration

Abstract

This paper investigates the relative contribution of convection, diffusion and charge effects during transport of dissolved organic compounds in aqueous solution using nanofiltration membranes. Measurements were carried out in a crossflow nanofiltration unit and in diffusion cells using four organic compounds (tryptophane, raffinose, benzylidene acetone and mandelic acid) in aqueous solution and five commercial nanofiltration membranes (UTC-20, Desal-HL-51, NTR-7450, NF270 and NF-PES-010). Diffusion was analyzed by the Donnan model for diffusion across a charged membrane. The charge effect was studied by changing the pH of the feed solution, which influences both the membrane and compound charge. It could be concluded that convection is the dominant transport mechanism for the organic compounds used. Furthermore, solute transport depends on the compounds properties. Uncharged compounds show a slight decrease of retention at high pH, which can be explained by an enlargement of the membrane pores due to an increased repulsion between the charged acidic groups. For charged compounds an increase of the retention is observed with increasing pH due to an increased repulsion between the charge of both compound and membrane. Finally, diffusion through membranes with tight pores (reflected by the MWCO) becomes more important as the molecular weight of the compound increases because steric hindrance in narrow pores mainly affects convective transport.