Membrane transport of organics. III. Permeation of some carboxylic acids through bipolar polymer membrane

The permeation of acetic (AA), propionic (PA), lactic (LA), oxalic (OA), ci tric (CA), and tartaric (TA) acids through the bipolar ion-exchange membran e Neosepta BP-1 (Tokuyama Corp.) was studied. It was found that the fluxes (J, mol cm(-2) s(-1))and mass-transfer coefficients (k, cm s(-1)) increase in the following order: CA < OA < LA < TA < PA less than or equal to AA. Th e transport processes in the Neosepta BP-1 membrane are concentration-depen dent and can be described phenomenologically using I-Fick's law for diffusi on. The permeation phenomena correspond to the solution-diffusion model sim ilarly as to the permeation of carboxylic acids through strongly acidic cat ion-exchange membranes. However, in competitive AA-PA transport experiments , typically for strongly basic membranes, the separation ability of the BP- 1 membrane with a preference toward AA was observed. The selectivity coeffi cients alpha (AA)(PA) calculated as the ratio of the respective mass-transf er coefficients vary in the range from 1.31 +/- 0.2 to 2.1 +/- 0.6. These v alues depend on the feed composition and the system arrangement, which mean s that alpha (AA)(PA) is always higher for the system with the anion-exchan ge layer is in contact with a feed solution. Rather low fluxes of PA, AA, a nd other acids, as compared to some monopolar membranes (Neosepta AFN-7, Na fion-120, Flemion), are promising for the application of the bipolar membra ne in an electrodialytic separation of carboxylic acids from their aqueous solutions or mixtures. (C) 2001 John Wiley & Sons, Inc.