Dual-mode transport of inorganic acids through polybenzimidazole (PBI) membrane

The diffusion and permeation of sulfuric, perchloric, and phosphoric acids through the polybenzimidazole (PBI) membrane were evaluated by using the time-lag method. The results showed that the permeation of these acids in the polymeric film is due to normal Fickian diffusion. Increasing the acidity and concentration of each inorganic acid upstream increased the diffusion coefficients. Permeation results confirmed that the dual-mobility model was applicable. The dual-mobility model constants were found by using a least square program via fitting experimental data to dual-mobility equation. The results have indicated that the partial immobilization of Langmuir sorption coefficient, F, decreases with molecular size of acids. Besides, the diffusion coefficient of acid molecules in Henry's law, DD, increases with increasing acidity of the acids studied. Comparison of permeability coefficients of acids in PBI membrane indicates that permeability increases with acidity. Therefore, diffusion is considered the dominant factor in this transport mechanism.