Parallel transport of BSA by surface and pore diffusion in strongly basic chitosan



The parallel transport of a protein by surface and pore diffusion within a highly porous ion exchanger is studied by measuring equilibria and uptake curves for adsorption of bovine serum albumin (BSA) on two different strongly basic chitosan ion exchangers (hard gels): Ch-2503 and Ch-2507 at pH 6.9 and 298 K. Experimental equilibrium isotherms are correlated by the Langmuir equation. Intraparticle effective diffusivities of BSA (Deff) are determined from the homogeneous Fickian diffusion model, increasing with the bulk phase concentration increase of BSA (C0). It suggests the existence of parallel diffusion. The surface diffusivity DS for the parallel diffusion model is determined from the Deff, and DS = 0.47 × 10−13 m2 · s−1 (Ch-2507) and 2.4 × 10−13 m2 · s−1 (Ch-2503) were obtained. Pore diffusivities based on the pore diffusion control (D′P) are obtained by matching the shrinking core model with the experimental uptake curves. D′P decreases with increasing C0. Since D′P in Ch-2507 is constant when C0 ≥ 1 kg·m−3, the constant value 2.7 × 10−11 m2 · s−1 is taken as the accurate pore diffusivity (DP). As D′P in Ch-2503 does not approach a constant value, DP = 1.0 × 10−11 m2 · s−1 is determined by matching the parallel diffusion model with the uptake data, and the model calculated using experimental values of DS and DP agrees reasonably well with the uptake data for Ch-2507 and Ch-2503. This theoretical approach may be applied not only for intraparticle diffusion of proteins but any adsorbates in porous materials.