Permeability of bacterial cellulose membranes

Mass transfer experiments were conducted to determine the transport and interaction parameters of selected molecules in hydrated bacterial cellulose (BC) membranes. The objective was to determine physiochemical characteristics and elucidate the mechanisms governing transport in relation to the membrane structure. Pore and sorption models developed previously for the analysis of transport in hydrogel membranes were relevant to the cellulose membrane system, including: (1) interfacial phenomena between the bulk fluid and the outer membrane surfaces and/or along a pore wall, (2) sorption into the membrane matrix itself with diffusion possibly affected by immobilization at specific interactive sites, (3) free and/or fixed site diffusion within the matrix and if appropriate, through the porous regions, whether as distinct pores, micro-channels or other non-homogeneous/discrete areas and (4) chemical reactions that Could alter the nature of the diffusing species or the media itself. Since all these mechanisms may be active, our classification, and thus characterization, was based upon whichever mechanism dominates. The diffusion coefficients of various dextrans (44-260 kDa), measured using a horizontal flow diffusion chamber, were compared to their corresponding values in water to demonstrate hindered diffusion. The degree of hindrance, psi, was the same for all four dextrans indicating that the parameter is not a function of pore or micro-channel size but rather due to the presence of membrane fibers. Three representative marker molecules (Vitamin B 12, lysozyme and bovine serum albumin) were evaluated using the same apparatus combined with desorption experiments to measure permeation (P) and effective diffusion (D-eff) coefficients. Partition coefficients, H, were subsequently calculated and verified experimentally. Estimates of D-eff in the BC membranes were made through a weighted average using literature values for the diffusion of the three molecules in water and regenerated cellulose along with measured H and psi values. The experimental results are in excellent agreement with these estimates indicating the presence of dual transport mechanisms, for solute transport through the continuous water phase and cellulose matrix, with some hindrance of molecular diffusion via fiber obstruction. With Vitamin B 12 and lysozyme (the two smallest solutes), equilibrium interactions such as adsorption and solubility are also important. These results help clarify the potential utility of these novel bio-derived membranes in a variety of possible separations scenarios. (c) 2005 Elsevier B.V. All rights reserved.