Adsorption kinetics in the polyethylenimine–cellulose fiber system

The rate of adsorption of fractionated polyethylenimine (PEI) from water onto regenerated cellulose fibers was studied as a function of the polymer diffusion coefficient. Differences in polymer molecular weight, salt concentration, and pH were employed to vary the diffusion coefficient which was measured independently by a free‐diffusion technique. The sorption rate was measured at the same conditions and found to increase with decreasing molecular weight, increasing polymer concentration, decreasing salt concentration, and increasing pH. A simplified rate equation based on diffusion control with Langmuirian adsorption in stirred solution was developed by utilizing the concept of a Nernst diffusional film. The equation was successful in predicting the relationship between adsorption rate and diffusion coefficient for most cases studied. It was found, however, that a very large barrier to mass transfer retards the adsorption rate. For the system studied it was concluded that this barrier is a result of diffusion into and subsequent adsorption onto the internal porous structure of the cellulose.