Modelling the effect of pre-swelling on adsorption dynamics of dyes by chitosan derivatives

It is well known that chitosan-based materials serve as efficient adsorbents for dyes. This type of materials undergoes swelling with finite rate, when they are immersed in water, until to reach an equilibrium (steady state) volume. A problem that has been overlooked in the literature is the interaction between the two phenomena of swelling and adsorption. The characteristic times of two phenomena are comparable, so the swelling state of the adsorbent particle affects adsorption kinetics. In the present work, the interaction between these two phenomena is studied for several combinations of chitosan-based adsorbents and dyes, using experimental and theoretical tools. Several experiments for different polymeric adsorbents (chitosan derivatives) and dyes (reactive and basic) were performed. It is clearly shown that the adsorption kinetics are sensitive to the time of the adsorbent immersion in water. A unified swelling-adsorption model is developed. This model is shown to be able to describe/reproduce the experimental data and allow their proper interpretation in terms of physicochemical processes.

► Interaction between dye adsorption and swelling dynamics in chitosan adsorbents.
► Pre-immersion of adsorbents in water before typical adsorption kinetic experiments.
► Immersion (called pre-swelling time) has large influence on adsorption kinetics.
► Combined model: (i) lumped model for adsorption kinetics, (ii) simplified model for adsorbent's water distribution.