Swelling–adsorption interactions during mercury and nickel ions removal by chitosan derivatives

•Swelling kinetics for Ni(II) and Hg(II) adsorption.•Chitosan grafted derivatives as adsorbents materials.•Enhanced capacity and rapid adsorption by adsorbents.•Diffusion-based adsorption model was successfully developed. Chitosan derivatives are extensively used as adsorbents for dyes and metal ions. These materials undergo swelling after their immersion in water. The maximum adsorption potential corresponds to completely swollen material. The process rates of swelling and adsorption are comparable leading to a dependence of the apparent adsorption rate on immersion history of the material. The way of performing the adsorption experiments for this type of materials is not clear in literature raising questions on the validity of the results. To clarify the situation a total of six adsorbate–adsorbent systems is studied here with respect to the effect of the swelling degree on the adsorption kinetics. Mercury (Hg(II)) and nickel (Ni(II)) ions were selected to be the model pollutants of this work and cross-linked chitosan microspheres grafted with chlorosulfonic acid (CSSULF) or ethylenimine (CSPEI) as adsorbent materials. It is shown that the adsorption kinetics is sensitive to the time of the adsorbent immersion in water. An attempt for development of a combined swelling–adsorption model is made. It is shown that the interaction between swelling and adsorption is more complex than a single water content dependence of diffusion coefficient. The evolution of diffusion coefficient that reproduces the experimental data is derived. Differences to the behavior between the three material used, are indicated and discussed.