Insights on the statistical physics modeling of the adsorption of Cd2+ and Pb2+ ions on bentonite-chitosan composite in single and binary systems

In this study, a bentonite-chitosan composite was tested for the single and binary adsorption of Cd2+ and Pb2+ ions at constant pH (6) and different temperatures (30, 40, and 50 °C). For both adsorbates, the adsorption capacities decreased in binary systems due to an antagonistic adsorption effect. Statistical physics models were used to analyze the adsorption isotherms and to attribute microscopic interpretations to the experimental evidences at a molecular level. Specifically, a monolayer model with one energy and an exclusive extended monolayer model were applied to single and binary adsorption data, respectively. The crossing of the experimental and modelling results proved that Cd2+ ion was more adsorbed in the binary system. The modelling analysis allowed determining that the numbers of ions linked per adsorbent site varied significantly in binary system with respect to single-compound ones, thus confirming that an inhibition effect occured. Finally, the adsorption energy was estimated for both adsorption systems indicating the occurrence of an endothermic adsorption. Moreover, adsorption energies showed concordant trends with the corresponding adsorption capacities of both Cd2+ and Pb2+, hence being able to describe the adsorption phenomena.