Novel model analysis for multimechanistic adsorption processes: Case study: Cadmium on nanochitosan

This study involves the development of a novel model encompassing two distinct mechanisms for the adsorption of cadmium ions onto nanochitosan to predict the composite equilibrium isotherm. The application of the multistage Freundlich isotherm analysis demonstrates that two regions are involved in the equilibrium process. This can be attributed to two types of adsorption mechanism, namely, chelation of the cadmium ions with the lone electron pairs on the free amino groups of chitosan and the second phase mechanism involves the surface complexation of the cadmium ions forming complexes of which more than one option is possible. Analysis of the high uptake loading enables the amount of cadmium adsorbed by each mechanism to be determined from the two regions of the Freundlich isotherm; the amino-chitosan-cadmium chelation capacity is 1.749 mmol Cd and the surface complexation capacity is 0.184 mmol/g. From these data the two individual isotherms for each mechanism are evaluated using a selection of different isotherm combinations to develop a better and more detailed representation of the multimechanism adsorption process. Based on the lowest SSE value, the best combination was Sips-Sips followed by the Sips-Langmuir for the chelation and surface complexation mechanisms respectively. Finally, a second optimisation modelling process has been applied using two generic isotherm equations to determine the isotherm constants without any Freundlich interstage loading capacities. The differences between the two approaches are significant, confirming that experimental adsorption data are required for each mechanistic stage independently.