Application of the Maxwell–Stefan approach to ion exchange in microporous materials. Batch process modelling

A model comprising external and intraparticle mass transfer resistances has been developed to describe ion exchange in microporous materials. The Maxwell–Stefan approach has been adopted due to their well documented advantages over Nernst–Planck relationships, particularly the facts of taking into account non-idealities, ion–ion and ion–solid interactions, and being easily applied to multicomponent systems.The model was tested with data available on literature, namely batch experiments on mercury (II) removal from aqueous solution using ETS-4 microporous titanosilicate (pore diameters between 3–4 Å). Calculated results point out it provides excellent fittings (AAD=4.93%AAD=4.93%; 44 data points) and exhibits fine predictive capability. Actually, the model is able to simulate ion exchange process with average deviations well inside the experimental accuracy (5–8%), using model parameters correlated from data measured under different operating conditions. This feature was analysed with three independent sets of data, and the average absolute deviations found increased only to 4.94%, 5.32%, and 7.72%. Such behaviour may be attributed to the sound physical principles of Maxwell–Stefan theory.The Nernst–Planck and the pseudo second-order kinetic models have been adopted for comparison. The Nernst–Planck based model provides higher deviation (AAD=5.57%)AAD=5.57%) but offers good representations also; the last one is totally unable to describe solution concentration along time (AAD=60.93%)AAD=60.93%), though it is one of the most applied equation in the field.