Further Insight in the High Selectivity of Pb(2+) Removal over Cd(2+) in Natural and Dealuminated Rich-Clinoptilolite.

This research aims to understand the experimental results on the high selectivity of Pb(2+) removal over Cd(2+) in natural and dealuminated rich-clinoptilolite. For this purpose, we have considered the results of experimental and Density Functional Theory (DFT)-based simulated annealing (SA) on sorption of Pb(2+) and Cd(2+) from aqueous solution. The dealumination process of natural clinoptilolite (Nat-CLI) was done by H(2)SO(4) solutions at different concentrations (0.1-1.0 M). The results show that the maximum sorption capacity (q(,max)) of Pb(2+) and Cd(2+) varied from 224.554 × 10(-3) to 53.827 × 10(-3) meq/g, and between 39.044 × 10(-3) to 20.529 × 10(-3) meq/g, respectively, when the values of Si/Al ratio change from 4.36 to 9.50. From a theoretical point of view, the global minimum energies of natural and dealuminated clinoptilolites before and after sorption of Pb(2+) and Cd(2+) were calculated by an SA method, where heating-cooling cycles were modeled by ab initio Molecular Dynamics followed by energy minimization. The theoretical results confirmed that for all Si/Al ratios, the sorption of Pb(2+) and Cd(2+) takes place, and for dealuminated systems, the exchange energy outcomes are more favorable for the Pb(2+) cations. Since such energy differences are very small, it is not explained from a thermodynamic point of view. On the other hand, it could be understood from a kinetic perspective. In this way, we set that the atomic structural properties of the zeolite modify the first hydration coordination sphere of metal cations.