A DFT Perspective on Cd²⁺ Removal by CaCO₃/FAU Zeolite Frameworks: Bridging Theory and Experiment

The efficient removal of toxic heavy metals from wastewater remains a critical environmental challenge. Faujasite (FAU) zeolite is highly effective for this purpose due to its high surface area, microporous structure, and exceptional ion exchange capacity. This study employs density functional theory (DFT) calculations to investigate the thermodynamics and geometry of Cd2+ capture by two distinct processes: (i) ion exchange and (ii) CaCO3/FAU adsorption. We systematically identify preferential binding sites and calculate binding energies for Cd2+ within the FAU framework. As a key result, the current theoretical modeling confirms previous experimental observations that dispersed CaCO3 units on the surface of faujasite enhance Cd2+ adsorption compared to aggregated units. The computed binding energies for the dispersed configuration range from -54 kcal/mol to -60 kcal/mol, depending on the number of CaCO3 units. These findings provide a fundamental theoretical perspective and represent a first step toward understanding Cd2+ removal by modified zeolites.