Fast adsorption of Mn2+ and Zn2+ on commercial LTA zeolite: kinetics, equilibrium, and preliminary reuse assessment

Abstract

Commercial LTA-type zeolite was evaluated for the adsorption of Mn2+ and Zn2+ from aqueous solutions, with emphasis on the relationship between framework properties and adsorption behavior. Despite its very low BET surface area, the zeolite showed rapid and highly efficient uptake of both metal ions, reaching near-equilibrium within the first minute. Kinetic modeling indicated site-controlled adsorption consistent with pseudo-second-order behavior, while equilibrium data were described by the Langmuir model, confirming monolayer adsorption on uniform exchange sites. Intraparticle diffusion played a secondary role, and Zn2+ exhibited faster site occupation than Mn2+. Stoichiometric analysis of Na+ release during adsorption, together with blank experiments, demonstrated that metal uptake occurs mainly through Na+/divalent ion exchange within the zeolite framework, with partial release of loosely bound sodium. This ion-exchange mechanism explains the high adsorption capacities obtained despite the very low external surface area, since performance is governed by framework charge density rather than surface adsorption. Germination assays with Cucumis sativus showed no acute phytotoxic effects for metal-loaded zeolites, indicating potential for safe reuse as a micronutrient source. These results demonstrate that commercial LTA zeolite enables fast and efficient metal removal and provide a proof-of-concept for sustainable post-adsorption valorization of the spent material.