The Nanopore Inner Sphere Enhancement Effect on Cation Adsorption: Sodium, Potassium, and Calcium

Nanopores in minerals can have a significant impact on cation adsorption, known as the nanopore inner-sphere enhancement (NISE) effect. Four zeolites representing three size classes of nanoporosity were used in this study to further elucidate the NISE phenomenon and describe its effects on the adsorption of Na, K, and Ca. Sodium and K ions have the same charge, but different ionic diameters. Sodium and Ca ions have similar ionic diameters, but different charges. Adsorption envelopes were created for these three cations on the four zeolites: zeolite Y, ZSM-5, ferrierite, and mordenite. Comparisons of the magnitudes of the adsorption of Na, K, and Ca on the four zeolites indicated that all cations adsorbed weakly on zeolite Y (large nanopores), Na and K adsorbed strongly on ZSM-5 and ferrierite (medium nanopores) while Ca adsorbed weakly, and all three cations adsorbed strongly on mordenite (small nanopores). It is noteworthy that in the medium nanopores, the two monovalent cations Na and K adsorbed more strongly than Ca, a divalent cation. The NISE effect is responsible for the changes in the relative strength of each cation's adsorption. This is an unusual adsorption mechanism and is counter to the traditional understanding of ion exchange reactions.