An investigation of strong sodium retention mechanisms in nanopore environments using nuclear magnetic resonance spectroscopy

Recent experimental research into the adsorption of various cations on zeolite minerals has shown that nanopore channels of approximately 0.5 nm or less can create an effect whereby the adsorption of ions, especially those that are weakly hydrated, can be significantly enhanced. This enhanced adsorption occurs due to the removal of hydrating water molecules which in turn is caused by the nanopore channel's small size. A new adsorption model, called the nanopore inner-sphere enhancement (NISE) effect, has been proposed that explains this unusual adsorption mechanism. To further validate this model a series of nuclear magnetic resonance (NMR) spectroscopy studies is presented here. NMR spectra were gathered for Na adsorbed on three zeolite minerals of similar chemical composition but differing nanoporosities: zeolite Y with a limiting dimension of 0.76 nm, ZSM-5 with a limiting dimension of 0.51 nm, and mordenite with a limiting dimension of 0.26 nm. The NMR experiments validated the predictions of the NISE model whereby Na adsorbed via outer-sphere on zeolite Y, inner-sphere on ZSM-5, and a combination of both mechanisms on mordenite. The strong Na adsorption observed in these nanoporous minerals conflicts with sodium's general designation as a weak electrolyte.