The Nanopore Inner-Sphere Enhancement (NISE) Effect and its Role in Sodium Retention

Date of Completion

January 2012

Keywords

Agriculture, Soil Science|Environmental Sciences|Geochemistry

Degree

Ph.D.

Abstract

The nanopore inner-sphere enhancement (NISE) effect is a new theory which describes cation adsorption and ion exchange behavior inside confining environments such as the nanopores of zeolite minerals. The NISE theory proposes that weakly hydrated ions may dehydrate and adsorb to mineral surfaces in these environments via a strong inner-sphere mechanism. Conversely, strongly hydrated ions are not as capable of taking advantage of this dehydration mechanism due to their strong bonds with the water molecules in their hydration spheres. Adsorption data showed that Na, K, and Ca adsorbed weakly on a zeolite with large nanopores and strongly on a zeolite with small nanopores. On a zeolite with intermediate nanopores, Na and K adsorbed strongly but Ca adsorbed weakly. The ability of monovalent ions to strongly out compete divalent ions for adsorption sites in nanopore channels raises the possibility of the NISE effect playing a role in the strong retention of Na in clay-rich sodic soils. Spectroscopic investigations using nuclear magnetic resonance showed that Na adsorbed via an inner-sphere mechanism in small and medium nanopores and via an outer-sphere mechanism in large nanopores. An electron paramagnetic resonance study showed that Mn adsorbed via an outer-sphere mechanism in large nanopores and via an inner-sphere mechanism in small nanopores. A calorimetry study similarly showed that the exchange of outer-sphere Ca by inner-sphere Na in the medium nanopores was highly energetic and not easily reversible. The exchange between the two outer-sphere ions in the large nanopores was not energetic enough to exceed background fluctuations while the exchange between the two inner-sphere ions in the small nanopores was equally energetic and reversible.^ These investigations all confirmed the predictions made by the NISE theory regarding the adsorption mechanisms of monovalent and divalent ions in confining nanopore channels. A column experiment testing the effect of increasing clay content in a sand/clay mixture on ion retention showed that the adsorption of Ca increased sharply when the clay percentage increased from 25% to 30%. This indicates the possibility that the NISE effect may occur in clay-rich sodic soils.^

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