The Nanopore Inner Sphere Enhancement Effect on Cation Adsorption: Sodium and Nickel
- C. P. Schulthess *a,
- R. W. Taylorb and
- D. R. Ferreirac
This study compared the adsorption behavior of Na+ and Ni2+ by zeolite minerals that differed only in the physical size of the nanopores in their interstitial regions. Any differences observed in the adsorption of these ions were interpreted in terms of how the physical variations of their nanopores might be involved. The adsorption strength of Ni2+ was weak with zeolite Y and ZSM-5 but strong with mordenite. The adsorption strength of Na+ was weak with zeolite Y but strong with ZSM-5 and mordenite. Even when Na+ adsorbed weakly, its adsorption strength was comparable to that of Ni2+ Similarly, when both adsorbed strongly, they both adsorbed with nearly the same strength. The adsorption reaction is a three-way competition between H+, Na+, and Ni2+ for the adsorption sites. We propose a new theory to explain the increase in adsorption by the ZSM-5 zeolite for Na+ and by mordenite for Na+ and Ni2+ The new theory states that a dehydrated inner sphere adsorbing ion will be stable if either the ion's attraction to the surface site is stronger than toward the bulk water or the ion's attraction to the bulk water is offset by hindering the presence of water in the region. This occurs when the nanopore diameters are larger than the ionic diameter of the ion but much smaller than the hydrated diameter of the ion, which occurs with interstitial cavities that are not greater than approximately 0.3 to 0.5 nm in size in at least one dimension.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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