Caffeine Adsorption on Natural and Synthetic Smectite Clays: Adsorption Mechanism and Effect of Interlayer Cation Valence
H Sakuma and K Tamura and K Hashi and M Kamon, JOURNAL OF PHYSICAL CHEMISTRY C, 124, 25369-25381 (2020).
There is currently significant interest in adsorbents for selective removal of caffeine from water. Here, we examined caffeine adsorption in a natural montmorillonite (Mnt) smectite clay mineral. A simple exchange of the original monovalent sodium interlayer cations with polyvalent aluminum cations increased the caffeine adsorption rate of the smectite by approximately 1.6 times (maximum adsorption from a 1.5 mmol L-1 aqueous solution of caffeine: Na+Mnt, 57.1%; Al(3+)Mnt, 91.2%). These adsorptions were well fitted by the Langmuir adsorption model indicating, that the adsorptions were limited by equilibrium. Humidity- controlled X-ray diffraction confirmed that caffeine adsorption occurred mainly in the interlayer nanospace. Na-23 and Al-27 magic-angle spinning-nuclear magnetic resonance (MAS-NMR) analyses revealed an attractive interaction between the interlayer cations and the caffeine molecules. The Al-27 MAS-NMR spectrum of Al(3+)Mnt indicated that most of the interlayer Al3+ was coordinated with six water molecules. From these findings, we developed an interaction model that simulated caffeine adsorption by Al(3+)Mnt. Molecular dynamics simulation revealed that adsorption occurred via electrostatic interactions between the caffeine molecules and the interlayer cations, not the basal planes of the clay. Optimization of the electrostatic interactions and nanospace available for the adsorption will be crucial for developing highly efficient, selective, clay-mineral-based adsorbents of caffeine molecules.
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