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Effect of Cations on Interlayer Water Dynamics in Cation-Exchanged Montmorillonites Studied by Nuclear Magnetic Resonance and X‑ray Diffraction Techniques

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journal contribution
posted on 26.03.2020, 20:13 by Sun Rongwei, Takehiko Tsukahara
To ensure the safety of geological disposal of radioactive wastes, understanding the migration behavior of radioactive species in montmorillonite clays has become increasingly important. However, there are still many indeterminate aspects about the influence of cation species and humidity on the interlayer water dynamics and swelling properties of montmorillonite clays. In this work, by using X-ray diffraction (XRD) and proton nuclear magnetic resonance spin–lattice relaxation rate (1/T1) techniques, we aimed to clarify the relation between water layer thickness and molecular dynamics in various cation-exchanged montmorillonites (Mn+-MMTs; Mn+ = Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, La3+, Sm3+, and Lu3+) in the temperature range of −40 to 50 °C. XRD measurements showed that layer thickness in Mn+-MMTs increased with increasing hydrated water according to the order of structure-breaking chaotropic K+, Rb+, and Cs+ ions, structure-making kosmotropic and borderline Li+, Na+, and Ba2+ ions, divalent ions, and trivalent lanthanide ions. It was found that the 1/T1 values are linearly correlated with d001 basal spacing and increase in the interlayer thickness induces acceleration of the motion of the interlayer water molecules. The temperature dependence of the 1/T1 values verified that interparticle water and interlayer water, regardless of whether it was frozen or unfrozen, coexist above approximately −5 °C, but only unfrozen interlayer water with poor hydrogen bonding rearrangement properties exists below −5 °C. Moreover, the T1-distribution results suggest that motions of the water molecules in the Mn+-MMTs can be uniquely characterized by a value of about 0.2 ms under supercooled environments because of dominant water–surface interactions.