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

Abstract

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/T 1) techniques, we aimed to clarify the relation between water layer thickness and molecular dynamics in various cation-exchanged montmorillonites (M n+-MMTs; M n+ = 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 M n+-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/T 1 values are linearly correlated with d 001 basal spacing and increase in the interlayer thickness induces acceleration of the motion of the interlayer water molecules. The temperature dependence of the 1/T 1 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 T 1-distribution results suggest that motions of the water molecules in the M n+-MMTs can be uniquely characterized by a value of about 0.2 ms under supercooled environments because of dominant water–surface interactions.

Atomistic Insights into the Interlayer Cation and Water Structures of Na-, K-, and Cs-Birnessite

“…Experimental data for K- and Cs-buserite (i.e., 2 W states) are unavailable for comparison with the calculated interlayer cation positions in the 2 W hydration state. However, a cation distribution similar to our MD results was reported in swollen montmorillonite (MMT), in which K + cations were located near the silicate surface, whereas the Na + cations resided at the midplane in the 2 W state interlayer. , In experiments, Cs-MMT appeared to absorb only up to 1 W of water in air, but an MD simulation study predicted two split densities of Cs + when MMT was swollen to the 2 W state, as in our MD simulations of the Cs-buserite phase.…”

Effect of Temperature on the Diffusion and Sorption of Cations in Clay Vermiculite

“…For example, as the temperature decreases, the distribution of interlayer water tends to be more uniform. 24 In this case, the diffusion coefficients of water and cations increase with increasing temperature. It is known that temperature has a stronger effect on water molecules.…”

“…As a result, a layered crystal structure is formed. 24 Therefore, the diffusion rate of interlayer cations strongly depends on the content of water molecules in the interlayer space (or is associated with the properties of clay swelling). In humid conditions, interlayer cations can interact with water molecules, forming various complexes depending on their charge and ionic radius.…”

“…Therefore, in recent years, many scientists have studied the effect of elevated temperatures of deep geological reservoirs on the processes of interaction between clay and various cations, including radioactive ones. − Temperature affects different processes in different ways. For example, as the temperature decreases, the distribution of interlayer water tends to be more uniform . In this case, the diffusion coefficients of water and cations increase with increasing temperature.…”

“…Equilibrium is achieved through checks and balances between water molecules and cations between the clay layers. As a result, a layered crystal structure is formed . Therefore, the diffusion rate of interlayer cations strongly depends on the content of water molecules in the interlayer space (or is associated with the properties of clay swelling).…”