Abstract
Crystalline hydrates of swelling clay minerals (smectites) exhibit a strong coupling between their ion exchange and hydration/dehydration reactions. The uptake or removal of water from smectite interlayers as a result of a change in the environmental conditions also leads to the partitioning of cations. Three factors, the solid ion composition, the solid basal spacing/water content, and the aqueous solution composition, are all implicated in controlling the thermodynamics of ion exchange. However, conventional approaches to measuring the exchange free energy cannot separate the influence of each of these individual factors. Here, we explore the energetics of the swelling and ion exchange reactions in montmorillonite using a potential of mean force approach and the thermodynamic integration method within molecular simulations. We investigate the influence of solution and clay composition on the spontaneity of the reactions, focusing on the 2 water-layer hydration state. The swelling simulations provide the equilibrium water content, interlayer water structure, and basal spacings, while thermodynamic integration of sodium–potassium exchange in the aqueous solution and solid phase are combined to calculate ion exchange free energies as a function of solution composition. Results confirm the tendency of the clay to collapse to lower hydration states as the concentration of the solution increases. Changes to the equilibrium water content, even at fixed hydration states, and the composition of the mixed electrolyte solution play a critical role in driving ion exchange and the selectivities of the clay to the exchanged cation, while the composition of the solid phase is shown to be insignificant. These findings underscore the extreme sensitivity of clay swelling and ion exchange thermodynamics to small (tenths of an Angstrom) deviations in layer spacing.
Highlights
The adsorption/desorption of micronutrients and the accumulation/dilution of industrial contaminants in soils that eventually impact the health and safety of the biosphere are controlled by ion exchange processes
We observe only a shallow potential well associated with the 4 W hydration state for the clay particle in the 1.0 M and 1.9 M solutions and the minima are not statistically significant based on the error estimates
These profiles indicate that clay compaction is favored in the presence of concentrated solutions, which has been supported by recent experimental and computational studies [17,46,47], and that the degree of compaction depends on the solution chemistry
Summary
The adsorption/desorption of micronutrients and the accumulation/dilution of industrial contaminants in soils that eventually impact the health and safety of the biosphere are controlled by ion exchange processes. Ion exchange processes are extensively used for the selective removal of heavy metals from wastewater, water softening, and the recovery of Li+, Na+, etc. Smectites (swelling clays), which are critical components of engineered geological barriers for the safe, longterm storage of nuclear waste, exhibit high cation exchange capacity (CEC). Montmorillonites (MMT – a type of smectite clay mineral) are effective barriers because their expansive behavior, upon contact with water, fills the voids between the container and the wall rock providing sufficient sealing of the nuclear waste repository. The dominant hydration content in the clayrich barrier correspond to the 2- (2 W) and 3-water layer (3 W) swelling states. The type and composition of cation in the interlayer determines the equilibrium water content and basal spacing for a specific hydration state [8,15]
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