Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays

Molecular insights into the temperature and pressure dependence of mechanical behavior and dynamics of Na-montmorillonite clay

Sodium montmorillonite (Na-MMT) clay mineral is a common type of swelling clay that has potential applications for nuclear waste storage at high temperatures and pressures. However, there is a limited understanding of the mechanical properties, local molecular stiffness, and dynamic heterogeneity of this material at elevated temperatures and pressures. To address this, we employ all-atomistic (AA) molecular dynamics (MD) simulation to investigate the tensile behavior of Na-MMT clay over a wide temperature range (500 K to 1700 K) and pressures (200 atm to 100 000 atm). The results show that increasing the temperature significantly reduces the tensile modulus, strength, and failure strain, while pressure has a minor effect compared to temperature, as seen in the normalized pressure-temperature plot. Mean-square displacement (MSD) analysis reveals increased molecular stiffness with increasing pressure and decreasing temperature, indicating suppressed atomic mobility. Our simulations indicate temperature-dependent dynamical heterogeneity in the Na-MMT model, supported by experimental studies and quantified local molecular stiffness distribution. These findings enhance our understanding of the tensile response and dynamical heterogeneity of Na-MMT clay under extreme conditions, aiding the development of clay minerals for engineering applications such as nuclear waste storage and shale gas extraction.

Interactions between micaceous minerals weathering and cesium adsorption

The environmental behavior of radioactive cesium (RCs) in contaminated areas is generally governed by soil and sediment components and natural weathering conditions. In this study, desorption tests and spectroscopic approaches were used to explore the interaction between the weathering of micaceous minerals (i.e., biotite and phlogopite) and the adsorption of Cs⁺ and the critical role of weathering in the environmental behavior of RCs. Results showed that the reaction sequence between weathering and Cs⁺ adsorption significantly affected the surface species of Cs⁺ and the structure of biotite and phlogopite. Regardless of whether it occurred before, after, or during Cs⁺ adsorption, weathering generated more high-affinity adsorption sites, namely, interlayer sites (ITs) and frayed edge sites (FESs), to different extents, and then facilitated the uptake of Cs⁺ at FESs and ITs on micaceous minerals in a poorly exchangeable state. Cs⁺ stabilized the micaceous mineral structure once it was absorbed within collapsed interlayers by hindering cation exchange and preventing further destruction during weathering. As important weathering factors, high temperature and Ca²⁺ content promoted the binding of Cs⁺ in the interlayers of biotite and phlogopite by enhancing interlayer cation exchange. These findings are beneficial for a better understanding of the environmental behaviors of RCs in the hydrosphere and pedosphere.

Molecular Origin of Wetting Characteristics on Mineral Surfaces

Accurate determination of the wetting characteristics on mineral surfaces is critical for many natural processes and industrial applications where multiphase flow in porous media is involved. The wetting behaviors on mineral surfaces are controlled by water-mineral interactions, giving rise to various wetting characteristics, including contact line advancement, formation of precursor films, etc. However, a fundamental understanding of wetting characteristics on different mineral surfaces is still lacking at the molecular level. Here, utilizing a comprehensive set of molecular dynamics simulations, we investigate the wetting characteristics of water on various mineral surfaces and obtain the corresponding water-mineral interaction properties (including the areal density of water-mineral interaction energy and the work of adhesion of the water-mineral interface), mineral wettability, and structural and diffusion properties of water molecules near the surface. We show that the diffusion properties of water molecules on mineral surfaces play an important role in wetting characteristics. We find that the contact line tends to advance forward in the jumping mode or the rolling mode during the wetting process, which depends on the diffusion capacity of the water molecules on mineral surfaces. The corresponding evolution of the solid-liquid friction coefficient during dynamic spreading is also analyzed. We further demonstrate the strong impact of isomorphic substitution and charge-balancing counterions on wetting characteristics on the surfaces of clay minerals. It is shown that the introduction of charge-balancing counterions can shift the mineral surface from strongly hydrophilic to strongly hydrophobic and lead to completely different wetting characteristics. Our results provide a clearer picture of the molecular underpinnings in mineral wetting phenomena and deepen the understanding of the control of water-mineral interactions on the wetting properties.

Hydrothermal-treatment desorption of cesium from clay minerals: The roles of organic acids and implications for soil decontamination

The adsorption and desorption of cesium (Cs) on clays of contaminated soil in a rhizosphere zone can be greatly affected by various biogeochemical processes, the timespans of which are usually months to years. Herein, we present several representative scenarios of the binding of Cs on diverse sites of vermiculitized biotite by controlled Cs adsorption to particles of different sizes. We investigated whether and how the fixed Cs in the different scenarios is desorbed by ambient and hydrothermal treatments with several low-molecular-weight organic acids (LMWOAs). The results showed that the sorbed Cs was discriminatively retained in the un-collapsed, partially collapsed, and thoroughly collapsed structures of vermiculites. The desorption of the sorbed Cs by hydrothermal LMWOAs extractions was easily realized in the un-collapsed structure, but was limited or minimal in the partially collapsed and thoroughly collapsed structures. The Cs desorption varied in accord with the LMWOA species applied and increased with the acid concentration, temperature, and number of treating cycles. The analysis of Cs-desorbed specimens confirmed their partial destruction and interlayer expansion, suggesting that the underlying mechanism of Cs removal by LMWOAs involves not only acid dissolution and complexation but also the accelerated weathering of clays within a short time under hydrothermal conditions. Our findings contribute novel insights into the mobility, bioavailability, and fate of Cs in contaminated soils and its removal from these soils for environmental restorations.

Selective removal of radiocesium from micaceous clay for post-accident soil decontamination by temperature-controlled Mg-leaching in a column

The effective and efficient removal of radioactive Cs from contaminated soil is highly urgent for the nuclear post-accident remediation. In present study, we achieved rapid Cs desorption from both a typical micaceous clay (i.e., vermiculitized biotite, VB) and actually contaminated soil by high-speed ion exchange through temperature-controlled continuous leaching with Mg-solutions in a column reactor. Cs-sorbed VB was firstly employed as a soil surrogate to explore the macro-Cs desorption process and micro-mechanism in detail. Results showed that VB sandwiched the adsorbed Cs to its interlayers within collapsed structure (10.7 Å) and prevent Cs release even by abundant extraction with H₂O at 250 °C or Mg²⁺ at 25 °C. However, Mg²⁺-extracted Cs desorption boosted significantly with elevating temperatures and 100 % of sorbed-Cs was removed from Cs-VB leached above 150 °C. Further structural and composition analysis of the leached specimen ensured that solvated Mg²⁺ preferentially entered into Cs⁺-collapsed interlayers at 150 °C than K⁺-interlayers above 200 °C, leading to prior complete Cs removal over K from VB at lower temperatures. By contrast, the Cs-contaminated soil reduced by ∼39 % but ∼82 % of its initial radioactivity after equally leaching with same volumes of Mg²⁺-solution at 150 and 200 °C, respectively. These temperature-controlled Cs desorption validated that radioactive Cs in actual soil indeed be tightly trapped by micaceous clays nearly in the Cs-K co-collapsed interlayers, to which its extraction by other cations can conditionally occur above enough high leaching temperatures. These superior features would inspire new insights for the design of novel practical technologies for treatment and decontamination of the nuclear post-accident soils.

Sorption and desorption of radiocesium by muscovite separated from the Georgia kaolin

Radiocesium (¹³⁷Cs) sorption by moderately weathered, sand-sized muscovite, obtained as a byproduct of kaolin ore processing, was observed at various concentrations of added stable Cs (0-100 μmol/L) over a 130 d period. After 18 h of batch sorption with 1 mmol/L NaCl as background electrolyte, conditional ¹³⁷Cs K_d values were near 2000 L/kg across the entire range of added stable Cs. Over four succeeding months, the K_d values increased by large factors for suspensions with little added Cs but increased only slightly for the suspensions with the most added Cs. The large grains of muscovite used in this study behaved distinctly differently than previously studied, much finer illite in that highly Cs-selective but low-abundance cation exchange sites appeared to be unavailable to the aqueous Cs during the first few days of the experiment. Diffusion pathways to highly Cs-selective sites were thought to be much longer in the muscovite than in frayed edges of illite, causing the highly Cs-selective sites to be isolated from the bulk solution. The longer diffusion pathways may be due to much greater stiffness of the material bounding interlayer wedges in the muscovite than in illite. This isolation from solution led to slow uptake at trace levels of Cs though the final K_d values (after 130 d) at those levels were comparable to those found for some illite. After 130 days, the original solutions were replaced by new electrolyte solutions containing no Cs, to observe ¹³⁷Cs desorption over another 130 d period. There was no indication of desorption of ¹³⁷Cs from the slowly accessible, highly Cs-selective sites apparently reached by most of the ¹³⁷Cs during sorption at the low Cs levels. The byproduct mica from kaolin processing might serve effectively as a chemically stable sorbent to isolate accidently released radiocesium and to hold it until the ¹³⁷Cs is virtually gone.

Radiocesium interaction with clay minerals: Theory and simulation advances Post-Fukushima

Insights at the microscopic level of the process of radiocesium adsorption and interaction with clay mineral particles have improved substantially over the past several years, triggered by pressing social issues such as management of huge amounts of waste soil accumulated after the Fukushima Dai-ichi nuclear power plant accident. In particular, computer-based molecular modeling supported by advanced hardware and algorithms has proven to be a powerful approach. Its application can now generally encompass the full complexity of clay particle adsorption sites from basal surfaces to interlayers with inserted water molecules, to edges including fresh and weathered frayed ones. On the other hand, its methodological schemes are now varied from traditional force-field molecular dynamics on large-scale realizations composed of many thousands of atoms including water molecules to first-principles methods on smaller models in rather exacting fashion. In this article, we overview new understanding enabled by simulations across methodological variations, focusing on recent insights that connect with experimental observations, namely: 1) the energy scale for cesium adsorption on the basal surface, 2) progress in understanding the structure of clay edges, which is difficult to probe experimentally, 3) cesium adsorption properties at hydrated interlayer sites, 4) the importance of the size relationship between the ionic radius of cesium and the interlayer distance at frayed edge sites, 5) the migration of cesium into deep interlayer sites, and 6) the effects of nuclear decay of radiocesium. Key experimental observations that motivate these simulation advances are also summarized. Furthermore, some directions toward future solutions of waste soil management are discussed based on the obtained microscopic insights.

Ion exchange selectivity in clay is controlled by nanoscale chemical-mechanical coupling

Ion exchange in nanoporous clay-rich media plays an integral role in water, nutrient, and contaminant storage and transport. In montmorillonite (MMT), a common clay mineral in soils, sediments, and muds, the swelling and collapse of clay particles through the addition or removal of discrete molecular layers of water alters cation exchange selectivities in a poorly understood way. Here, we show that ion exchange is coupled to the dynamic delamination and restacking of clay layers, which creates a feedback between the hydration state of the exchanging cation and the composition of the clay interlayer. Particles with different hydration states are distinct phases with unique binding selectivities. Surprisingly, equilibrium achieved through thermal fluctuations in cation concentration and hydration state leads to the exchange of both ions and individual MMT layers between particles, a process we image directly with high-resolution transmission electron microscopy at cryogenic conditions (cryo-TEM). We introduce an exchange model that accounts for the binding selectivities of different phases, which is likely applicable to many charged colloidal or macromolecular systems in which the structural conformation is correlated with the activities of water and counterions within spatially confined compartments.

Structure, Dynamics, and Stability of Water Molecules during Interfacial Interaction with Clay Minerals: Strong Dependence on Surface Charges

Water participates actively in a wide range of interfacial adsorption and reaction processes, and its structure, dynamics, and stability, all of which are crucial to these processes, have been addressed in the present study by means of molecular dynamics simulations. The structure and dynamic behaviors of interfacial water are distinct from those of bulk water and rely strongly on the amounts of surface charges in clay minerals; for example, H-bonds exist predominately among the interfacial water molecules for zero and low surface charges, whereas prevail between the water molecules and clay surfaces for high surface charges. Stabilities of the interfacial water molecules oscillate remarkably during an increase of surface charges that first decline and then are pronouncedly enhanced. Surface charges play a critical role during the interfacial clay/water interaction, and the interfacial water molecules are "liquid-like" at zero and low surface charges but "ice-like" at high surface charges. The present results greatly promote our understanding of clays/water interfaces that exist ubiquitously in environmental conditions.

Radiocesium interaction with clay minerals: Theory and simulation advances Post-Fukushima

Insights at the microscopic level of the process of radiocesium adsorption and interaction with clay mineral particles have improved substantially over the past several years, triggered by pressing social issues such as management of huge amounts of waste soil accumulated after the Fukushima Dai-ichi nuclear power plant accident. In particular, computer-based molecular modeling supported by advanced hardware and algorithms has proven to be a powerful approach. Its application can now generally encompass the full complexity of clay particle adsorption sites from basal surfaces to interlayers with inserted water molecules, to edges including fresh and weathered frayed ones. On the other hand, its methodological schemes are now varied from traditional force-field molecular dynamics on large-scale realizations composed of many thousands of atoms including water molecules to first-principles methods on smaller models in rather exacting fashion. In this article, we overview new understanding enabled by simulations across methodological variations, focusing on recent insights that connect with experimental observations, namely: 1) the energy scale for cesium adsorption on the basal surface, 2) progress in understanding the structure of clay edges, which is difficult to probe experimentally, 3) cesium adsorption properties at hydrated interlayer sites, 4) the importance of the size relationship between the ionic radius of cesium and the interlayer distance at frayed edge sites, 5) the migration of cesium into deep interlayer sites, and 6) the effects of nuclear decay of radiocesium. Key experimental observations that motivate these simulation advances are also summarized. Furthermore, some directions toward future solutions of waste soil management are discussed based on the obtained microscopic insights.

Complex Formation via Hydrogen bonding between Rhodamine B and Montmorillonite in Aqueous Solution

This study investigates the adsorption mechanism differences among four nitrogenous dyes, sulforhodamine G (SRG), uncharged/deprotonated rhodamine B (RhB), orange II (Or II) and methyl blue (MB) by montmorillonite (MMT). MMT adsorption capacity for cationic MB was three times that of uncharged RhB and anionic SRG, while anionic Or II was not absorbed. Colloidal MMT particles have two types of surfaces, basal and edge, that interact with nitrogenous dyes very differently. The surface acidity of MMT was characterized with the pyridine adsorption method using in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS). Adsorption of cationic MB was compared with the adsorption of RhB. In-situ attenuated total reflectance Fourier transform infrared (in-situ ATR-FTIR) spectroscopy indicated that a nitrogen atom on RhB complexes with a metal hydroxyl on an MMT edge through a water bridge. The highly polar edge hydroxyl is important to hydrogen bond formation. Cation ion exchange and washing experiments, as well as studies on the effect of temperature, pH and ionic strength on adsorption further clarified the adsorption mechanism. Our results provide insights into the effects of molecular structure on the adsorption of nitrogenous dyes by clay and the role of edge surfaces in the adsorption process.

Dynamical inversion of the energy landscape promotes non-equilibrium self-assembly of binary mixtures

When driven out of equilibrium, many diverse systems can form complex spatial and dynamical patterns, even in the absence of attractive interactions.

When driven out of equilibrium, many diverse systems can form complex spatial and dynamical patterns, even in the absence of attractive interactions. Using kinetic Monte Carlo simulations, we investigate the phase behavior of a binary system of particles of dissimilar size confined between semiflexible planar surfaces, in which the nanoconfinement introduces a non-local coupling between particles, which we model as an activation energy barrier to diffusion that decreases with the local fraction of the larger particle. The system autonomously reaches a cyclical non-equilibrium state characterized by the formation and dissolution of metastable micelle-like clusters with the small particles in the core and the large ones in the surrounding corona. The power spectrum of the fluctuations in the aggregation number exhibits 1/f noise reminiscent of self-organized critical systems. We suggest that the dynamical metastability of the micellar structures arises from an inversion of the energy landscape, in which the relaxation dynamics of one of the species induces a metastable phase for the other species.

Effects of NH4+, K+, Mg2+, and Ca2+ on the Cesium Adsorption/Desorption in Binding Sites of Vermiculitized Biotite

The reversibility of cesium adsorption in contaminated soil is largely dependent on its interaction with micaceous minerals, which may be greatly influenced by various cations. Herein, we systematically investigated the effects of NH₄⁺, K⁺, Mg²⁺, and Ca²⁺ on the adsorption/desorption of Cs⁺ into different binding sites of vermiculitized biotite (VB). Original VB was initially saturated by NH₄⁺, K⁺, or Mg²⁺; we then evaluated the adsorption of Cs⁺ on three treated VBs, and the desorption by extraction with NH₄⁺, K⁺, Mg²⁺, or Ca²⁺ was further evaluated. Our structural analysis and Cs⁺ extractability determinations showed that NH₄⁺ and K⁺ both collapsed the interlayers of VB, resulting in the dominant adsorption of Cs⁺ to external surface sites on which Cs⁺ was readily extracted by NH₄⁺, K⁺, Mg²⁺, or Ca²⁺ irrespective of their species, whereas Mg²⁺ maintained the VB with expanded interlayers, leading to the overwhelming adsorption of Cs⁺ in collapsed interlayer sites on which the Cs⁺ desorption was difficult and varied significantly by the cations used in extraction. The order of Cs⁺ extraction ability from the collapsed interlayers was K⁺ ≫ Mg²⁺ ≈ Ca²⁺ ≫ NH₄⁺. These results could provide important insights into Cs migration in soil and its decontamination for soil remediation.