Paraquat, one of the most widely utilized herbicides globally, causes a significant environmental challenge due to its poor degradation rate and tendency to adsorb into clay interlayers. Several remediation methods have been proposed but their effectiveness remains suboptimal. The primary reason for this is the lack of microscopic understanding of paraquat–montmorillonite interactions. In this work molecular dynamics simulations were applied to study the interlayer structures and mobility of paraquat intercalated montmorillonite. Two stable hydration states were identified from the calculated immersion energy curve, which corresponded to a water content of 185 mgwater/gclay and 278 mgwater/gclay (the most stable). Paraquats remained in direct contact with the clay surface in both the anhydrous and hydrated states. At the water content of 185 mgwater/gclay, paraquats formed π-π stacking while at 278 mgwater/gclay, they were separated by a layer of water. Paraquat showed very small self-diffusion coefficients in the interlayer space of montmorillonite, indicating rather limited motions. The results in this work provide a basis for a better understanding of the interaction of paraquat with clay minerals.

In order to understand the microscopic properties of alkylammonium-intercalated vermiculites, molecular dynamics simulations employing the clayff-CVFF force field were performed to obtain the interlayer structures and dynamics. The layering behavior of alkyl chains was uncovered. With the model used in the present study (1.2 e per unit cell), the alkyl chains formed monolayers with carbon-chain lengths of C6, bilayers from C7 to C10, and pseudo-trimolecular layers from C15 to C18. Intermediate states also existed between bilayer and pseudo-trimolecular layer states from C11 to C14. The ammonium groups had two locations: most ammonium groups were located over the six-member rings (~90%), and the rest above the substitution sites (~10%). The ammonium groups interacted with the vermiculite surface through H bonds between ammonium H atoms and surface O atoms. The ammonium groups were fixed firmly on surfaces and, therefore, had very low mobility. The alkyl chains were slightly more mobile. The similarities and differences between alkylammonium-intercalated vermiculites and smectites were revealed. The layering behaviors of alkyl chains were similar in alkylammonium-intercalated vermiculites and smectites: the alkyl chain behavior was controlled by both the amount of layer charge and the carbon chain length. The distributions of ammonium groups, however, were different, caused by the layer-charge distribution in vermiculites being different from that in smectites. The atomic-level results derived in the present study will be useful for future research into and the applications of organo-vermiculites.

The structure and dynamics of alkylammonium-intercalated smectites were simulated using molecular dynamics employing the clayff-CVFF force field, the reliability of which was firstly validated for these systems. The layering behaviors of alkyl chains confirm the scenarios of the monolayer, transition and bilayer configurations for short, medium-length and long carbon tails, respectively. In the systems without water, the alkylammonium groups are all anchored firmly above the surface six-member rings through H bonds between ammonium hydrogen and surface oxygen, and the alkyl tails are a little more mobile. With water involved, some ammoniums are dragged out of the potential barriers of the six-member rings by water molecules through the strong H bonds between water oxygen and ammonium hydrogen. The intercalated water scarcely affects the basal spacing, alkyl chain layering or alkylammonium dynamics. It is also found that the systems with alkyl chains of 11 to 14 exhibit the greatest density, resulting in the extremely limited mobility of the intercalated species.

Using first-principles molecular-dynamics simulations, probable inner-sphere complexes of Fe2+ adsorbed on the edge surfaces of clay minerals were investigated. Ferrous ions are important reductants in natural processes and their properties can be altered significantly by complexation on edge surfaces of clay minerals. However, the microscopic picture of adsorption sites and structures of Fe2+ is difficult to reveal with modern experimental techniques and, therefore, remains unclear. From the results of first-principles molecular-dynamics simulations, evidence has been provided that complexes on ≡Si—O sites were the most stable forms, which should be responsible for the experimentally observed pH-dependent uptake. Such complexation was found to be strong enough to distort the local coordination structures of Si—O tetrahedra in the substrate. Analyses showed that Fe2+—Owater coordination structures were dominated by the solvent with surface groups participating in the complexes via H bonding. The present study provided a microscopic basis for understanding the chemical processes involving surface-complexed Fe2+ ions.

In order to identify the influences imposed by Fe substitution, density functional theory-based Car-Parrinello molecular dynamics simulations were employed to study both oxidized and reduced Febearing smectites. The following basic properties were investigated: local structures in the clay layer, hydroxyl orientations, and the vibration dynamics of H and Si. Structural analyses indicated that the average Fe-O bond lengths are ~2.08 Å and 2.02 Å in the reduced and oxidized models, respectively, and the Fe substitutions did not affect the coordination structures of the Al-O and Si-O polyhedra. For hydroxyl orientations, Fe(III) substitution had no obvious influence but Fe(II) forces the coordinated hydroxyls to present a wide-angle distribution. Furthermore, the present work has shown that both substitutions can red-shift the hydroxyl in-plane bending mode. The analyses also revealed that Fe(III) substitution has no effect on the Si-O stretching, while Fe reduction causes a blue-shift of the out-of-plane stretching mode. The results provide quantitative constraints and clues for future research.

Aiming to identify the complexing mechanisms of heavy metal cations on edge surfaces of 2:1-type clay minerals, systemic first-principles molecular dynamics (FPMD) simulations were conducted and the microscopic structures and complex free energies were obtained. Taking Cd(II) as a model cation, the structures on both (010) and (110) edges of the complexes were derived for the three possible binding sites (≡SiO, ≡Al(OH)2/≡AlOH≡AlSiO, and vacant sites). The stable complexes adsorbed on the three binding sites on both terminations had similar structures. The free energies of the complexes on (010) edges were calculated by using the constrained FPMD method. The free energies of complexes on the ≡SiO and ≡Al(OH)2 sites were similar and they were both significantly lower than the free energy of the complex on the octahedral vacant site. In association with the concept of high energy site (HES) and low energy site (LES) in the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange (2SPNE SC/CE) sorption model, the vacant site was assigned to HES and the other two sites to LES, respectively.