Molecular Dynamics Simulation Study on Adsorption Characteristics of Illite for Hg²⁺

Abstract

The Three Gorges Reservoir area of the Yangtze River has formed vast riverine fallout zones as a result of its periodic water storage and flood discharge operations, and the main constituents of this area are quaternary loose clays. It is important to study the microscopic characteristics of clay minerals in these fallout zones and their adsorption properties of Hg²⁺ to guide the environmental safety of the fallout zones in the Three Gorges Reservoir area. In this context, the authors of this paper used X-ray diffraction (XRD) experiments to reveal the main clay mineral compositions in the fallout zones and then constructed the molecular model structures of the clay minerals based on molecular dynamics theory and studied the adsorption characteristics of these clay minerals with Hg²⁺ in depth. The results show that the main clay minerals in the Three Gorges Reservoir area fallout zone include illite, illite-mixed layer and green-mixed layer, in which the content of illite ranges from 21% to 54%. Taking illite as the study object, the heat of adsorption of Hg²⁺ in illite ranged from 14.83 kJ·mol⁻¹ to 31.92 kJ·mol⁻¹, which is a physical adsorption. The heat of adsorption was mainly affected by the water content and had little relationship with temperature. With the gradual increase in water content, the heat of adsorption gradually decreases. The adsorption amount of Hg²⁺, on the other hand, is jointly affected by water content and temperature and decreases with the increase in water content and temperature; under natural environmental conditions (P = 0.1 Mpa), the adsorption characteristics of Hg²⁺ in illite change with the change in water content. When the water content was between 0% and 6.95%, the increase in water content led to an increase in the interlayer spacing of illite, and the adsorption of Hg²⁺ in illite was in a monolayer state, with the adsorption peaks located from 4.5~5.5 Å. When the water content increased to 6.95% to 13.90%, the layer spacing of illite reached the maximum, and the adsorption of Hg²⁺ in illite transitioned from a monolayer to a bilayer, with the adsorption peaks located between 5 Å and 9~10 Å, respectively. When the water content was further increased to 13.90% to 20.85%, the increase in water content instead led to a slight decrease in the layer spacing of illite, showing a tendency of transitioning from a bilayer to a monolayer adsorption layer, which at the same time changed the number of adsorption layers of Hg²⁺; the study also revealed that the interaction between illite and Hg²⁺ was regulated by van der Waals and Coulomb forces, whereas the increase in temperature promoted the Hg²⁺ +diffusion, and an increase in water content inhibits the diffusion of Hg²⁺. In summary, these findings provide valuable theoretical support for solving the problem of Hg²⁺ pollution in the Three Gorges Reservoir Decline Zone.

1. Introduction

Subject to the alternating cycle of reservoir storage–flood discharge processes, the Three Gorges Reservoir area has formed a large dissipation zone [1]. As an important reservoir–bank transition area, the fallout zone is important for people’s production and life [2]. However, industrial sewage and domestic wastewater contain a large amount of heavy metal pollutants, which migrate into the receding zone through transformation, decomposition, diffusion and other methods, causing harm to the entire ecosystem [3]. Soil sediments in the Three Gorges Reservoir area are mainly contaminated by heavy metal elements (Cd, Pb, Cr, Hg, Cu, Zn, As), of which Hg and As are the most serious [4,5].

In the soils of the Three Gorges Reservoir area fallout zone, clay minerals are some of the main components, and the study of their properties is of great significance in solving the heavy metal pollution remediation problems in this area. With the continuous updating of scientific means, the study of clay minerals is no longer limited to traditional experimental methods, and scholars have gradually carried out in-depth investigation of their structures from the perspective of molecular simulation. For example, Ma et al. [6] systematically investigated the adsorption configurations, density distributions and adsorption energies of H₂O, CO₂, CH₄, N₂ and C₃F₈ in kaolinite by using molecular dynamics methods. They discussed the intermolecular binding energies, adsorption distances and free energies under different pressure and temperature conditions. Yang Youwei et al. [7] investigated the adsorption behavior of kaolinite on NH⁴⁺ by molecular simulation, analyzed the heat of adsorption and the amount of adsorption of kaolinite on NH⁴⁺ at different water contents and explored the effect of water molecules on the adsorption between kaolinite and NH⁴⁺. The study by Ren Yu et al. [8] focused on the molecular simulation of illite and explored the physicochemical and mechanical properties such as the basic layer spacing, interaction energy and diffusion coefficient of water molecules of illite under different conditions of temperature and pressure and interlayer water saturation, thus promoting a deeper understanding of the microstructure of illite. On the other hand, Benoit Carrier et al. [9] investigated the effects of different water contents, different temperatures and interlayer cations on the mechanical properties of montmorillonite. In addition, Najafi et al. [10] and Mosavi Mirak et al. [11] enhanced the adsorption capacity of clay minerals for pollutants in wastewater by introducing modified column-supported clays with elements such as Fe, Al and Ti, resulting in more efficient adsorption. The study of clay minerals is gradually moving from experimental to molecular simulation, which provides effective methods and approaches for a more accurate and in-depth understanding of their properties and potential application in environmental remediation.

Currently, there are many studies on the microscopic and gas adsorption properties of clay minerals, but molecular simulation of Hg²⁺ adsorption is still rare. Therefore, the authors carried out a systematic sampling of typical areas in the Three Gorges Reservoir area and relied on X-ray diffraction (XRD) experiments to obtain the main components of the clay minerals in the Three Gorges Reservoir area fallout zone with a combination of qualitative and quantitative methods and constructed the ilmenite model and used the Monte Carlo method to study the Hg²⁺ adsorption in different water contents in ilmenite. The mechanism of clay minerals regarding Hg²⁺ was studied from the perspective of molecular dynamics. The research results are expected to provide theoretical guidance for the prevention and control of Hg²⁺ pollution in the Three Gorges Reservoir Decline Zone.

2. Materials and Methods

2.1. Sample Collection

The Three Gorges Reservoir area, spanning from western Chongqing to eastern Hubei, exhibits distinct erosion and deposition zones. This study aims to investigate the characteristics of these zones by collecting soil samples from six areas, including Changshou District, Fuling District, Zhongxien County, Wanzhou District, Fengjie County, and Wushan County (Figure 1). The samples were obtained from locations along the Yangtze River, where fresh and humid samples appeared as light gray to dark gray, while dry samples were predominantly light gray to light yellow in color. X-ray diffraction analysis revealed that the erosion and deposition zone soil samples primarily consisted of quartz, feldspar and clay minerals [12]. Among the clay minerals, illite, kaolinite, illite–smectite mixed layer and chlorite–smectite mixed layer were the dominant components (

Table 1. Table of relative content of clay minerals in water-level fluctuation zone of Three Gorges Reservoir area.

Sample of the Three Gorges Reservoir Area’s Water-Level-Fluctuating Zone	Relative Content of Clay Minerals/%
	Illite	Kaolinite	Chlorite	Illite–Smectite Mixed Layer	Chlorite–Smectite Mixed Layer
Changshou District	44	8	11	16	21
Fuling District	50	9	14	24	3
Zhongxien County	44	7	9	37	3
Wanzhou District	54	10	14	13	9
Fengjie County	21	3	2	38	36
Wushan County	40	2	3	37	18

). Illite exhibited the highest relative content, ranging from 21% to 54%, followed by the illite–smectite mixed layer and chlorite–smectite mixed layer. Additionally, small amounts of kaolinite and chlorite were also present.

2.2. Experimental Methods

2.2.1. X-ray Diffraction Detection Analysis

To study the composition of clay minerals in the Yangtze River ablation zone, which were sent to the Unconventional Experimental Centre of China National Offshore Oil Corporation Energy Development Co., Ltd. (Located in Tianjin, China) for XRD experiments, the mineralogical compositions of six groups of samples were determined using a Rigaku Dmax-2500 X-ray diffractometer under the following conditions: a Cu target, Kα radiation, a 1 mm/8 mm/2.5°/Ni filter and a slit system with a DS of 1°. The operating voltage was 40 KV, the operating current was 30 mA and the scanning was carried out in the range of 3° to 45° (angle 2θ) at a step rate of 2 deg/min. The detection temperature was 21 °C and the humidity was controlled at 43%.

2.2.2. Construction of Illite Models

This research paper focuses on the geotechnical properties of illite, a prominent clay mineral found in the erosion and deposition zone of the Three Gorges Reservoir area. The study aims to investigate the adsorption behavior of Hg²⁺ on illite under varying water content conditions. To achieve this, a clay mineral pore model was constructed using Materials Studio 2020 software (Accelrys, San Diego, CA, USA). Illite, a layered silicate clay mineral containing potassium, exhibits a characteristic T-O-T layered structure with interlayer potassium serving as the center of symmetry. The single-cell model of illite was derived from spatial coordinate data by Drits possessing a monoclinic crystal system with space group C2/m, α = γ = 90°, β = 101.57°, a = 5.20 Å, b = 8.97 Å and c = 10.226 Å. In the silicon–oxygen tetrahedron of illite, every 8 Si atoms are replaced by Al atoms, thereby balancing the resulting negative charge with interlayer K⁺ [13].

Illite’s periodic interlayer structure was utilized to construct a clay mineral supercell with dimensions of 4a × 2b × c (Figure 2a). As the erosion and deposition zone serves as a transitional area between the reservoir and the bank, the differential hydration of water molecules, varying in content, influences the adsorption behavior of Hg²⁺ on illite. In order to construct illite models under different water content conditions, we added a certain number of water molecules (Figure 2b) to the illite model to turn it into a model of illite with different water content (

Table 2. Water saturation of different water molecules.

Number of Water Molecules	0	10	20	30
Water saturation/%	0	6.95	13.90	20.85

).These models were subsequently analyzed and simulated using computational methods.

2.2.3. Illite Model Optimization

This research paper focuses on the geotechnical properties of illite, a prevalent clay mineral in geotechnical engineering applications. The study aims to improve the stability and accuracy of the initially established illite model through geometric optimization. To achieve this, the Forcite module was employed, utilizing the steepest descent algorithm in the Geometry Optimization task. The optimization process aimed to minimize the structure energy and attain a more stable and optimal illite model. The maximum iteration step was selected, ensuring that the energy of the model system no longer decreased. The CLAYFF force field, specifically designed for clay minerals, was utilized, along with appropriate charge calculations [14]. Electrostatic interactions were computed using the Ewald sum method, while van der Waals interactions employed the atom-based method with a cutoff radius of 9.5 Å. Figure 3 shows the changes in the illite structural model before and after optimization, the molecules within the circles indicate that a positional shift has occurred during the optimization process to achieve the lowest energy state.

Table 3. Changes in energy before and after optimization of illite model (in brackets, the total uncertainty).

		Initial Structure/kJ·mol−1	Final Structure/kJ·mol−1
Valence energy	Bond	1022.46 (0.01)	36.28 (0.01)
	Valence electron total energy	1022.46 (0.01)	36.28 (0.01)
Non-bond energy	Van der Waals	1942.79 (0.01)	2559.86 (0.01)
	Electrostatic	−336,060.82 (0.2)	−348,844.97 (0.2)
	Three-Body	1104.12 (0.01)	115.03 (0.01)
	Non-bond total energy	−315,570.38 (0.22)	−323,186.92 (0.22)
Total energy of system		−314,547.92 (0.24)	−323,150.64 (0.24)

displays the energy comparison of the illite model before and after optimization. The energy of the optimized model reached the lowest point, indicating improved overall stability.

2.2.4. Adsorption Simulation Calculation

This research paper focuses on the adsorption simulation of Hg²⁺ on illite models with different water contents, contributing to the understanding of pollutant behavior in geotechnical engineering. The Hg²⁺ adsorption model was manually constructed in the Visualizer window, with charges added using the Forcite module. The Sorption module facilitated the Hg²⁺ adsorption calculation, allowing for free movement of the adsorbate. The Fixed Pressure task employed the Metropolis method, setting a constant pressure of 101 kPa, with temperatures varying from 278 K to 318 K. The simulation involved equilibration and production steps of 1.5 × 10⁶ steps, while other settings remained consistent with the model optimization. The simulation was conducted on four groups of illite models with different water contents. Subsequently, the movement trajectory of Hg²⁺ within illite was analyzed using the Dynamics function in the Forcite module. Molecular dynamics simulation employed the canonical ensemble (NVT) with the Nose temperature control system, a simulation time of 500 ps, a time step of 1 fs, the Universal force field and automatic charge allocation. Other parameters aligned with those used for model optimization.

3. Results and Discussion

3.1. Interlayer Spacing of Illite with Different Water Contents

This research paper focuses on the analysis of interlayer spacing and configuration of illite models with different water contents, contributing to the understanding of the swelling behavior of illite in contact with water. The interlayer spacing was calculated for four groups of illite models, representing water contents of 0%, 6.95%, 13.90% and 20.85% at normal temperature and pressure. The obtained interlayer spacings were 10.22 Å, 12.59 Å, 14.30 Å and 13.80 Å, respectively, indicating an increase of 0%, 23.19%, 39.92% and 35.03% compared to the dry state. These results demonstrated the accuracy of the constructed illite model [15]. In Figure 4, it is shown that the layer spacing of the illite model varies with the water content, confirming the swelling properties of illite in contact with water. Within the range of 0–20 water molecules, the interlayer spacing exhibited a positive correlation with the number of water molecules, indicating an increase in interlayer spacing as the number of water molecules increased. However, when the number of water molecules reached 20–30, the interlayer spacing decreased, suggesting that the interlayer spacing reached its limit at 20 water molecules. The decrease in interlayer spacing beyond this threshold was attributed to changes in the form of water molecules [16].

Formula (1) represents the calculation method for determining the interlayer spacing of illite [8]:

$$d=\frac{<V>}{<a><b><c>}$$

(1)

In the equation, <V> represents the simulated model volume, and <a>, <b> and <c> are the corresponding average values of the model cell parameters.

3.2. Adsorption Characteristics of Illite and Hg²⁺ under Different Water Contents

3.2.1. Adsorption Characteristics of Clay Minerals and Hg²⁺

The adsorption results of Hg²⁺ in various illite water models at different temperatures are simulated and shown in Figure 5.

This research paper focuses on the adsorption characteristics of Hg²⁺ on illite models with different water content to reveal the behavior of heavy metal contaminants in geotechnical engineering. The simulation results showed that the heat of adsorption of Hg²⁺ ranged from 14.83 kJ·mol⁻¹ to 31.92 kJ·mol⁻¹, indicating the presence of physical adsorption. The enthalpy of adsorption indicates the strength of adsorption from adsorbent to adsorbent. Greater adsorption enthalpy indicates stronger adsorption capacity, while lower adsorption enthalpy suggests weaker adsorption capacity. Figure 5a demonstrates that Hg²⁺ adsorption enthalpy is influenced by water content but not by temperature. As the water content increases, the adsorption enthalpy decreases. This can be attributed to the expansion of illite caused by the presence of water, leading to an increase in the interlayer distance. Consequently, the contact probability between Hg²⁺ and the illite surface decreases, resulting in reduced adsorption capacity and enthalpy. This finding is consistent with previous studies [17,18,19,20]. The phenomenon can be explained using adsorption theory, where a smaller interlayer distance indicates closer surfaces of the illite, leading to overlapping adsorption forces [21]. This overlapping force is stronger than the force between the adsorbate at a larger interlayer distance. Therefore, illite with a smaller interlayer distance exhibits higher Hg²⁺ adsorption capacity and larger adsorption enthalpy.

Figure 5b illustrates that the numerical value of Hg²⁺ adsorption is related to the water content and temperature of the illite model. When the water content is 0%, the numerical value of Hg²⁺ adsorption is highest, gradually decreasing with increasing water content. This phenomenon occurs because a higher water content results in a larger interlayer distance. However, the interlayer distance has an upper limit and cannot increase indefinitely. As a result, in a limited adsorption space, the interlayer cation K⁺ of the illite layer and the increasing water molecules occupy some space, reducing the adsorption space available for Hg²⁺ [22]. Additionally, water molecules entering the interlayer region can affect the interlayer force, thereby reducing Hg²⁺ adsorption [23]. The decrease in Hg²⁺ adsorption with increasing temperature is primarily due to the physical exothermic process of Hg²⁺ adsorption on the illite surface. Elevated temperatures intensify the kinetic energy of Hg²⁺ within the clay mineral, enhancing its movement and indirectly influencing the magnitude of Hg²⁺ adsorption [24,25].

Based on the simulation results, the Hg²⁺ adsorption capacity in the 13.90% water content illite model ranged from 0.09 to 0.2 mmol·g⁻¹. These results closely align with the ion adsorption outcomes and experimental values obtained by other researchers in models with a water content of 12%, validating the reliability of the simulation results [7,26,27].

3.2.2. Spatial Distribution of Hg²⁺ in Illite

In order to deeply investigate the relationship between the spatial distribution changes in Hg²⁺ in the illite model and the temperature and water content, we carried out the molecular dynamics simulation of illite adsorption of Hg²⁺ and plotted the corresponding concentration distribution curves and adsorption density maps, which are presented in Figure 6 and Figure 7. Figure 6 and Figure 7 show the effects of temperature and water content on the adsorption of Hg²⁺ by illite. From Figure 6, it can be seen that the change of temperature has no significant effect on the shape of the concentration distribution curve of Hg²⁺ adsorbed by illite, while the change of water content leads to a significant change of the curve. The effect of water content on the adsorption of Hg²⁺ by illite is more clearly shown in Figure 7, where the distribution of Hg²⁺ (small red particles) between the illite layers changes from a monolayer to a bilayer as the water content increases. However, combining the results in Figure 6 and Figure 7, we clearly see that under natural environmental conditions (P = 0.1 Mpa), different water contents significantly affected the concentration distribution of Hg²⁺ adsorbed in illite. Hg²⁺ adsorption concentration changes with water content can be divided into three stages: in the first stage, when the water content was from 0% to 6.95%, the spacing of illite layers increased from 10.22 Å to 12.59 Å. Under the condition of 0% water content, an adsorption layer was formed with its peak located in the range of 4.5~5.5 Å, which is the center of the layer spacing. With the gradual increase in water content, the illite layer spacing widened, and the adsorption layer gradually shifted from a monolayer to a bilayer, although the change was less pronounced. The second stage occurs in the range of water content from 6.95% to 13.90%. With the increase in water content, the two adsorption layers became more obvious. At 13.90% water content, the illite layer spacing reaches a maximum value of 14.30 Å and the two adsorbed layers are most pronounced. Due to the overall symmetry of illite, these two adsorption layers also show a symmetrical distribution between them, with the first peak occurring at 5 Å and the second peak from 9~10 Å. The two adsorption layers also show a symmetrical distribution between the two adsorption layers. The water content in the third stage ranges from 13.90% to 20.85%. In this stage, with the increase in water content, the spacing between the illite layers no longer continued to increase, but instead began to decrease from 14.30 Å to 13.80 Å. The peaks of the two adsorption layers showed that one was larger than the other, which showed a tendency towards merging into a single adsorption layer. In summary, the results clearly indicate that the distribution of Hg²⁺ adsorption concentration in illite is closely related to the water content of illite. More specifically, the change in water content directly determines the size of the layer spacing of illite. With the gradual increase in layer spacing, the adsorption behavior of Hg²⁺ gradually evolved from the initial monolayer adsorption to bilayer adsorption. As the layer spacing gradually decreased, the adsorption behavior of Hg²⁺ also showed a tendency to change from bilayer adsorption to monolayer adsorption [7].

This research paper investigates the intermolecular interactions between illite and Hg²⁺ using the radial distribution function. The radial distribution function provides insights into the probability of finding particles at a given distance from a central particle, elucidating the interaction characteristics between different water contents of illite and Hg²⁺ [28]. Formula (2) represents the radial distribution function [29].

$$g_{\alpha \beta }\left(r\right)=\frac{n_{\beta }}{4{\rho }_{\beta }\pi r^{2}dr}$$

(2)

In the equation, n_β is the radius; number of other particles is in the range of r → r + dr; ρ_β is the crystal density, g/cm³; r is the distance to study particles.

Molecular dynamics simulations were conducted to study the radial distribution functions of the O atom on the surface of the illite water model and Hg²⁺ at different temperatures. The results are summarized in

Table 4. Peak value g(r) and radius r of radial distribution function.

Water Content	Temperature/K	Highest Peak Value g(r)	r/Å	Water Content	Temperature/K	Highest Peak Value g(r)	r/Å
0%	278	1.38	7.71	6.95%	278	1.35	6.93
	288	1.32	7.41		288	1.37	8.35
	298	1.39	8.07		298	1.43	8.41
	308	2.61	7.23		308	1.40	7.61
	318	1.41	6.39		318	1.39	9.61
13.90%	278	1.38	11.43	20.85%	278	1.39	7.25
	288	1.39	8.81		288	1.40	7.31
	298	1.34	11.03		298	1.40	8.97
	308	1.35	11.41		308	1.42	9.63
	318	1.47	8.43		318	1.54	7.17

. In the radial distribution function graph, the highest peak g(r) corresponds to the radius r. If the radius r exceeds 3.5 Å, it signifies that the interaction between the studied entities is mediated by Coulomb and van der Waals forces. Conversely, if the radius r is less than 3.5 Å, it suggests that hydrogen bonds and chemical bonds contribute to the interaction between the entities [30]. Analysis of Table 4 reveals that the highest peak values for the different water-containing illite models at varying temperatures correspond to a radius r greater than 3.5 Å, indicating that illite adsorbs Hg²⁺ primarily through van der Waals and Coulomb forces.

3.2.3. The Self-Diffusion Coefficient of Hg²⁺ in Illite

This research paper investigates the self-diffusion coefficient of Hg²⁺ in hydrated illite through molecular dynamics simulations. The self-diffusion coefficient represents the relationship between particle position change and time during migration in space, reflecting particle-to-particle and particle-to-pore surface collisions. The three-dimensional Einstein equation is used to derive the self-diffusion coefficient, as expressed in Formula (3) [31].

$$D=\underset{t\to \infty }{\lim }\frac{1}{6t}\left\{{\left[r\left(t\right)-r\left(0\right)\right]}^2\right\}$$

(3)

In the equation, {[r(t) − r(0)]²} represents the mean square displacement of the particle center of mass, which is determined through molecular dynamics analysis. The self-diffusion coefficient D is calculated as the slope of the mean square displacement raised to the power of 1/6.

Molecular dynamics simulations were performed to study the self-diffusion coefficient of Hg²⁺ adsorbed in hydrated illite at different temperatures. The results are presented in Figure 8. The figure illustrates that the self-diffusion coefficient of Hg²⁺ ranges from 0.37 × 10⁻⁹ m²·s⁻¹ to 1.45 × 10⁻⁹ m²·s⁻¹. The value increases with rising temperature and decreases with increasing water content. The increase in value with temperature can be attributed to the elevated kinetic energy of Hg²⁺, intensifying its movement and causing it to escape from the adsorption site. This weakens the interaction force between illite and Hg²⁺, thereby promoting an increase in the self-diffusion coefficient of Hg²⁺. Conversely, the introduction of water molecules in illite leads to a decrease in the self-diffusion coefficient of Hg²⁺, as the added water molecules interact with Hg²⁺ and inhibit its diffusion [22].

4. Conclusions

(1) Through X-ray diffraction (XRD) analysis of six sets of samples from the Three Gorges Reservoir Decline Zone, illite was found to be one of the main constituents, with its relative content ranging from 21% to 54%. In order to study the adsorption characteristics of illite, we used molecular simulation (Materials Studio 2020 software from China University of Mining and Technology, Xuzhou, China) to build a model of illite and added water molecules according to different water contents to obtain the structural model of illite at different water contents. The layer spacing of illite was 10.22 Å, 12.59 Å, 14.3 Å and 13.8 Å at water contents of 0%, 6.95%, 13.8% and 20.85%, and the percentage of layer spacing increase was 0%, 23.19%, 39.92% and 35.03%, respectively.

(2) The results of the adsorption simulation showed that the adsorption heat distribution of illite with Hg²⁺ ranged from 14.83 kJ·mol⁻¹ to 31.92 kJ·mol⁻¹, which is a physical adsorption. In the illite model, the heat of adsorption of Hg²⁺ was mainly affected by the water content, and the heat of adsorption of Hg²⁺ decreased with the increase in water content. Meanwhile, the adsorption of Hg²⁺ was jointly affected by the water content and temperature, and the increase in water content or the increase in temperature would lead to the decrease in the adsorption of Hg²⁺ by illite, indicating that higher water content and temperature would reduce the adsorption capacity of illite for Hg²⁺.

(3) The results of molecular dynamics simulations showed that the concentration of Hg²⁺ in illite with different water contents exhibited three stages. At a water content of 0% to 6.95%, Hg²⁺ was mainly adsorbed in the interlayer of illite, forming a monolayer adsorption layer with its peak value located from 4.5~5.5 Å. When the water content increased to 6.95~13.90%, the interlayer spacing of illite reached a maximum at 13.90% water content, and the adsorption layer transitioned from a monolayer to a bilayer, with peaks located between 5 Å and 9~10 Å, respectively. However, when the water content continued to increase to 13.8–20.85%, the layer spacing of illite decreased instead of increasing the water content, and the bilayer peak of the adsorbed layer appeared to be asymmetric, which showed the transition from a bilayer to monolayer. In addition, the radial distribution function indicated that the adsorption of Hg²⁺ by illite was mainly connected to van der Waals and Coulomb forces, while the self-diffusion coefficients of Hg²⁺ in illite ranged from 0.37 × 10⁻⁹ m²·s⁻¹ to 1.45 × 10⁻⁹ m²·s⁻¹, and the increase in temperature promoted the diffusion of Hg²⁺, while the increase in water content inhibited the diffusion of Hg²⁺.