Exploration of Methylmercury Adsorption on Montmorillonite Surfaces Through Density Functional Theory

Abstract

To propel the development of a robust methylmercury immobilisation technology, CH₃Hg⁺ adsorption on montmorillonite surfaces was simulated herein using density functional theory. This study involved a thorough molecular-level analysis, including factors such as electron potential energy, molecular orbital configurations, stable adsorption configurations, adsorption energies, charge distributions, and density of states. The principal findings are summarised as follows: (1) CH₃Hg⁺ adsorption on the (001) surface was characterised by an adsorption energy ranging from −27 to −51.7 kJ/mol. In this case, Hg was attracted to the involved silicon–oxygen ring cavities. Meanwhile, on the (010) surface, CH₃Hg⁺ exhibited an adsorption energy ranging between −119.4 and −154.3 kJ/mol. In this case, Hg was attracted to hydroxyl groups such as ≡Al(OH)(OH₂) and ≡Si(OH), forming a covalent bond with the oxygen atom of these groups. (2) Comparative analysis revealed that the adsorption energy of CH₃Hg⁺ on the (010) surface surpassed that on the (001) surface. On the (001) surface, electrostatic interactions were the predominant factor influencing adsorption, while on the (010) surface, electrostatic and covalent bonding interactions were important. Notably, the strength of electrostatic interactions was greater on the (001) surface than on the (010) surface. (3) The formation of covalent bonds between CH₃Hg⁺ and the (010) surface was primarily attributed to the overlap of electron cloud between Hg and surface O atoms. In particular, the interaction between the s orbital of Hg and the p orbital of O facilitated the formation of a σ bond. Overall, these findings provide a theoretical framework for the advancement of efficient in situ immobilisation technologies for methylmercury.

1. Introduction

Mercury contamination in soil predominantly arises from various anthropogenic activities, including coal combustion and mining operations, alongside mineral resource utilisation [1]. Notably, mercury is highly toxic and converts into methylmercury (CH₃Hg⁺) via bacterial metabolism. This intensifies its toxicity and exacerbates its adverse effects on human health and the environment [2].

Several methodologies have emerged for the remediation of mercury-contaminated soils, including phytoremediation, thermal desorption, and immobilisation. Amongst these techniques, immobilisation, in particular, is considered promising for treating soil with low concentrations of Hg [3]. Clay and its derivatives have been identified as favourable remediation agents, demonstrating a high capacity for immobilising the heavy metal ions present in soil [4]. Wang [5] utilised calcium-carbonate-enriched clay minerals and diammonium phosphate as immobilisation agents to realise efficient Hg immobilisation in soil matrices.

Similarly, Li [6] used illite–smectite clay with bone chars added in a certain proportion as a curing agent to achieve substantially low Cd contents in soil and brassica. The modification of clay minerals enhances their specific surface areas and ion-exchange capabilities, augmenting their capacity to bind heavy metal ions [7]. In one study, humic-acid-modified montmorillonite demonstrated a reduction of 65.9% in the Hg concentration in a leachate [8]. The immobilisation of heavy metals by clay minerals primarily occurs through mechanisms such as sorption, precipitation, and adsorption [9,10].

Owing to the advancements in computer technology, the adsorption mechanism of heavy metal ions on clay minerals can be analysed from a microscopic molecular perspective using simulations. Du [11,12] employed density functional theory (DFT) to simulate the adsorption of chromium (Cr(III)) and cadmium (Cd(II)) on clay mineral surfaces. The (010) surface displayed a greater adsorption energy compared to the (001) surface, with variations in pH levels and ion concentrations influencing the adsorption sites and ionic forms present on these surfaces. DFT is effective for investigating molecular-level interactions between heavy metal ions and clay minerals, facilitating effective understanding of these complex interactions.

As a clay mineral, montmorillonite exhibits excellent adsorption capacity for heavy metal ions [13]. In particular, the regulability of its interlayer domain can be modified using physical and chemical methods to enhance its adhesion to heavy metal ions [14]. To improve the fixing effect of montmorillonite on methylmercury, the interface of montmorillonite must be purposefully regulated. Therefore, exploring the microscopic action mechanism between methylmercury and the surface of montmorillonite is imperative. However, research focusing on the interaction mechanisms between methylmercury and montmorillonite remains limited. Hence, we employed DFT to perform a comprehensive analysis of the molecular-level characteristics of methylmercury adsorption on various montmorillonite surfaces. The analysis involved factors such as electron potential energy, molecular orbital configurations, adsorption configurations, adsorption energies, charge distributions, and density of states. The insights provided herein serve as a theoretical foundation for developing efficient in situ immobilisation technologies for methylmercury.

2. Modelling and Calculations

2.1. Material Surface Models

The surface models of montmorillonite were constructed according to Peng’s methods [15,16]. The specific procedures employed are outlined below. Initially, Wardle’s model [17] was employed for simulating montmorillonite crystal cells, which were designed as supercells with dimensions of 2 × 1 × 1. Subsequently, aluminium atoms within the involved aluminium oxide octahedron were substituted by magnesium atoms, resulting in a chemical formula of Na_0.5Al_3.5Mg_0.5Si₈O₂₀(OH)₄. This adjustment was consistent with required lattice substitution ratios specific to montmorillonite. Then, the initial cell structures were optimised and the surface models were developed. To investigate the microscopic adsorption mechanisms of methylmercury on montmorillonite crystal faces, surface models were extracted from the lattice phase along the (001) and (010) surfaces. Further, each surface model included a 15 Å thick vacuum layer introduced as the normal to the surface.

The surface models were then optimised, generating refined montmorillonite surface models (Figure 1). As shown in Figure 1A, the montmorillonite (001) surface exhibits a tetrahedral–octahedral–tetrahedral structure comprising two layers of SiO₄ tetrahedra, sandwiching an AlO₆ octahedron. Within the AlO₆ octahedron, Mg²⁺ replaces Al³⁺, creating a negative surface charge. Na⁺ is used to balance this charge. A silicon–oxygen ring is observed on the (001) surface. On the (010) surface (Figure 1B), the exposed Si atoms form coordination bonds with OH groups, forming ≡Si–OH structures. Moreover, Al atoms coordinate with OH groups to form ≡Al–OH or ≡Al–(OH)(OH₂) structures. The constructed montmorillonite surfaces for the (001) and (010) planes comprise 82 and 105 atoms, respectively.

2.2. Calculation Methods

The initial unit cells of montmorillonite were optimised using the Dmol3 module in Material Studio software (Version 2017, Accelrys Corporation, San Diego, CA, USA) based on DFT [18,19]. This optimisation was performed using the Perdew–Burke–Ernzerhof exchange–correlation functional within the generalised gradient approximation framework [20] and involved Tkatchenko–Scheffler (TS) dispersion corrections. Full electron calculations were performed, and the double-numerical polarised atomic orbital basis set was used. During computation, the electron spin remained unrestricted, with the initial spin configuration being adopted at the outset. Further, the Broyden–Fletcher–Goldfarb–Shanno optimisation algorithm was employed, with the self-consistent field convergence criterion set to 1 × 10⁻⁶ eV/atom.

For geometric optimisation, the following convergence criteria were defined: a maximum atomic displacement of 5 × 10⁻³ Å, maximum interatomic force of 2 × 10⁻³ Ha/Å, and total energy change in the system of 1 × 10⁻⁵ Ha. All calculations were executed in the reciprocal space, with a 1 × 2 × 1 sampling point K in the Brillouin zone [21] selected. The optimised crystal parameters for montmorillonite were as follows: a = 5.24, b = 9.08, and c = 10.96 Å, with angles α = 86.30°, β = 100.5°, and γ = 90.14°. These values closely align with the experimental data reported by Beermann [22]. The surface models and adsorption systems were optimised using the same exchange–correlation functional and convergence criteria that were applied to the bulk volume.

To prevent the adsorption systems from operating in a vacuum environment, the Conductor-like Screening Model (COSMO) was chosen. Adsorption was simulated in an aqueous medium with a dielectric constant of 78.54. Charge neutrality within the adsorption systems was achieved through lattice substitution or the incorporation of chloride ions. Furthermore, K points were restricted to the Г point. CH₃Hg⁺ units were optimised within a cubic space of 15 Å × 15 Å × 15 Å, and the criteria for optimisation were the same as those used for the surface models.

2.3. Calculation of Adsorption Energies

The calculation of the adsorption energy facilitated an assessment of the interaction strength between CH₃Hg⁺ and various adsorption sites on a surface. The formula used for determining the adsorption energy of CH₃Hg⁺ on the montmorillonite surfaces was as follows:

$$E_{\mathrm{ads}}=E\left[{\mathrm{CH}}_3{\mathrm{Hg}}^{+}-\mathrm{MMT}\right]-E\left[\mathrm{MMT}\right]-E\left[{\mathrm{CH}}_3{\mathrm{Hg}}^{+}\right]$$

(1)

where E[CH₃Hg⁺–MMT] represents the total energy of the systems after CH₃Hg⁺ adsorption on the montmorillonite surfaces. Further, E[MMT] indicates the energy of the pristine montmorillonite surface. In addition, E[CH₃Hg⁺] reflects the energy of CH₃Hg⁺. E_ads represents the adsorption energy of CH₃Hg⁺ on the montmorillonite surfaces. The sign of E_ads reflects the nature of the adsorption process: a negative value indicates that CH₃Hg⁺ adsorption is an exothermic reaction, occurring spontaneously. Further, a large absolute value of E_ads indicates high adsorption stability. Conversely, a positive value indicates an endothermic reaction, indicating a nonspontaneous and unstable adsorption process.

3. Results and Discussion

3.1. Analysis of Surface Adsorption Sites

To determine the adsorption potential of the montmorillonite surfaces, we simulated electrostatic potentials (ESPs) and frontier orbitals of the adsorption systems. Figure 2 depicts the active adsorption sites between methylmercury and the montmorillonite surfaces. The Hg atom in CH₃Hg⁺ is characterised by considerable electropositivity (Figure 2A). Meanwhile, the O atom on the (001) surface (Figure 2B) and the hole within the Si–O ring demonstrate notable electronegativity. This observation aligns with the electrostatic potential characteristics of the illite (001) surface, where the surface O atom and Si–O ring hole exhibit negative electrostatic potential [23]. Frontier orbitals of a molecule are indicators of the interaction strength between the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO). As illustrated in Figure 2C, the LUMO of CH₃Hg⁺ is predominantly localised around the Hg atoms. Meanwhile, the HOMO orbitals of the (010) surface are primarily distributed around the oxygen atoms within the hydroxyl groups (Figure 2D), especially hydroxyl oxygen atoms adjacent to lattice substitutions.

Thus, the Hg atom in CH₃Hg⁺ exhibits pronounced electropositivity and a notable LUMO, functioning as an adsorption terminus for interaction with the montmorillonite surfaces. Electrostatic potential analysis reveals that the oxygen atoms and Si–O ring holes on the (001) surface act as adsorption sites. For further analysis, the oxygen top sites are designated as T1 and T2 and oxygen hole sites are designated as H1–H3 (Figure 2E). Furthermore, based on frontier orbital analysis, the oxygen atoms of the hydroxyl groups act as adsorption sites; hence, the oxygen top sites are designated as T1–T3, the oxygen bridge sites are designated as B1–B5, and the oxygen hole sites are designated as H1–H3. The specific arrangement of these sites is depicted in Figure 2F.

3.2. Adsorption Configuration Analysis

Through refinement of the surface model with various adsorption sites, we identified stable adsorption configurations and the corresponding adsorption energies (Figure 3). The adsorption behaviour of CH₃Hg⁺ on the (001) plane can be summarised as follows.

(1)

The (001)–H1(T1, T2) configuration represents a stable arrangement formed when CH₃Hg⁺ is adsorbed at the H1, T1, and T2 sites. The CH₃ group is oriented upwards, while the Hg atom is positioned downwards. The calculated adsorption energy for this configuration is −51.7 kJ/mol.

(2)

The (001)–H2 and (001)–H3 configurations predominantly involve CH₃Hg⁺ being attracted to O atoms on the surfaces while simultaneously being repelled by Si atoms. Hence, CH₃Hg⁺ is adsorbed at a certain angle, resulting in low adsorption energies of −27.1 and −37.5 kJ/mol, respectively. Thus, the adsorption energy varies between −27 and −51.7 kJ/mol.

The adsorption energy of the most stable configuration for water molecules on the (001) surface is 62.7 kJ/mol [15]. Accordingly, in aqueous environments, water molecules exhibit a strong affinity towards the surface and are preferentially adsorbed. Consequently, water molecules go between CH₃Hg⁺ and the (001) surface, pushing the ion away from the surface. The adsorption configurations of CH₃Hg⁺ on the (010) plane are as follows.

(1)

In the (010)–H3 configuration, CH₃Hg⁺ is positioned between the hydroxyl groups ≡Al(OH₂), ≡Al(OH), and ≡Si(OH), with the Hg atom being attracted to the oxygen atom of these groups.

(2)

Other adsorption configurations, namely (010)–Bn (where n = 1–5), (010)–Hn (where n = 1–2), and (010)–Tn (where n = 1–4), demonstrate a similarity: CH₃Hg⁺ is adsorbed above the oxygen atom of ≡Al(OH) and the Hg atom forms a covalent bond with this oxygen atom. The adsorption energy of CH₃Hg⁺ on the (010) surface varies between −119.4 and −154.3 kJ/mol. Meanwhile, the adsorption energy of water molecules on the (010) surface ranges from −30.72 to −82.56 kJ/mol [15].

These energy values indicate that in aqueous environments, CH₃Hg⁺ directly interacts with the (010) surface. Cd²⁺, another divalent heavy metal ion, exhibits an adsorption energy ranging from −178.53 to −254.67 kJ/mol on the (010) plane and forms two covalent bonds with oxygen atoms [12]. Thus, the methylation of Hg²⁺ may potentially attenuate its interaction with the (010) surface.

The lowest-energy stable adsorption configurations on the surfaces were selected for in-depth analysis to investigate the interaction mechanisms between CH₃Hg⁺ and the surfaces. Figure 4 illustrates the adsorption configurations (001)–H1 and (010)–B2, with

Table 1. Configuration parameters of (001)–H1 and (010)–B2.

Parameters	(001)–H1	(010)–B2
Hg–OS1 bond length (Å)	2.762	2.413
Hg–OS2 bond length (Å)	3.361	3.739
Hg–OS3 bond length (Å)	2.869	4.224
Hg–OS4 bond length (Å)	3.482	3.901
Hg–OS5 bond length (Å)	2.999	2.770
Hg–OS6 bond length (Å)	3.224	2.469
Adsorption energy (kJ/mol)	−51.7	−154.3

detailing the corresponding configuration parameters. Figure 4A displays the (001)–H1 configuration, wherein CH₃Hg⁺ is adsorbed above the Si–O ring hole. The distances between the Hg atom and surface oxygen atoms (O_Sn; n = 1–6) range from 2.762 to 3.224 Å. Owing to electrostatic, polarisation, and dipole effects, Hg²⁺ attracts water molecules, generating a hydration layer around it. The interaction distances between Hg²⁺ and the oxygen atoms of the first layer of water molecules span between 2.42 and 3.58 Å [24]. Consequently, Hg is attracted towards surface O_Sn (n = 1–6) and positioned above the ring hole.

Figure 4B presents the (010)–B2 configuration, wherein CH₃Hg⁺ is adsorbed between the O_Sn units (n = 1–6). Here, the distances between the Hg atom and O_Sn units (n = 2–6) span between 2.469 and 4.224 Å. CH₃Hg⁺ forms a covalent bond (bond length = 2.413 Å) with O_S1 because of mutual attraction. In addition, owing to attraction from CH₃Hg⁺, the (OH₂) ligands of ≡Al(OH)(OH₂) desorb from the surface and transform into H₂O. The energy barrier for the desorption of (OH₂) ligands from the surface spans between 8.2 and 21 kJ/mol [25], indicating that the (OH₂) ligands can easily desorb from ≡Al(OH) when attracted by CH₃Hg⁺. Notably, the adsorption energy of the most stable configuration for CH₃Hg⁺ on the (010) surface is −154.3 kJ/mol, markedly higher than that on the (001) surface (−51.7 kJ/mol). This difference is attributed to covalent bonding interactions between Hg and O_S1 on the (010) surface.

3.3. Charge Analysis

To further investigate the adsorption mechanisms, we conducted charge analyses on the stable adsorption configurations (Figure 5). A detailed examination of the Hg–O_S1 interaction was performed to derive electron density profiles for Hg and O_S1. In the (001)–H₁ configuration, there was no overlap of the electron densities of Hg and O_S1 (Figure 5A). Meanwhile, in the (010)–B configuration, an overlap between the electron densities of Hg and O_S1 was observed. The partial electron densities of the two atoms on the (010) surface considerably overlap. This indicates that some electrons are shared between the two atoms, resulting in the formation of a stable covalent bond. This overlap indicates the formation of covalent bonds between Hg and O_S1 on the (010) plane.

Table 2. Mulliken population value and charge of the stable adsorption configurations.

Parameters	(001)–H1	(010)–B2
Hg–OS1 Mulliken population value	0.03	0.12
CH3Hg charge	0.83	0.59

presents the Mulliken population and charge of the stable adsorption configurations. The Mulliken population value for the interaction between Hg and O_S1 on the (001) surface is 0.03, and that on the (010) surface is higher at 0.12. A higher Mulliken population value indicates stronger covalent bonding and weaker ionic characteristics. Therefore, there is a strong covalent bond between Hg and O_S1 on the (010) plane. The size of the layout reflects the degree of overlap between the electron clouds, and Hg and O_S1 experience a stronger covalent interaction on the (010) plane than on the (001) plane, aligning with the principle of electron density overlap. Ca(OH)⁺ adsorbs on montmorillonite surfaces and transfers electrons to neighbouring oxygen atoms, showing a reduced charge from 0.70 to 0.81 [16]. Similarly, CH₃Hg⁺ exhibits charges of 0.83 and 0.59 when adsorbed on the (001) and (010) surfaces, respectively. The adsorption system maintains electrical neutrality; as the cationic charge increases post-adsorption, the electrostatic interaction between the cation and the surface intensifies. This indicates that the electrostatic influence of CH₃Hg⁺ is more pronounced on the (001) surface than on the (010) surface.

3.4. Analysis of Partial Density of States (PDOS)

The formation of covalent bonds is characterised by the symmetric distribution and overlap of atomic orbitals [26,27]. To further investigate the bonding mechanisms, we selected the Hg–O_S1 interactions under the stable adsorption configurations (001)–H1 and (010)–B2 for analysing the density of states. Figure 6 shows the density of states between Hg and O_S1 under the stable adsorption configurations. Figure 6A shows the density of states for the (001)–H1 configuration. The p orbital density of states of O_S1 shows two sharp peaks, indicating strong electron localisation and low reactivity of O_S1. The overlap between the s orbital of Hg and p orbital of O_S1 occurs in an energy range of −1–−2.5 eV, leading to bond formation. Meanwhile, anti-bonding interactions occur in an energy range of 2.5–6.5 eV. Notably, a large energy interval of orbital overlap in the density of states leads to strong interactions between the orbitals. As the anti-bonding interaction strength between the s orbital of Hg and p orbital of O_S1 in the (001) plane is greater than the bonding interaction strength, Hg and O_S1 do not form a bond. The densities of states of Hg and O_S1 in the (010)–B2 configuration are displayed in Figure 6B. The p orbital density of states of O_S1 exhibits a gentle peak, indicating strong electron delocalisation and high reactivity of O_S1. Notably, the s orbital of Hg overlapped with the p orbital of O_S1 in an energy range of 0–−10 eV to form a bond in a range of 2.5–6.0 eV to form an anti-bond. As the bonding interaction strength was much higher than the anti-bonding interaction strength, a σ bond formed between Hg and O_S1.

4. Conclusions

Herein, the adsorption behaviour of CH₃Hg⁺ on montmorillonite (001) and (010) surfaces was probed through DFT simulations, which yielded the following insights.

CH₃Hg⁺ was adsorbed on the two surfaces, with the Hg atom directly interacting with the surfaces. The calculated CH₃Hg⁺ adsorption energy on the (001) surface varied between −27 and −51.7 kJ/mol, where Hg was attracted to the silicon–oxygen ring cavities. Meanwhile, on the (010) surface, the CH₃Hg⁺ adsorption energy ranged from −119.4 to −154.3 kJ/mol, where Hg was pulled towards the hydroxyl groups (≡Al(OH)(OH₂) and ≡Si(OH)), forming a covalent bond with the oxygen atom of the ≡Al(OH) group.

The interaction between CH₃Hg⁺ and the (001) surface was purely electrostatic. Meanwhile, in the case of the (010) surface, electrostatic interactions were accompanied by covalent bonding. Notably, electrostatic interactions were more pronounced on the (001) surface. The covalent bonding of CH₃Hg⁺ to the (010) surface was primarily attributed to the overlap between the electron densities of the Hg atom and the O atom of the ≡Al(OH) group. The interaction involved the overlap of the s orbital of Hg with the p orbital of O, generating a σ bond.

The study reveals the following: (1) a stable covalent bond exists on the (010) surface, which facilitates the stable adsorption of methylmercury. Therefore, by adjusting the particle size of montmorillonite, the area of the exposed (010) surface can be increased to achieve higher adsorption of methylmercury. (2) Electrostatic adsorption occurs at the (001) surface, which exerts a weak effect on methylmercury, and the (001) surface area increases when the montmorillonite mineral is separated at the interlayers. Therefore, the incorporation of active agents within the interlayer space is recommended to stabilise the interlayer domain of montmorillonite and increase the number of adsorption active sites.