# Bridging the Gap between Molecular Simulations and Surface Complexation Modeling for Heterogeneous Surfaces: A Case Study with Uranium and Arsenic Adsorption on Clay Minerals

Pengyuan Gao, Xiandong Liu, Zhijun Guo, Christophe Tournassat

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Pengyuan Gao, Xiandong Liu, Zhijun Guo, Christophe Tournassat. Bridging the Gap between Molecular Simulations and Surface Complexation Modeling for Heterogeneous Surfaces: A Case Study with Uranium and Arsenic Adsorption on Clay Minerals. Environmental Science and Technology, 2024, 58 (51), pp.22733-22743. 10.1021/acs.est.4c07338. insu-04866027

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# Bridging the Gap between Molecular Simulations and Surface Complexation Modeling for Heterogeneous Surfaces: A Case Study with Uranium and Arsenic Adsorption on Clay Minerals

Pengyuan Gao, Xiandong Liu,* Zhijun Guo, and Christophe Tournassat

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Cite This: *Environ. Sci. Technol.* 2024, 58, 22733–22743

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## ACCESS

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Supporting Information

**ABSTRACT:** Among all natural submicrosized phases, clay minerals are ubiquitous in soils and sedimentary rocks in nature as well as in engineered environments, and while clay minerals' adsorption properties have been studied extensively, their unique level of surface reactivity heterogeneities necessitates further investigation at the molecular level to understand and predict the influence of these heterogeneities on their macroscopic properties. In this study, we investigated the surface structures and desorption-free energies of U(VI) species ($UO_2^{2+}$) and As(V) species ($H_2AsO_4^-$ and $HAsO_4^{2-}$) complexed at different edge surface reactive sites of a cis-vacant montmorillonite layer using first-principles molecular dynamics (FPMD). We show that U(VI) forms bidentate and tridentate complexes on montmorillonite edge

surfaces, whereas As(V) monodentate complexes are the most stable. Then, we constrained a state-of-the-art surface complexation model (SCM) with surface acid-base chemistry and surface complexation properties obtained from the molecular-level simulation results. While our results highlighted the complexity of the interfacial chemistry controlling the complexation of inorganic contaminants on clay mineral surfaces, they also evidenced the reliability of an integrated workflow using modern multiscale simulation techniques to address the challenge of predicting heterogeneities in mineral surface reactivity.

**KEYWORDS:** first-principles molecular dynamics, surface complexation modeling, clay, montmorillonite, cis-vacant, uranium, arsenic

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## 1. INTRODUCTION

Recognition of the harmful effects of pollution on human health and longevity, as well as on the Earth's fragile ecosystems, is growing.¹⁻⁴ Strategies for controlling and remediating environmental pollution are a global issue that remains to be addressed. Because of the ubiquitous occurrence of high adsorption capacity clays in soils and sediments, clay minerals have been used to control toxic substances in Earth's critical zone, such as biogeochemical surface environments, isolation barriers in radioactive waste repositories, and remediation agents for contaminated soils.⁴⁻⁷ Montmorillonite, a typical 2:1-type clay mineral with a TOT (tetrahedral-octahedral-tetrahedral) layered structure, exhibits two kinds of surfaces, which are basal and edge surfaces.⁸ Basal surfaces interact with adsorbates mainly through physical interactions. In contrast, amphoteric functional groups at edge surfaces bind adsorbates through chemical bonding³,⁹ (Figure 1). However, due to the intricate and diverse nature of clay edges, the mechanisms of contaminant retention remain debated, such as the unique level of clay edge surface reactivity heterogeneities and the role of electrostatics and bonding interactions in the adsorption process.

Uranium and arsenic are among the most important contaminants due to their high toxicity and wide distribution.¹⁰,¹¹ Because arsenic concentrations in uranium ores can be as high as 10 wt %,¹² uranium and arsenic would be released together during the mining process. U(VI) and As(V) exhibit predominant stability under natural oxidized conditions.¹³⁻¹⁶ Consequently, uranium and arsenic contamination of groundwater resources threatens the health of millions of people worldwide. The maximum contaminant level (MCL) is set at 30 µg/L for uranium and 10 µg/L for arsenic in drinking water.¹⁷ However, more than 50 countries, spanning South Asia, Africa, America, and some European countries, have drinking water exceeding these limits.¹⁷,¹⁸

The modeling and prediction of adsorption processes have significant academic and practical value in environmental

Received: July 17, 2024

Revised: November 29, 2024

Accepted: December 2, 2024

Published: December 10, 2024

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<figure><img src="image_9.png" /><figcaption>**Figure 1.** Interaction mechanisms and research methodologies of metal ions with clay minerals. One edge-sharing octahedral sheet (cyan) connects to two corner-sharing tetrahedral sheets (navy) to form the 2:1 TOT (tetrahedral–octahedral–tetrahedral) layered structure. The adsorption of metal ions (blue) in complexes with water molecules (red and white) can occur on edge surfaces through chemical bonds in inner-sphere mode. On the basal surface, an outer-sphere surface complex or interlayer cation (gold) bridging complex can be formed. The distribution coefficient ($K_D$) in the adsorption data shows retention processes as a function of pH and constant metal ion concentration that can be fitted by the surface complexation model. Molecular simulations and in situ spectroscopic analysis reveal the adsorption mechanism of metal ion complexes on clay mineral surfaces. The effective extrapolation of molecular-level information from spectroscopic and simulation techniques facilitates the implementation of practical applications in macroscopic studies.</figcaption></figure>

science.¹⁹,²⁰ An in-depth understanding of these processes helps reveal the transport, distribution, and ultimate fate of contaminant elements in the environment and thus is essential for environmental management and the development of pollution control strategies.²¹⁻²³ The adsorption of metal cations and metalloid anions on natural clay minerals has been investigated through batch experiments for decades, elucidating the basic principles governing adsorption on their surfaces.²³⁻²⁹ Numerous adsorption data have been obtained,³⁰,³¹ and the physicochemical adsorption mechanisms have been addressed repeatedly using spectrometric and diffractometric techniques.³,³² EXAFS (extended X-ray absorption fine structure) and TRLFS (time-resolved laser-induced fluorescence spectroscopy, for elements with fluorescent properties) provide information on surface species on clays.²¹,³² These studies have confirmed that metal ions form outer-sphere complexes (e.g., by cation exchange in the interlayer space) and inner-sphere complexes (e.g., complexation on the edge surfaces)³ (Figure 1). According to the neighbor atoms' shell configurations, surface complexes were observed to form at edge octahedral aluminol and tetrahedral silanol sites.³³,³⁴ However, the characterization of the exact type of reacting surface is challenging due to the averaged nature of the information. To overcome this issue, periodic density functional theory (DFT) calculations have been used to characterize the structures of surface complexes on specific clay edge surface sites by optimizing geometries.³⁵,³⁶ While this modeling approach provides an atomic-level understanding of the types of surface sites and bonding mechanisms, it does not make it possible to evaluate site affinities accurately because of the lack of consideration of solvation and temperature effects.

Adsorption can occur at different sites on the layered structure of clay minerals, and the preferred site depends on factors such as the nature of the adsorbate, ionic strength, and pH. For instance, the formation of outer-sphere surface

complexes, which involves long-range interactions, is influenced by the surface potential and is thus highly dependent on ionic strength, whereas the formation of inner-sphere surface complexes involves chemical bonding, which often involves the displacement of H⁺ from the adsorbing site, and is thus highly dependent on pH conditions. Surface complexation models (SCMs) have been applied successfully in the prediction of wet chemisorption data for metal contaminants,⁷ but they rely on strong model assumptions and are tested only by goodness-of-fit to adsorption data. Therefore, the parameters utilized in SCMs must rely on strong physicochemical significance. Undoubtedly, it is inevitable that methodologies not equipped with realistic structures will produce ambiguous results. Consequently, while the overall mechanisms are now well-established and existing adsorption models can fit macroscopic adsorption, the prediction of the adsorption capabilities of each type of clay mineral surface site remains challenging. In situ spectroscopic techniques provide actual surface species characteristics, which can be included in SCMs.²¹,³²,³⁷,³⁸ Molecular simulation has proved to be a complementary reliable investigation method, especially in situations where spectroscopic experimental insight is insufficient.³⁵,³⁶,³⁹⁻⁴¹ For example, the first-principles molecular dynamics (FPMD) simulation method enables investigation into the free energies associated with metallic ion adsorption on various edge surface sites, thereby revealing their binding affinity⁴⁰⁻⁴² and making the multiscale approach linking nano-, micro-, and macro-scale studies a reality in clay science⁴³⁻⁴⁵ (Figure 1). Several advances have already been made through a multiscale approach combining first-principles molecular dynamics simulations and surface complexation modeling (FPMD-SCM),⁴⁰,⁴⁵⁻⁴⁷ which increased confidence for further studies.

In situ analysis showed that the cis-vacant structure of montmorillonites, a noncentrosymmetric structure that edges perpendicular to the [010] and [0&#x0304;10], [1&#x0304;10] and [1&#x0304;1&#x0304;10] are
different, predominates in natural samples rather than the centrosymmetric trans-vacant structure.⁴⁸⁻⁵¹ However, most of the existing atomistic studies of metal ion adsorption on clays have been based on the trans-vacant structure, which potentially leads to incorrect predictions of their properties. Especially, previous studies pointed out that the different local structures (trans- and cis-vacant) lead to distinct surface sites and acid-base chemistry (i.e., acidity constants, pKₐ) on edge sites, which affect the adsorption properties.⁴⁶,⁴⁷ Thereby, the evaluation of the adsorption of contaminants on montmorillonite should be made on models built with a cis-vacant structure. Molecular simulations demonstrated that edge surfaces of cis-vacant clay layers are positively or negatively charged depending on surface crystallographic orientations,⁴⁶,⁴⁷ while trans-vacant clay layers are always negatively charged in pH conditions relevant to the environment. This property may have a first-order influence on the adsorption of anionic contaminants such as arsenic oxyanions or uranium carbonate/hydroxyl complexes. To the best of our knowledge, atomic-scale studies have not been reported for adsorption on the edge surfaces of cis-vacant clay layers.

In this study, we carried out a molecular simulation study to systematically investigate the complexation structures and desorption-free energies of U(VI) cation species (UO₂²⁺) and As(V) anion species (H₂AsO₄⁻ and HAsO₄²⁻) adsorbed on different orientations of cis-vacant clay edges. By using the derived atomic-level information, including surface acid-base chemistry for cis-vacant TOT layers and surface complexation structures, a state-of-the-art SCM was developed to reveal the mechanisms behind the experimental adsorption data of contaminants on clay minerals with a bottom-up method.

## 2. METHODOLOGY

**2.1. Computational Models.** The periodic slab models on surfaces perpendicular to [010], [01̄0], [110], and [11̄0] of cis-vacant montmorillonite were constructed based on our previous work.⁴⁶,⁴⁷ The surface models include two unit cells. A solution region of 20 Å above the surface was filled with 130 water molecules, which approximate the density of bulk water at ambient conditions, according to previous reports.⁴⁶,⁴⁷ Additionally, 10 water molecules were inserted into the interlayer to create a monolayer hydrate. Finally, an orthorhombic box with dimensions of 12.45 × 10.44 × 33.56 Å was utilized.

In the initial surface models, previous studies concluded that the functional groups ≡Si(OH), ≡Al(OH)₂/≡Al(OH) (OH₂), and ≡Si(O₃)₂Al/≡SiO₃(O₃)Al sites on surfaces perpendicular to [010] and [01̄0] and ≡Si(O₃)HAl, ≡Si(OH), ≡Al(OH₂), and ≡Al(OH)₂/≡Al(OH)(OH₂) on surfaces perpendicular to [110] and [11̄0] are reactive at common pH values.⁴⁶ Hence, the investigated surface sites for UO₂²⁺ included ≡SiO, ≡Si(O₃)₂Al, ≡Si(O₃)Al, ≡Al(OH) (OH₂)/≡Al(OH)₂, and ≡Al(OH) on cis-vacant edge surfaces. UO₂²⁺ was placed at the complexing sites to form multidentate surface complexes, including surface complexes at the ≡SiO(O₃)₂Al site on surfaces perpendicular to [010], at ≡Al(OH)₂ and ≡Al(OH)SiO sites on surfaces perpendicular to [01̄0], at ≡Al(OH)O₃ and ≡SiO₃O₃ sites on surfaces perpendicular to [110], and at the ≡Al(OH)SiO site on surfaces perpendicular to [11̄0] (Figure S1). For H₂AsO₄⁻ and HAsO₄²⁻, monodentate complexes were formed by replacing the water group at the ≡Al(OH)(OH₂) site on surfaces perpendicular to [01̄0] and [11̄0] (Figure S2).

**2.2. First-Principles Molecular Dynamics.** The FPMD simulations were performed using the CP2K/Quickstep package, based on the Perdew-Burke-Ernzerhof functional and a hybrid Gaussian plane-wave scheme (GPW).⁵²,⁵³ The Goedecker-Teter-Hutter (GTH) pseudopotentials were chosen to describe the core electrons.⁵⁴ The wave functions were expanded in optimized double-ζ Gaussian basis sets, and the plane waves were expanded with a cutoff energy of 400 Ry.⁵⁵ Dispersion corrections were applied in all calculations with the Grimme-D3 method.⁵⁶,⁵⁷

Born-Oppenheimer molecular dynamics simulations were performed in the canonical (NVT) ensemble, employing Nóse-Hoover thermostats with a time step of 0.5 fs at a finite temperature of 300 K.⁵⁸ The wave function optimization tolerance was 10⁻⁶. For each system, we performed an equilibration run of 3 ps and a production run of 10.0 ps.

**2.3. Free Energy Calculations.** Constrained FPMD simulations were conducted to quantitatively assess the stability of each surface complex and determine the respective free energy of desorption. The free energy (ΔG) profile was obtained through the integration of the average forces (F) along a specified reaction coordinate (x).⁵⁹,⁶⁰

$$ \Delta G(x) = - \int_{x_0}^{x} F(x') dx' $$

For UO₂²⁺ surface complexes, the coordination number (CN) of U with O atoms at the surface adsorption sites was chosen as the reaction coordinate. At the ≡SiO(O₃)₂Al site on the surface perpendicular to [010], the UO₂²⁺ desorption process occurred from the tridentate inner-sphere complex to the outer-sphere complex. On surfaces perpendicular to [01̄0], [110], and [11̄0], UO₂²⁺ desorption processes occurred from the bidentate inner-sphere complex to the outer-sphere complex at the ≡Al(OH)₂, ≡Al(OH)SiO, ≡Al(OH)O₃, and ≡SiO₃O₃ sites. The CN was defined as

$$ CN = \sum_{i} \frac{1 - (r_{i}/r_{c})^{12}}{1 - (r_{i}/r_{c})^{24}} $$

where rᵢ is the distance between U and O atoms of surface adsorption sites, and rₐ is the cutoff value. Here, rₐ was set to 3.1 Å, which makes the definition of CN for uranyl close to the actual coordination situation.

For H₂AsO₄⁻ and HAsO₄²⁻ surface complexes, the distance of the Al-As bond was selected as the reaction coordinate for the process from the monodentate complex at the ≡Al(OH)(OH₂) site on surfaces perpendicular to [01̄0] and [11̄0] to the outer-sphere complex.

**2.4. Surface Complexation Modeling.** An in-house version of PHREEQC, wherein the electrostatic diffusion layer model (DLM) takes into account the spillover of electrostatic potential from basal surfaces, was utilized to construct the U(VI) and As(V) surface complexation models.²⁷,⁴⁵,⁶¹ The THERMOCHIMIE v.11a thermodynamic database was used for the solute species,⁶¹ which is available at https://www.thermochimie-tdb.com/. The surface species, including the surface site type, site acidity constants, and site affinities, were constrained by FPMD simulations. PHREEQC scripts are provided in the Supporting Information.

## 3. RESULTS AND DISCUSSION

**3.1. Surface Complexation Structures of U(VI) and As(V) on Cis-Vacant Clay Edges.** On cis-vacant clay edge
<figure><img src="image_10.png" /><figcaption>**Figure 2.** Equilibrium configurations of U(VI) and As(V) complexation on edge surfaces of the cis-vacant TOT clay layer. (a) Surface complexation structures of UO₂²⁺ on surfaces perpendicular to [010], [01̄0], [110], and [1̄10] on a cis-vacant layer. (b) Surface complexation structures of H(AsO₄)²⁻/H₂(AsO₄)⁻ on surfaces perpendicular to [010] and [1̄10] on a cis-vacant layer.</figcaption></figure>

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Color scheme

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surfaces, several reactive sites have been proposed for complexing ions.⁴⁶,⁴⁷ All possible U(VI) and As(V) surface complexes on different cis-vacant edges were first probed with FPMD simulations.

According to the simulations, U(VI) and As(V) remain adsorbed in the form of inner-sphere complexes on edge surfaces on a cis-vacant layer with various surface orientations (Figures S3 and S4). UO₂²⁺ formed a stable tridentate complex on the ≡SiO(O₃)₂Al site on the surface perpendicular to [010] on a cis-vacant layer (Figure 2a). This adsorption mode can be considered as being similar to vacancy sites on the surface perpendicular to [010] on a trans-vacant layer, on which Ni²⁺ adsorption was studied using similar techniques.⁴¹ However, the vacancy site of the trans-vacant edge consists of two aluminum octahedral lattices, making it more suitable for metal ions with small radii. In contrast, the vacancy ≡SiO(O₃)₂Al site of the cis-vacant edge has better equatorial coordination and adaptability for acyl groups such as UO₂²⁺. Two equivalent U–Oₐ bonds with an average bond length of 2.42 Å and one longer U–Oₗ bond of 2.53 Å were found for the tridentate uranyl surface complex on the ≡SiO(O₃)₂Al site on a surface perpendicular to [010] on a cis-vacant layer (Table S1).

Furthermore, UO₂²⁺ formed bidentate complexes at ≡Al(OH)₂, ≡Al(OH)SiO, ≡Al(OH)Oₐ, and ≡SiOOₐ sites on surfaces perpendicular to [01̄0], [110], and [1̄10] on a cis-vacant layer, which was consistent with uranyl complexation on trans-vacant model edges.⁴⁰ The U–O radial distribution functions (RDF) and accumulating coordination number (CN) of the tridentate and bidentate surface complexation structures exhibit similarity, featuring two acyl oxygen atoms at a distance around 1.8 Å and equatorial 5-coordinated bound oxygen atoms ranging from 2.2 to 2.7 Å (Figure S5). For the

special tridentate uranyl surface complex, the equilibrium U–Oₗ distance is 2.58 Å, making it difficult to be distinguished from coordinated water (around 2.60 Å for U–Oₗwater), indicating that spectroscopic techniques might not be able to identify this tridentate complex on ≡SiO(O₃)₂Al sites on a cis-vacant layer. Additional U–O/Al/Si distances were also provided for comparison with the majority of EXAFS experiments (Table S2) and previous DFT calculations (Table S3).³³⁻³⁵,³⁷,³⁸ These studies enhanced our understanding of possible complex sites and major surface species of UO₂²⁺. However, EXAFS has considerable uncertainty in interpreting the second adjacent atom involved in the surface complex, and previous atomic-level calculations were all based on the trans-vacant layer structure model, which may lead to inaccurate comparisons.

Under weakly acidic conditions, the ≡Al(OH)(OH₂) and ≡SiOH sites are dominant on the cis-vacant clay edges. H(AsO₄)²⁻/H₂(AsO₄)⁻ was able to form monodentate inner-sphere complexes by replacing the water group at the ≡Al(OH)(OH₂) site on surfaces perpendicular to [01̄0] and [1̄10] due to the weak binding of the water molecule (Figure 2b). H(AsO₄)²⁻ and H₂(AsO₄)⁻ species have similar ~1.88 Å Al–OₗAs distances on the ≡Al(OH)(OH₂) site on surfaces perpendicular to [01̄0] and [1̄10]. The Al–As distances ranged from 3.30 to 3.38 Å (Table S4), which was consistent with EXAFS results (~3.30 Å) in most As(V) adsorption studies⁶³⁻⁶⁷ on minerals other than montmorillonite. Unfortunately, spectroscopic and microscopic characterizations are lacking for As(V)-montmorillonite systems.

## 3.2. Stabilities of U(VI) and As(V) on Cis-Vacant Clay Edges.
The stabilities of the U(VI) and As(V) surface complexes were probed by calculating the free energy changes
<figure><img src="image_13.png" /><figcaption>**Figure 3.** Profiles of free energy of U(VI) and As(V)desorption from edge surface sites of the cis-vacant TOT clay layer. (a) Metal ion desorption reaction process on the clay edge. (b) UO₂²⁺ on surfaces perpendicular to [010], [0&#x20D7;10], [1&#x20D7;10], and [1&#x20D7;10] on a cis-vacant layer. (c) H(AsO₄)₂⁻/ H₂(AsO₄)⁻ on surfaces perpendicular to [01&#x20D7;0] and [1&#x20D7;10] of the cis-vacant clay model. The statistical error in free energy of desorption was estimated by integrating the statistical errors in the mean forces, which were estimated as the semidifference between the average forces using only the first half or the second half of the trajectory.</figcaption></figure>

from the inner-sphere states to the outer-sphere states on a cis-vacant layer (Figure 3a).

The desorption-free energy value of the tridentate uranyl surface complex on the ≡SiO(On)₂Al site on the surface perpendicular to [010] on a cis-vacant layer was 36.4 kcal/mol (Figure 3b). The changes in free energy gradually increased from CN = 3 to CN = 1, while it changed dramatically from the monodentate structure on the ≡SiO site to the outer-sphere complex, indicating weaker binding of the U atom with apical bridge oxygen (On). Meanwhile, each apical O site exhibited a free energy of ~8 kcal/mol on the ≡SiO(On)₂Al site, which is higher than that of the apical O site on the ≡Al(OH)On and ≡SiOOₐ sites on the surface perpendicular to [110] of the trans-vacant model (around 2 kcal/mol).⁴⁰ The observation reflected the additional stability provided by the vacancy sites. On the surface perpendicular to [0&#x20D7;10], the desorption processes (Figure S6) of uranyl from ≡Al(OH)₂ and ≡Al(OH)SiO sites were similar to that on the surface perpendicular to [010] of the trans-vacant model.⁴⁰ All of the monodentate complexes brought local stability in the curve. Meanwhile, the free energies from ≡Al(OH)₂ and ≡Al(OH)SiO sites were 25.0 and 25.9 kcal/mol on a cis-vacant layer, respectively. Similarly to its adsorption on the surface perpendicular to [1&#x20D7;10] of the trans-vacant model,⁴⁰ UO₂²⁺ also formed bidentate surface complexes at ≡Al(OH)On and ≡

SiOOₐ sites on the surface perpendicular to [1&#x20D7;10] of the cis-vacant model, which had desorption-free energies of 25.7 and 26.6 kcal/mol, respectively. The free energies were similar to those from the ≡Al(OH)₂ and ≡Al(OH)SiO sites on the surface perpendicular to [0&#x20D7;10] on the cis-vacant layer (25.0 and 25.9 kcal/mol). The desorption processes on apical bridge oxygen (On) still exhibited weak binding strength, with a slow energy increase from CN = 2 to CN = 1, which was consistent with the surface perpendicular to [1&#x20D7;10] of the trans-vacant model on ≡Al(OH)On and ≡SiOOₐ sites, as well as the surface perpendicular to [010] of the cis-vacant model on the ≡SiO(On)₂Al site. The free energy value of UO₂²⁺ at the ≡Al(OH)SiO site on the surface perpendicular to [1&#x20D7;10] (26.5 kcal/mol) was comparable to that of previous bidentate surface complexes on cis-vacant model edges. The desorption-free energy curves for H(AsO₄)²⁻ and H₂(AsO₄)⁻ were obtained as the Al-As distance increased from 3.4 to 5.0 Å at ≡Al(OH)(OH₂) sites on surfaces perpendicular to [0&#x20D7;10] and [1&#x20D7;10] on a cis-vacant layer (Figure 3c). In the desorption process, H(AsO₄)²⁻/H₂(AsO₄)⁻ were replaced with solvent water molecules, forming outer-sphere complexes at 4.6 Å (Figure S7). The free energies of H(AsO₄)²⁻/H₂(AsO₄)⁻ desorption were 10.7/9.7 and 17.0/14.8 kcal/mol on the surfaces perpendicular to [0&#x20D7;10] and [1&#x20D7;10], respectively.
All desorption-free energy values of UO₂²⁺ on cis- and trans-vacant edge surface sites are collected in Table 1. As we see,

<table><caption>**Table 1. Free Energy of UO₂²⁺ Desorption from Cis- and Trans-Vacant Clay Edge Surface Sites<sup>a</sup>**</caption><thead><tr><th rowspan="2">surface</th><th colspan="2">cis-vacant</th><th colspan="2">trans-vacant</th></tr><tr><th>sites</th><th>ΔG<br>(kcal/mol)</th><th>sites</th><th>ΔG<br>(kcal/mol)</th></tr></thead><tbody><tr><td>⊥[010]</td><td>≡SiO(O<sub>2</sub>)<sub>2</sub>Al</td><td>36.4 ± 3.7</td><td>≡Al(OH)<sub>2</sub><br>≡AlOHSiO</td><td>21.5 ± 1.7<br>21.1 ± 2.4</td></tr><tr><td>⊥[0&#x0304;10]</td><td>≡Al(OH)<sub>2</sub><br>≡AlOHSiO</td><td>25.0 ± 2.4<br>25.9 ± 1.6</td><td></td><td></td></tr><tr><td>⊥[110]</td><td>≡Al(OH)O<sub>a</sub><br>≡SiO<sub>o</sub>O<sub>a</sub></td><td>25.7 ± 1.5<br>26.6 ± 1.9</td><td>≡Al(OH)O<sub>a</sub><br>≡SiO<sub>o</sub>O<sub>a</sub></td><td>20.0 ± 1.5<br>19.8 ± 1.3</td></tr><tr><td>⊥[&#x0304;110]</td><td>≡AlOHSiO</td><td>26.5 ± 1.7</td><td></td><td></td></tr></tbody></table>

<sup>a</sup>The free energies of uranyl desorption with the trans-vacant model were taken from Zhang et al.<sup>40</sup>

the free energies on all of the investigated sites on the trans-vacant layer were similar, which meant similar thermodynamic stabilities. For the cis-vacant model, all bidentate surface complexes were also similar, but with higher values compared to the sites with the trans-vacant model, which means that cis-vacant edge sites had higher thermodynamic stability for uranyl. Thus, several bidentate surface complexes may be responsible for a single-site reactivity in currently available SCMs.<sup>27,40</sup> In addition, the tridentate uranyl surface complex on the surface perpendicular to [010] of the cis-vacant model had the highest desorption-free energy value (36.4 kcal/mol), which may correspond to a strong site in previous SCMs.<sup>25,68</sup> Specifically, at the molecular level, the tridentate surface complex exhibits a lower capacity and a higher adsorption affinity in comparison to other bidentate surface complexes (Table 1). The desorption-free energy values of H(AsO₄)²⁻/ H₂(AsO₄)⁻ on cis-vacant edge surface sites are summarized in Table 2. H(AsO₄)²⁻ and H₂(AsO₄)⁻ adsorption on identical

<table><caption>**Table 2. Desorption-Free Energy Values of H(AsO₄)²⁻/ H₂(AsO₄)⁻ on Cis-Vacant Clay Edge Surface Sites**</caption><thead><tr><th rowspan="2">surface</th><th colspan="2">H(AsO₄)²⁻</th><th colspan="2">H₂(AsO₄)⁻</th></tr><tr><th>sites</th><th>ΔG<br>(kcal/mol)</th><th>sites</th><th>ΔG<br>(kcal/mol)</th></tr></thead><tbody><tr><td>⊥[0&#x0304;10]</td><td>≡Al(OH)<br>(OH<sub>2</sub>)</td><td>10.7 ± 1.4</td><td>≡Al(OH)<br>(OH<sub>2</sub>)</td><td>9.7 ± 1.8</td></tr><tr><td>⊥[&#x0304;110]</td><td>≡Al(OH)<br>(OH<sub>2</sub>)</td><td>17.0 ± 1.1</td><td>≡Al(OH)<br>(OH<sub>2</sub>)</td><td>14.8 ± 1.3</td></tr></tbody></table>

edges exhibited comparable binding affinities, while surfaces perpendicular to [0&#x0304;10] and [1&#x0304;10] displayed distinct binding capacities. Additional hydrogen bond interactions between H(AsO₄)²⁻ and H₂(AsO₄)⁻ with adjacent silicon hydroxyl groups on the surface perpendicular to [1&#x0304;10] were also observed (Figure 2b), whereas these did not exist on the surface perpendicular to [0&#x0304;10], thereby explaining the higher desorption-free energy of As(V) on the surface perpendicular to [1&#x0304;10] on the cis-vacant layer.

**3.3. Predictive SCM Based on FPMD for U(VI) and As(V) Adsorption on Heterogeneous Clay Mineral Surfaces.** In our SCM scheme, one edge surface functional group corresponds to a combination of one octahedral and two tetrahedral sites on different surface orientations together with their associated OH groups, which enables a connection to be established with molecular simulations.<sup>40</sup> The surface functional group can be represented as ≡[edge]SOH₄⁺, with a maximum of 4 H⁺ per site and a maximum charge of +1 in the absence of isomorphic substitution in the surface functional group. The surface complexation reaction was then defined as

$$ \equiv[\text{edge}]\text{SOH}_4^+ + \text{M}^n \\ \Leftrightarrow \equiv[\text{edge}]\text{SOH}_{4-a}[\text{M}]^{n+1-a} + a\text{H}^+ \log K_R $$

The acid-base properties (log K<sub>H,Sites</sub>) of montmorillonite had been estimated previously using FPMD calculations (Table S5).<sup>46,47</sup> Additionally, the acidity constants of uranyl and arsenate in aqueous solution had also been obtained.<sup>62,69-71</sup> Next, it is essential to evaluate the surface complexation reaction constants (log K<sub>R</sub>) of metal ions on specific sites. In our FPMD calculations, the results correspond to the reaction:

$$ \equiv[\text{edge}]\text{SOH}_{4-a}^{1-a} + \text{M}^n \Leftrightarrow \equiv[\text{edge}]\text{SOH}_{4-a}[\text{M}]^{n+1-a} $$

It should be noted that the free energies of desorption obtained from FPMD calculations cannot be directly applied to determine surface complexation reaction constants because the corresponding equilibrium constants were dependent on the electrostatic model, which is not thermodynamically significant.<sup>7</sup> Nevertheless, the values of free energy of desorption can also reveal the fundamental patterns in binding constants, which enabled the identification of surface complexation reactions that correspond to newly imposed constraints (log K<sub>C</sub>). Therefore, log K<sub>R</sub> could be approximated as follows:

$$ \log K_R \sim \log K_C + \log K_{H,Sites} $$

<table><caption>**Table 3. Reactions for U(VI) and As(V) Adsorption on Montmorillonite Based on the Cis-Vacant Clay Model<sup>a,b</sup>**</caption><thead><tr><th>surface</th><th>sites</th><th>reaction (Si<sup>T</sup>-Al<sup>Oc</sup>-Si<sup>T</sup>)</th><th>log K<sub>H,Sites</sub></th><th>log K<sub>C</sub></th><th>log K<sub>R</sub></th></tr></thead><tbody><tr><td colspan="6">U(VI) surface complexation reactions</td></tr><tr><td>⊥[010]</td><td>≡SiO(O<sub>2</sub>)<sub>2</sub>Al</td><td>≡[010]SOH<sub>4</sub><sup>+</sup> + UO<sub>2</sub><sup>2+</sup> ⇌ ≡[010]SOH[UO<sub>2</sub>]<sup>0</sup> + 3H<sup>+</sup></td><td>0.8 ± 1.1, -5.9 ± 1.3, -6.8 ± 1.0</td><td>9.2</td><td>-2.7 ± 3.4</td></tr><tr><td>⊥[110]</td><td>≡Al(OH)O<sub>a</sub></td><td>≡[110]SOH<sub>4</sub><sup>+</sup> + UO<sub>2</sub><sup>2+</sup> ⇌ ≡[110]SOH<sub>2</sub>[UO<sub>2</sub>]<sup>+</sup> + 2H<sup>+</sup></td><td>11.7 ± 1.3, -5.7 ± 1.4</td><td>8.7</td><td>14.7 ± 2.7</td></tr><tr><td>⊥[&#x0304;110]</td><td>≡AlOHSiO</td><td>≡[&#x0304;110]SOH<sub>4</sub><sup>+</sup> + UO<sub>2</sub><sup>2+</sup> ⇌ ≡[&#x0304;110]SOH<sub>2</sub>[UO<sub>2</sub>]<sup>+</sup> + 2H<sup>+</sup></td><td>-7.0 ± 1.6, -7.4 ± 1.4</td><td>8.7</td><td>-5.7 ± 3.0</td></tr><tr><td colspan="6">As(V) surface complexation reactions:</td></tr><tr><td>⊥[0&#x0304;10]</td><td>≡Al(OH)(OH<sub>2</sub>)</td><td>≡[0&#x0304;10]SOH<sub>4</sub><sup>+</sup> + H(AsO₄)²⁻ ⇌ ≡[0&#x0304;10]SOH<sub>3</sub>[H(AsO₄)]<sup>2-</sup> + H<sup>+</sup></td><td>-5.7 ± 1.4</td><td>3.3</td><td>-2.4 ± 1.4</td></tr><tr><td></td><td>≡Al(OH)(OH<sub>2</sub>)</td><td>≡[0&#x0304;10]SOH<sub>4</sub><sup>+</sup> + H₂(AsO₄)⁻ ⇌ ≡[0&#x0304;10]SOH<sub>3</sub>[H₂(AsO₄)]⁻ + H<sup>+</sup></td><td>-5.7 ± 1.4</td><td>3.3</td><td>-2.4 ± 1.4</td></tr><tr><td>⊥[&#x0304;110]</td><td>≡Al(OH)(OH<sub>2</sub>)</td><td>≡[&#x0304;110]SOH<sub>4</sub><sup>+</sup> + H(AsO₄)²⁻ ⇌ ≡[&#x0304;110]SOH<sub>3</sub>[H(AsO₄)]<sup>2-</sup> + H<sup>+</sup></td><td>-7.0 ± 1.6</td><td>3.8</td><td>-3.2 ± 1.6</td></tr><tr><td></td><td>≡Al(OH)(OH<sub>2</sub>)</td><td>≡[&#x0304;110]SOH<sub>4</sub><sup>+</sup> + H₂(AsO₄)⁻ ⇌ ≡[&#x0304;110]SOH<sub>3</sub>[H₂(AsO₄)]⁻ + H<sup>+</sup></td><td>-7.0 ± 1.6</td><td>3.8</td><td>-3.2 ± 1.6</td></tr></tbody></table>

<sup>a</sup>The specific edge surface area was set to 9 m²·g⁻¹ and the relative abundance of edges perpendicular to [010]:[0&#x0304;10]:[110]:[&#x0304;110] was set to 0.15:0.15:0.35:0.35, respectively. <sup>b</sup>The errors of log K<sub>R</sub> were derived from log K<sub>H,Sites</sub> based on Gao et al.<sup>46,47</sup>
<figure><img src="image_14.png" /><figcaption>**Figure 4.** U(VI) and As(V) adsorption distribution coefficients result as a function of pH. The symbols correspond to the experimental data, $C_{U(VI)}= 1.0 \times 10^{-6}$ mol/L, $m/V_{U(VI)}= 0.5$ g/L, $C_{As(V)}= 4.0 \times 10^{-7}$ mol/L, $m/V = 25$ g/L, $I = 0.1$ mol/L. The solid and dotted lines were calculated from SCMs. (a) U(VI) SCM with parameters predicted from FPMD results. (b) Same U(VI) SCM with the consideration of surface binuclear uranyl reactions and a DIC concentration of $6 \times 10^{-5}$ mol·L$^{-1}$. (c) As(V) SCM with parameters predicted from FPMD results. (d) Same As(V) SCM with additional cation-bridging complexes on basal surfaces (related parameters were adjusted to fit the data).</figcaption></figure>

According to the site charge-pH distribution on cis-vacant clay edges (Figure S8), the $\equiv$SiO$(O_a)_2$Al site on a surface perpendicular to [010], the $\equiv$Al(OH)O$_a$ site on a surface perpendicular to [110], and the $\equiv$Al(OH)SiO site on a surface perpendicular to [1$\overline{1}$0] are completely formed at pH 8.6, 9.6, and 9.8, respectively, within the common pH range.

The electrostatic interaction potentials between the three sites and UO$_2^{2+}$ in 0.1 M NaCl were estimated based on the edge surface potential (Figure S9). The electrostatic interaction potential energies for the $\equiv$SiO$(O_a)_2$Al, $\equiv$Al(OH)O$_a$, and $\equiv$Al(OH)SiO sites are 7.5, 6.9, and 5.2 kcal/mol (Table S6), respectively, indicating that uranyl has distinct electrostatic interactions on different sites. From previous free energy perspectives (Table 1), all bidentate surface complexes at cis-vacant edge sites had similar free energies, and the tridentate surface complex exhibited the highest desorption-free energy value. Thus, the bonding strength of uranyl at the three sites can be identified by calculating the free energies that eliminate the electrostatic interaction component (Table S6). The results suggested that the tridentate surface complex on the $\equiv$SiO$(O_a)_2$Al site has the highest bond strength (28.9 kcal/mol), while the bidentate surface complexes on the $\equiv$Al(OH)SiO site (21.3 kcal/mol) has a higher bond strength than the $\equiv$Al(OH)O$_a$ site (18.3 kcal/mol). These findings reveal that the electrostatic potentials and bonding interactions combine to contribute to the comparable free energies of uranyl upon adsorption on cis-

vacant edges. Therefore, $\equiv$SiO$(O_a)_2$Al, $\equiv$Al(OH)O$_a$, and $\equiv$Al(OH)SiO sites were used as preferred adsorption sites for U(VI) modeling, with log $K_C$ values of 9.2, 8.7, and 8.7, respectively (Table 3). The log $K_C$ values for As(V) adsorption on surfaces perpendicular to [0$\overline{1}$0] and [1$\overline{1}$0] on cis-vacant layer edges were evaluated independently to be 3.3 and 3.8 at the $\equiv$Al(OH)(OH$_2$) sites, respectively (Table 3). This is consistent with our FPMD simulations, which showed that the binding strength on the surface perpendicular to [1$\overline{1}$0] was higher than that on [0$\overline{1}$0], while the binding strengths of H(AsO$_4$)$^{2-}$ and H$_2$(AsO$_4$)$^{-}$ were similar (Table 2).

Therefore, in our SCMs, all parameters employed were of strong physicochemical significance, derived from experimental data or ab initio calculations, including edge surface distributions,⁷² isomorphic substitution content,⁷³ surface site density,²⁷,⁴⁰,⁴⁵⁻⁴⁷ specific surface area,⁸ acidity constants data set of specific clay model,⁴⁶,⁴⁷ surface species types, and complexation stability (this study). All of the SCM parameters are listed in Table S7.

**3.4. Reassessment of the Physicochemical Significance of SCMs.** The acid-base reaction constants determine the shape of the adsorption curves, and their values exhibit no sensitivity to the SCM results within the error margin.⁴⁰ In contrast, the affinity constant governs the maximum height of the adsorption curve and demonstrates a high degree of sensitivity to parameter values. Hence, batch adsorption data were necessary to benchmark the SCM within the error range
of affinity constants to gain an in-depth understanding of the adsorption processes (Figure 4). The adsorption data for U(VI) and As(V) were obtained from the literature, with the corresponding experimental conditions provided in Table S8.<sup>27,29</sup>

For U(VI), a sharp increase in U(VI) adsorption was observed from pH 4 to pH 6, followed by a limited decrease in U(VI) adsorption at pH > 6. By adjustment of the affinity constant within the error range, the U(VI) SCM aligned with experimental data below pH 6 (Figure 4a). However, it significantly overestimated the adsorption curves in the pH range of 6–9 and underestimated them above pH 9 (Figure 4a). Reassessment of the physicochemical significance of the U(VI) adsorption data is therefore imperative for calibration. Batch adsorption experiments from the literature were carried out under CO₂-free conditions, but dissolved inorganic carbon (DIC) measurements indicate that carbonate was not fully excluded from the solutions, despite the efforts to achieve this goal.<sup>27</sup> In addition, the formation of multinuclear uranyl surface complexes has been demonstrated at high pH in previous studies.<sup>27,40,74–76</sup> Addressing this, the U(VI) adsorption curve was adequately reproduced by further introducing surface binuclear uranyl reactions on the ≡ Al(OH)SiO site on the surface perpendicular to [110] and a DIC concentration of 6 × 10<sup>-5</sup> mol·L<sup>-1</sup> (Figure 4b). At pH values higher than 6, the presence of DIC reduces the adsorption of U(VI) because of the competition of U(VI)-carbonate aqueous complexes.<sup>27,37,77</sup> Notably, within the limits of data accuracy and the low carbonate concentration, there was no need to include the formation of uranyl-carbonate surface complexes in the model to simulate the experimental data (Figure S10), which is consistent with the conclusions reported in Tournassat et al.<sup>27</sup> At pH > 9, the formation of multinuclear uranyl surface species leads to an increase in adsorption (Figure 4b). Moreover, a reference SCM that included cation exchange reactions was constructed without changing any parameters to access the cation exchange reaction contribution (Figure S11). The results suggested that cation exchange had a negligible influence on the overall adsorption under the tested conditions.

Regarding the bell-shaped curve of arsenate (As(V)) adsorption data observed at pH below 7,²⁹ the SCM showed a good fit but underestimated values at strongly acidic and basic pH, as depicted in Figure 4c. Notably, prior studies have suggested that As(V) can form cation-bridging complexes on basal surfaces.²⁸ Incorporating this reaction enabled us to accurately model the As(V) adsorption data over the whole pH range (Figure 4d and Table S7).

## 4. ENVIRONMENTAL IMPLICATIONS

Here, we report a comprehensive investigation integrating molecular simulations, thermodynamic modeling, and experimental data to elucidate the adsorption behavior of metallic contaminants by clay minerals. As a case study, we choose typical U(VI) cation species (UO₂²⁺) and As(V) anion species (HAsO₄²⁻ and H₂AsO₄⁻) as the adsorbates to investigate the adsorption structures and abilities on different orientations of cis-vacant montmorillonite. The surface complexation structures of UO₂²⁺ were found to agree with the commonly observed spectroscopic and computational results, exhibiting a bidentate complexation mode. Additionally, a special tridentate surface complex was also proposed, which deserves future spectroscopic verification. HAsO₄²⁻/H₂AsO₄⁻ were examined

as monodentate surface complexes on the edges. All of the free energies of desorption of UO₂²⁺ and H₂AsO₄⁻/HAsO₄²⁻ from the edge sites were evaluated, yielding positive results that indicate their ability to form stable inner-sphere surface complexes on the cis-vacant edges.

According to SCMs, for U(VI), the adsorption was mainly contributed by surfaces perpendicular to [110] below pH 5.5, while above pH 5.5, surfaces perpendicular to [010] and [110] were dominating (Figure 4a). According to the edge surface properties with a cis-vacant model, at pH below 6–7, edge surfaces are positively charged on surfaces perpendicular to [010] and [110] and negatively charged on surfaces perpendicular to [010] and [110],<sup>46,47</sup> which provide a comprehensive elucidation of the adsorption behavior of U(VI). The aforementioned statement serves as an explanatory framework for the adsorption of other metal cations on clays. Also, because the surfaces perpendicular to [010] and [110] are positively charged below pH 7, they are the primary adsorption surfaces for metalloid anions such as As(V) (Figure 4c). Hence, clay edge surface chemistry, including surface acid-base chemistry and surface complexation properties, plays a crucial role in determining the environmental fate of contaminants. This reveals that specific crystallographic orientation governs the adsorption of cationic or anionic substances on cis-vacant clay, which offers valuable insights for future modeling studies. In summary, a multiscale adsorption model of U(VI) and As(V) on a cis-vacant clay layer, including micromechanisms and macromodels, elucidates the fundamental adsorption laws of metallic/metalloid pollutants and provides a theoretical basis for optimizing the environmental performance aspects of clay-based environmental materials.

At the most fundamental level, our study contributes to renewed methodologies for investigating the fate of various pollutants in natural environments. The multiscale FPMD-SCM approach couples atomic-level information from FPMD simulations with SCM, which makes it possible to reveal and predict the environmental behavior of pollutants under different scenarios. Specifically, our prediction model combines multiple parameters based on the actual physicochemical significance, avoiding empirical parameters to correctly reflect the behavior mapping of contaminates in clay-rich environments.

<figure><img src="image_15.png" /></figure>

### ASSOCIATED CONTENT

<figure><img src="image_16.png" /></figure>

#### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c07338.

FPMD and SCM results, and PHREEQC input file scripts to reproduce SCM results (PDF)

<figure><img src="image_17.png" /></figure>

### AUTHOR INFORMATION

#### Corresponding Author

Xiandong Liu — State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering and Frontiers Science Center for Critical Earth Material Cycling, Nanjing University, Nanjing, Jiangsu 210023, P. R. China;
● orcid.org/0000-0003-3966-3998; Email: xiandongliu@nju.edu.cn

#### Authors

Pengyuan Gao — State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering and
Frontiers Science Center for Critical Earth Material Cycling, Nanjing University, Nanjing, Jiangsu 210023, P. R. China; [orcid.org/0000-0002-8499-9274](orcid.org/0000-0002-8499-9274)

Zhijun Guo - Radiochemistry Laboratory, School of Nuclear Science and Technology and Frontiers Science Center for Rare Isotopes, Lanzhou University, Lanzhou 730000, China; [orcid.org/0000-0002-2371-8854](orcid.org/0000-0002-2371-8854)

Christophe Tournassat - ISTO, UMR 7327, CNRS, BRGM, OSUC, Université d'Orléans, 45071 Orléans, France; Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States; [orcid.org/0000-0003-2379-431X](orcid.org/0000-0003-2379-431X)

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.est.4c07338

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work benefitted from financial support from the National Natural Science Foundation of China (Grant Nos. 42125202, 42402043, and 12175094), the Postdoctoral Fellowship Program of CPSF (GZC20240674), the China Postdoctoral Science Foundation (Grant No. 2024M761386), funding from the EC Horizon 2020 project EURAD under grant agreement 847593 (WP FUTURE), and a grant overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" program, LabEx VOLTAIRE, 10-LABX-0100 at ISTO. We are grateful to the CaSciMoDoT-Leto Supercomputing facilities of Region Centre Val de Loire in France and the High-Performance Computing Center (HPCC) of Nanjing University for providing computing time on its blade cluster system.

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