Understanding the Adsorption Mechanism of Phenol and Para-Chlorophenol onto Sepiolite Clay: A Combined DFT Calculations, Molecular Dynamics Simulations, and Isotherm Analysis

Abstract

This study integrates molecular dynamics (MD) simulations and density functional theory (DFT) computations to elucidate the unique adsorption characteristics of phenol and para-chlorophenol onto sepiolite by examining structural deformation, electronic properties, and adsorption energetics. The hydroxyl group (-OH) of phenol mainly determines its adsorption process since it has a quite negative Mulliken charge (−0.428) and significant electrophilic reactivity (fi⁺ = 0.090), therefore enabling strong hydrogen bonding with the silanol (-SiOH) groups of sepiolite. By π-π interactions with the electron-rich siloxane (-Si-O-Si-) surfaces, the aromatic carbons in phenol improve stability. The close molecular structure allows minimum deformation energy (E_def = 94.18 kcal/mol), hence optimizing alignment with the sepiolite surface. The much negative adsorption energy (E_ads = −349.26 kcal/mol) of phenol supports its further thermodynamic stability. Conversely, because of its copious chlorine (-Cl) component, para-chlorophenol runs against steric and electrical obstacles. The virtually neutral Mulliken charge (−0.020) limits electrostatic interactions even if the chlorine atom shows great electrophilicity (fi⁺ = 0.278). Chlorine’s electron-withdrawing action lowers the hydroxyl group’s (fi⁺ = 0.077) reactivity, hence lowering hydrogen bonding. Moreover, para-chlorophenol shows strong deformation energy (E_def = 102.33 kcal/mol), which causes poor alignment and less access to high-affinity sites. With less negative than phenol, the adsorption energy for para-chlorophenol (E_ads = −317.53 kcal/mol) indicates its reduced thermodynamic affinity. Although more evident in para-chlorophenol because of the polarizable chlorine atom, van der Waals interactions do not balance its steric hindrance and reduced electrostatic interactions. With a maximum Qmax = 0.78 mmol/g, isotherm models confirm the remarkable adsorption capability of phenol in contrast to Qmax = 0.66 mmol/g for para-chlorophenol. By hydrogen bonding and π-cation interactions, phenol builds a dense and structured adsorption layer, and para-chlorophenol shows a chaotic organization with reduced site use. Supported by computational approaches and experimental validation, the results provide a comprehensive knowledge of adsorption mechanisms and provide a basis for the design of adsorbents catered for particular organic pollutants.

1. Introduction

Water pollution from organic contaminants has emerged as a significant global concern due to its serious environmental and health consequences. Phenol and its halogenated derivatives, notably para-chlorophenol, are particularly alarming pollutants due to their extensive industrial application and persistence in aquatic habitats. These compounds are frequently present in effluents from industries like pharmaceuticals, petrochemicals, and agriculture, posing considerable ecological threats and public health dangers [1,2,3]. Their toxicity, resistance to biodegradation, and propensity for bioaccumulation require the creation of effective removal technologies from wastewater. Adsorption is one of the most efficient and adaptable methods for eliminating organic contaminants from water. It is preferred for its simplicity, cost efficiency, and capacity to address a broad spectrum of pollutants [4,5,6]. The efficacy of the adsorption process is significantly influenced by the selection of adsorbent. Natural clays, including sepiolite, have garnered significant interest owing to their elevated surface area, distinctive structural characteristics, and chemical stability [7,8]. Sepiolite is a fibrous magnesium silicate clay adsorbent, distinguished by its microporous channels and many silanol (-SiOH) groups, rendering it an exceptional option for the adsorption of polar and nonpolar organic molecules [9,10]. Several comprehensive studies have been conducted on sepiolite as an adsorbent, but the chemical mechanisms governing its interactions with certain organic molecules are still inadequately understood. Phenol and para-chlorophenol are exemplary model compounds for examining these pathways because of their differing structural and electrical characteristics. The small size of phenol and its hydroxyl (-OH) group promote strong hydrogen bonding and electrostatic interactions, whereas the bulky chlorine (-Cl) substituent in para-chlorophenol causes steric hindrance and electronic polarization effects [11,12]. These distinctions render them significant for comprehending how molecule shape, electronic characteristics, and steric variables affect adsorption behavior.

Progress in computational chemistry, especially density functional theory (DFT) and molecular dynamics (MD) simulations has allowed researchers to elucidate the atomic-level interactions that dictate adsorption processes [13,14]. Integrating these computational methods with experimental adsorption research enables a more thorough comprehension of adsorption mechanisms. Fukui indices and Mulliken charge assessments obtained from DFT calculations are important in identifying reactive sites on both the adsorbate and adsorbent, while MD simulations provide valuable information regarding adsorption energetics and structural dynamics [15,16].

This work seeks to reconcile theoretical approaches by examining the adsorption mechanisms of phenol and para-chlorophenol on sepiolite. Using a synthesis of DFT calculations, MD simulations, and isotherm modeling, we clarify the impact of electrical characteristics, molecule structure, and steric parameters on adsorption efficiency. These findings further the overarching objective of enhancing adsorbent design for the effective elimination of organic contaminants from wastewater.

2. Materials and Methods

2.1. Adsorbent Model Preparation

The adsorbent model, as shown in Figure 1, was prepared using Biovia Materials Studio 2020, by cleaving the sepiolite structure along the (110) plane, a surface chosen for its high reactivity and extensive use in adsorption studies [9]. The bulk structure of sepiolite was constructed based on experimentally derived lattice parameters [1], with orthorhombic unit cell dimensions of a = 13.4 Å, b = 26.8 Å, c =5.28 Å and angles α = β = γ = 90°. Atomic positions were assigned according to crystallographic data for Mg₈Si₁₂O₃₀(OH)₄. (H₂O)₄∙8H₂O [17] captured the fibrous channels formed by Mg-O-Si chains. To ensure sufficient surface area and reduce periodic boundary artifacts, the unit cell was scaled into a supercell by extending it U = 4 along the a-axis and V = 1 along the b-axis, resulting in overall dimensions of 53.6 Å × 26.8 Å × 5.28 Å. The scaling along the a-axis was specifically implemented to increase surface area while minimizing interactions between adsorbates, preserving the natural periodicity of the (110) plane through the choice of V = 1. To isolate the slab from periodic images and prevent artificial interactions, a vacuum layer of 30 Å was introduced along the c-axis. The crystal slab was modeled with a thickness of 30 Å, equivalent to approximately five-unit cells along the c-axis. This slab was symmetrically centered within the simulation box to eliminate edge effects and maintain structural alignment.

2.2. DFT Computational Details

The molecular structures of phenol and para-chlorophenol were investigated using density functional theory (DFT) with the B3LYP hybrid functional and the 6-311G basis set [18,19], selecting of B3LYP based on its extensively benchmarked against experimental thermochemical and spectroscopic data for organic and hydrogen-bonded systems, striking a reliable balance between accuracy and computational cost. The 6-311G basis set ensures that key features such as charge distribution and hydrogen-bond interactions are well described without the prohibitive expense of larger basis sets [20,21], producing optimized geometries illustrated in Figure 2. All DFT calculations were performed using the Gaussian 09W software package, with Gauss View 5 employed for analyzing and visualizing frontier molecular orbitals (FMO) [22]. Key calculated electronic parameters, including the energies of the highest occupied (E_HOMO) and lowest unoccupied molecular orbitals (E_LUMO), facilitated the computation of several global reactivity descriptors. These descriptors encompass the energy gap (ΔE), ionization energy (I), electron affinity (A), chemical potential (μ), hardness (η), softness (S), electronegativity (χ), global electrophilicity index (ω), nucleophilicity (ε), and back-donation energy (ΔE _{back-donation}) [23,24,25,26], as derived from equations in

Table 1. DFT computational equations.

Ionization energy (I)	$I=-EHOMO$
Electron affinity (A)	$A=ELUMO$
Chemical potential (μ)	$\mathsf{\mu }=-$χ = $-{\displaystyle \frac{\mathrm{I}+\mathrm{A}}{2}}$
Hardness (η)	$\mathsf{\eta }={\displaystyle \frac{\left(\mathrm{I}-\mathrm{A}\right)}{2}}$
Softness (S)	$\mathrm{S}={\displaystyle \frac{1}{\mathsf{\eta }}}$
Global electrophilicity index (ω)	$\omega ={\displaystyle \frac{{\chi }^2}{2\eta }}={\displaystyle \frac{{\mu }^2}{2\eta }}$
Nucleophilicity (ε)	$\epsilon ={\displaystyle \frac{1}{\omega }}$
Back-donation energy (ΔE back-donation)	${\mathsf{\Delta }\mathrm{E} }_{\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}\text{-}\mathrm{d}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}={\displaystyle \frac{1}{8}}\ast \left(EHOMO-LUMO\right)$

.

To evaluate the local reactivity indicators of both compounds, we focused on the analysis of partial Mulliken atomic charges, electrostatic potential maps, the spatial distributions of HOMO and LUMO orbitals, and Fukui indices.

2.3. Adsorption MD Simulation Preparation

The molecular dynamics simulations were carried out using the COMPASSIII force field to ensure an accurate representation of interactions. Electrostatic forces were calculated using the Ewald summation method, while van der Waals interactions were handled through the atom-based summation approach [27,28]. To determine the most energetically favorable adsorption sites, the Adsorption Locator tool was employed, which identified low-energy configurations. Additionally, the sorption module was used to simulate adsorption isotherms across a range of fugacity conditions using the configurational bias Monte Carlo method [29]. The simulations were performed under the NVT ensemble at a temperature of 298 K, with a Berendsen thermostat maintaining thermal stability throughout the process. A cubic simulation box with dimensions of 30 Å × 30 Å × 30 Å was used to ensure adequate solvation of the sepiolite slab and the adsorbates. The solvent density was set to 1.0 g/cm³, replicating bulk water conditions; the system contained 500 water molecules, maintaining a neutral medium at a pH of 7. This choice was made to prevent competitive hydrogen adsorption on the sepiolite surface under acidic conditions and to avoid the deprotonation of phenol and chlorophenol, which would lead to the formation of phenolate and para-chlorophenolate species in the basic medium. Such species are known to introduce electrostatic repulsion with the negatively charged sepiolite surface, potentially reducing adsorption efficiency [30,31]. Previous experimental studies have demonstrated that the highest adsorption efficiency for phenol and para-chlorophenol occurs in neutral media (pH 6–7). This approach allowed for precise modeling of the adsorption behavior of phenol and para-chlorophenol on the sepiolite surface, providing comprehensive insights into the underlying mechanisms governing these interactions.

3. Results

3.1. DFT Results and Analysis

To gain a deeper understanding of the electronic properties and reactivity of phenol and para-chlorophenol, density functional theory (DFT) calculations were conducted. The analysis focuses on the frontier molecular orbitals, global reactivity descriptors, and related electronic parameters, as summarized in Figure 3 and

Table 2. The parameters derived from DFT equations.

Parameters	Phenol	Para-Chlorophenol
EHOMO	−6.783	−6.856
ELUMO	−0.431	−0.5091
ΔEGAP	6.352	6.077
I	6.783	6.586
A	0.431	0.509
χ	3.607	3.547
μ	−3.607	−3.547
η	3.176	3.038
S	0.31	0.33
ω	2.04	2.07
ε	0.49	0.48
ΔE back-donation	−0.794	−0.759

. The electronic properties of phenol and para-chlorophenol were analyzed using density functional theory (DFT) to understand their potential adsorption behavior on sepiolite. The parameters derived from the HOMO-LUMO energies and reactivity indices are presented in Table 2. Phenol has a slightly larger HOMO-LUMO gap (ΔE_gap = 6.352 eV) compared to para-chlorophenol (ΔE_gap = 6.077 eV), which could suggest that phenol may exhibit lower polarizability and a more stable electronic structure. This characteristic might favor strong, directional interactions such as hydrogen bonding. In contrast, the chlorine substituent in para-chlorophenol raises the HOMO energy (−6.586 eV compared to −6.783 eV for phenol), potentially enhancing its electron-donating ability. Its lower LUMO energy (−0.509 eV versus −0.431 eV for phenol) indicates an improved ability to accept electrons. These factors might allow para-chlorophenol to interact with electron-deficient sites more effectively [32,33]. Regarding global reactivity indices, phenol shows slightly higher electronegativity (χ = 3.607 vs. 3.547 for para-chlorophenol), which could indicate a stronger overall attraction to electron-rich sites. However, para-chlorophenol exhibits higher softness (S = 0.33 eV vs. 0.31 eV for phenol) and lower chemical hardness (η = 3.038 eV vs. 3.176 eV for phenol), suggesting greater polarizability and a potentially more adaptable electronic structure. These characteristics may facilitate interactions with diverse adsorption sites but could also imply reduced stability in specific configurations [34,35]. Both molecules demonstrate properties that could influence their adsorption behavior in different ways. The electronic structure of phenol may favor more stable and directional interactions, while para-chlorophenol’s greater polarizability and electron-donating capacity could allow for adaptable and varied adsorption interactions.

3.2. Molecular Electrostatic Potential (MEP) Maps

The Molecular Electrostatic Potential (MEP) maps, shown in Figure 4, reveal the electrostatic distribution over the molecular surfaces of phenol and para-chlorophenol, highlighting regions critical for reactivity. The color gradient represents electron density: red for electron-rich regions prone to electrophilic attack, blue for electron-deficient areas susceptible to nucleophilic attack, and green or yellow for balanced regions with minimal reactivity [36,37]. For phenol, the oxygen atom in the hydroxyl group is surrounded by a prominent red region, indicating its electron-rich nature and strong potential for hydrogen bonding or electrostatic interactions. The hydroxyl hydrogen appears blue, reflecting its electron-deficient character, making it a site for nucleophilic interactions. The aromatic ring shows delocalized π-electron density, shaded green or yellow, suggesting limited reactivity but possible involvement in van der Waals or π-cation interactions. In para-chlorophenol, the chlorine atom reduces the electron density of the hydroxyl oxygen, weakening its hydrogen bonding potential. The aromatic ring displays noticeable polarization, with a slight reddening near the para-carbon due to resonance effects and blue shifts on adjacent positions. This asymmetry alters the molecule’s reactivity, introducing steric and electronic effects that moderate its interaction strength compared to phenol. Phenol’s stronger electron-rich regions make it more reactive for hydrogen bonding and electrostatic interactions, whereas para-chlorophenol’s reactivity is reduced due to steric hindrance, electron-withdrawing, and polarization effects caused by the chlorine substituent. This contrast directly influences their adsorption behavior on sepiolite.

3.3. Mulliken Charges and Fukui Indices

Mulliken charges and Fukui indices are critical descriptors for identifying reactive sites in molecules. They allow for an evaluation of the electrophilic and nucleophilic tendencies at specific atoms, providing insights into how these molecules interact with their environment. The results of the Mulliken charge analysis and the Fukui indices (fi⁺, fi⁻, fi°) for phenol and para-chlorophenol are presented in Figure 5 and

Table 3. The Mulliken charge analysis and the Fukui indices (fi⁺, fi⁻, fi°) for phenol and para-chlorophenol.

Phenol					Para-Chlorophenol
Atom	Charge	fi+	fi−	fi∘	Atom	N	fi+	fi−	fi°
1C	−0.09858	0.119488	0.001053	0.120541	1C	−0.08987	0.023476	−0.01381	0.009662
2C	−0.09028	0.045774	0.097578	0.143352	2C	−0.08196	0.034383	0.07010	0.104487
3C	−0.07859	0.070632	0.109257	0.179889	3C	−0.08360	0.041943	0.07469	0.116637
4C	0.113792	0.079907	0.000894	0.080801	4C	0.283182	0.067005	0.01401	0.081022
5C	−0.07859	0.070632	0.109257	0.179889	5C	−0.08360	0.041942	0.07469	0.116637
6C	−0.09028	0.045774	0.097578	0.143352	6C	−0.08196	0.034383	0.07010	0.104487
7H	0.096526	0.093138	0.091068	0.184206	7H	0.111408	0.084951	0.11736	0.202313
8H	0.099092	0.08087	0.103984	0.184854	8H	0.103076	0.090871	0.12457	0.215447
9H	0.101857	0.082546	0.102757	0.185303	9H	0.103076	0.09087	0.12457	0.215446
10H	0.101857	0.082546	0.102757	0.185303	10H	0.111408	0.084951	0.11736	0.202313
11H	0.099092	0.08087	0.103984	0.184854	11O	−0.59202	0.077178	0.04015	0.117329
12O	−0.42810	0.090302	0.041483	0.131785	12H	0.321034	0.049966	0.037224	0.08719
13H	0.252234	0.057519	0.038353	0.095872	13Cl	−0.02014	0.278081	0.148948	0.427029

. Typically, the atom with the highest fi⁺ value is preferred for nucleophilic attack (acceptance of electron), while the atom with the highest fi⁻ value is favored for electrophilic attack (donation of electron) [38,39].

For the hydroxyl oxygen, 12O in phenol has a charge of −0.428 and an electrophilic Fukui index (f⁺) of 0.090, indicating moderate electrophilic reactivity and a capacity for hydrogen bonding. In contrast, the hydroxyl oxygen of para-chlorophenol, 11O, exhibits a more negative charge of −0.592 and a lower f⁺ value of 0.077, suggesting reduced hydrogen bonding potential. The enhanced negative charge on 11O might favor electrostatic interactions, but steric hindrance from the chlorine substituent could limit effective binding. The chlorine atom, 13Cl, in para-chlorophenol acts as an electrophilic hotspot, with a significant f⁺ value of 0.278 and a Mulliken charge of −0.020. This site could engage with nucleophilic regions of a substrate, but its para position and steric bulk might restrict strong directional interactions. Polarization effects caused by 13Cl are evident in the aromatic ring. For instance, the adjacent carbon, 4C, in para-chlorophenol shows a positive charge of 0.283 compared to 0.114 for 4C in phenol. This polarization enhances the nucleophilicity of carbons such as 4C in para-chlorophenol, with an f⁻ value of 0.014, compared to 0.001 for phenol’s 4C, suggesting slightly increased reactivity at this site. The hydrogen atoms bonded to the oxygen, such as 7H in phenol and 7H in para-chlorophenol, also differ in charge and reactivity. In phenol, 7H has a Mulliken charge of 0.097 and an f⁺ value of 0.093, while in para-chlorophenol, 7H shows a higher charge of 0.111 but a lower f⁺ of 0.085. This indicates that hydrogen bonding potential is weaker in para-chlorophenol, likely due to the electron-withdrawing effect of the chlorine substituent. Overall, 12O in phenol appears more capable of engaging in directional hydrogen bonding, while 11O in para-chlorophenol favors electrostatic interactions. The chlorine atom, 13Cl, introduces electrophilic competition but also disrupts interaction efficiency due to steric effects. The adjacent carbon atoms, particularly 4C, show enhanced reactivity in para-chlorophenol due to polarization, while hydrogens such as 7H in phenol demonstrate slightly stronger bonding potential. These electronic features highlight distinctive interaction tendencies for each molecule, which could influence their adsorption or reactivity in subsequent studies.

4. Adsorption MD Simulation Results

The adsorption energetics of phenol and para-chlorophenol on sepiolite were evaluated under neutral conditions to explore their interaction mechanisms and positions, as shown in Figure 6. The total energy (E_total), adsorption energy (E_ads), rigid adsorption energy (E_rigid), and deformation energy (E_def) are summarized in

Table 4. The adsorption energetics of phenol and para-chlorophenol on sepiolite results.

Compound	Total Energy (kcal/mol)	Adsorption Energy (kcal/mol)	Rigid Adsorption Energy (kcal/mol)	Deformation Energy (kcal/mol)
Phenol	−349.26	−341.40	−435.58	94.18
Para-chlorophenol	−327.39	−317.53	−419.86	102.33

.

Phenol exhibits a more negative E_total (−349.26 kcal/mol) compared to para-chlorophenol (−327.39 kcal/mol), indicating its adsorption is thermodynamically and binding strength more favorable. The E_ads (the energy released when a molecule binds to the surface) are significantly more negative for phenol (−341.40 kcal/mol) than for para-chlorophenol (−317.53 kcal/mol). This suggests stronger binding interactions between phenol and the sepiolite [40,41]. The E_rigid (energy assuming no structural deformation) is more negative for both compounds than their actual adsorption energy, highlighting the energy penalty associated with molecular deformation during adsorption. Para-chlorophenol requires greater E_def (102.33 kcal/mol) than phenol (94.18 kcal/mol) to adapt to the sepiolite surface geometry [42].

Figure 7 shows the variations in van der Waals and electrostatic interaction energies during the adsorption of phenol and para-chlorophenol onto the adsorbent surface across the simulation steps. The electrostatic interaction energies for phenol are notably stronger (more negative), reaching approximately −170 kcal/mol, compared to para-chlorophenol, which peaks at around −140 kcal/mol. This indicates that electrostatic forces play a stronger role in the adsorption of phenol, likely due to its higher polarity and stronger interactions with the negatively charged surface of the adsorbent. In contrast, the van der Waals interaction energies show a slightly better performance for para-chlorophenol. This is attributed to the bulky chlorine atom in para-chlorophenol, which enhances dispersion forces and strengthens van der Waals interactions. The presence of chlorine, however, introduces steric effects that slightly reduce para-chlorophenol electrostatic interactions when compared to phenol.

5. Adsorption Isotherm Model

The adsorption isotherms of phenol and para-chlorophenol on sepiolite were analyzed using both the Freundlich and Langmuir models. The nonlinear Freundlich model provided the best fit to the data, with parameters (K_f^Phenol = 0.44 kPa⁻¹, n^phenol = 11, R² = 0.99 and K_f^{para-ChloroPhenol} = 0.37 kPa⁻¹, n^{para-ChloroPhenol} = 11, R² = 0.98). These high R² values reflect the nonlinear Freundlich model’s ability to describe the multilayer adsorption mechanism and the heterogeneous distribution of adsorption sites on sepiolite. In contrast, the nonlinear Langmuir model, which assumes monolayer adsorption, showed poorer fits, with parameters (K_L^Phenol = 0.08 kPa⁻¹, Qm_L^phenol = 0.75 mmol/g, R² = 0.73 and K_L^{para-ChloroPhenol} = 0.24 kPa⁻¹, Qm_L^{para-ChloroPhenol} = 0.62 mmol/g, R² = 0.80) (Figure 8). This sigmoidal pattern is characteristic of multilayer adsorption on a heterogeneous surface with a finite number of active sites. The plateau observed at higher fugacities (900–1000 kPa) indicates the saturation of sepiolite’s adsorption sites, beyond which no additional adsorption occurs as a complete multilayer form. Phenol demonstrates a higher maximum adsorption capacity (Q_max = 0.78 mmol/g) compared to para-chlorophenol (Q_max = 0.66 mmol/g). This result highlights phenol’s stronger interaction with sepiolite.

6. Proposed Mechanism

The adsorption of phenol and para-chlorophenol onto sepiolite is dictated by complex molecular interactions at the atomic level, involving site-specific affinities, molecular geometry, and the unique surface chemistry of sepiolite. The following mechanism integrates insights from isotherm models, density functional theory (DFT) calculations, and reactivity indices to provide a detailed understanding of the adsorption behavior of these molecules.

Phenol Adsorption: The hydroxyl (-OH) group of phenol serves as the primary adsorption site. Its highly negative Mulliken charge (−0.428) and significant electrophilic Fukui index (fi⁺ = 0.090) facilitate strong hydrogen bonding with the silanol (-SiOH) groups on the sepiolite surface. Additionally, the aromatic carbons (particularly atoms 3 and 5) exhibit high nucleophilic Fukui indices (fi⁻ = 0.109), suggesting potential π-π interactions with the electron-rich siloxane (-Si-O-Si-) surfaces of sepiolite. The compact molecular structure of phenol (94.11 g/mol) and the flexibility of its -OH group allow it to undergo minimal structural deformation (E_def = 94.18 kcal/mol) upon adsorption. This facilitates efficient penetration into sepiolite microporous channels, optimizing hydrogen bonding and improving overall adsorption efficiency. Phenol’s strong interactions with the sepiolite surface are further supported by its thermodynamic stability, as indicated by its highly negative adsorption energy (−349.26 kcal/mol).

Para-chlorophenol Adsorption: In para-chlorophenol, the presence of a bulky chlorine (-Cl) substituent introduces steric hindrance and reduces the molecule’s ability to interact with the sepiolite surface effectively. The chlorine atom (atom 13) has a relatively high electrophilic Fukui index (fi⁺ = 0.278), but its near-neutral Mulliken charge (−0.020) limits its capacity for significant electrostatic interactions. Consequently, para-chlorophenol relies primarily on weaker van der Waals forces or hydrophobic partitioning for adsorption. The hydroxyl oxygen (atom 11) in para-chlorophenol retains a strongly negative Mulliken charge (−0.592) but exhibits reduced electrophilic reactivity (fi⁺ = 0.077) compared to phenol. This reduction is likely due to the electron-withdrawing effects of the chlorine group, which decrease the overall reactivity of the -OH group. Furthermore, the larger steric bulk of para-chlorophenol increases structural deformation energy (E_def = 102.33 kcal/mol), forcing the molecule into less favorable configurations and limiting its access to high-affinity sites within the sepiolite micropores.

Phenol demonstrates a more efficient adsorption mechanism than para-chlorophenol, as evidenced by its higher adsorption energy and stronger interaction with the sepiolite surface. The localized reactivity of phenol (fi⁺ = 0.090) aligns well with the hydrophilic surface of sepiolite, resulting in a compact and well-ordered adsorption layer. Phenol molecules orient their -OH groups toward the silanol or cation-exchange sites and align their aromatic rings parallel to the surface, maximizing hydrogen bonding and π-cation interactions. In contrast, para-chlorophenol adopts a disordered adsorption configuration. The bulky -Cl groups protrude away from the surface to minimize steric strain, leading to lower packing density and weaker van der Waals interactions. Despite para-chlorophenol lower HOMO-LUMO gap (ΔE_GAP = 6.077 eV) and higher global electrophilicity index (ω = 2.071), its adsorption is thermodynamically less favorable due to inefficient site utilization and weaker binding forces.

The adsorption of phenol onto sepiolite is driven by the strong affinity of its hydroxyl group for silanol sites, its compact molecular structure, and its ability to form robust hydrogen bonds and π-cation interactions. Para-chlorophenol, on the other hand, faces steric and electronic challenges that reduce its adsorption efficiency. The proposed mechanism schema demonstrated in Figure 9 and

Table 5. Comparison of phenol and para-chlorophenol adsorption mechanisms on sepiolite.

Interaction Type	Trend (Phenol vs. Para-Chlorophenol)	Molecular Basis	Effect on Mechanism	Impact on Adsorption
Hydrogen Bonding	Phenol: Strong stabilization (−170 kcal/mol).	Phenol -OH group forms strong hydrogen bonds with -SiOH groups. Chlorine in para-chlorophenol withdraws electron density, reducing -OH polarity.	Phenol aligns optimally for hydrogen bonding, enhancing interaction strength. Para-chlorophenol -OH is less nucleophilic, weakening its H-bonding ability.	Phenol: Stronger adsorption. Para-chlorophenol: Weaker adsorption.
π-Cation Interactions	Phenol: Significant contribution. Para-chlorophenol: Reduced contribution.	Phenol electron-rich aromatic ring interacts strongly with Mg2⁺ ions in sepiolite. Chlorine in para-chlorophenol reduces electron density on the aromatic ring.	Phenol π-cloud engages Mg2⁺ effectively, stabilizing adsorption. Para-chlorophenol depleted π-cloud weakens π-cation attraction.	Phenol: Enhanced adsorption. Para-chlorophenol: Reduced adsorption.
Van der Waals	Phenol: Moderate contributions. Para-chlorophenol: Stronger contributions.	Phenol -OH group is compact and less polarizable. Chlorine in para-chlorophenol is larger and highly polarizable, enhancing dispersive forces.	Para-chlorophenol bulky -Cl group increases the surface contact area, enhancing van der Waals forces. Phenol relies more on electrostatic interactions.	Phenol: Minor role. Para-chlorophenol: Compensates for weaker electrostatics.
Steric Effects	Phenol: Minimal hindrance. Para-chlorophenol: Significant hindrance.	Phenol -OH group is compact and does not impede surface alignment. Para-chlorophenol bulky -Cl group disrupts molecular orientation near the surface.	Para-chlorophenol struggles to align optimally for hydrogen bonding and π-interactions. Phenol maintains close surface contact without steric clashes.	Phenol: Favors adsorption. Para-chlorophenol: Hinders adsorption.
Aromatic Interactions	Phenol: Strong stacking with the silicate framework. Para-chlorophenol: Weaker stacking.	Phenol aromatic ring retains full electron density for edge-to-face stacking with sepiolite silicate framework. Chlorine distorts ring symmetry and electron density.	Phenol stabilizes through effective aromatic stacking. Para-chlorophenol-distorted ring symmetry reduces stacking efficiency.	Phenol: Strengthens adsorption. Para-chlorophenol: Weakens adsorption.

shows the comparison of phenol and para-chlorophenol adsorption mechanisms on sepiolite.

7. Comparative Geometric Analysis

Figure 10 demonstrates the geometric analysis. Starting by the adsorption of phenol onto sepiolite, the C4–C5 bond remains at 1.397 Å before and after adsorption, preserving ring planarity and indicating that the ring itself is undistorted, while the 12O–13H bond lengthens from (0.94 Å before adsorption) to (1.207 Å after adsorption) as the oxygen 12O engages in a strong H–O⋯H hydrogen bond with a hydrogen of silanol at a distance of 2.29 Å. A secondary contact between a ring hydrogen (10H) and surface oxygen at 2.43 Å further reinforces this interaction. By contrast, para-chlorophenol’s Cl–C bond (13Cl–1C) increases only slightly from (1.173 Å before adsorption) to (1.178 Å after adsorption) and its C4–C5 distance remains unchanged, indicating preserved aromatic geometry. However, its O–H⋯O hydrogen bond is much weaker, as shown by a stretched H⋯O(silanol) separation of (3.63 Å 11O–H of the sepiolite surface). To compensate, the chlorine 13Cl atom positions itself 2.41 Å from an Mg²⁺ site, forming a moderate electrostatic or dispersion contact. Together, these measurements illustrate that phenol achieves robust anchoring through strong hydrogen bonding, whereas para-chlorophenol settles for a weaker elongated O–H bond, underscoring the steric and electronic penalties of the chlorine substituent.

8. Comparison of Phenol/Chlorophenol Adsorption Capacities Across Different Clay

The comparison of maximum phenol/chlorophenol adsorption experimental capacities of different clay is presented in

Table 6. Comparison of maximum phenol/chlorophenol adsorption capacities.

Adsorbent	Phenol Qe (mmol/g)	Chlorophenol Qe (mmol/g)	References
Palygorskite	1.49	/	[43]
Ca-Montmorillonite	0.37	/	[44]
Kaolin	/	0.65	[45]
Red clay	0.89	/	[46]
Na-Montmorillonite	/	0.23	[47]
Magadiite	0.55	/	[48]
Meta kaolinite	0.32	/	[49]
Na-Bentonite	1.09	/	[50]
Sepiolite	/	0.3	[51]
HDTMA-Sepiolite	/	0.7	[51]
Sepiolite	0.78	0.66	This work

.

9. Conclusions

• Phenol demonstrates enhanced adsorption efficiency on sepiolite relative to para-chlorophenol, attributed to robust hydrogen bonding and π-cation interactions, as indicated by DFT calculations;
• The hydroxyl group in phenol has a pronounced negative Mulliken charge (−0.428) and notable electrophilic reactivity (fi⁺ = 0.090), hence augmenting its interaction with silanol (-SiOH) groups;
• The substantial chlorine substituent of para-chlorophenol diminishes its hydrogen bonding capacity and imposes steric hindrance, as evidenced by its less reactive hydroxyl group (fi⁺ = 0.077) and almost neutral Mulliken charge (−0.020);
• Phenol exhibits reduced deformation energy (+94.18 kcal/mol), facilitating superior structural alignment with sepiolite’s active sites, in contrast to para-chlorophenol (+102.33 kcal/mol);
• Theoretical isotherm findings validate phenol’s superior adsorption capacity (Qmax = 0.78 mmol/g), indicating effective site usage and alignment;
• Nonlinear Freundlich-type behavior for both molecules indicates multilayer adsorption, with phenol establishing a denser and more organized adsorption layer.

This study synthesizes DFT insights, Fukui indices, Mulliken charge analysis, and MD simulation data to provide a complete framework for elucidating adsorption mechanisms and informing the design of customized adsorbents.