📄 chemistry

Surface Diffusion–Driven Adsorption of Pyrene on Al- MCM-41: An Integrated Experimental and Phenomenological Modeling Study

This study demonstrates that Al-MCM-41 is an effective adsorbent for removing pyrene from petroleum effluents, revealing through integrated experiments and phenomenological modeling that surface diffusion is the dominant mass-transfer mechanism, particularly at higher adsorption loadings.

Original authors: Rafael T. Landim, Solange A. Quintella, Antônio S. Araújo, Joaquin Silvestre-Albero, Célio L. Cavalcante, F. Murilo T. Luna

Published 2026-09-14

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CC BY 4.0

Original authors: Rafael T. Landim, Solange A. Quintella, Antônio S. Araújo, Joaquin Silvestre-Albero, Célio L. Cavalcante, F. Murilo T. Luna

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Pollutants known as polycyclic aromatic hydrocarbons, or PAHs, are a persistent and dangerous class of chemicals found in the environment. These molecules, which include substances like pyrene, often originate from the burning of coal, the processing of oil, and the incineration of waste. Because they are toxic and can cause long-term harm to living things, removing them from water and soil is a critical goal for environmental safety. One of the most effective ways to clean these contaminants is through adsorption, a process where a solid material acts like a sponge, trapping the harmful molecules on its surface and holding them there. While traditional materials like activated carbon have been used for this purpose, scientists are constantly searching for advanced materials that offer larger surface areas and more efficient pathways for these molecules to travel, ensuring they are captured quickly and completely.

In a recent study, researchers investigated a specific engineered material called Al-MCM-41 to see how well it could capture pyrene, a representative member of the PAH family. This material is a type of mesoporous silica, meaning it is built with a network of tiny, uniform tunnels that are larger than those found in standard sponges but still microscopic in scale. The scientists created a version of this material that includes aluminum, which introduces specific chemical sites on the tunnel walls that can interact more strongly with aromatic molecules. To understand exactly how this material works, the team conducted experiments in two different ways. First, they mixed the solid material with liquid solutions containing pyrene in a batch process to see how much of the pollutant the material could hold at different temperatures. Second, they pushed the liquid through a column packed with the material to watch how the pollutant moved through the system over time, simulating a real-world filtration setup.

The results from the batch experiments revealed that the material is highly effective at trapping pyrene, but the amount it can hold depends on the temperature. The researchers found that the material captured more pyrene at lower temperatures, specifically at 30 degrees Celsius, suggesting that the process is driven by physical forces rather than a chemical reaction that releases heat. By analyzing the data, they calculated that the energy required to hold the pyrene molecules on the surface was 136.1 kilojoules per mole. This value is significantly higher than what is typically seen with standard activated carbon, indicating that the aluminum-containing material provides a much stronger grip on the pollutant. The team also observed that the material's surface is not perfectly uniform; instead, it has regions with slightly different energies, which helps explain why the material performs so well across a range of conditions.

To understand how the pyrene molecules move inside the material, the researchers combined their column experiments with computer simulations. They modeled the journey of the molecules as they traveled through the liquid and then into the solid material. The simulations showed that the movement of the pyrene is driven by two main mechanisms: the molecules drifting through the empty spaces inside the pores, and the molecules sliding along the inner walls of those pores. At lower concentrations of pyrene, the movement through the empty spaces was the dominant factor. However, as the concentration of pyrene increased, the sliding motion along the surface became much more important. In fact, the speed at which the molecules slid along the surface increased dramatically, by nearly two orders of magnitude, as more of the surface became covered with pyrene. This finding highlights that the material does not just sit passively; the surface itself actively facilitates the transport of the pollutant once it begins to accumulate.

The study concluded that Al-MCM-41 is a promising candidate for removing polycyclic aromatic hydrocarbons from contaminated fluids. The material combines a large surface area, a well-organized network of tunnels, and specific chemical sites that attract the pollutant. The research demonstrated that while the material works well at capturing the molecules, the way those molecules move once they are inside is complex and changes depending on how much of the surface is occupied. By understanding these details, engineers can better design systems to clean up environmental contaminants. The work provides a clear picture of how this advanced material functions, offering a solid foundation for developing more effective strategies to protect water and soil from the harmful effects of industrial pollutants.

Technical Summary: Surface Diffusion–Driven Adsorption of Pyrene on Al-MCM-41

Problem Statement
Polycyclic aromatic hydrocarbons (PAHs) are persistent environmental contaminants with documented toxic, mutagenic, and carcinogenic effects, frequently found in petroleum-derived effluents. While adsorption is a recognized low-energy removal technology, optimizing these processes requires a detailed understanding of retention mechanisms within mesoporous materials. Traditional adsorbents like activated carbon have limitations, prompting the investigation of ordered mesoporous materials such as MCM-41. However, a comprehensive understanding of the diffusion mechanisms—specifically the interplay between pore and surface diffusion—for bulky PAH molecules like pyrene on aluminum-modified MCM-41 (Al-MCM-41) remains an area requiring further integration of experimental data and phenomenological modeling.

Methodology
The study employed an integrated experimental and computational approach to evaluate Al-MCM-41 as an adsorbent for pyrene in an iso-octane solvent system.

Material Synthesis and Characterization: Al-MCM-41 (Si/Al ratio ≈ 5) was synthesized hydrothermally using CTMABr as a template. The material was characterized via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption-desorption (BET/BJH), and scanning electron microscopy (SEM) to confirm the ordered mesoporous hexagonal structure, surface acidity, and textural properties.
Batch Experiments: Equilibrium adsorption data were collected at 30, 45, and 60 °C. The data were fitted to Langmuir and Toth isotherm models to determine maximum adsorption capacities and surface heterogeneity parameters. The heat of adsorption was estimated using the van't Hoff equation, accounting for solvent co-adsorption effects.
Fixed-Bed Column Experiments: Dynamic adsorption was studied using a Varian ProStar 210 HPLC system at 30 °C. Breakthrough curves were generated for two inlet pyrene concentrations (0.17 and 3.57 mmol L⁻¹).
Phenomenological Modeling: A pore and surface diffusion model was developed and solved numerically using the gPROMS simulator. The model incorporated axial dispersion, external film mass transfer, and intraparticle transport (pore and surface diffusion). Surface diffusivity ( $D_S$ ) was estimated by optimizing the model against experimental breakthrough data. The Darken equation was applied to correct surface diffusivity values based on the thermodynamic factors derived from both Langmuir and Toth isotherms.

Key Results

Material Properties: The synthesized Al-MCM-41 exhibited a type IV isotherm with a narrow pore size distribution centered at approximately 2.8 nm, a specific surface area of 540 m² g⁻¹, and a total pore volume of 0.560 cm³ g⁻¹. XRD and FTIR confirmed the successful incorporation of aluminum into the silica framework, generating acidic sites while preserving the hexagonal mesostructure.
Equilibrium Behavior: Adsorption was found to be exothermic, with capacity decreasing as temperature increased. The Toth model provided a superior fit ( $R^2$ up to 0.998) compared to the Langmuir model, suggesting surface energetic heterogeneity due to aluminum incorporation. The maximum adsorption capacity at 30 °C was estimated at 0.804 mmol g⁻¹ (Toth) and 0.912 mmol g⁻¹ (Langmuir).
Thermodynamics: The apparent heat of adsorption was calculated to be 136.1 kJ mol⁻¹. This value is significantly higher than that reported for pyrene on activated carbon (44.9 kJ mol⁻¹) and comparable to or higher than other PAHs on MCM-41, attributed to the interaction between the aromatic $\pi$ -electrons of pyrene and the acidic sites on the Al-MCM-41 surface.
Diffusion Mechanisms: The phenomenological model accurately reproduced the breakthrough curves (RMSE < 8.2×10⁻²). A critical finding was the concentration dependence of surface diffusivity ( $D_S$ ). While pore diffusion ( $D_p$ ) dominated at lower concentrations, $D_S$ increased by nearly two orders of magnitude as the adsorbed amount increased, becoming a dominant mass transfer mechanism at higher loadings.
Correction Factors: The application of the Darken correction revealed that the choice of isotherm model significantly influences the estimation of corrected surface diffusivity ( $D_{S0}$ ). The Toth-based correction yielded more modest variations in $D_{S0}$ across the loading range, reflecting the energetic heterogeneity of the surface, whereas the Langmuir-based correction showed a sharper increase.

Significance and Claims
The authors claim that this study provides essential insights into the adsorption and diffusion phenomena of PAHs on Al-MCM-41. The primary contribution is the demonstration that surface diffusion is a critical mechanism, particularly at higher adsorbate loadings, and that its magnitude is strongly dependent on surface coverage. By integrating experimental breakthrough data with a phenomenological model, the study validates Al-MCM-41 as a viable adsorbent for pyrene removal. The research highlights that the presence of aluminum-induced acidic sites enhances the interaction energy and adsorption capacity compared to pure silica or activated carbon. Furthermore, the work underscores the necessity of using appropriate isotherm models (such as Toth for heterogeneous surfaces) and thermodynamic corrections (Darken) to accurately predict mass transfer parameters, which is vital for the design and optimization of industrial adsorption processes for environmental remediation.

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