Mechanistic Insights into the Adsorption of Cationic Crystal Violet onto Sepiolite: A Comprehensive Study Combining Experimental and Theoretical Approaches

Abstract

This study investigates the removal of Cationic Crystal Violet (CCV) from aqueous solutions using Sepiolite through a comprehensive approach combining experimental adsorption processes, density functional theory (DFT), and molecular dynamics simulations. Sepiolite was characterized using FTIR, XRD, SEM, and BET analysis, which revealed a high specific surface area of 296.60 m²/g. Various process parameters, including adsorbent dose, pH, temperature, and contact time, were examined. Batch experiments at 22°C and pH 7 achieved a high removal efficiency of 99.84%. Equilibrium isotherms study showed that Sips, Langmuir, and Freundlich modelsyielded satisfactory correlation coefficients (R² > 0.93). Kinetic modeling indicated that the pseudo-second-order model best described the adsorption kinetics (R² = 1). Thermodynamic assessments revealed the exothermic nature of the adsorption process. Computational analyses through DFT calculations and MEP mappingshowed the existence of highly reactive nucleophilic sites centered on the nitrogen atoms. Mulliken charges and Fukui indices elucidated the presence of different intermolecular interactions, where hydrogen bounds are predominant between 20N and 21N atoms and hydroxyl groups of the adsorbent. Also; n-π and π-π interactions were detected between siloxane groups and 20N and 21N atoms; and 8C respectively.Molecular dynamics simulations applied on CCV molecule near the (110) surface of Sepiolite hasfurther reinforced the system's stability, as evidenced by the adsorption energy value (-260.97 kcal/mol) corresponding to the most stable CCV-Sepiolite configuration. The combined experimental and computational results provide new insightson the adsorption mechanismdriven by chemisorption and physisorption.These resultsenable thetheoretical predictionof the suitability of Sepiolite to adsorb other molecules and optimize their adsorption, minimizing material waste, and guide future experiments by understanding interactions at the molecular level.