Adsorption of acid Blue 25 dye by CPAA coated on SBA-15 in aqueous solution: A density functional theory study

•Adsorption mechanism of Acid Blue 25 from aqueous solution on a SBA-15/CPAA was studied.•Calculations with DFT were used to explain Acid Blue 25 adsorption on SBA15/CPAA.•The comparison between experimental and theoretical studies on thermodynamic parameters was done. In this research, the geometric and electronic structure of the Acid Blue 25 dye as adsorbate and SBA-15 mesoporous silica functionalized with cross-linked polyacrylic acid (CPAA) polymer (SBA-15/CPAA) as adsorbent are studied using density functional theory (DFT) in the aqueous phase. In order to determine the best site of the dye for interacting with adsorbent, the atomic Hirshfeld charges and frontier orbitals are investigated. The result of this investigation confirms that the sulfonate group of the dye is the best nucleophilic site. The adsorbent has two protonated sites (–NH2+ and = OH+) to attract dye anion. The evaluation of calculated adsorption energy, quantum descriptors such as electronic chemical potential (μ) and chemical hardness (η), dipole momentum and solvent effect are shown that the (–NH2+) group in comparison with (=OH+) group attracts the sulfonate group of the dye with more potent force and forms more stable complex. The adsorbent and adsorbate species have the full charge before adsorption. The results of computed transferred charge and the bond order determined that the interaction is mainly electrostatic. In this work, thermodynamic parameters such as enthalpy, entropy, and Gibbs free energy changes are calculated. The results of these parameters showed that the adsorption is an exothermic and spontaneous process. Besides, the obtained values of thermodynamic parameters are in the range of electrostatic interaction.