A Functional Nacrite–ODA Hybrid for Auramine O Removal: Adsorption Mechanisms Revealed by Experiments and DFT

Abstract

Cationic dye contamination in aqueous effluents remains a persistent environmental challenge, motivating the development of efficient and reusable adsorbents. In this study, nacrite organically modified with octadecylamine (Nacrite–ODA) was evaluated for the removal of Auramine O (AO) from aqueous solutions. Comprehensive physicochemical characterization, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and N2 adsorption–desorption isotherms (Brunauer–Emmett–Teller, BET), confirmed successful organo-functionalization, interlayer expansion, and a significant increase in specific surface area, all of which enhanced the accessibility of active sites toward AO adsorption. Adsorption kinetics were well described by the pseudo-second-order model, while equilibrium data were best fitted by the nonlinear Sips isotherm, indicating adsorption onto energetically heterogeneous sites. Intraparticle diffusion analysis further indicated that both surface adsorption and pore diffusion contributed to controlling the adsorption rate. Thermodynamic parameters (negative ΔG° and ΔH°) confirmed that AO uptake is spontaneous and exothermic. To gain molecular-level insight into the adsorption mechanism, density functional theory (DFT) calculations were performed, revealing energetically favorable AO+ configurations and close intermolecular contacts at the Nacrite-ODA interface. Complementary analyses of Mulliken charges, Fukui indices, and spin density identified electronically distinct regions of AO, notably electron-rich nitrogen atoms and a localized spin density at C16, consistent with electrostatic attraction and noncovalent hydrophobic, dispersion, and hydrogen-bonding interactions with the adsorbent surface. Reusability tests showed that Nacrite-ODA retained approximately 80% of its initial adsorption capacity after five adsorption–desorption cycles. Together, these experimental and computational results establish Nacrite-ODA as an effective and regenerable adsorbent, while providing mechanistic insight into the noncovalent basis of cationic dye–clay interactions.