Organically modified sodium montmorillonite for enhanced adsorption of xylenol orange from aqueous solutions: kinetic, thermodynamic, and mechanistic study

Xylenol orange (XO) is a persistent anionic azo dye that resists conventional treatment. Cetyltrimethylammonium bromide (CTAB) and binding poly (vinylpyrrolidone) (PVP) were sequentially intercalated onto Na-montmorillonite (NaMt) to prepare a CTAB/PVP-modified Na-montmorillonite (CP-NaMt), and its adsorption toward XO was evaluated. The nanostructured adsorbent was characterized by chemical analysis, morphological, phase analysis, and point-of-zero-charge measurements. The characterization indicated successful organic intercalation accompanied by slight basal-spacing expansion, partial layer exfoliation and the development of a hierarchical micro-mesoporous structure. The Brunauer–Emmett–Teller specific surface area increased from 46.5 to 88.7 m 2 ·g −1 . Batch tests identified pH = 5 as optimal, consistent with XO speciation and the positively charged surface of CP-NaMt. The adsorption process reached equilibrium rapidly, with a maximum capacity of 93.85 mg·g −1 achieved within 40 min at 30 °C. Kinetics followed a pseudo-second-order model and intraparticle-diffusion analysis indicated multistage transport beginning with a film-diffusion regime. Equilibrium isotherms were better described by the Langmuir model. Thermodynamic analysis indicated a spontaneous, endothermic process with an entropy gain that facilitates desolvation and mass transport. Regeneration with ethanol maintained ≥ 87.4% removal over five cycles. Spectroscopic evidence and control experiments supported a mechanism dominated by electrostatic ion pairing at quaternary ammonium sites, further stabilized by hydrogen bonding and hydrophobic partitioning and π-π stacking within organic microdomains. This low-cost, regenerable clay platform shows potential for polishing dye-laden wastewater.