Detailed study of Safranin-O adsorption on sepiolite clay: Kinetics, thermodynamics, isotherms and theoretical calculations for optimal water treatment efficiency

Abstract

Large-scale pollution requires industrial waste management policy. This study describes the adsorptive removal process for the cationic dye Safranin-O (Sfr), known for its high toxicity, using a natural, chemically unmodified clay classified as a fibrous clay, namely sepiolite. This adsorbent was thoroughly characterized using a variety of techniques, including Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with X-ray energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray fluorescence (FX), zero charge point analysis (pHZCP) and Zeta potential. The kinetic study showed that adsorption followed pseudo-second-order kinetics associated with a mixed diffusion process including bulk and intra-particle diffusion. The adsorption process reached equilibrium within 300 min. Sorption isotherm models fitted the experimental data in the order: M-J (four parameters) > F-S V (five parameters) > F-S IV (four parameters) > Sips (three parameters) ≈ Hill (three parameters) > R-P (three parameters) > Freundlich (2 parameters) > Langmuir (2 parameters). The maximum adsorption capacity estimated by the Langmuir model was approximately 233.81 mg/g. Analysis of the thermodynamic parameters highlighted adsorption as a spontaneous, physical and exothermic process by nature, underlining the preponderance of randomness at the adsorbent surface. In addition, DFT calculations followed by some electronic parameters linked to the Sfr dye were studied. Topological characteristics such as Localised Orbital Locator (LOL), Electron Localisation Function (ELF), Visualizing Molecular Dynamics (VMD), and Reduced Density Gradient (RDG) were discussed using the multiwave function (Multiwfn) in liquid phase. Monte Carlo (MC) simulation models were conducted to probe the interaction and mechanism between the fibrous clay fraction surface and the Sfr dye. Overall, sepiolite, due to its abundance, low cost, efficiency and environmental safety, is emerging as a promising adsorbent for the treatment of wastewater containing harmful dyes.