Removal of methylene blue and rose bengal dyes from aqueous solutions using 1-naphthylammonium tetrachloroferrate (III)

In this work, an ionic liquid (IL), 1-naphthylammonium tetrachloroferrate (III), was synthesized and characterized using different characterization techniques. The IL existed as solid at room temperature and hence it was explored as an adsorbent for the removal of methylene blue (MB) and rose bengal (RB) dyes from aqueous solutions. The adsorption capacity of the adsorbent at equilibrium was found to be 6.443 mg/g for MB and 20.70 mg/g for RB. Furthermore, the adsorption isotherms were fitted into Langmuir isotherm model indicating that the two dyes were homogenously adsorbed on the surface of the material and the adsorption kinetics followed pseudo-second-order kinetic model with R2 value above 0.99. In addition, density functional theory (DFT) calculation confirmed the experimental findings showing energy gap (ΔE) of 3.572 eV and 4.084 eV for MB and RB respectively, and after complexation, a shift of 1.261 eV was observed in MB compared to 0.484 eV in RB in basic medium, indicating higher tendency of MB adsorption. The adsorption energies exhibited a greater affinity for MB with 13.2 kcal/mol, while RB in acidic condition with a value of 20.2 kcal/mol and 2.806 Å binding distances for RB which is lower than the Van der Waals radii, 2.990 Å showing Lewis acid-base interaction.