Kinetics and thermodynamics of toluidine blue and methyl violet 2B adsorption onto modified moringa oleifera seed hulls composite

Abstract In the current study, the use of a novel adsorbent material (MO@HA) obtained from compositing Moringa oleifera seed hulls with hydroxyapatite (HA) for the removal of Toluidine Blue (TB) and Methyl Violet 2B (MV 2B) is reported. The prepared absorbents were characterized by TGA/DSC, XRD, and TEM/EDX. MO@HA composite material exhibited a good thermal stability, a high oxygen content, and an atomic percentage of calcium of 1.86%. The adsorption kinetics and thermodynamics were investigated at different concentrations (50–300 mg/L) and temperatures (293–333 K). The composite was developed to investigate the kinetics models and thermodynamics studies of the adsorption of TB and MV 2B onto MO@HA and evaluate their rate-limiting steps. Adsorption kinetics were analyzed using different kinetic models to determine the rate-controlling mechanisms of the adsorption process. Based on the correlation coefficient, the results demonstrated that the adsorption of TB (0.9813 ≤ R 2 ≤ 0.9984) and MV 2B (0.9835 ≤ R 2 ≤ 0.9922) onto the composite adsorbent followed the pseudo-first-order model, compared to pseudo-second-order, intra-particle diffusion, Elovich, and Boyd models. Thermodynamic parameters were evaluated to assess the spontaneity and energy changes associated with adsorption. According to the thermodynamic parameters of MV 2B (ΔG° varied from − 1.802 to -15.203 kJ mol −1 , ∆H° = 96.38 kJ mol −1 , and ∆S° = +335.02 J mol −1 K −1 ) and TB (ΔG° varied from − 0.941 to -13.950 kJ mol −1 , ∆H° = 94.36 kJ mol −1 , and ∆S° = +325.24 Jmol −1 K −1 ) across all the experimental temperatures, adsorption processes were feasible, spontaneous, favourable and endothermic. The functionalization of Moringa oleifera seed hulls with hydroxyapatite significantly enhanced the dye removal efficiency compared to unmodified adsorbents, making this composite a promising material for environmental remediation applications. The sticking probabilities of TB and MV 2B on the surface of MO@HA were high (S* ≪ 1), indicating that adsorption process was physical in nature.