New insight into the eco-friendly synthesis of MgO NPs using olive pomace extract: combined experimental and DFT study for Congo red adsorption.

In this study, magnesium oxide nanoparticles (MgO NPs) were synthesized via a green method using olive pomace extract as a natural reducing and stabilizing agent. XRD, FTIR, SEM-EDX, TG/DTG, and BET analyses confirmed the formation of crystalline, quasi-spherical MgO with a cubic structure and an average crystallite size of ∼13 nm. The MgO NPs were applied for the removal of Congo Red (CR) dye from aqueous solutions, achieving 91.5% removal within 30 min and over 95% under optimal conditions (pH ≈ 7, dose = 0.2 g, C 0 = 20 mg L-1, T = 298 K). Equilibrium data fitted the Dubinin-Radushkevich isotherm, and kinetics followed a pseudo-second-order model, indicating chemisorption as the dominant mechanism. Thermodynamic parameters indicated spontaneous (ΔG° = -31.16 to -31.43 kJ mol-1) and exothermic (ΔH° = -27.21 kJ mol-1) adsorption, with increased disorder at the solid-liquid interface (ΔS° = +13.50 J (mol-1 K-1)) and a slight decrease in adsorption at higher temperatures. DFT and MD studies provided molecular-level insights, showing strong electrostatic and charge-transfer interactions between CR sulfonate groups and Mg2+ sites, supported by a low HOMO-LUMO gap (∼2.83 eV) and favorable adsorption configurations. These results demonstrate that green-synthesized MgO NPs from olive pomace are efficient and environmentally friendly adsorbents for dye removal from wastewater.