Experimental and theoretical insight into the adsorption of phenol and 2,4-dinitrophenol onto Tithonia diversifolia activated carbon

Adsorption of phenol and 2,4-dinitrophenol onto Tithonia diversifolia activated carbon (0.445 cm^-3g⁻¹ pore volume and 854.44 m²g⁻¹ BET surface area) were studied by the experimental batch method and theoretical density functional theory. A maximum of 99.98% and 97.81% removal efficiency was attained for phenol and 2,4-dinitrophenol respectively at optimised conditions. The pseudo-second-order model described the adsorption kinetics well, while the Langmuir model best-elucidated the adsorption isotherm with a maximum adsorption capacity of 50.552 mg g⁻¹ for phenol and 42.607 mg g⁻¹ for 2,4-dinitrophenol. According to the calculated thermodynamic parameters, the adsorption process was spontaneous and endothermic. The process performance was further validated using real wastewater, and the removal efficiency of 84–90% was attained for both analytes. Theoretical investigations through density functional theory calculations suggested that the presence of oxygenated functional group on activated carbon surface decreased the adsorbate-adsorbent interaction. Meanwhile, a comparative study of the adsorption energies of various interactions indicated that phenol interacted more strongly with the activated carbon as compared to DNP. Regeneration studies indicated the reusability of the exhausted carbon up to the fifth cycle with significant removal efficiency.