Kinetic and Computational Study of Degradation of Two Azo Dyes, Metanil Yellow and Orange II, by Iron Oxide Nanoparticles Synthesized Using Hylocereus undatus

[Image: see text] Wastewater treatment is an unavoidable necessity and requires immediate action with the aim of saving future generations from water crises and ensuring the sustainability of clean and drinkable water. In the present work, green synthesis of iron oxide nanoparticles (IONPs) was carried out from fruit extract of Hylocereus undatus to observe its photocatalytic activity towards two azo dyes, namely, Metanil yellow and Orange II dye. IONPs served as an efficient photocatalyst for the degradation of Metanil yellow dye, and the reaction followed pseudo-first-order kinetics. The effect of the initial dye concentration, amount of photocatalyst added, and effect of pH on photocatalytic degradation of Metanil yellow and Orange II dye using IONPs was studied. The results were compiled to reveal the most effective conditions for degradation to occur. A computational study of two dyes using DFT (Density Functional Theory) calculations were also performed to calculate thermodynamic properties such as the free energy, dipole moment, HOMO–LUMO energy gap, chemical potential, global hardness, softness, global electrophilicity index, ionization potential, electron affinity, etc. These parameters were used to describe the reactivity of dye toward the active photocatalytic species responsible for the degradation of dye. Natural population analysis was also performed, and Fukui indices were also calculated to explain the possible attacking sites of dyes by active photocatalytic species.