Computational Study of the Mechanistic Pathway Of Hydroxyl Radical-Initiated Degradation of Disperse Red 73 Dye

The mechanisms of hydroxyl radical (·OH) degradation of disperse red 73 (DR73) dye were investigated using density functional theory (DFT) calculations. Comparative feasibility of ·OH attack at the azo (N=N) site or on a > C–N group was examined based on their energies with a view to determine the more preferred pathway for DR73 degradation and predict its degradation products. Further decomposition of the cleavage products by ·OH radical through processes such as deamination and loss of molecular nitrogen was also examined. The results showed that ·OH radical attack on DR73 is more favourable via the azo (N=N) site. However, subsequent rupture of the N=N bond was found to be kinetically and thermodynamically less favourable compared to the C–N bond rupture arising from the attack of the radical on a ring carbon attached to the azo group. Introduction of water (as a solvent) was found to produce an accelerating and stabilising effects on the N=N bond cleavage mechanism, but an inhibitory and destabilising effects on the C–N counterpart. Deamination and nitrogen evolution reactions of the primary degradation products upon further ·OH radical attack were found to be kinetically and thermodynamically feasible.