Mechanistic insights into acid orange 7 azo dye (AO7) reduction using DFT calculations

Abstract

A major environmental issue is the occurrence of azo-dye chemicals in textile wastewater. Improving the biological process of dye removal requires a good understanding of the degradation reaction mechanism. In this work, we selected the acid orange 7 (AO7) as the azo dye model and the couple redox AQS/AHQS as a redox mediator (RM). The Gibbs free energy and the redox potential of AO7 were predicted through the Density Functional Theory (DFT). The comparison of our results with experimental data shows that the DFT method is very effective in predicting the redox potential of AO7. Then, to propose a new reaction mechanism for the biological degradation of AO7 with the couple AQS/AHQS, the local and global reactivity descriptors were computed with a hybrid PBE0 functional and dnp basis set. The global descriptors showed that AHQS behaves as a nucleophile against AO7 and as an electrophile against AO7 hydrazo-intermediate (AO7-HI). Fukui function analysis shows that the reaction mechanism of AO7 reduction occurs in two steps: a nucleophilic attack leads to the formation of AO7-HI, followed by an electrophilic attack that leads to the formation of aromatic amines. We found that the proposed reaction mechanism obtained by theoreticalcomputation is in good agreement with other mechanisms of azo dyes reduction proposed in the literature.