Evaluation of Key Intermediates in Azo Dye Degradation by Advanced Oxidation Processes: Comparing Anilines and Phenols

Anilines and phenols are common intermediates in dye degradation. However, there is limited information on selecting anilines or phenols as priority intermediates in vivo and silico. Therefore, we selected 12 common azo dyes and 7 derivatives to investigate the formation and fate of intermediates in a Fenton-like process. The result showed that the correlation coefficient between the logarithm of decolorization rate (log10 k) and generation of anilines (r = −0.838) was higher than that of phenols (r = −0.768). Furthermore, one-way analysis of variance revealed that dyes with −NH2 and −OH significantly contributed to aniline generation (p < 0.05). Specifically, bisazo dyes with −OH and monoazo dyes with −NH2 were prone to generating anilines as intermediates. The generation mechanism involved hydroxyl radical oxidation and azo reduction, with hydroxyl radical oxidation playing a more prominent role in azo-dye degradation. Based on the Fukui index, hydroxyl radicals were likely to oxidize the −N═N– bond of azo dyes, rapidly forming anilines. Subsequently, anilines were likely to undergo further oxidation into phenols. Additionally, the degradation of substituted derivatives followed the substituent effect. This systematic investigation into the generation and transformation of intermediates during azo-dye degradation can provide new insights into controlled pollutants in textile wastewater treatment.