Theoretical investigation on the mechanism of the OH-initiated degradation process of reactive red 2 azo dye

Abstract

The degradation treatments of azo dyes using various advanced oxidation processes (AOPs) have attracted considerable attention. Recently, our research group reported the degradation of reactive red 2 (RR2) azo dye in the newly designed ozone aerated internal micro-electrolysis filter (OIEF) system, the treatment performance of which is excellent. However, the reaction channels along which the RR2 dianion degrades remain to be deciphered at the molecular reaction level. Here, we report the degradation mechanism of the RR2 dianion by means of the density functional theory. The OH-initiated model in aquatic conditions has been adopted in the quantum chemical calculations due to the key role of the hydroxyl radical in the AOPs. According to our calculation results, there are three possible C-attack electrophilic substitution channels for the RR2 dianion and two C-attack channels for its hydrazone tautomer. It has been found that in each possible channel, the primal step is the initial association between the hydroxyl radical and one of the sulfonic groups, leading to the formation of a pre-reaction complex which is provided with a distinct hydrogen bond feature where the hydroxyl radical serves as a proton donor. The tautomerization between the azo and hydrazone forms was also found to have an influence on the distribution of degradation intermediates and products. The theoretical results we present are consistent with previous theoretical studies and our experimental findings and may shed some light on the improvements of the degradation treatments for RR2 and other structurally similar azo dye pollutants.