New Insights on ARS Degradation Mechanisms: Initial Reaction Calculation Based on DFT Method

Advanced oxidation is a highly efficient method for the degradation of alizarin red S (ARS). The process generates both free and non-free radicals which are the main active substances in the oxidative degradation of ARS. Based on the fact that the degradation mechanism is still unclear on the atomic scale, the initial reaction pathways and mechanism of \(SO_4^{ \bullet - }\) and ·OH and 1O2 with ARS are explored in this paper. The results of quantum chemical calculations demonstrate that the energy barrier (ΔG≠) of \(SO_4^{ \bullet - }\) addition reactions to C-sites attached to the hydroxyl substituents < ·OH addition < other reactions. Additionally, free radicals prefer to attack α-sites than adjacent β-sites. Among all reactions channels, the initial addition of \(SO_4^{ \bullet - }\) to the C5 α-site has the lowest energy barrier of 5.63 kcal/mol, highest apparent rate constants, and branching radio of 4.74 × 108 M−1 s−1 and 93.81% at 298 K, respectively. Thus, it is the most dominant initial reaction channel.