Transformation of phenol and nitrobenzene by superoxide radicals: Kinetics and mechanisms

In this study, the kinetics and mechanisms of superoxide radical ( O 2 · - ${\mathrm{O}}_2^{·-}$ )-mediated transformation of two monosubstituted aromatic contaminants with diametrically opposed electronic effect, namely phenol (PhOH) and nitrobenzene (NB), were investigated through combined experimental and theoretical approaches. The reaction rate constant between O 2 · - ${\mathrm{O}}_2^{·-}$ and PhOH was determined to be 269 M −1 s −1 via competition kinetic method. Such weak role of O 2 · - ${\mathrm{O}}_2^{·-}$ was confirmed by density functional theory (DFT) calculations at SMD/CAM-B3LYP/6-311++G(2d,2p)//CAM-B3LYP/6-31+G(d,p) level of theory. However, NB exhibited a higher reactivity towards O 2 · - ${\mathrm{O}}_2^{·-}$ than PhOH, demonstrating the selectivity of O 2 · - ${\text{O}}_{\text{2}}^{·-}$ to electron withdrawing group. Despite the difference in reactivity, O 2 · - ${\text{O}}_{\text{2}}^{·-}$ shows very limited removal capacity for PhOH and NB. These results were also in reasonably good agreement with an indication from a quantitative structure–property relationship model. With these quantitative and mechanistic results, we provided a more comprehensive understanding on the O 2 · - ${\text{O}}_{\text{2}}^{·-}$ -mediated transformation of organic pollutants bearing different electron donating/withdrawing groups.