Kinetics studies and mechanistic considerations on the reactions of superoxide radical ions with dissolved organic matter

Abstract Superoxide ion (O 2 •− ) is one of the short lived reactive oxygen species (ROS) formed in aquatic environments. The reactions of O 2 •− with the model dissolved organic matter (DOM) were studied using a chemiluminescent analysis method under relevant environmental conditions. The reaction of O 2 •− with DOM produced reduced DOM (DOM •− ) by fast one-electron-transfer in the initial stage. This process resulted an initial “loss” in the O 2 •− decay kinetics. DOM •− is unstable which will continue react with O 2 •− generating H 2 O 2 to complete a catalytic dismutation cycle. Based on analyzing the observed pseudo-first order O 2 •− decay rates ( k pseudo ), the quasi-steady-state concentration of DOM •− is found to be equal to the initial loss of O 2 •− . Thus, the rate constant for DOM •− with HO 2 • /O 2 •− is derived to be (1.1–1.9) × 10 6  M −1  s −1 in the temperature range of 7.8–41.4 °C. Meanwhile, the apparent rate constant for DOM with O 2 •− in a flow cell during a short time (2.25 s) is measured as (1.5–3.3) × 10 3 M C −1 s −1 in the temperature range of 8.2–38.6 °C. These temperature dependent O 2 •− reaction rate constants present an apparent activation energy of (19.6 ± 2.9) kJ mol C −1 for DOM, while that of DOM •− (12.5 ± 3.5 kJ mol −1 ) is lower. For the pseudo-first order decay rate of O 2 •− , the catalyzed-dismutation by metal components ranges from 13 to 23%; the contribution by aromatic ketones of DOM is estimated to be 10–13% by using NaBH 4 reduction method. The residual contribution might mainly occur at the quinone-like groups, which contributed 64%–77% to the total dismutation. The pH effects on the apparent catalytic rate constants dominate the reaction of O 2 •− with DOM. The present work suggests that DOM is an important sink for O 2 •− in aquatic environments. Furthermore, we proposed that the reaction of O 2 •− with DOM could be a potential source of DOM •− in natural water.