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

Abstract

Superoxide ion (O2•−) is one of the short lived reactive oxygen species (ROS) formed in aquatic environments. The reactions of O2•− with the model dissolved organic matter (DOM) were studied using a chemiluminescent analysis method under relevant environmental conditions. The reaction of O2•− with DOM produced reduced DOM (DOM•−) by fast one-electron-transfer in the initial stage. This process resulted an initial “loss” in the O2•− decay kinetics. DOM•− is unstable which will continue react with O2•− generating H2O2 to complete a catalytic dismutation cycle. Based on analyzing the observed pseudo-first order O2•− decay rates (kpseudo), the quasi-steady-state concentration of DOM•− is found to be equal to the initial loss of O2•−. Thus, the rate constant for DOM•− with HO2•/O2•− is derived to be (1.1–1.9) × 106 M−1 s−1 in the temperature range of 7.8–41.4 °C. Meanwhile, the apparent rate constant for DOM with O2•− in a flow cell during a short time (2.25 s) is measured as (1.5–3.3) × 103 MC−1 s−1 in the temperature range of 8.2–38.6 °C. These temperature dependent O2•− reaction rate constants present an apparent activation energy of (19.6 ± 2.9) kJ molC−1 for DOM, while that of DOM•− (12.5 ± 3.5 kJ mol−1) is lower. For the pseudo-first order decay rate of O2•−, 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 NaBH4 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 O2•− with DOM. The present work suggests that DOM is an important sink for O2•− in aquatic environments. Furthermore, we proposed that the reaction of O2•− with DOM could be a potential source of DOM•− in natural water.