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

Superoxide ion (O-2(center dot-)) is one of the short lived reactive oxygen species (ROS) formed in aquatic environments. The reactions of O-2(center dot-) with the model dissolved organic matter (DOM) were studied using a chemiluminescent analysis method under relevant environmental conditions. The reaction of O-2(center dot-) with DOM produced reduced DOM (DOM center dot-) by fast one-electron-transfer in the initial stage. This process resulted an initial "loss" in the Or decay kinetics. DOM is unstable which will continue react with O-2(center dot-) generating H2O2 to complete a catalytic dismutation cycle. Based on analyzing the observed pseudo-first order O-2(center dot-) decay rates (kpseudo), the quasi-steady-state concentration of DOM is found to be equal to the initial loss of O-2(center dot-). Thus, the rate constant for DOM center dot- with HO2 center dot/O-2(center dot-) is derived to be (1.1 -1.9) x 10(6) M-1 s(-1) in the temperature range of 7.8-41.4 degrees C. Meanwhile, the apparent rate constant for DOM with Or in a flow cell during a short time (2.25 s) is measured as (1.5-3.3) x 10(3) MC-1 S-1 in the temperature range of 8.2-38.6 degrees C. These temperature dependent Or reaction rate constants present an apparent activation energy of (19.6 +/- 2.9) kJ mol(C)(-1) for DOM, while that of DOM center dot- (12.5 +/- 3.5 kJ mol(-1)) is lower. For the pseudo-first order decay rate of Or, 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 O-2(center dot-) with DOM. The present work suggests that DOM is an important sink for O-2(center dot-) in aquatic environments. Furthermore, we proposed that the reaction of O-2(center dot-) with DOM could be a potential source of DOM center dot- in natural water. (C) 2018 Elsevier Ltd. All rights reserved.