CHEMISTRY OF SUPEROXIDE ION-RADICAL (O-2-) IN SEAWATER .1. PK-STAR-ASW (HOO) AND UNCATALYZED DISMUTATION KINETICS STUDIED BY PULSE-RADIOLYSIS

Superoxide, the one-electron reduction product of O2 and an important transient in photochemical and possibly other redox processes in natural waters, decays by autoredox dismutation: 2 O2- + 2 H+ → H2O2 + O2. The optical, acid-base, and self-decay properties of O2- in seawater were studied by electron pulse radiolysis-kinetic spectrophotometry, using added chelators to suppress transition metal catalysis. We find that (1)pKa* (HO2)=4.60±0.15,(2)[O2-] decay above pH 6 is strictly second order and pH dependent:-[O2-]/dt=kdis[O2 -]OOH]≈2k2[O2-] 2[H2] (3)k2sw=5±1x1012[H+] (S=30-36 0 00,assuming γH+=1). The pKa rate law, and rate constant are extremely similar to those in pure water; theoretically predicted 3-5-fold slower rates in seawater were not confirmed. In noncoastal high-salinity surface waters additional pseudo first-order O2- decay terms were undetectable (< 1.5 s-1); estuarine water samples exhibited pseudo first-order O2- decay terms of 1-5 s-1. These additional paths could compete significantly with uncatalyzed dismutation in surface seawater even upon 1000-fold dilution, which suggests that oceanic O2- decay may be controlled by unknown processes in addition to second-order dismutation. © 1990.