Chemistry of superoxide radical in seawater: CDOM associated sink of superoxide in coastal waters

Colored dissolved organic matter (CDOM) and humic substances contain a nonmetallic redox-cycling component capable of catalyzing superoxide (O-2(-)) dismutation. First-Order rate coefficients (k(pseudo)) measured for this O-2(-) sink in a number of coastal and Chesapeake Bay water samples range up to 1.4 s(-1), comparable in magnitude to catalyzed dismutation by Cu species. A significant (r(2) = 0.73) correlation is observed between k(pseudo) and the optical absorption and salinity of individual coastal water samples, suggesting an association with non-marine-derived CDOM. The activity of this sink is not changed by acidification or boiling of samples but is removed by photooxidation, indicating that it is an organic compound, but that it is neither enzymatic nor likely to consist of tightly bound metals. The stoichiometry of hydrogen peroxide formation from O-2(-) decay indicates that this sink is capable of a redox cycle catalyzing the dismutation of O-2(-) This CDOM sink combined with the organic copper sink previously described will produce a steady-state superoxide concentration in coastal waters that is 100-1000-fold lower than that predicted from bimolecular dismutation alone. Catalyzed O-2(-) decay was also observed in a variety of humic and fulvic acid samples, possibly occurring through quinone functionalities. Although the presence of quinone moieties in humic and fulvic acids has been demonstrated, there do not appear to be good correlations between several measures of quinone content and the O-2(-) dismutation rates of these samples.