Photoproduction of One-Electron Reducing Intermediates by Chromophoric Dissolved Organic Matter (CDOM): Relation to O2(-) and H2O2 Photoproduction and CDOM Photooxidation.

A molecular probe, 3-amino-2,2,5,5,-tetramethy-1-pyrrolydinyloxy (3ap), was employed to determine the formation rates of one-electron reducing intermediates generated photochemically from both untreated and borohydride-reduced Suwanee River fulvic and humic acids (SRFA and SRHA, respectively). This stable nitroxyl radical reacts rapidly with reducing radicals and other one-electron reductants to produce a relatively stable product, the hydroxylamine, which can be derivatized with fluorescamine, separated by HPLC and quantified fluorimetrically. We provide evidence that O2 and 3ap compete for the same pool(s) of photoproduced reducing intermediates, and that under appropriate experimental conditions, the initial rate of hydroxylamine formation (RH) can provide an estimate of the initial rate of superoxide (O2(-)) formation. However, comparison of the initial rates of H2O2 formation (RH2O2) to that of RH show far larger ratios of RH/RH2O2 (∼6-13) than be accounted for by simple O2(-) dismutation (RH/RH2O2 = 2), implying a significant oxidative sink of O2(-) (∼67-85%). Because of their high reactivity with O2(-) and their likely importance in the photochemistry of CDOM, we suggest that coproduced phenoxy radicals could represent a viable oxidative sink. Because O2(-)/phenoxy radical reactions can lead to more highly oxidized products, O2(-) could be playing a far more significant role in the photooxidation of CDOM than has been previously recognized.