Photoproduction of One-Electron Reducing Intermediates by Chromophoric Dissolved Organic Matter (CDOM): Relation to O_2~- and H_2O_2 Photoproduction and CDOM Photooxidation

摘要

A molecular probe, 3-amino-2,2,5,5,-tetramethy-l-pyrrolydiny- loxy (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 hydroxyiamine, which can be derivatized with fluorescamine, separated by HPLC and quantified fluorimetrically. We provide evidence that O_2 and 3ap compete for the same pool(s) of photoproduced reducing intermediates, and that under appropriate experimental conditions, the initial rate of hydroxylamine formation (R_H) can provide an estimate of the initial rate of superoxide (O_2~-) formation. However, comparison of the initial rates of H_2O_2 formation (R_(H2O2)) to that of R_H show far larger ratios of R_H/R_(H2O2) (～6-13) than be accounted for by simple O_2~- dismutation (R_H/R_(H2O2) ＝ 2), implying a significant oxidative sink of O_2~- (～67-85%). Because of their high reactivity with O_2~- and their likely importance in the photochemistry of CDOM, we suggest that coproduced phenoxy radicals could represent a viable oxidative sink Because O_2~-/phenoxy radical reactions can lead to more highly oxidized products, O_2~- could be playing a far more significant role in the photooxidation of CDOM than has been previously recognized.