Insights into the Photoproduction Sites of Hydroxyl Radicals by Dissolved Organic Matter in Natural Waters

The hydroxyl radical ((OH)-O-center dot) is the most reactive oxidant produced in natural waters. Photoproduction by chromophoric dissolved organic matter (CDOM) is one of its main sources, but the structures responsible for this production remain unknown. Here, a series of substituted phenol model compounds are examined to test whether these structures could act as a source of (OH)-O-center dot. We find that many of these compounds do produce (OH)-O-center dot with quantum yields (Phi) ranging from similar to 10(-4) to similar to 10(-2). In particular, two compounds that have hydroxy groups and carboxyl groups in a para relationship (4-hydroxybenzoic acid and 2,4-dihydroxybenzoic acid) exhibit relatively high Phi values, similar to 10(-2). For 2,4-dihydroxybenzoic acid, the formation of (OH)-O-center dot was confirmed through the use of competition kinetics and reaction with methane. We conclude that these types of structures, which may derive from polyphenolic source materials such as lignins, tannins, and humic substances, could be an important source of (OH)-O-center dot in natural waters.