Role of the photo-Fenton reaction in the production of hydroxyl radicals and photobleaching of colored dissolved organic matter in a coastal river of the southeastern United States

Photochemical reactions involving colored dissolved organic matter (CDOM) in natural waters are important determinants of nutrient cycling, trace gas production and control of light penetration into the water column. In this study the role of the hydroxyl radical (•OH) in CDOM photodegradation was explored as well as the contribution of photo-Fenton chemistry to •OH formation. Photochemically produced •OH was observed under aerobic and dioxygen-depleted conditions in highly colored, acidic natural water samples obtained from a freshwater reach of the Satilla River, a river in the southeastern United States. Net aerobic •OH formation along with the production of hydrogen peroxide and Fe(II) provided evidence of photo-Fenton produced •OH. A reduction in •OH production in the presence of iron chelators further suggests the importance of iron and the photo-Fenton reaction in this water. Apparent quantum yield values for the photochemical production of •OH were determined from 300–320 nm. In addition, the relationship between •OH photoproduction and effects of irradiation on the optical properties of CDOM was examined. Changes in the light absorption and fluorescence properties of water samples from the Satilla River and other natural waters were compared to •OH production rates. The ability of constituents of Satilla River water, principally the dissolved organic matter, to scavenge •OH was also considered. Results indicate that the photo-Fenton reaction accounts for more than 70% of total photochemical •OH production in Satilla River water. Given the significant levels of •OH produced in this water, it is possible that •OH influences CDOM photobleaching.