Photochemical formation of hydroxyl radical by constituents of natural waters

A new method is employed to determine the rates of photochemical hydroxyl radical (OH) formation in aqueous solutions and in natural waters under both aerobic and anaerobic conditions. Quantum yields for OH formation from the photolysis of nitrate and nitrite obtained by this method a re in good agreement with previous measurements. Photolysis of Suwannee River fulvic acid (SRFA) solutions produced the hydroxyl radical under anaerobic conditions in proportion to the SRFA concentration. Under aerobic conditions, the quantum yields for OH formation were slightly higher and exhibited a different wavelength dependence than those obtained under anaerobic conditions. Experiments employing catalase indicate that Fenton chemistry can account for at most 50% of the total signal under aerobic conditions for SRFA irradiated at 310 and 320 nm. These results indicate the presence of a dioxygen-independent pathway of hydroxyl radical production that cannot be assigned to nitrate/nitrite photolysis or to Fenton chemistry. Results from the preliminary application of this method to natural waters are also presented.