Hydroxyl Radical Production via the Photo-Fenton Reaction in the Presence of Fulvic Acid

The photo-Fenton reaction, the reaction of photoproduced Fe(II) with H2O2 to form the highly reactive hydroxyl radical (OH*), could be an important source of OH* in sunlit natural waters. To determine if the photo-Fenton reaction is significant in mildly acidic surface waters, we conducted experiments simulating conditions representative of natural freshwaters using solutions of standard fulvic acid and amorphous iron oxide at pH 6.0. A probe method measuring 14CO2 produced by the reaction of 14C-labeled formate with OH* was used to detect small OH* production rates without otherwise influencing the chemical reactions occurring in the experiments. Net H2O2 accumulation was simultaneously measured using an acridinium ester chemiluminescence method. Measured losses of H2O2 by reaction with Fe(II) in dark experiments produced approximately the expected quantities of OH*. The difference between H2O2 accumulation in the presence and absence of Fe(III) in fulvic acid solutions exposed to light was interpreted as the loss of H2O2 by reaction with photoproduced Fe(II), consistent with measured OH* production rates. The Fe ligand desferrioxamine mesylate eliminated both OH* production and H2O2 photoloss induced by Fe. Our results imply that when Fe is a major sink of H2O2, the photo-Fenton reaction is likely to be the most important source of OH*, leading to a significant sink of organic compounds in a wide variety of sunlit freshwaters.