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The purpose of this study is to examine the mechanism of photo-oxidation of natural dissolved organic matter (DOM) in the presence of iron. This process is of interest in natural waters for several reasons:  as a significant sink of DOM in sunlit surface waters; as a source and sink of reactive oxygen species (HO2/O2 •-, hydrogen peroxide, and HO•) and as a factor controlling iron speciation. Studies were conducted in laboratory model systems containing fulvic acid and lepidocrocite (γ-FeOOH) particles at pH 3 and pH 5, irradiated with simulated sunlight. Measured concentrations of dissolved Fe(II), total dissolved Fe, and hydrogen peroxide were interpreted as the net effects of competing reactions reducing and oxidizing Fe and producing and destroying hydrogen peroxide. A kinetic model constructed using information gained from separate experiments in simpler systems was used to assess the relative importance of individual reactions. Comparison of photoreductive dissolution rates in aerated and de-aerated systems at pH 3 and pH 5 indicated that the decrease in rate with increasing pH is mostly due to a decrease in the concentration of surface Fe(III)−fulvate complexes and that, in the presence of oxygen, some of the surface Fe(II) is re-oxidized (not necessarily by oxygen) before detachment can take place. Kinetic modeling indicated that fast redox cycling of Fe occurs at both pH values. The dark reduction of Fe(III) by fulvic acid and photochemical ligand-to-metal charge transfer reactions of dissolved Fe(III)−fulvate complexes play almost equally significant roles in the reduction of dissolved Fe(III). The main oxidants of dissolved Fe(II) are HO2/O2 •- (produced via reduction of O2 by photo-excited fulvic acid) and hydrogen peroxide [the product of Fe(II) reaction with HO2/O2 •-].

Iron redox cycling can catalyze the oxidation of humic substances and increase the rate of oxygen consumption in surface waters rich in iron and organic carbon. This study examines the role of Fenton's reaction [oxidation of Fe(II) by hydrogen peroxide] in this catalytic cycle. A number of competing processes were observed in model systems containing dissolved Fe, hydrogen peroxide, and Suwannee River fulvic acid. First, the effective rate constant of Fenton's reaction increased with increasing fulvic acid concentration, indicating the formation of Fe(II)-fulvate complexes that react more rapidly with hydrogen peroxide than Fe(II)-aquo complexes. This effect was significant at pH 5 but negligible at pH 3. A second effect was scavenging of the HO • radical produced in Fenton's reaction by fulvic acid, forming an organic radical. The organic radical reduced oxygen to HO 2 • /O 2 •-, which then regenerated hydrogen peroxide by reaction with Fe(II). Finally, Fe(III) was reduced by a dark reaction with fulvic acid, characterized by an initially fast reduction followed by slower processes. The behavior of Fe(II) and hydrogen peroxide over time in the presence of fulvic acid and oxygen could be described by a kinetic model taking all of these reactions into account. The net result was an iron redox cycle in which hydrogen peroxide as well as oxygen were consumed (even though direct oxidation of Fe(II) by oxygen was not significant), and the oxidation of fulvic acid was accelerated.