Iron redox cycling in surface waters: Effects of humic substances and light

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 reoxidized (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(·-)].