The iron oxide-catalyzed production of hydroxyl radical (*OH) from hydrogen peroxide (H2O2) has been used to oxidize organic contaminants in soils and groundwater. The goals of this study are to determine which factors control the generation rate of *OH (VOH) and to show that if VOH and the rate constants of the reactions of *OH with the system's constituents are known, the oxidation rate of a dissolved organic compound can be predicted. Using 14C-labeled formic acid as a probe, we measured VOH in pH 4 slurries of H2O2 and either synthesized ferrihydrite, goethite, or hematite or a natural iron oxide-coated quartzitic aquifer sand. In all of our experiments, VOH was proportional to the product of the concentrations of surface area of the iron oxide and H2O2, although different solids produced *OH at different rates. We used these results to develop a model of the decomposition rate of formic acid as a function of the initial formic acid and hydrogen peroxide concentrations and of the type and quantity of iron oxide. Our model successfully predicted the VOH and organic compound oxidation rates observed in our aquifer sand experiment and in a number of other studies but overpredicted VOH and oxidation rates in other cases, possibly indicating that unknown reactants are either interfering with *OH production or consuming *OH in these systems.

Superoxide and other reactive oxygen species (ROS) originate from several natural sources and profoundly influence numerous elemental cycles, including carbon and trace metals. In the deep ocean, the permanent absence of light precludes currently known ROS sources, yet ROS production mysteriously occurs. Here, we show that taxonomically and ecologically diverse heterotrophic bacteria from aquatic and terrestrial environments are a vast, unrecognized, and light-independent source of superoxide, and perhaps other ROS derived from superoxide. Superoxide production by a model bacterium within the ubiquitous Roseobacter clade involves an extracellular oxidoreductase that is stimulated by the reduced form of nicotinamide adenine dinucleotide (NADH), suggesting a surprising homology with eukaryotic organisms. The consequences of ROS cycling in immense aphotic zones representing key sites of nutrient regeneration and carbon export must now be considered, including potential control of carbon remineralization and metal bioavailability.

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 •-].

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