Rates of hydroxyl radical generation and organic compound oxidation in mineral-catalyzed Fenton-like systems

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 (V-OH) and to show that if V-OH 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 C-14-labeled formic acid as a probe, we measured V-OH 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, V-OH 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 V-OH and organic compound oxidation rates observed in our aquifer sand experiment and in a number of other studies but overpredicted V-OH and oxidation rates in other cases, possibly indicating that unknown reactants are either interfering with .OH production or consuming .OH in these systems.