Rates of Hydroxyl Radical Generation and Organic Compound Oxidation in Mineral-Catalyzed Fenton-like Systems

TLDR

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 study aims to identify the factors controlling •OH generation rate and to demonstrate that, knowing •OH production and its reaction rate constants with system constituents, the oxidation rate of a dissolved organic compound can be predicted. Using ¹⁴C‑labeled formic acid as a probe, we measured •OH in pH 4 slurries of H₂O₂ with synthesized ferrihydrite, goethite, hematite, or natural iron‑oxide‑coated quartzitic aquifer sand, and then developed a model of formic‑acid decomposition as a function of initial formic‑acid and H₂O₂ concentrations and iron‑oxide type and quantity. •OH was proportional to the product of iron‑oxide surface area and H₂O₂ concentrations, with different solids producing •OH at varying rates, and our model accurately predicted •OH and organic‑compound oxidation rates in the aquifer sand experiment and several other studies but overpredicted them in some cases, suggesting unknown reactants may interfere with or consume •OH.

Abstract

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.