Influence of electrostatics on the oxidation rates of organic compounds in heterogeneous Fenton systems

Electrostatic effects influence the oxidation rates of charged dissolved organic compounds in systems where the hydroxyl radical ((OH)-O-.) is produced by the iron oxide-catalyzed decomposition of hydrogen peroxide (H2O2). Experiments were performed using goethite and the (OH)-O-. probes C-14-labeled formic acid, 2-chlorophenol (2-CP), and nitrobenzene. At pH 4 and an ionic strength of 0.01 M, formic acid (pK(a) = 3.745) detected a steady-state concentration of (OH)-O-. ([(OH)-O-.](ss), calculated as a solution average) approximately 50 times higher than the two neutral probes did in the same systems, indicating significant enrichment of formate at the surface of the positively charged iron oxide where the (OH)-O-. is being produced. Increasing the pH and ionic strength decreased formic acid oxidation rates by factors consistent with predicted decreases in electrostatic effects. In the presence of high 2-CP concentrations, the [(OH)-O-.](ss) measured by formic acid decreased with time, and goethite coagulation increased, due to loss of positive charge on the oxide surface as the oxidation products of 2-CP complexed surface Fe species. The [(OH)-O-.](ss) detected by 2-CP did not change significantly, indicating that neither goethite coagulation nor surface complexation of 2-CP oxidation products interfered with the rate of (OH)-O-. generation; however, such an effect could have occurred in experiments using dissolved Fe instead of goethite. Model predictions of organic compound oxidation rates in mineral-catalyzed Fenton-like systems were improved by taking electrostatic effects into account.