Influence of Electrostatics on the Oxidation Rates of Organic Compounds in Heterogeneous Fenton Systems

Abstract

Electrostatic effects influence the oxidation rates of charged dissolved organic compounds in systems where the hydroxyl radical (*OH) is produced by the iron oxide-catalyzed decomposition of hydrogen peroxide (H2O2). Experiments were performed using goethite and the *OH probes 14C-labeled formic acid, 2-chlorophenol (2-CP), and nitrobenzene. At pH 4 and an ionic strength of 0.01 M, formic acid (pKa = 3.745) detected a steady-state concentration of *OH ([*OH]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 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]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]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 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.

Interfacial mechanisms of heterogeneous Fenton reactions catalyzed by iron-based materials: A review

“…Because electrostatic attractions between charged organic compounds and charged iron oxide surfaces are reported to affect organic oxidation reactions (Kwan and Voelker, 2004;Zhang et al, 2010), pH and ion strength may influence the oxidation rates (Kwan and Voelker, 2004). Moreover, both H + and ligands can promote the dissolution of some iron-based materials, inducing a homogeneous Fenton mechanism (He et al, 2015;Sun et al, 2014;Xue et al, 2009c).…”

Temperature Dependence of the Oxidation of 2-Chlorophenol by Hydrogen Peroxide in the Presence of Goethite

“…Fenton systems in particular have been found to be very active for the oxidation of 2-chlorophenol (2-CP), and as a result these have been well-studied [2][3][4]. In the case of heterogeneous Fenton systems, goethite (a-FeOOH) has been shown to catalyze the oxidation of 2-chlorophenol in the presence of hydrogen peroxide [5][6][7][8]. Goethite is an ideal catalyst for aqueous phase reactions because it is prevalent in nearly all soils and it is relatively inexpensive.…”

Impacts of Goethite Particles on UV Disinfection of Drinking Water

“…In the inactivation experiments, the concentrations and turbidities of goethite particles are the same and thus their capacity for providing bacteria with protection from UV exposure by absorbing, scattering, and blocking UV light should be similar. The presence of goethite particles could potentially enhance the formation of radicals under UV irradiation (17), thus improving the efficiency of UV disinfection. Our experimental data have, however, shown that an increase in goethite particle concentrations decreases UV disinfection efficiency.…”