Enhanced reactivity of superoxide in water-solid matrices.
Superoxide is unreactive in deionized water, but aqueous systems containing added solvents, including H2O2 at >100 mM, show significantly increased reactivity of superoxide with oxidized organic compounds such as highly chlorinated aliphatics. The potential for solid surfaces to similarly increase the reactivity of superoxide in water was investigated. Heterogeneous birnessite (gamma-MnO2)-catalyzed decomposition of H2O2 promoted the degradation of the superoxide probe hexachloroethane (HCA) at H202 concentrations as low as 7.5 mM, while no measurable HCA degradation was found in parallel homogeneous iron(III)-EDTA-H2O2 systems at H2O2 concentrations <100 mM. Electron spin resonance spectroscopy confirmed that superoxide was the dominant reactive species generated in the birnessite-catalyzed decomposition of H2O2. Increased superoxide reactivity was also found in aqueous superoxide-glass bead heterogeneous systems, and the rates of HCA degradation increased as a function of the surface area of the glass beads. The results of this research show that, similar to the addition of solvents, the presence of surfaces also enhances the reactivity of superoxide in water, possibly by altering the superoxide solvation shell. On the basis of these findings, superoxide generated in catalyzed H2O2 propagations (CHP; modified Fenton's reagent) used for in situ chemical oxidation (ISCO) may have greater reactivity with highly oxidized contaminants than previously thought.