| Oxidation of dissolved Hg(0) (Hg(0)aq) in aquatic systems is an important component of the aquatic redox chemistry of Hg. Hydroxyl free radical (·OH) is one of the most reactive environmental oxidants. Fenton's reagents (Fe(II) + H2O2) can provide ·OH in natural waters to induce various redox reactions. The present research was aimed to providing some kinetic knowledge and mechanistic understanding of the aqueous oxidation of Hg(0)aq by ·OH generated through the Fenton reaction.;Our kinetic study shows that Hg(0)aq can be oxidized rapidly under the dark condition in the presence of Fenton's reagents and the oxidation kinetics depends on the initial concentration ratio ([Fe(II)]0:[H 2O2]0) and amount of Fenton's reagents and also on the solution pH. The Hg(0)aq oxidation exhibits an apparent zero-order kinetics at the relatively low molar ratios of [Fe(II)]0:[H 2O2]0 (≤1). If the concentrations of H 2O2 are much greater than the concentrations of Fe(II), however, the observed kinetics of the Hg(0)aq oxidation is apparent first-order followed by an apparent zero-order kinetics. At the high molar ratios of [Fe(II)] 0:[H2O2]0, an apparent zero-order kinetics occurs first and then the kinetic curve reaches a plateau. The Hg(0) aq oxidation rate increases with the amount of Fenton's reagents. The ferrous system of Fenton's reagents exhibits higher Hg(0)aq oxidation rates compared with the ferric system. The oxidation of Hg(0)aq appears to depend on the pH with the highest oxidation rate occurring at pH ∼2.7.;The mechanism of the Hg(0)aq oxidation by Fenton's reagents is inferred using the kinetic information obtained. At the low ratios of [Fe(II)] 0:[H2O2]0 (< 1), Hg(0)aq mainly competes with H2O2 to react with ·OH, and HO 2· participates in the propagating radical chain reactions. At the high ratios, however, Hg(0)aq competes with Fe(II) to react with ·OH, and the reaction between Fe(III) and H2O 2 propagates the radical chain reactions; the chain reactions terminate in a very short time. At the intermediate ratio of [Fe(II)]0:[H 2O2]0, Hg(0)aq competes with Fe(II) and H 2O2 for ·OH. Since the reaction between ·OH and Fe(II) is faster than the one between ·OH and H2O 2, more H2O2 than Fe(II) stays in the Fenton system. The Hg(0)aq oxidation can decrease water-to-air transfer of Hg(0), but it can also provide Hg(II), which is the precursor for methyl Hg (CH 3Hg+). |
