Discovery of the Activated-Carbon Radical AC+ and the Novel Oxidation-Reactions Comprising the AC/AC+ Cycle as a Catalyst in an Aqueous Solution

The oxidation reaction of the cobalt(II) complex [Co(edta)]2− to [Co(edta)]− (edta4− = ethylenediaminetetraacetate ion) occurred efficiently by hydrogen peroxide or peroxodisulfate with activated carbon (AC) in aqueous media of pH 4.7, where the AC acts as a catalyst of the electron-transfer mediator. The oxidation reaction constitutes a chain reaction comprising the AC/AC+ cycle, in which the oxidation reaction is initiated by an electron-transfer from AC to hydrogen peroxide or peroxodisulfate to make radicals of AC+ and OH•, or SO4−•. These radicals could rapidly oxidize [Co(edta)]2− to [Co(edta)]−. The rate of the oxidation reaction is of first-order with respect to the concentrations of hydrogen peroxide (or peroxodisulfate) and the AC amounts added, and of zero-order to [Co(edta)]2− under the conditions. Further, the rate was unrelated to the dissolved O2. In the case of hydrogen peroxide, it could be decomposed by the AC, even in the absence of [Co(edta)]2−, according to the first-order with respect to the hydrogen peroxide concentration; the rate constant was 2-fold of the rate constant in the presence of [Co(edta)]2−. However, in the case of peroxodisulfate, no appreciable decomposition of the S2O82− was found in the absence of [Co(edta)]2−, and the decomposition rate in the presence of [Co(edta)]2− was 104-fold faster than the rate of hydrogen peroxide decomposition under the same conditions. Consequently, it is concluded that the AC acts in a way similar to iron(II) ion in the Fenton-reagent.