Dioxygen-mediated oxidation of hydroquinone with cobalt ions in a bicarbonate aqueous solution for the production of active radicals

The catalytic activity and mechanism of Co2+ in HCO3- aqueous solution on the oxidative transformation of 1,4-hydroquinone (H(2)Q) to 1,4-benzoquinone (BQ) with molecular oxygen as the oxidant is investigated. The Co(II)-HCO3- system is much more efficient in the oxidation of H(2)Q than HCO3-, Co(II)-OH- or other metal ion-HCO3- systems due to the fact that the formed [Co(II)(HCO3-)](+) complex can facilitate the generation of reactive oxygen species. In HCO3- solution, H2Q can react with dissolved oxygen to form the semiquinone radical (SQ(center dot-)). As a chain carrying radical, SQ(center dot-) continues to transfer an electron to molecular oxygen to yield the superoxide radical, followed by the generation of H2O2. The resulting H2O2 is further utilized as an oxidant by the Co(II)-HCO3- complex in situ with the formation of the (OH)-O-center dot radical, which is highly active for H(2)Q oxidation and even for dye degradation.