Overlooked Formation of Carbonate Radical Anions in the Oxidation of Iron(II) by Oxygen in the Presence of Bicarbonate

Iron(II), (Fe(H2O)(6)(2+), (Fe-II) participates in many reactions of natural and biological importance. It is critically important to understand the rates and the mechanism of Fe-II oxidation by dissolved molecular oxygen, O-2, under environmental conditions containing bicarbonate (HCO3-), which exists up to millimolar concentrations. In the absence and presence of HCO3-, the formation of reactive oxygen species (O-2(center dot-), H2O2, and HO*) in Fe-II oxidation by O-2 has been suggested. In contrast, our study demonstrates for the first time the rapid generation of carbonate radical anions (CO3 center dot-) in the oxidation of Fe-II by O-2 in the presence of bicarbonate, HCO3-. The rate of the formation of CO3 center dot- may be expressed as d[CO3 center dot-]/dt=[Fe-II[[O-2][HCO3-](2). The formation of reactive species was investigated using H-1 nuclear magnetic resonance (H-1 NMR) and gas chromatographic techniques. The study presented herein provides new insights into the reaction mechanism of Fe-II oxidation by O-2 in the presence of bicarbonate and highlights the importance of considering the formation of CO3 center dot- in the geochemical cycling of iron and carbon.