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

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