Influence of phosphate on the oxidation kinetics of nanomolar Fe(II) in aqueous solution at circumneutral pH

The oxygenation kinetics of nanomolar concentrations of Fe(II) in aqueous solution have been studied in the absence and presence of millimolar concentrations of phosphate over the pH range 6.0–7.8. At each phosphate concentration investigated, the overall oxidation rate constant varied linearly with pH, and increased with increasing phosphate concentration. A model based on equilibrium speciation of Fe(II) was found to satisfactorily explain the results obtained. From this model, the rate constants for oxygenation of the Fe(II)-phosphate species FeH2PO4+, FeHPO4 and FePO4− have been determined for the first time. FePO4− was found to be the most kinetically reactive species at circumneutral pH with an estimated oxygenation rate constant of (2.2 � 0.2) � 10 M−1 s−1. FeH2PO4+ and FeHPO4 were found to be less reactive with oxygen, with rate constants of (3.2 � 2) � 10−2 M−1 s−1 and (1.2 � 0.8) � 10−1 M−1 s−1, respectively.