Effect of chloride driven copper redox cycling on the kinetics of Fe(II) oxidation in aqueous solutions at pH 6.5-8.0

The kinetics of Fe(II) oxidation by nanomolar concentrations of Cu(II) under both oxygenated and deoxygenated conditions at circumneutral pH (6.5-8.0) and varying NaCl concentrations (0-0.7 M) have been investigated. In the absence of O2, oxidation of Fe(II) by Cu(II) followed second-order kinetics with the rate of oxidation increasing with increasing pH and chloride concentration. In the presence of O2, while the rate of Fe(II) oxidation by both O2 and Cu(II) still increased rapidly with increasing pH, the effect of chloride was reversed with a faster overall removal of Fe(II) observed in the absence of chloride. Even though Fe(II) reacts slowly with Cu(II) in the absence of chloride, rapid re-formation of Cu(II) renders Cu(II) a more effective oxidant under this condition. Formation of Cu(I) and its concomitant transformation indicated that Cu(I) is a product in the oxidation of Fe(II) by Cu(II). A simple kinetic model has been developed which adequately describes the oxidation of Fe(II) by Cu(II) over a range of experimental conditions.