The role of copper and oxalate in the redox cycling of iron in atmospheric waters

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Abstract

During daytime, the redox cycling of dissolved iron compounds in atmospheric waters, and the related in-cloud transformations of photooxidants, are affected by reactions of Fe and Cu with hydroperoxy (HO2) and superoxide (O2) radicals and the photoreduction of Fe(III)-oxalato complexes. We have investigated several of the important chemical reactions in this redox cycle, through laboratory simulation of the system, using γ-radiation to produce HO2/O2. At concentrations comparable to those measured in atmospheric waters, the redox cycling of Fe was dramatically affected by the presence of oxalate and trace concentrations of Cu. At concentrations more than a hundred times lower than Fe, Cu consumed most of the HO2/O2, and cycled between the Cu(II) and Cu(I) forms. Cu+ reacted with FeOH2+ to produce Fe(II) and Cu(II), with a second order rate constant of approximately 3 × 107 M−1s−1. The presence of oxalate resulted in the formation of Fe(III)-oxalato complexes that were essentially unreactive with HO2/O2. Only at high oxalate concentrations was the Fe(II)C2O4 complex also formed, and it reacted relatively rapidly with hydrogen peroxide (k = (3.1 ± 0.6) × 104 M−1s−1). Simulations incorporating measurements for other redox mechanisms, including oxidation by ozone, indicate that, during daytime, Fe should be found mostly in the ferrous oxidation state, and that reactions of FeOH2+ with Cu(I) and HO2/O2, and to a lesser degree, the photolysis of Fe(III)-oxalato complexes, are important mechanisms of Fe reduction in atmospheric waters. The catalytic effect of Cu(II)/Cu(I) and Fe(III)/Fe(II) should also significantly increase the sink function of the atmospheric liquid phase for HO2 present in a cloud. A simple kinetic model for the reactions of Fe, Cu and HO2/O2, accurately predicted the changes in Fe oxidation states that occurred when authentic fogwater samples were exposed to HO2/O2.

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