THE ROLE OF COPPER AND OXALATE IN THE REDOX CYCLING OF IRON IN ATMOSPHERIC WATERS

During daytime, the redox cycling of dissolved iron compounds in atmos pheric waters, and the related in-cloud transformations of photooxidan ts, are affected by reactions of Fe and Cu with hydroperoxy (HO2) and superoxide (O2-) radicals and the photoreduction of Fe(III)-oxalato co mplexes. We have investigated several of the important chemical reacti ons in this redox cycle, through laboratory simulation of the system, using gamma-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 concentrati ons 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 x 10(7) m-1 s-1. The pr esence of oxalate resulted in the formation of Fe(III)-oxalato complex es 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) x 10(4) M-1 S-1). Simulations incorporating measurements for other redox mecha nisms, including oxidation by ozone, indicate that, during daytime, Fe should be found mostly in the ferrous oxidation state, and that react ions of FeOH2+ with Cu(I) and HO2/O2-, and to a lesser degree, the pho tolysis of Fe(III)-oxalato complexes, are important mechanisms of Fe r eduction in atmospheric waters. The catalytic effect of Cu(II)/Cu(I) a nd 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 pred icted the changes in Fe oxidation states that occurred when authentic fogwater samples were exposed to HO2/O2-,