Dl. Sedlak et J. Hoigne, THE ROLE OF COPPER AND OXALATE IN THE REDOX CYCLING OF IRON IN ATMOSPHERIC WATERS, Atmospheric environment. Part A, General topics, 27(14), 1993, pp. 2173-2185
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-,