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The oxidation kinetics of nanomolar concentrations of Cu(I) in NaCl solutions have been investigated over the pH range 6.5-8.0. The overall apparent oxidation rate constant was strongly affected by chloride, moderately by bicarbonate, and to a lesser extent by pH. In the absence of bicarbonate, an equilibrium-based speciation model indicated that Cu(+) and CuClOH(-) were the most kinetically reactive species, while the contribution of other Cu(I) species to the overall oxidation rate was minor. A kinetic model based on recognized key redox reactions for these two species further indicated that oxidation of Cu(I) by oxygen and superoxide were important reactions at all pH values and chloride concentrations considered, but back reduction of Cu(II) by superoxide only became important at relatively low chloride concentrations. Bicarbonate concentrations from 2 to 5 mM substantially accelerated Cu(I) oxidation. Kinetic analysis over a range of bicarbonate concentrations revealed that this was due to formation of CuCO(3)(-), which reacts relatively rapidly with oxygen, and not due to inhibition of the back reduction of Cu(II) by formation of Cu(II)-carbonate complexes. We conclude that the simultaneous oxygenation of Cu(+), CuClOH(-), and CuCO(3)(-) is the rate-limiting step in the overall oxidation of Cu(I) under these conditions.

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Two major pathways are reported to account for photochemical reduction of Fe­(III) in sunlit surface waters, namely, ligand-to-metal charge transfer (LMCT) and superoxide-mediated iron reduction (SMIR). In this study, we investigate the impact of Fe­(III) speciation (organically complexed (Fe­(III)­L versus iron oxyhydroxide (AFO)) on Fe­(III) reducibility by photogenerated superoxide (O2 • –) and LMCT. To simulate conditions typical of fresh, estuarine, and coastal waters, we have used Suwannee River Fulvic Acid (SRFA) as a representative of the natural organic matter likely to associate with Fe­(III). Our results show that the photolabile Fe­(III)­SRFA complex is reduced rapidly by LMCT, while O2 • – does not play a role in reduction of these entities. In contrast, the relatively less photolabile AFO is reduced by both O2 • – and LMCT. The reduction of AFO by O2 • – occurs following the dissolution of AFO, and hence, the contribution of O2 • – to reductive dissolution of AFO is dependent on conditions such as the age of the AFO and initial AFO concentration affecting the rate of dissolution of AFO. Our results further show that while colloidal Fe­(III) (in this work, particles >0.025 μm) is reduced by O2 • –, there is no involvement of O2 • – in dissolved Fe­(III) reduction. Overall, our results show that superoxide-mediated iron reduction will be important only in natural waters containing limited concentrations of Fe binding ligands.