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Abstract
The redox chemistry of copper in natural aquatic systems is of great interest due to its significance to the biogeochemistry of copper, an essential micronutrient, and also due to the formation of reactive oxygen species (ROS) during these redox reactions. Despite this, many knowledge gaps pertaining to the redox transformations of copper exist, particularly in systems containing oxygen where copper species may be simultaneously oxidized and reduced. In order to improve our understanding of copper redox chemistry under conditions typical of oxic natural waters, the oxidation kinetics of nanomolar concentrations of Cu(I) in NaCl solutions was firstly 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. Two kinetic models have been developed, using both the speciation-based approach and the so-called “CuL” approach with both models validating the conclusions that i) the oxidation of Cu(I) by oxygen and superoxide are important pathways at all pH values and chloride concentrations considered and ii) the simultaneous oxygenation of Cu+, CuClOH– and CuCO3– is the rate-limiting step in the overall oxidation of Cu(I) with the back reduction of Cu(II) by superoxide only becoming important at low chloride concentrations.
As electron-transfer mediators, quinone moieties play important roles in essential biogeochemical processes and serve as a vital link in the movement of electrons through cells and tissues. Copper has previously been shown to catalyze the oxidation of hydroquinones and enhance the hydroquinone-induced cytotoxicity. As such, in the second phase of the project, the oxidation of 1,4-hydroquinone (H2Q) and 1,4-naphthohydroquinone (NH2Q) in the absence and presence of nanomolar concentrations of Cu(II) in NaCl solutions at circumneutral pH has been investigated. While the autoxidation of H2Q is slow under the conditions studied, the autoxidation of NH2Q is significant with the apparent oxidation rate increasing with increasing pH and the initial NH2Q concentration. The results also show that under aerobic conditions, copper is capable of catalyzing the oxidation of H2Q and NH2Q, with concomitant oxygen consumption and peroxide generation. The presence of O2 facilitated the catalytic role of Cu(II) with regeneration of Cu(II) via continuous oxidation of Cu(I) and rapid removal of semiquinone radicals resulting in the generation of superoxide. Detailed kinetic models have been developed to describe the predominant mechanisms operative in these systems. The half-cell reduction potentials of different redox couples indicated good agreement between thermodynamic and kinetic considerations of various key reactions involved, further providing confirmation of the veracity of the proposed models describing the interactions of copper and quinone species in circumneutral pH saline solutions.