PH-dependence of production of oxidants (Cu(III) and/or HO•) by copper-catalyzed decomposition of hydrogen peroxide under conditions typical of natural saline waters

Abstract

The oxidation and reduction kinetics of copper in natural aquatic systems determines its prevailing redox state, with implication to its toxicity, bioavailability and solubility. When reduced copper(I) co-occurs with ubiquitous hydrogen peroxide (H2O2), their reaction forms an oxidizing intermediate able to affect redox transformation of copper, other transition metals and organic carbon. In this work the kinetics and mechanism of reactions of nanomolar concentrations of copper (Cu) with H2O2 in bicarbonate-buffered 0.7 M NaCl solutions over the pH range from 6.0 to 8.0 have been investigated and a kinetic model developed to assist in understanding the results obtained. While the rate of cuprous ion (Cu(I)) peroxidation did not vary markedly with pH, the rates of both cupric ion (Cu(II)) reduction by H2O2 and the production of oxidative intermediates showed strong pH dependence with both rates observed to increase with increasing pH. The increasing proportion of H2O2 present as the hydroperoxyl anion (HO2−) with increasing pH plays an important role in these trends. However, in order to adequately describe the behavior observed, it is also necessary to invoke a pH-dependent intrinsic conditional rate constant for the reaction between Cu(II) and HO2−. The nature of the oxidants formed when Cu(I) is oxidized by H2O2 has been probed by measuring the hydroxylation of phthalhydrazide and the degradation of formate in the absence and presence of probe compounds with well-established reactivity toward hydroxyl radicals (HO•). The results obtained suggest that a higher oxidation state of copper, Cu(III), rather than HO• is the key oxidant formed over the pH range of 6.0–8.0. Simulations under conditions relevant to natural saline environments reveal that increase in pH results in acceleration in the H2O2-driven cycling of Cu between +I and +II oxidation states but does not significantly influence the rate of production of Cu(III) with important implication for the geochemistry of copper, other redox active elements and oxidizable organic compounds.