Oxidation of Cu(I) in Seawater at Low Oxygen Concentrations

The oxidation of nanomolar copper­(I) at low oxygen (6 μM) concentrations was studied as a function of pH (6.7–8.2), ionic strength (0.1–0.76 M), total inorganic carbon concentration (0.65–6.69 mM), and the added concentration of hydrogen peroxide, H₂O₂ (100–500 nM) over the initial 150 nM H₂O₂ concentration in the coastal seawater. The competitive effect between H₂O₂ and O₂ at low O₂ concentrations has been described. Both the oxidation of Cu­(I) by oxygen and by H₂O₂ had a reaction order of one. The reduction of Cu­(II) back to Cu­(I) in the studied seawater by H₂O₂ and other reactive oxygen intermediates took place at both high and low O₂ concentrations. The effect of the pH on oxidation was more important at low oxygen concentrations, where δlog k/δpH was 0.85, related to the presence of H₂O₂ in the initial seawater and its role in the redox chemistry of Cu species, than at oxygen saturation, where the value was 0.6. A kinetic model that considered the Cu speciation, major ion interactions, and the rate constants for the oxidation and reduction of Cu­(I) and Cu­(II) species, respectively, was applied. When the oxygen concentration was lower than 22 μM and under the presence of 150 nM H₂O₂, the model showed that the oxidation of Cu­(I) was controlled by its reaction with H₂O₂. The effect of the pH on the oxidation rate of Cu­(I) was explained by its influence on the oxidation of Cu­(I) with O₂ and H₂O₂, making the model valid for any low oxygen environment.