Oxidation of Cu(I) in seawater at low oxygen concentrations

Abstract

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.