The oxidation of Cu(I) with H 2O 2 in natural waters

Abstract

The oxidation of Cu(I) with H 2O 2 in seawater and in NaCl solutions has been measured as a function of pH (6 to 9), temperature (5 to 45°C), ionic strength (0.5 to 6 m) and composition. The seawater rate constants, K ( M −1 s −1), for the oxidation d[ Cu( I)]/ dt = − k[ Cu( I)][ H 2 O 2] have been fitted to the equation log k = 11.55 − 2250/T − 3.71I 1 2 + 2.06I with a σ = 0.04. The energy of activation was 43.1 ± 1.0 kJ mol −1. At a constant ionic strength ( I = 1, 3, and 6 m) in NaCl-NaClO 4 mixtures, the Cl − dependence has been attributed to the oxidation of various forms of Cu(I) in the solution. At a fixed Cl − concentration, the addition of Mg 2+ causes the rate to decrease, and the addition of HCO − 3 causes the rate to increase. The increase due to HCO − 3 may be due to the formation of CuHCO 0 3 which has a faster rate of oxidation than CuCl 0. The decrease caused by Mg 2+ may be due to the slow exchange of MgEDTA or MgCO 3 with Cu 2+, which may cause the overall oxidation rates of Cu(I) to be slower due to back reactions of Cu(II) with H 2O 2. Empirical equations for the rate of oxidation of Cu(I) with H 2O 2 in natural waters are given. The H 2O 2 results are compared with the kinetics of Cu(I) oxidation with O 2 in natural waters. At the levels of O 2 (200 μM) and H 2O 2 (0.1 μM) in surface sea waters, the oxygen oxidation is 130 times faster than peroxide oxidation. In rainwaters, however, the concentration of hydrogen peroxide (100 μM) is great enough so that H 2O 2 is the dominant oxidant for Cu(I).