EFFECTS OF PH, CHLORIDE, AND BICARBONATE ON CU(I) OXIDATION KINETICS AT CIRCUMNEUTRAL PH

The redox chemistry of copper species in the upper water column plays a significant role in its speciation, transport and bioavailability. In natural waters, copper typically occurs in either the cuprous (Cu(I)) or cupric (Cu(II)) oxidation state. Most previous studies have focused primarily on Cu(II), principally because Cu(I) is easily oxidized to Cu(II) by oxygen or other oxidants. However, a growing body of evidence indicates that a number of potentially important reactions, many of which are photochemically induced, may lead to Cu(I) formation and result in a significant steady-state concentration of Cu(I) in natural waters.

In this study, the oxidation kinetics of nanomolar concentrations of Cu(I) in NaCl solutions have been investigated over the pH range 6.5-8.0.The overall apparent oxidation rate constant was strongly affected by chloride, moderately by bicarbonate and, and to a lesser extent, by pH. In the absence of bicarbonate, an equilibrium-based speciation model indicated that Cu⁺ and CuClOH^- were the most kinetically reactive species, while the contribution of other Cu(I) species to the overall oxidation rate was minor. A kinetic model based on recognized key redox reactions for these two species further indicated that oxidation of Cu(I) by oxygen and superoxide were important reactions at all pH values and [Cl^-] considered, but back reduction of Cu(II) by superoxide only became important at relatively low chloride concentrations. Bicarbonate concentrations from 2-5 mM substantially accelerated Cu(I) oxidation. Kinetic analysis over a range of bicarbonate concentrations revealed that this was due to the formation of CuCO₃^-, which reacts relatively rapidly with oxygen, and not due to inhibition of the back reduction of Cu(II) by formation of Cu(II)−carbonate complexes. We conclude that the simultaneous oxygenation of Cu⁺, CuClOH^- and CuCO₃^- is the rate-limiting step in the overall oxidation of Cu(I) under these conditions. Determination of values for apparent and intrinsic rate constants for the oxidation of those critical Cu(I) species by O₂ over the pH range 6.5-8.0 should greatly assist in understanding and predicting inorganic Cu(I) and Cu(II) transformations in natural waters.