Species kinetics and heterogeneous reactivity of dissolved Cu in natural freshwaters

Using high specific activity 64Cu2+ as radiotracer, the distribution kinetics among Cu species were established in natural organic-rich freshwaters under steady-state conditions, i.e., with minimal disturbance of existing equilibria. Study sites with contrasting suspended particulate matter (SPM) characteristics were investigated. Our analytical protocol allowed the differentiation between the following Cu species: SPM associated Cu, dissolved reactive (free and labile) Cu, and organically complexed Cu. The data obtained were successfully evaluated by compartmental analysis, which showed the importance of organically complexed Cu in freshwaters, and the dominant role of the interactions between organically complexed Cu and SPM in a SPM-rich water. The kinetic 54Cu measurements indicated that the attainment of equilibrium between dissolved reactive and organically complexed Cu took ca. <1-2 h, and 4-15 h for the interaction between dissolved organically complexed and SPM associated Cu. The kinetic study was augmented by voltammetric measurements of the dissolved (stable) Cu equilibrium speciation conditions in the natural waters. These measurements showed that the waters contained very low cupric ion concentrations (10(-12)-10(-15) M), with more than 99.9% of the dissolved Cu complexed by strong organic ligands (conditional stability constants: 10(13.4)-10(15.4)).