The role of organic matter in controlling copper speciation in precipitation

Organic complexation has been suggested to be an important mechanism by which the biogeochemistry of transition metals is altered in natural waters. We have conducted complexing ligand titrations on rainwater, collected during the winter and spring from a semi-urban U.K. location, using cathodic stripping voltammetry with tropolone as the added ligand. The results show that organic ligands capable of binding copper are present in all the precipitation samples measured. As a consequence of the method used, only those copper-natural ligand complexes with conditional stability constants (1 : 1 stoichiometry-log KCuL′) between 11 and 14 have been identified. Within this stability region, ligand concentrations vary from 10.2 to 34.5 nM, very similar to the total copper levels which range between 10.2 and 33.2 nM. While total copper has a concentration of the order of 10−8 M, free Cu2+ ions exist at levels of just 10−11−10−12 M, showing that copper is largely complexed in the rainwater samples measured. The existence of such low free ion concentrations must be considered when assessing the catalytic role of rain and aerosol trace metals in atmospheric reactions and determining the impact of atmospheric inputs on surface water biogeochemistry. Conditional stability constants increase with the detection window used (from log KCuL′) = 11.4 at a detection window centred at log αCuAL = 2.68 to log KCuL′) = 12.57 at log αCuAL = 4.47). As the method is specific to a limited range of conditional stabilities, dependent on the strength and amount of competitive ligand used, this concentration dependence suggests the presence of a wider range of natural ligands than those identified here. The source of the organic complexant in precipitation is unclear. We postulate, based on stable isotope measurements made on rains collected at the same location (Cornell et al., 1995), that the organic ligands are likely to be terrestrial in origin.