Effect of pH on measurement of strong copper binding ligands in lakes

We investigated the effect of pH on Cull binding by natural organic ligands in two New Zealand lakes on the South Island, New Zealand, using competitive ligand equilibration with salicylaldoxime and detection by cathodic stripping voltammetry. When the pH of a lake-water sample was adjusted to different values in the range 6.3-8.0, the conditional Cu2+ binding constant K" was found to increase, with a slope log K" versus pH of nearly +2, as would be expected for functional groups having a proton dissociation constant pK(a) > 9. In support of this, a comparison of log K" values measured on samples taken from surface and subsurface waters of two New Zealand alpine lakes over a 2-yr period showed a very similar log K"-pH dependence. These results imply that the functional group chemistry of strong Cu2+-binding ligands in such lakes is relatively uniform and may involve phenolic OH groups. In Lake Hayes, the Cu-binding ligand concentration [L](T) exceeded that of total dissolved Cu, [Cu](T) at almost all times of the year and all depths. However, in Lake Manapouri, little evidence of Cu-binding ligand was observed during late summer in the mixed layer, which suggests a seasonal cycle in Cu-binding ligands that is perhaps driven by enhanced ultraviolet irradiation in summer or by seasonal changes in phytoplankton community structure. This may have important consequences for the toxicity of Cu2+ to organisms in these lakes during summer.