Kinetics of Trace Metal Complexation: Ligand-Exchange Reactions

Ligand-exchange reactions of copper-nitrilotriacetate (CuNTA) and copper-humate complexes with a fluorescent ligand, calcein, were examined and compared. The reverse reactions of copper-calcein with ethylenedi-aminetetraacetate (EDTA) and NTA were also studied; the correspondence of equilibrium constants and kinetic rate constants is discussed. Reactions with model ligands proceeded both by complete dissociation of the initial complex (disjunctive pathway) and by direct attack of the incoming ligand on the initial complex (adjunctive pathway). Reactions of humate-bound copper were also consistent with this mechanistic interpretation. The contributions of the adjunctive and disjunctive pathways depended on the Cu-to-humate loading; both pathways were important at the Cu-to-DOC loadings typical of natural waters. At higher values, disjunctive ligand exchange predominated. This pathway could be interpreted in terms of dissociation of Cu from a humate-binding site and the rate constant related to the conditional stability constant for Cu-humate binding.