Slow coordination reactions in seawater

The rates of reaction of transition metals with free or protonated ligands are intrinsically fast. Yet the observed rate of formation of metal complexes with strong ligands in seawater containing a mixture of ligands is remarkably slow. When Cu(II) is added to mixtures of natural and synthetic ligands, the equilibrium distribution of metal species may be established only after hours, days, or even months. Initial reaction of the stronger ligand with the added metal is retarded in the presence of seawater concentrations of Ca. Thus Cu complexes are initially formed with the weaker ligands. Final equilibrium metal speciation is attained slowly through a series of ligand- and metal-exchange reactions. Observations of slow kinetics of coordination reactions in model systems demonstrate that re-equilibration of a natural system undergoing perturbations of metal or ligand concentrations, under natural or analytical conditions, cannot be assumed to be rapid. This study also suggests that the concentration of strong complexing agents in seawater may be underestimated in measurements of metal complexation that involve metal additions.