Chemodynamics of Soft Nanoparticulate Complexes: Cu(II) and Ni(II) Complexes with Fulvic Acids and Aquatic Humic Acids

The dynamics of metal complexation by small humic substances (fulvic acid and aquatic humic acid, collectively denoted as “fulvic-like substance”, FS) are explored within the framework of concepts recently developed for soft nanoparticulate complexants. From a comprehensive collection of published equilibrium and dissociation rate constants for CuFS and NiFS complexes, the association rate constant, k a, is determined as a function of the degree of complexing site occupation, θ. From this large data set, it is shown for the first time that k a is independent of θ. This result has important consequences for finding the nature of the rate limiting step in the association process. The influence of electric effects on the rate of the association process is described, namely (i) the accelerating effect of the negatively charged electrostatic field of FS on the diffusion of metal ions toward it, and (ii) the extent to which metal ions electrostatically accumulate in the counterionic atmosphere of FS. These processes are discussed qualitatively in relation to the derived values of k a. For slowly dehydrating metal ions such as Ni­(H2O)6 2+ (dehydration rate constant, k w), k a is expected to derive straight from k w. In contrast, for rapidly dehydrating metal ions such as Cu­(H2O)6 2+, transport limitations and electric effects involved in the formation of the precursor outer-sphere associate appear to be important overall rate-limiting factors. This is of great significance for understanding the chemodynamics of humic complexes in the sense that inner-sphere complex formation would not always be the (sole) rate-limiting step.