hydrolysis rate constant of 2,4-DOE were observed. These observations are mutually consistent with a rapid, reversible partitioning of 2,4-DOE between water and "dissolved" humic substances. The humic-bound 2,4-DOE is not adsorbed to glass and is protected from alkaline hydrolysis. On the basis of this model, with aquatic humus concentrations expressed as g of C/g of H20, the distribution coefficient of 2,4-DOE between water and aquatic humus (Kb) was calculated from both the glass adsorption data and the alkaline hydrolysis data, yielding log Kb values of 4.4 and 4.8, respectively.From the magnitude of the distribution coefficient of 2,4-DOE between water and "dissolved" humic substances, a significant interaction is generally expected for hydrophobic compounds in natural waters. Rates of alkaline hydrolysis, volatilization, etc., would be expected to be reduced in proportion to the fraction of a hydrophobic solute that is associated with humic substances.If any general acid-base catalysis of the hydrolysis reaction is attributable to humic substances, that contribution is completely masked by the partitioning phenomenon, which strongly decreases the rate of alkaline hydrolysis of 2,4-DOE.

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Freshwaters are recognized as dynamic systems that may be far-removed from equilibrium. A kinetic approach using the competing ligand exchange method with Chelex 100 as the competing ligand and inductively coupled plasmamass spectrometry to measure the dissociation kinetics was used to investigate the chemical speciation of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II) in model solutions of a well-characterized fulvic acid (Laurentian fulvic acid) and a freshwater sample collected from the Grand River (Ontario, Canada). The kinetic distribution of the metal species were quantitatively characterized by their first-order dissociation rate constants. This kinetic speciation approach has the advantage of providing an objective method for estimating the dissociation rate constants without any a priori assumptions about the number of kinetically distinguishable components or the shape of the distribution. Three factors were found to influence the kinetics of trace metal competition in the freshwater environment: (i) metal-to-ligand ratio, (ii) ionic potential (z2/r), and (iii) ligand field stabilization energy. The results illustrate the importance of considering the valence-shell electron configuration in predicting the kinetics of trace metal competition in the freshwater environment. The markedly slow dissociation kinetics of Ni(II) and Cu(II) species suggest that the usual equilibrium assumption for freshwaters may not be valid. This study has demonstrated the ability of the kinetic model to correctly predict the relative rates of trace metal reactions, indicating that the kinetic model provides a chemically significant description of the kinetic processes in natural waters.

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