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Record number 431313
Title Chemodynamics of metal complexation by natural soft colloids: Cu(II) binding by humic acid
Author(s) Town, R.M.; Duval, J.F.L.; Buffle, J.; Leeuwen, H.P. van
Source The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 116 (2012)25. - ISSN 1089-5639 - p. 6489 - 6496.
Department(s) Physical Chemistry and Colloid Science
Publication type Refereed Article in a scientific journal
Publication year 2012
Keyword(s) physicochemical heterogeneity - solvent exchange - copper-binding - simple ligands - fulvic-acids - ion-binding - relaxation - substances - kinetics - model
Abstract The chemodynamics of Cu(II) complexation by humic acid is interpreted in terms of recently developed theory for permeable charged nanoparticles. Two opposing electric effects are operational with respect to the overall rate of association, namely, (i) the conductive enhancement of the diffusion of Cu2+, expressed by a coefficient f(e nu), which accounts for the accelerating effect of the negative electrostatic field of the humic particle on the diffusive transport of metal ions toward it, and (ii) the ionic Boltzmann equilibration with the bulk solution, expressed by a factor f(B), which quantifies the extent to which Cu2+ ions accumulate in the negatively charged particle body. These effects are combined in the probability of outer-sphere metal site complex formation and the covalent binding of the metal ion by the complexing site (inner-sphere complex formation) as in the classical Eigen mechanism. Overall "experimental" rate constants for CuHA complex formation, k(a), are derived from measurements of the thermodynamic stability constant, K*, and the dissociation rate constant, k(d)(*), as a function of the degree of metal ion complexation, theta. The resulting k(a) values are found to be practically independent of theta. They are also compared to theoretical values; at an ionic strength of 0.1 mol dm(-3), the rate of diffusive supply of metal ions toward the particles is comparable to the rate of inner-sphere complex formation, indicating that both processes are significant for the observed overall rate. As the ionic strength decreases, the rate of diffusive supply becomes the predominant rate-limiting process, in contrast with the general assumption made for complexes with small ligands that inner-sphere dehydration is the rate-limiting step. The results presented herein also resolve the discrepancy between experimentally observed and predicted dissociation rate constants based on the above assumption.
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