Kinetics of Heavy Metal Dissociation from Natural Organic Matter: Roles of the Carboxylic and Phenolic Sites.

We developed a unifying model for the kinetics of heavy metal dissociation from natural organic matter (NOM) in this study. The kinetics model, integrated with the equilibrium model WHAM 7, specifically considered metal ion reactions with various NOM sites formed by the carboxylic and phenolic sites. The association and dissociation rate coefficients for metal reactions with various NOM sites were constrained by WHAM predicted equilibrium distribution coefficients at specific reaction conditions. We developed the relationship for the dissociation rate coefficients among different binding sites for each metal, which was internally constrained by the metal binding constants. The model had only one fitting parameter, the dissociation rate coefficient for the metal complexes formed with two weak carboxylic sites, and all other parameters were derived from WHAM 7. The kinetic data for metal dissociation from NOM were collected from the literature, and the model was able to reproduce most of relevant data analyzed. The bidentate complexes appeared to be the predominated species controlling metal dissociation under most environmental conditions. The model can help to predict the reactivity and bioavailability of heavy metals under the impact of multiple competing ligands including NOM.