Equilibrium and kinetics of zinc complexation with dissolved natural organic matter in fresh water

Zinc (Zn) complexation with NOM in natural fresh waters was investigated. Modeling of Zn-NOM complexation and predicting the zinc speciation in the natural water systems were achieved based on experimental results. Characterizations of Zn DOM complexation with representative DOM samples were conducted to evaluate the robustness of the current representation of these interactions in the WHAM (Windermere Humic Aqueous Model)-based models. Square wave anodic stripping voltammetry (SWASV) was employed to measure labile zinc activities and free zinc ion activities were computed from the fraction of labile zinc. Zn and proton binding by NOM samples from the different rivers and lakes showed similarity, while the NOM sample from the wastewater treatment discharge demonstrated different Zn and proton binding characteristics. WHAM VI predictions are consistent with the experimental curves for the NOM samples from rivers and lakes in the high Zn concentration range but underestimate free Zn activities at low Zn concentrations. Under the same conditions, the free Zn activities of the DE-WWOM titration were overestimated by WHAM VI. Fitting the Zn titration curves with FITEQL 4.0 showed that at least two distinctive classes of Zn binding sites are required to fit well the entire curves. A 2-site discrete site model was developed to describe Zn-NOM complexation. The model assumes that only more strongly acidic carboxylic groups account for Zn binding, whereas weaker acidic phenolic groups do not. Ionic strength has a significant effect on Zn-NOM complexation, increasing ionic strength resulted in weakened apparent Zn-NOM complexation. Ca competes with Zn for binding sites on NOM and high Ca concentrations reduced Zn-NOM complexation. Zn-NOM complexation reactions are very fast; equilibrium is achieved within a few minutes under the Zn and Ca concentrations studied.