Metal (Pb, Cd, and Zn) Binding to Diverse Organic Matter Samples and Implications for Speciation Modeling
Smith, D. Scott
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This study evaluated the influence of dissolved organic matter (DOM) properties on the speciation of Pb, Zn and Cd. Six DOM samples were categorized into autochthonous and allochthonous source based on their absorbance and fluorescence properties. The concentration of free metal ions (CM2+) measured
by titration using Absence of Gradient and Nernstian Equilibrium Stripping (AGNES) was compared with that predicted by Windermere Humic Aqueous Model (WHAM). At the same binding condition (pH, dissolved organic carbon, ionic strength, and total metal concentration) the allochthonous DOM showed a higher level of Pb binding than the autochthonous DOM (84- to 504-fold CPb2+ variation). This dependency, however, was less pronounced for Zn (12- to 74-fold CZn2+ variation) and least for Cd (2- to 14-fold CCd2+ variation). The WHAM performance was affected by source variation through the active DOM fraction (F). The commonly used F = 1.3 provided reliable CPb2+ for allochthonous DOMs and acceptable CCd2+ for all DOM, but significantly underpredicted CPb2+ and CZn2+ for autochthonous DOM. Adjusting F improved CM2+ predictions, but the optimum F values were metal-specific (e.g., 0.03 - 1.9 for Pb), as showed by linear correlations with specific optical indexes. The results indicate a potential to improve WHAM by incorporating rapid measurement of DOM optical properties for site-specific F.