Competitive adsorption of heavy metals in humic substances by a simple ligand model

Proton and metal ion binding to humic acids has been recognized as an important factor in controlling metal speciation and mobility in aqueous and soil environments. The binding and competition behavior of humics has not been fully described, however, due to the polydisperse mixtures of natural organic polyelectrolytes with different functional groups. The simplified discrete binding group type of model (Model A) was used in this study. Model A was applied to describe three single metals and their competitive binding on the humic acids. Model A considers only a single carboxyl group type and a single phenolic hydroxyl group type with a variable electrostatic interaction factor that is expressed as a polynomial equation. Metal binding experiments were performed using a batch-type dialysis equilibration method, and mass and charge balance expressions were used to calculate individual components of the system. The fit of Model A suggested that the major binding site for the three metal ions tested (Cd, Zn, and Cu) was a bidentate chelate of the carboxyl groups. Model A described two- and three-metal competition on the humic acids using the intrinsic stability constant for the single metal ion. The model used in this study adequately described the competition of two metals; however, it showed weaknesses when applied to more complex systems, especially when metals with substantially different binding characteristics were present. Model A presented a better prediction with adjusted Cu concentration (2:1:1 Cu/Cd/Zn).