Analysis of Trace Metal Humic Acid Interactions Using Counterion Condensation Theory

PORASSO, RODOLFO D.; JULIO C. BENEGAS; MARC A. G. T. VAN DEN HOOP; SERGIO PAOLETTI

ENVIRONMENTAL SCIENCE & TECHNOLOGY

AMER CHEMICAL SOC

Año: 2002 vol. 36 p. 3815 - 3821

0013-936X

Recent extensions of counterion condensation theory, originally developed for well-defined linear polyelectrolytes, enable us to analyze the interaction of trace metals with humic acid. In the present model, the heterogeneity of the macromolecule is taken into account as well as thechemical binding of the considered metal ions to the humic material. Experimentally, potentiometric titrations have been performed for humic acid in solution in the presence of different environmentally important (heavy) metals (Ca, Cd, Cu, Ni, and Pb) at various metal concentrationsby titrating with potassium hydroxide without additional salt. From proton release data obtained for the initial point in the titration, it was estimated that the interaction of the different metals with the humic acid in terms of binding strength increased in the order Ca < Cd ≈ Ni < Pb ≈ Cu. These results were confirmed by model analysis. Experimentally obtained apparent dissociation constants were in good agreement for the humic acid systems containing Ca, Cd, and Ni at concentrations ranging from 0 up to 0.75 × 10-3 mol L-1 and polymer dissociation degree fromabout 0.1 up to approximately 0.8. Also for the Cu/humic acid and Pb/humic acid systems, the agreement between experimental data and calculated data was satisfactory at the lowest metal concentrations over the complete titration curve. For elevated levels of Cu and Pb, the agreement between experimental data and theoretical calculationsbecomes less satisfactory at low degrees of dissociation of the humic acid. This distortion of the potentiometric curves is probably due to changes in the intrinsic pK of the functional groups due to metal binding. This complex process is not included in present polyelectrolytic models.