Binding constants of lead by humic and fulvic acids studied by anodic stripping square wave voltammetry

A better understanding of metal ion binding to soil organic substances is of fundamental importance in geochemical modeling of environments. Fulvic acids (FA) and humic acids (HA) make up an important part of soil organic matter, and their binding capacity affects the fate of metal ions and plays an important role in their mobility. Binding constants of Pb(II) to HA and FA were evaluated by anodic stripping square wave voltammetry (ASSWV) where the binding reaction was performed at pH 4.5 in a medium of 0.020 mol l⁻¹ KNO₃. Results showed that ASSWV technique was well suited for the estimation of the binding capacity of a natural organic matter towards heavy metals. Based on the voltammetric titration curves, binding constants of Pb(II) complexes formed with HA and FA were 0.78 × 10⁶ and 0.15 × 10⁶ mol⁻¹ l, which indicated that complex of Pb²⁺ with HA was more stable than with FA. The average molecular weight of HA and FA prepared from soil samples were also found to be 1821 g mol⁻¹ and 805 g mol⁻¹, respectively.