Measurement of lead complexation by humic acids and humic acid analogues using competitive ligand exchange

In soil and water, metal complexation by humic substances has been demonstrated to have great importance in determining the bioavailability of many trace metals including lead. The binding of lead by humic acids has important implications for lead toxicity, and remediation. In this study we demonstrate the use of 4-(2-pyridylazo)-resorcinol (PAR) as a competitive ligand for the purposed of determining sorption isotherms for Pb²⁺ on humic acids and humic acid analogs. Equilibration rates at pH 8 were very rapid and the sorption isotherms measured were fit to the Langmuir isotherm equation and values for the stability constants (K_HA) and complexing site concentrations (L_t) are reported. At a PAR concentration of 0.24 mM and humic acid concentrations of 25-200 mg/L, the mass normalized log K_HA values for the humic substances ranged from 7.2 to 7.9, while the log L_t values ranged from -2.8 to -3.8. At lower PAR concentrations both K_HA and L_t tended to increase in magnitude indicating the role of the PAR concentration in establishing the measurement window for the interaction of Pb²⁺ with humic materials.