Effect of Humic and Fulvic Acid Concentrations and Ionic Strength on Copper and Lead Binding

We investigated the influence of humic and fulvic acid concentration (in the range of 1-1000 mg/L) on the binding of the two trace metals Cu(II) and Pb(II). The ability of the non-ideal competitive adsorption (NICA)-Donnan model to correctly predict Cu and Pb binding at low humic or fulvic acid concentration and lower ionic strength (0.01 M NaNO3), based on model parameters obtained from experiments conducted at high humic or fulvic acid concentrations (approximately 1000 mg/L) and higher ionic strength (0.1 M NaNO3), was tested. The binding of Cu and Pb to humic and fulvic acid in 0.01 M NaNO3 was determined over wide ranges in proton and metal ion activities using three different methods: ligand exchange-adsorptive differential pulse cathodic stripping voltammetry at low humic or fulvic acid concentrations (1-3 mg/L), differential pulse anodic stripping voltammetry at intermediate humic or fulvic acid concentrations (10-20 mg/L), and ion-selective electrodes at high humic or fulvic acid concentrations (approximately 1000 mg/L). The results demonstrate that binding isotherms for Cu and Pb can be measured at low humic or fulvic acid concentration using suitable voltammetric techniques. The binding isotherms for Cu and Pb to humic and fulvic acid obtained at constant pH values in the range of pH 4-8 are shown to be independent of humic and fulvic acid concentration. The NICA-Donnan model, calibrated for Cu and Pb binding using data measured at high humic and fulvic acid concentrations and an ionic strength of 0.1 M, accurately predicts Cu and Pb binding at low humic and fulvic acid concentrations and lower ionic strength (0.01 M). We conclude that NICA-Donnan parameters obtained by fitting experimental data measured with ion-selective electrodes at high humic or fulvic acid concentrations can be used for geochemical modeling of soils and aquatic environments with much lower concentrations of humic or fulvic acids.