Cadmium, lead, and copper binding to humic acid and fulvic acid extracted from an ombrotrophic peat bog

We studied the binding of Pb(II), Cd(II), and Cu(II) to humic acid and fulvic acid extracted from an ombrotrophic peat bog, by metal titration experiments at pH 5.5 and 6.0, and 0.1 M KNO3 ionic strength. Free metal ion concentrations in solution were measured using ion selective electrodes. The amounts of base required to maintain constant pH conditions were used to calculate H+ / M2+ molar exchange ratios For all three metal ions the amount of cation bound to the humic acid was greater than the amount bound to the fulvic acid. The isotherms for Pb showed saturation, with a maximum adsorption capacity almost the same as that observed for Cu. In the case of Cd, the saturation was not reached in such a clear way. At the lowest concentrations of metal, the slope of the log–log plot of the cadmium binding isotherm was intermediate between that of copper (the lowest) and that of the lead (the highest). The proton to metal exchange ratios followed the order Cu2+ > Pb2+ > Cd2+. The binding data were quantitatively described with the NICA-Donnan model, which allowed characterization only of the carboxylic type binding sites. The values of the binding constants were higher for the humic acid than for the fulvic acid, and for Pb than for Cd. The values obtained for the exponent ni allowed analysis of the stoichiometry of the binding reactions.