Influence of molar mass distribution on the complexation of heavy metals by humic material

The association of the heavy metals zinc(II) and cadmium(II) with humic acids of different molar mass distributions has been studied by differential pulse anodic stripping voltammetry. The various humic acid samples were obtained using an experimental procedure in which an untreated Fluka humic acid sample was fractionated by pH variation. Flow held-flow fractionation was used to characterize the humic acid samples with respect to the molar distributions, which appeared to be relatively wide for all samples. The voltammetric analysis of the binding properties of the various samples for zinc(II) and cadmium(II) yielded similar values for the mean stability (K) over bar of the metal complex which thus appears to be independent of the molar mass distribution of the sample. Differences in the shape of the voltammetric complexation curves were the result of differences in the mean diffusion coefficients of the humics involved.