Speciation and solubility of Cu, Ni and Pb in contaminated soils

A long-term incubation experiment was established to assess the solid ⇅ solution equilibria of (Cu2+), (Pb2+) and (Ni2+) in soil pore water. The experiment comprised 23 soils spiked with 135, 300 and 75 mg kg−1 of Cu, Pb and Ni, respectively, added as nitrate salts in solution. Samples of soil pore water were extracted several times during the incubation period of 818 days and concentrations of Cu, Pb, Ni, dissolved organic carbon and major cations and anions were measured. Similar measurements were carried out on a smaller selection of historically contaminated soils to extend the range of data and assess compatibility of the measurements with the incubated soils. The chemical speciation program ‘WHAM VI’ was used to speciate metal ions in solution. A pH-dependent Freundlich equation was used to describe free ion activities, p(M2+), for Cu, Pb and Ni using total adsorbed metal, soil pH, soil organic carbon content and ionic strength as determining variables. For all three metals the greatest improvements in model fit were found when metal ions were assumed to be adsorbed exclusively on soil organic carbon rather than on the ‘whole soil’. Further improvements in the description of p(Cu2+) and p(Ni2+) were found when the ionic strength of the soil pore water was included within the model formulation. Residual standard deviations (–log10(M2+) units) for the best-fit models were 0.36, 0.53 and 0.29 for p(Cu2+), p(Pb2+) and p(Ni2+), respectively. The effects of progressive fixation of Pb, Cu and Ni on model parameterization, during the course of the experiment, were found to be small. Independent datasets from both published and unpublished sources were used to compare experimental protocols and validate the model for the determination of (M2+) in soil pore water.