Retention and distribution of three heavy metals in a carbonated soil: comparison between batch and unsaturated column studies

The uptake of 5×10−4 mol/l solutions of Cd, Pb and Zn by a fluvio glacial deposit with high carbonate content (227 mg/g) was studied in column and batch experiments. The heavy metals were added to soil samples and the retention examined using a speciation scheme with five fractions: exchangeable, acid-soluble, reducible, oxidizable and residual. In batch experiments, the three metal ions were completely bound by the soil, mostly to the acid-soluble fraction: 62% for Cd, 96% Pb and 86% Zn. The exchangeable fraction was of minor importance, with 27% Cd, 4% Pb and 11% Zn. The thermodynamic data suggested that lead and cadmium were mainly associated with carbonate ions while the zinc was found in two forms, zinc hydroxide and zinc carbonate. The influence of initial metal concentration was examined for Cd; an increase in metal concentration led weaker retention. The column experiments showed progressive saturation of the soil, with lead being retained better than cadmium or zinc. The measured retentions of all three metals were lower for columns than in batches. This difference was not explainable by thermodynamic data or by reaction kinetics alone. We believe it is mainly due to preferential flows in column experiments, favouring metal ion elution.